Multicellular Organisms

Key Area 2.4

Variation and Inheritance

Variation

We are learning to

•Describe what variation is and categorise variation as continuous or discrete.

I can

• Define the term „continuous variation‟ with examples

• Define the term „discrete variation‟ with examples

• Define the term „allele‟

Variation

No two members of the same species are exactly alike.

Variation

• Differences that occur between individual members of a species are called variation.

• The combing of genes during sexual reproduction contributes to variation in a species.

Types of variation

• There are 2 types of variation:

– 1: Continuous variation (polygenic

inheritance)

– 2: Discrete variation (single gene inheritance)

Variation task

seed shape in peas (round or wrinkled)

body mass in humans

milk yield in cattle

wing length in fruit flies

length of index finger in humans

ear lobe type in humans (attached or unattached)

eye colour in fruit flies (red or white)

body length in trout

tongue rolling ability in humans

resting heart rate in humans

From the list below, categorise the characteristics into discrete or continuous variation.

Discrete versus continuous variation

Discrete variation Continuous variation

seed shape in peas body mass in humans

wing length in fruit flies milk yield in cattle

ear lobe type in humans length of index finger in humans

eye colour in fruit flies body length in trout

tongue rolling ability in humans

resting heart rate in humans

1. CONTINUOUS • No distinct groups

e.g. height, pulse, weight

• Usually involves numbers

• Usually illustrated by a line graph

2. DISCRETE • 2 or more distinct groups

e.g. eye colour, tongue rolling, ear lobes

• Illustrated by a bar graph

Polygenic Inheritance

• Most characteristics shown by plants and animals show polygenic inheritance.

• This means they are controlled by many genes that act together.

• Polygenic characteristics show continuous variation.

• Examples of this include height, weight, skin colour and hand span in humans.

Starter Question : A pupil took thumb prints from 30 pupils in her class. She found that there was four fingerprint types:

14 pupils had a loop pattern and only 3 pupils an arch pattern. She counted 7 pupils who had a tented arch pattern and the rest had a whorl pattern. a) What type of variation do fingerprints illustrate? b) How would you illustrate this information? (as a bar

chart or line graph)

Arch Loop Whorl Tented arch

Inheritance

We are learning

•The vocabulary that we will need to describe how genes are inherited.

I can

Define the terms allele, homozygous, heterozygous, genotype, phenotype, dominant and recessive.

Family resemblance

Why do members of the same family often look similar?

Genetic material When cells divide, it is essential that genes are copied into the new cells.

Genes are the basic unit of inheritance, and are responsible for the characteristics of an organism.

Genes are located on chromosomes, each of which is made from a very long, tightly coiled molecule of DNA. You inherit chromosomes from your parents. How many from each?

Different versions of genes Each different version of a gene is called an allele.

allele for

brown

eyes

allele for

blue

eyes

Homozygous alleles If the alleles for a characteristic in a homologous pair are the same, the organism is said to be homozygous for that characteristic.

allele for

brown eyes

allele for

brown eyes

allele for

blue eyes

allele for

blue eyes

Heterozygous alleles

The characteristic expressed by heterozygous alleles will depend on which allele is dominant and which allele is recessive.

What colour eyes will this heterozygous pair of alleles produce?

allele for

brown eyes

allele for

blue eyes

?

Dominant or recessive?

Dominant alleles are always expressed in a cell‟s phenotype. Only one copy of the dominant allele needs to be inherited in order for it to be expressed. Dominant alleles (e.g. brown eyes) are represented by an upper case letter (e.g. „B‟).

The phenotype for a particular characteristic depends on which allele is dominant and which allele is recessive.

Recessive alleles are only expressed in a cell‟s phenotype if two copies of it are present. If only one copy is present, its effect is „masked‟ by the dominant allele. Recessive alleles (e.g. blue eyes) are represented by a lower case letter (e.g. „b‟).

What eye colour? The allele for brown eyes is dominant over the allele for blue eyes.

The individual will have brown eyes, because the allele for brown eyes masks the allele for blue eyes.

allele for

brown eyes

allele for

blue eyes

So, what colour will the eyes be of an individual who is heterozygous for eye colour?

Key words Phenotype

Genotype

Allele

Dominant

Recessive

Homozygous

Heterozygous

•This allele determines the development of a

characteristic

•The symbols used to identify genetic information (e.g

Bb)

•This allele will determine a characteristic only if

there are no dominant ones

•This word refers to a pair of chromosomes being

made of two different alleles of a gene

•The characteristics a genotype (e.g. Bb) produce

•This word refers to a pair of chromosomes being

made of two of the same alleles of a gene

•Different forms of the same gene

Key words Phenotype

Genotype

Allele

Dominant

Recessive

Homozygous

Heterozygous

•This allele determines the development of a

characteristic

•The symbols used to identify genetic information (e.g

Bb)

•This allele will determine a characteristic only if

there are no dominant ones

•This word refers to a pair of chromosomes being

made of two different alleles of a gene

•The characteristics a genotype (e.g. Bb) produce

•This word refers to a pair of chromosomes being

made of two of the same alleles of a gene

•Different forms of the same gene

Starter

Starter

Polygenic

Monohybrid crosses

We are learning to

• Use monohybrid crosses to predict the characteristics inherited and in what expected ratios.

I can

• Use monohybrid crosses or genetic diagrams to work out inheritance in a number of examples.

Gregor Mendel

• In the early 19th century and Austrian monk called Gregor Mendel studied inheritance in garden pea plants.

Mendel‟s experiments Over seven years, Mendel experimented on more than 28,000 pea plants! Why were his experiments so successful?

Pea plants grow quickly.

Pea plants are available in pure-breeding (homozygous) strains.

Many pea plant characteristics show discrete variation; they are either one form or another, with no intermediates. This means that their phenotypes are easily distinguishable.

Monohybrid crosses

There are two alleles controlling pea shape. This means there are three possible genotypes that the F2 generation of plants could inherit, leading to two possible phenotypes.

SS

ss

Ss

smooth

wrinkly

smooth

Genotype

heterozygous

homozygous dominant

homozygous recessive

Phenotype

The type of experiment that Mendel carried out, investigating just a single characteristic, is called a monohybrid cross.

The likelihood of a trait being produced during a monohybrid cross can be mapped out using a Punnett Square.

25/04/2019

Try some yourself! 1) In mice, white fur is dominant (W). What type of offspring would you expect from a cross between a heterozygous individual and one with grey fur? Explain your answer with a genetic diagram (Use the letter W for your genotypes)

2) A homozygous long-tailed cat is crossed with a homozygous short-tailed cat and produces a litter of 9 long-tailed kittens. Show the probable offspring which would be produced if two of these kittens were mated and describe the characteristics of the offspring (hint: work out the kitten‟s genotype first).Use the letter T for your genotypes.

Task

• Sponge bob genetics

• Beaker babies task

Predicted Vs Reality

• When we use punnet squares to estimate the ratio of phenotypes and genotypes, the number we calculate is an expected ratio.

• In reality the outcome can be quite different as fertilisation is a random process.

Starter

Starter

Heterozygous

Recessive

Monohybrid

Genetics: Family trees

We are learning to

• Describe patterns of inheritance using family trees

I can

• interpret family trees and use them to identify the genotypes of parents and offspring.

Human inheritance • Unlike pea plants, humans do not breed to suit

geneticists.

• They also produce too few offspring to allow reliable conclusions to be drawn about phenