GENETICS - StudyTime NZ · Here at StudyTime, we’re pretty much GCs (good citizens), so to help...

NCEA | Walkthrough Guide

GENETICS

DISTANCE/TIME GRAPHS AND AVERAGE SPEED

dist.miles

timehours

12

34

5

8

6

4

2

Smaller Surface → Larger pressure

Level 1SCIENCE

AcidBase

WaterSalt

HCl + NaOHH2O + NaCl

TT TtTt

Tt

Tt

TT

t

t

tt

Introduction 3

Cells and DNA 4

What is a cell? 4Structure of DNA 7Genes, Alleles and Chromosomes 11Genotype and phenotype 14Quick Questions 16

Introducing Mutations 16

Effects of mutations 17What the heck is meiosis? 19How mutations affect sex and non-sex cells 20Quick Questions 21

Determining Phenotypes and Genotypes 22

Punnett squares 22Pedigree chart 24Quick Questions 26

Variation 26

Sexual and asexual reproduction 26Population variation 28Quick Questions 30

Key Terms 31

Level 1 Science | Genetics

Level 1 Science | Genetics | © Inspiration Education Limited 2018. All rights reserved.3


INTRODUCTIONThis standard is all about DNA.

What is DNA? You’ve heard your teacher say it a million times, you’ve seen it drawn on the whiteboard, and if you’re lucky, you’ve heard it mentioned on Jurassic Park.

But do you really know what it is?

Whatever you answered, you’re in luck! This standard is pretty much all about DNA. What DNA means, what it does - and how it results in you looking a whole lot different to the rest of your classmates.

If that sounds a little ambitious, don’t fear! In this study guide, we’re going to take you on a journey of what it means to live life as a double stranded DNA helix.

What will you learn in this walkthrough guide?

We’re going to start by zooming out a little, and introducing you to cells. Next, we’ll bring in DNA itself, including what its role is and what makes it truly important to humans. We’ll then talk about its structure, and how it coils up into special structures called chromosomes. From there, we’ll tell you how you can divide DNA up into genes, including the different options called alleles - and how these result in special descriptions called genotypes and phenotypes. We’ll finish off this section by discussing how mutation comes along and, just when you think you’ve got it all nailed, changes everything up.

Because things aren’t fun when you keep them to yourself, we’ll discuss how genes are passed on, and how we can work this out using punnett squares and pedigree charts. We’ll finish with a comparison of sexual and asexual reproduction, and how all of this comes together to create a diverse population.

Sound fun? Let’s get into it! By the time you close this book, you’ll know everything your level one exam will ever want you to know about DNA. So let’s get cracking!

A word on exam strategy.

Genetics is generally a pretty jargon intensive topic; you need to know some pretty big words! The key is to push through that phase of “OMG, are they even speaking English” and try and break down each word one-by-one.

Here at StudyTime, we’re pretty much GCs (good citizens), so to help you out, we’ve

4 Level 1 Science | Genetics | © Inspiration Education Limited 2018. All rights reserved.


CELLS AND DNAWhat is a Cell?

Every living thing is made up of cells!

The exciting thing about this standard is that it’s your very first look into the big field of biology. And the first thing you should know about biology, is that it’s the study of life.

Everything that lives (whether it’s you, your cat, or those herbs your Mum is insisting on trying to grow) is covered by the subject of biology. Fortunately (or unfortunately), this standard doesn’t ask you to delve into what helps Mum’s basil plant actually grow - but it does ask you to understand the one thing all of these living beings have in common. What do all these marvellous things have in common?

If you said that all of these living organisms are made up of cells, you’d be right.

But what is a cell?

Cells are the smallest unit of life.

If you took any living thing, and looked at it closely under a microscope, you would notice that it is made up of hundreds of thousands of tiny pieces, completely invisible to the naked eye. We call this a cell, and it’s the smallest chunk that a living thing can be broken down into.

made this guide in plain English as much as we can. We’ve also included a glossary for some of the key terms that you’ll need to master for your exam.

If learning key words first off scares you (or bores you), then focus on understanding the concepts the first time around, and then memorise the definitions so you can explain it the way the NCEA wants you to.

However, the language we use isn’t always something you can directly write in yourexam! When this is the case, we offer a more scientific definition or explanation (in ahandy blue box) underneath. These boxes are trickier to understand on your first readthrough, but contain language you are allowed to write in your exam.Look out forthem to make sure you stay on target!



All cells contain a nucleus

Each of these cells - whether they make up the stalk of a basil plant or the neck of a giraffe, are packed in tightly together, and have one important thing in common. They all contain a nucleus.

CellNucleus

For this standard, the nucleus is the most important part of the cell. The nucleus is the first technical word we’ll introduce to you in this standard - but just think of it as a big container inside the cell, holding all of the important DNA.

The nucleus is a dense double-membraned organelle found within cells, which contains the genetic material: the DNA.

DNA

All animals, humans and plants are made up of millions of tiny cells. Each of these cells have a container inside them called a nucleus. And inside the nucleus is our DNA. There we go, home sweet home! Now that we know where in the body we are looking, let’s talk about what DNA actually is.

DNA is a molecule which carries all of the genetic instructions for your cells

That’s the million dollar question! As you may have guessed, DNA is actually an acronym. The bad news is, it stands for Deoxyribonucleic Acid - which is a lot harder to remember. The good news is, you definitely don’t have to. So, unless you want to show off to your parents or that special someone (like the author of this guide did when they were 16), forget I said anything - and refer to it as DNA from now on.

All you really have to remember, is that DNA is a set of instructions for your cells. Coincidentally, that’s why every cell in your body has a nucleus with an identical copy



of the DNA. Because each cell in your body needs a copy of the instructions.

DNA contains all of the information needed to produce traits

But why do our cells need instructions? Good question. Have you ever wondered why our eyes look different to our nose? And on that track, why some people have blue eyes, whilst others’ are brown?

Hold that thought

Imagine if you gave a whole bunch of pre-schoolers the same set of lego bricks. You gave one of the students instructions to build a hospital, another to build a house, and a third to build one of those super cool pirate ships.

Each of the students began with the same materials - but the instructions meant that they could build different things with them.

The DNA is the instructions. Whilst DNA doesn’t build an eye for you, it gives the cell instructions on what an eye looks like - and specifically, what your particular eye looks like.

So, the first important role of DNA is to make who you are! It provides a whole bunch of instructions which help you construct an eye that is uniquely yours. It contains all of the information necessary to produce physical features - which for now, we’ll call traits.

A trait is a feature of an organism that can vary between organisms

The Makingof a Blue Eye

by DNA

DNA is responsible for passing information to the next generation

The other role of DNA is to pass these instructions down to your children. Ever wondered why your eyes look like your Mum’s? That’s because some of the instructions used to create her eye were given to you when you were born. That’s right, some of the DNA from your mum can be found in your cells, inside your nucleus. We’ll get into that later.



STOP AND CHECK:

Turn your book over and see if you can remember:

The two main roles of DNA.

Structure of DNA

DNA’s structure is super important to its function

By now, you should have started to realise that DNA is actually a very important molecule with two very important roles.

The other really important thing to understand about DNA is that its structure is very important to the way it functions (or works).

DNA has a double helix structure

The structure can be a bit of to get your head around, so let’s start big and work our way down.

You may have seen the structure of DNA before. It’s often described as a ‘double helix molecule’. But what does this mean?

I’ll break it down.

Double = There are two strandsHelix = The two strands are twisted together

Hold up, where did the strands come from? Remember how we said DNA is really just a whole bunch of instructions?

DNA is made up of specific sequences of letters

Well, it turns out these instructions aren’t written out like pages of a book - but instead, in long strands of information. These strands are actually really easy to understand - because they are written using only four letters: A, C, G and T.

That’s right, all of the information needed to make up everything that you are is written out using A, C, G and T. If you were to read this sequence yourself, it wouldn’t make any sense. But luckily, our bodies are super smart - and this sequence makes a lot of sense to them!



The most crucial thing about this sequence is the order of the letters. Special machines in our cells are able to read the order of these letters, and know exactly how to build everything it takes to form a fully functioning human being.

Well, it’s not actually a sequence of letters, but a sequence of ‘bases’

The next thing to remember, is that the ‘A’s, ‘C’s, ‘T’s and ‘G’s aren’t actually written as letters, but special molecules called ‘bases’.

Bases are tiny molecules, linked together in DNA to create a sequence of information that the cells can read.

So, the bases are the most important part of DNA. But, they’re not the only part. Each base is connected to two more things - a sugar molecule, and a phosphate molecule. The sugar and the phosphate are stolen from the food we eat, and connect to each base to give it strength, as well as help link it to other bases.

Each strand of DNA is made up of a big chain of nucleotides

When the base is nice and connected up, we call it a nucleotide.

Here’s what a nucleotide looks like:

Pbase: A, C, G or T

sugar group

phosphate group

A nucleotide contains a specific base, phospate group and sugar group bound together to form part of a DNA helix.

Now, we know already that the importance of DNA is linked to the sequence of bases, so we know that one nucleotide is useless on its own.

So, we know we need to start connecting our nucleotides together. That’s where the phosphate and sugar come in!



Whilst the base is worrying about providing the cell with instructions, the phosphate group from one nucleotide attaches somewhere to the sugar molecule of another nucleotide, and so on. So, DNA is just a pattern of connected nucleotides. Here’s what that looks like:

P

P

Now, we’re starting to get something that looks like a strand!

Now that we’ve got that sussed, let’s swing back to our double helix. The double helix is just two of these strands twisted together.

Bases are like exclusive best friends

We’ve gone over how nucleotides attach to each other, but how does this doubling up work? In order to get DNA strands to connect to each other, the bases must connect in a game we call ‘base pairing’.

Base pairing is exactly how it sounds. Each base ‘pairs up’ with a friend in another strand of DNA, until we have a long sequence of pairs.

Although it sounds nice and wholesome, there is one rule you have to follow when drawing two sides of DNA paired up.

The A and T bases can pair up and bond together, while the C and G bases can pair up and bond together. But A and C, A and G, T and C, or T and G don’t like to mix and mingle. They’re exclusive best friends. Pretty rude, right?

A T

C G

This exclusive base pairing is called complimentary base pairing.



Bases pairing up leads to double stranded madness

Once the nucleotides are all linked together through bases, and everyone is obeying the rules, we get something that looks like this:

P

P P

P

A T

C G

C G

Base Pairing

Whew! We’re getting there! Let’s summarise what we know about the DNA structure, with a nice NZ building metaphor.

Imagine a ladder: you’ve got two edges, or sides, connected by the multiple rungs, or steps, going upwards. DNA is a bit like a ladder, as there are two long strands of DNA which are connected together. The edges of the ladder are the sugar phosphate backbone. The rungs of the ladder are the complimentary base pairs.

It’s a funky, impractical ladder, though, as the double strands of DNA continuously twist around each other, giving that “helical” shape. Don’t worry about why it looks like this, it is partly because it is a very stable and compact structure.

paired bases(the “rungs”)

DNA backboneDNA backbone

(sugar and phosphate groups)

paired bases(the “rungs”)



So, DNA is a long molecule that is made up of nucleotides and each nucleotide has a sugar, a phosphate and a base. These nucleotides match up with other nucleotides that have a base that they like to hang out with, A with T and G with C.

The sequence of AT and GC base pairings are the instructions for life, we have a lot of DNA, and it is all stored in the nucleus of our cells (sometimes called the ‘brain’ of the cell), by reading the instructions a cell can make and do everything that it needs to.

These instructions are also passed down to the next generation, this is why we usually look similar to our parents and often have the same coloured eyes.

In the next section we are going to talk about how long sequences of DNA actually code or provide instructions for a partiular trait or characteristic

STOP AND CHECK:


The shape of DNA. What makes up a DNA molecule. Which bases are friends, and which ones want nothing to do with one another?

Try to explain it in your own words.

Genes, Alleles and Chromosomes

So we know a bit about DNA now!

We know that DNA is a series of instructions. We know that these instructions are written using a very specific sequence of base pairs. It turns out that we have a lot of DNA and that long sections of DNA that code for particular traits are called genes.

When a cell goes to make a copy of itself, it needs the instructions to be organised, just like how organising your room makes it easier to find things! The way a cell does this is by winding the DNA into tight structures called chromosomes, which contain all of our genes. Humans have 23 pairs of chromosomes, 46 in total. 23 of these chromosomes come from your mother and the other 23 come from your father.

A cell with 46 chromosomes (a pair of each chromosome) is called diploid.

So, chromosomes are made of tightly coiled DNA and sections of this DNA are called genes, which specify a particular trait.



Traits are physical attributes.

Traits can be anything from eye colour to hair colour, and so on. Each trait is written as a particular sequence of DNA bases. What this means is that the information to produce each trait is found on a particular region of DNA, and has a specific DNA sequence. Each chromosome gives the cell not just one set of instructions - but instructions to build a whole bunch of different components. Each of these components is called a trait.

We can break up a sequence of bases into genes.

The region of DNA that codes for a trait is called a gene. In the diagram below, you can see how the big sequence of bases can be broken down into areas based on what the instructions code for. For example, there is a gene for eye colour.

Chromosome

gene gene

A A C G G R C T T G G C C A C A G T T A C G

A gene is always found in the same place - and therefore on the same chromosome, in every individual. That’s why we can break a big sequence of bases into genes - because they’ll always be in the same place in the different cells.

Genes can take two (or sometimes more) different forms.

If everybody had the exact same sequence for each and every gene we’d all be the same person: you wouldn’t be able to tell the difference between your best friend, your worst enemy, or your grandma, which would make life a tad difficult.

Thankfully, not everyone’s sequences (or instructions) in the sequence of DNA are the same! A different DNA sequence produces variations in each gene, making slightly varied traits. These variant forms of genes are called “alleles”.

DNA

replicatedchromosome



A gene is a region of DNA which codes for a specific trait. An allele is a version of this gene that has a slightly different base pair sequence. E.g. brown or blue eyes.

Put another way, an allele is a ‘version’ of a particular gene. For example, we might have a gene that codes for hair colour and the alleles for the hair colour might be blonde, brunette or red hair.

If you zoomed in on one of my cells, right into the nucleus, into one of my chromosomes and found the allele for one of my traits, it is very likely that you’ll find a different order of bases than if you performed the same process in Mila Kunis.

This means Mila Kunis and I have different alleles.

(Which I’m not too happy about.)

=

STOP AND CHECK: Turn your book over and see if you can remember:

The definition of a gene. The relationship between genes and alleles; what makes them different?


allele 1

samechromosome

di�erent base sequence

T

A

G

A

C

A

A

A

A

A

A

A

C

A

C

A

A

A

T

A

allele 2T

A

G

A

C

A

T

A

G

C

G

C

C

A

C

A

A

A

T

A



Genotype and Phenotype

Genotypes explain why you and your Mum have the same (or different) eye colour

Remember a while ago I told you that the the reason why your eyes look like your Mum’s is because some of her DNA was passed on to you?

Well, I wasn’t lying! In fact, the reason for this is that your Mum passed one of her alleles (or series of unique DNA instructions) dictating what her eyes look like down to you.

However, it’s not quite that simple. Because it takes two parents to form a new individual, every new organism ends up inheriting two copies of each chromosome - and therefore two copies of every allele.

Sometimes, these two alleles aren’t the same. For example, you might have the allele that gives you detached earlobes or the allele that gives you attached earlobes.

The combination of alleles you have is called the “genotype”. A genotype simply describes all of the alleles present in an organism for each trait.

Genotype = Dd

Detached earlobe

D =

Attached earlobe

d =

Some alleles are equal, but some are more equal than others

Now, if you get the allele for detached earlobes and the allele for attached earlobes, you usually don’t have one ear lobe that’s detached and the other which is attached. Instead, you have two detached ones. The world inside the cell is cut-throat with not every allele having equal rights.

Therefore, when you inherit two alleles, one of them is expressed, and the other one is silenced. The one that ends up being expressed is called the dominant allele, while we call the other the recessive allele.



Important things to know when talking about genotypes

When writing answers in your exam, remember these things:

The dominant allele is always expressed.The recessive allele is always ‘masked’ by a dominant allele - so is only expressed when the genotype contains two recessive alleles.

In practice, genotypes are drawn using a letter to represent each allele.

By convention, dominant alleles are represented by an uppercase letter while recessive alleles are represented by the same letter in its lowercase form.

Going back to the idea of a genotype, there are basically 3 different genotypes possible:

1. Two dominant alleles – this is called homozygous dominant. 2. One dominant allele and one recessive allele – this is called heterozygous. 3. Two recessive alleles – this is called homozygous recessive.

The genotype for each trait tells the cell what proteins to make, and how to make a person. What the trait comes out looking like – blue eyes, red hair, attached earlobes, a weird bendy thumb, curly hair, and so on – is called the phenotype. Another way to think about the phenotype is as the ‘result’ of the genotype.

Genotype Phenotype

Genotype Protein Phenotype

Genotype PhenotypeBb

A good way to remember what the phenotype means, is to think “P” for “physical”; the physical expression of the genotype.

How genotypes link to phenotypes

If somebody is homozygous dominant they’ll have the dominant phenotype, and if they’re homozygous recessive, with no dominant alleles around to bully them, they’ll have the recessive phenotype. With the heterozygous genotype, the dominant allele masks the recessive allele and so you end up with the dominant phenotype as well.



STOP AND CHECK:


The difference between genotype and phenotype. The difference between dominant and recessive alleles.


Quick Questions

What is the function of DNA? Have a go at explaining the structure of DNA: both its shape and what it’s made of. If you’re feeling artistically-inclined, try drawing its structure.

What are genes and alleles? What is the difference between them? What determines the genotype and phenotypes for a particular trait or characteristic? Don’t forget to define these two terms!

INTRODUCING MUTATIONSNow that we’ve defined an allele, it’s time to find out what happens when things don’t go exactly to plan. Mutations form a crucial part of this topic, as well as a big part in the differences between organisms we view all the way through biology.

Mutations are permanent changes to the DNA base sequence

Picture yourself typing the instructions for those pre-schoolers that we gave the lego to at the start of this guide.

You’re typing up the recipe to your favourite piece of the house, but in all the excitement you begin to type faster and faster, so fast that you don’t notice all the typos you’ve made. Who has time to proofread anyway? People buy your book, but because of all the typos, the student building the house ends up creating a door that is twice as big as the frame.

Although cells and DNA are pretty incredible, they definitely aren’t perfect. Sometimes more DNA or cells are required by the organism, so the DNA is copied - or ‘replicated’. During this process, things can go wrong.

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Mutations are the typos made in the DNA sequence.

Mutations are random, irreversible changes to the DNA base sequence.

normal DNA

wrong base

G

A

C

A

C

A

T

A

A

A

T

A

C

A

mutated DNAG

A

C

A

C

A

T

G

A

T

T

A

C

G

STOP AND CHECK:


What a mutation is.


Effects of Mutations

When you make a typo in real life, there are a few things that could happen:

Most of the time it doesn’t really matter because you can still make out the word.If you’re unlucky the typo really messes things up and the word doesn’t make any sense whatsoever. Very occasionally, by either forgetting a letter, adding an extra one in, or putting in the wrong one, you accidentally create a completely different word.

This is a lot like mutations. Think back to the previous section: the specific DNA base sequence determines the allele for a particular trait, and ultimately affects what the organism looks or acts like.

Mutations are small changes with big consequences

Remember that an allele is the unique series of bases in our DNA which decides a version of what a trait is going to look like.

So, what happens when we change the sequence?

Well, we certainly can’t claim to have the same allele. When this change happens, we



actually end up with something completely different.

In fact, mutations are the way that new alleles are made. A lot of the time, these new alleles aren’t helpful - and end up giving us a door that doesn’t fit the frame. But sometimes, they end up creating a new allele that is helpful and sometimes the new allele makes no difference at all.

In fact, mutations are our main source of new DNA.

How alleles are passed on

Don’t worry, we’re not talking about ‘the birds and the bees’ here. Well we kind of are, but in a biological sort of way...

So far, we’ve mentioned that DNA is passed on, but I’ve been a bit elusive so far as to how this actually happens.

So, if you’re feeling a bit in the dark, don’t worry, we’re about to delve into what it really takes to pass alleles on to future generations.

Cells are made up of 46 chromosomes - half from Mum and half from Dad

We’ll start off by reminding you what a cell is. Remember that we have been referring to cells as the things that make up organisms - that, as well as fulfilling other important roles, cells hold all of the DNA needed to create unique individuals.

Remember that we have also talked about the fact that our cells contain 46 chromosomes - 23 from one parent, and 23 from the other. These cells are called diploid.

Well, I am about to blow your minds wide open.

In turns out there’s a sneaky, special type of cell that have been purposefully left out until now. Gametes are special types of cells, and, because of this, are commonly known as sex cells.

You may have heard of sex cells referred to their even more common names - sperm cells in the male, and egg cells in the female.

Gametes are pretty similar to other cells, however, they have one key difference. They only have 23 chromosomes. Put another way, they only have one set of alleles.

23 23



So why would this be?

To think about this, I’m going to tell you about a process called fertilisation.

How babies are made, the biology way

When creating a little human, sperm needs to fertilise an egg to produce a special structure called a zygote. This zygote divides itself over and over again to produce an embryo, fetus and then a baby. Because you need one sperm and one egg, these gametes need to have half the number of chromosomes, so that when they combine the zygote will have a normal number.

Sperm egg zygote

division division division

So, gametes need to have half the number of chromosomes as a regular body cell. But, how does this happen?

It happens through a process called meiosis.

STOP AND CHECK:


What a gamete is, and why they only have 23 chromosomes.


What the Heck is Meiosis?

In meiosis, a regular body cell (with 46 chromosomes) gets a signal telling it that gametes are needed.

It then responds by duplicating everything inside of itself to give itself a grand total of 92 chromosomes.

This may seem a little counter-productive, but bear with me. Because, it then splits itself perfectly in two to create two ‘daughter cells’, each identical to the original cell. This process is a great way of creating an identical cell, however, there is still too many for the cell to qualify as a gamete.



In order to create two gametes, each cell splits itself in half a second time to create a grand total of four daughter cells, each with 23 chromosomes in them. When a cell only has one set of chromosomes, they are called haploid.

OVARY(female)

TESTES(male)

2 sets ofChromosomes

1 set ofChromosomes

meiosisEGG

SPERMmeiosis

:

:

Once a sperm and an egg come together, a zygote forms

Once we have gametes, formed through meiosis, we are able to join them up to form a zygote.

A zygote, by definition is made up of 23 chromosomes from one parent (from one gamete), and 23 from the other (the other gamete). How many is that in total? 46! Therefore, our zygote forms the very first cell of a new human being! This zygote also has a new combination of alleles, because the alleles that come from the parent cells are random, this is known as independent assortment.

This zygote goes on to replicate itself over and over again, creating cells that are made up of half of the alleles from one parent, and half from the other.

STOP AND CHECK:


How many times the cell divides in meiosis, and why this is necessary to create gametes?

How meiosis results in cells with a new mix of alleles


How mutations affect sex and non-sex cells

Now that we’re comfortable with gametes, let’s have a think about how mutations are passed on.



Sex cells are gametic cells, non-sex cells are somatic cells

I’ve told you that you can refer to sex cells as gametes, but what about everything else? Feel free to refer to all other cells from now on as somatic cells.

Now, it’s not just cells that get their own classification system.

Mutations can either be somatic, occurring in somatic/body cells, or gametic, occurring in gametic/sex cells.

Mutations in somatic cells aren’t passed onto the next generations, but mutations in gametic cells are.

When a mutation occurs in a somatic cell it will only affect cells in that area. This means that they only affect the individual in which the mutation occurred and can’t be passed on. A somatic cell is never used to pass DNA onto a baby, so therefore, any changes to their DNA do not end up in the offspring and are not inherited.

Gametic mutations occur in the cells that are about to become the gametes: the egg or the sperm. Since the gametes are involved in reproduction and create the offspring, gametic mutations will be passed down to the offspring.

Since the offspring begins as a single cell, ALL cells of the offspring will have these. Therefore, gametic mutations will be present in every single somatic cell in the offspring as well.

STOP AND CHECK:


Why mutations on gametic and somatic cells affect the organism and its offspring differently.


Quick Questions

What is a mutation and how does it relate to the DNA sequence of an individual?

Why is it important that gametic and somatic cells are different? What happens during meiosis?

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DETERMINING PHENOTYPES AND GENOTYPESNow that we’ve got all of this information - and a pretty solid understanding of some core biology principles, it’s time we learnt how to present them.

In this section, we talk about two pretty important biological devices: Punnett squares and Pedigree charts. Here’s what we’re going to cover:

Figure out how to complete one of these “Punnett Squares” and then find a way to determine genotype and phenotype ratios. Interpret a pedigree chart and figure out who’s got the ‘bad genes’.

Punnett Squares

We’ve covered the fact that each individual has 2 alleles for every trait or characteristic. However during reproduction, the mother and father each pass on just 1 of these alleles to the offspring. This can often result in a number of different genotypes - and therefore phenotypes possible in the offspring - depending on which ones they are lucky enough to inherit.

Punnett squares are used to work out the probability of a characteristic being passed on to the offspring.

An empty punnett square looks a little like an empty noughts and crosses board, and is shown in the example below.

Mastering punnett squares in five easy steps

Here are some steps to filling one out!

1. Write out the genotype for each parent using an upper or lower case letter to represent each allele they can pass on.

2. In the upper margin, write each of the alleles the mother can pass on (the two alleles the mother has for the trait).

3. In the left-hand margin, write each of the alleles the father can pass on (the two alleles the father has for the trait).

4. Work through each of the inner squares, combining the alleles from above and to the left to form a genotype in each square.

5. Count the number of homozygous dominant, heterozygous and homozygous recessive genotypes and write out the results. This is called the genotype ratio.



1 2 3 4 5

Mother = BbFather = bb 2 : 2B b B b

b

b

B b

b

b

Bb

Bb

bb

bb

Calculating phenotypic ratios

When it comes to figuring out the phenotype ratio in the offspring, the offspring will either display the dominant trait or the recessive trait.

Remember, if the genotype is homozygous dominant or heterozygous, then the dominant trait will be expressed. Only when the genotype is homozygous recessive is the recessive trait expressed.

The phenotype ratio can be calculated by counting the number of offspring inside the punnett square that would physically express each trait.

HOMOZYGOUS RECESSIVE x HETEROZYGOUS(aa) (Aa)

a a

Aa Aa

aa

A

a aao�spring genotype

parent genotype

parent genotype

Now, you may be thinking: “I know families with 5 girls and 0 boys, or 4 boys and just 1 girl; this doesn’t match those Punnett Squares”.

Genotype and phenotype ratios are just probabilities and can’t 100% predict the genotype or phenotype of the next offspring

It’s really important to remember that the Punnett Squares and the genotype/phenotype ratios you calculate using them are just probabilities. They tell us the likelihood of seeing those genotypes or phenotypes in the offspring. When you produce a small number of offspring it’s possible that the true genotype/phenotype ratios won’t exactly match the theoretical ones.

As the number of offspring increases the more likely these two values will match. Think of it this way: We all know there is a 50-50 chance of a coin landing heads or tails. If you flip a coin three times and get three heads in a row, that wouldn’t be too



unusual though, right? This is the same for sex - when you only have a few offspring, sometimes they all just end up the same sex by chance. But if you flipped a coin 100 times, it would be pretty rare to get heads 100 times in a row. It is the same in biology. Organisms that have lots and lots of offspring at once like fish, are much more likely to have closer to 50% girls and 50% boy offspring.

STOP AND CHECK:

Turn over your book and see if you can determine the genotype and phenotype ratio for the following examples:

‘B’ is the dominant allele for brown eyes and ‘b’ is the recessive allele for blue eyes. Calculate the genotype and phenotype ratio for a BB x Bb cross.

‘H’ is the dominant allele for brown hair and ‘h’ is the recessive allele for blonde hair. Calculate the genotype and phenotype ratio for a Hh x hh cross.

‘D’ is the dominant allele for tall pea plants and ‘d’ is the recessive allele for small pea plants. Calculate the genotype and phenotype ratio for a Dd x Dd cross.

Then have a go and see if you can explain why the real phenotype ratio doesn’t always match the one calculated using Punnett Squares.

Pedigree Chart

Pedigree charts are a bit like a genetic family history. Everybody’s on it: your mum and dad, any siblings you have, your grandma and grandad, any cousins, and so on.

Basically, pedigree charts just show us the phenotypes of each individual for a particular trait, such as hair colour, eye colour, or even whether or not they have a certain disease. These can be used to guess the genotype of particular individuals.

It is important to remember that pedigree charts show us the results for one particular trait. Therefore, if we wanted to look at hair colour and height in a family, we would need to draw two pedigree charts.

The basics of a pedigree chart.

First let’s cover a few conventions to be aware of:

Females are represented by circles, while males are represented by squares. Horizontal lines between shapes represent parents (two people that sexual reproduction has occurred between).If children are produced they are shown branching off this horizontal line below the parents. Usually the circle or square is shaded if the individual has the characteristic/trait phenotype you are looking for. We call this being ‘affected’.



Female

MaleAffected

Important things to remember about pedigree charts.

When you look at a pedigree chart there are a number of things to think about: If an individual has the recessive trait they must be homozygous recessive. (As a dominant allele would mask any recessive alleles if it was present). A homozygous recessive child can only be produced if both parents are heterozygous or homozygous recessive. (As they both need to have a recessive allele to pass on) - draw a punnet square if you don’t believe us! If an individual has the dominant trait they are either homozygous dominant or heterozygous. If an individual with the dominant trait produces a child with the recessive trait, the parent must be heterozygous in order to pass on the recessive allele.

Let’s have a go at an example.

Say you were given a pedigree chart that looks like this, and told that the affected gene was the dominant one and asked to give the genotype of the circled parent:

= unaffected FEMALE= affected FEMALE

= unaffected MALE

= affected MALE

1. The first thing to do is check the phenotype of the individual. From the diagram, we can see that the individual has the dominant phenotype. This means that they must possess at least one dominant allele.

2. Unfortunately for us, because they carry the dominant phenotype, we are unable to tell whether the individual is homozygous dominant or heterozygous for the gene.



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3. To solve this, we can look at its offspring. The parent has offspring with the recessive phenotype, therefore we know that the offspring must have the homozygous recessive genotype.

4. In order to have the homozygous recessive genotype, the offspring must have inherited a recessive allele from each parent. This means that our unknown individual must possess at least one recessive allele in order to be able to pass it on.

5. Because we know the circeld parent must possess at least one of each allele, we are able to deduce that the unknown individual has the heterozygous genotype

STOP AND CHECK:


The conventions used in a pedigree chart. The 4 concepts that can help you determine the genotypes of most individuals in a pedigree chart.


Quick Questions Explain how Punnett Squares can be used to determine the genotype and phenotype ratios in the offspring of two parents.

Explain how Pedigree charts can be used to determine the genotype of individuals.

VARIATIONWe’ve been keeping things pretty small lately - DNA and cells are definitely difficult to see, and therefore can take a while to wrap your head around.

To keep things interesting, we’re going to flip the script for a bit, and look at things on a bigger scale. We keep on talking about how genotypes and alleles create differences. Well, it’s time for us to prove it.

Have a look at the people around you right now. Chances are, there are some key differences in your appearances, and the way you are acting. This is called genetic variation, and describes the differences caused by different sets of DNA in individuals.



Sexual and Asexual Reproduction

The basics of reproduction

This process of two gametes fusing together is a bigger part of a process called reproduction.

Now, we’ll leave the bigger part of this discussion up to your parents. But for now, just remember that in biology, reproduction is the process of creating new individuals.

The reproduction which occurs between humans is a special kind of reproduction called sexual reproduction.

Sexual reproduction is special because it involves two parents - and therefore, due to the process of two gametes joining up to form one zygote, results in offspring which will always have a different combination of alleles from its parents.

Some living things, such as bacteria, do things a little differently. Bacteria are shy, they like to keep to themselves, and so they reproduce without going out and mingling with others.

Bacteria carry out asexual reproduction, which means they only reproduce with themselves. Because only one set of DNA is being directly copied, asexual reproduction produces offspring that are all identical to each other and identical to the parent.

Asexual reproduction

You can probably guess that there will be very little genetic variation in these populations. Variation only comes from mutations in asexual populations.

STOP AND CHECK:


The difference between sexual and asexual reproduction. The sources of genetic variation for sexual reproduction and for asexual reproduction.




Population Variation

If you take a population, like the population of New Zealand or even just the population of your high school class, you’ll see heaps of variation. Some people will be taller than others; there will be blue eyes and brown eyes; there will be differences in skin colour and hair types; there will even be differences in traits like resting heart rate.

A population is a community of organisms that can interbreed with each other.

In biology, survival is key

Nature is all about the survival of the population. If you ask any animal, plant or bacteria, the meaning of life is to simply reproduce and make sure that the population continues to exist.

Populations of animals, plants, and bacteria all have a lot to worry about: will we survive the weather, will there be enough food, is anybody planning on eating us, and will we be able to find a nest, or shelter, in the current land availability crisis?

How environment affects survival

Unless humans do something stupid, these things are mostly predictable and only slow changes occur. Occasionally, there is a dramatic change in the organism’s environment, such as a natural disaster or a particularly harsh winter. These changes can wipe out a whole bunch of them. For example, a freezing cold winter one year could kill off a population of hairless critters, or if a certain plant suddenly disappears from the face of the planet and a population of grazing herbivores are too fussy to eat anything else.

Possible changes in the environment could include disease, famine, drought, extreme weather or deforestation.

Why genetic variation is important

This is where the genetic variation among a population becomes important. A population with low genetic variation is less likely to survive a sudden change in environment, while a population with high genetic variation is much more likely to survive. Remember, nature couldn’t care less about the individuals; it’s about the population as a whole. To understand this, let’s use an example:

Imagine a zombie outbreak occurs across the globe. If there was no genetic variation across all humans, either everybody will be immune or everybody will be non-immune to the zombie virus. If there is no immunity, then everybody becomes a zombie and



eventually dies: the human race ceases to exist. Thankfully, the human race has lots and lots of genetic variation, which means that there is a good chance that some people would be immune to the zombie virus. As long as these people are fast enough and not too tasty to zombies, then the human race will be able to continue existing.

No variation:

Variation:

This idea of certain individuals being more resistant to an environmental change and surviving long enough to reproduce and pass on their alleles is the basis of natural selection. This is often called “survival of the fittest”.

Basically, as these ‘fit’ individuals have the best survival rates, they are likely to produce the most offspring (which are likely to inherit these traits as the alleles are passed down). Therefore, the following generations will have more of this trait. This is evolution driven by a natural selection for certain traits.

Little Genetic Diversity

High Genetic Diversity

Thinking back to reproduction.

Just when you thought it was over! Let’s have one more think back to the previous section - where we talked about sexual and asexual reproduction.



Apart from some pretty clear logistics, remember that the main difference between asexual and sexual reproduction is that asexual reproduction results in offspring identical to the parent - whilst sexual reproduction results in offspring with a mix of alleles from the mother and father.

Because of this mix, each offspring ends up being genetically different to the parents, as well as the other offspring.

Now that we’ve geared you up with some new terminology, let’s call those differences variation!

So, linking that back to population genetics, we can see that sexual reproduction actually gives a massive advantage to a species - as it is a key part in ensuring there is variation when the environment changes!

So, why do bacteria still perform asexual reproduction? It turns out asexual reproduction has its perks too. Think about how much effort you put into finding a match. You have to agonise over text messages, think of good locations to meet up, and make sure you are always acting interested, but never too keen.

Asexual reproduction removes all need for this! Because they can reproduce without a mate, asexual reproduction aids bacteria in reproducing quickly and safely. A trade off they are willing to make to sacrifice a bit of genetic diversity!

STOP AND CHECK:


Why genetic variation is important in a population, using an example. What natural selection is.


Quick Questions

How does meiosis increase genetic variation? What are sexual and asexual reproduction? How do they differ? How does sexual reproduction create genetic diversity? Discuss the importance of genetic variation in a population using a made-up example.

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KEY TERMSAllele:

An alternative form of a gene, producing an alternative form of a particular characteristic or trait.

Chromosome: One really long DNA molecule found inside the nucleus of the cell.

DNA: A long molecule composed of nucleotides which carry the genetic information.

Gamete: Either the sperm or egg; cells which are involved in sexual reproduction.

Gene: A section of DNA, found at a particular chromosome location (locus), which encodes for a particular characteristic or trait.

Genotype: The combination of two alleles for a particular characteristic/trait or at a particular locus.

Mutation: A random, permanent and irreversible change to the DNA base sequence.

Natural Selection: “Survival of the Fittest”; the process where organisms better adapted to the environment are more likely to survive, reproduce and pass down their alleles to future generations.

Nucleotide: The repeating units of DNA composed of a sugar molecule, one of four bases (A, C, G or T), and a phosphate group.

Phenotype: The physical expression of the genotype. In other words, the physical appearance of the characteristic/trait.

Somatic Cells: The body cells, or any cell that is not a gamete.

GENETICS - StudyTime NZ · Here at StudyTime, we’re pretty much GCs (good citizens), so to help...

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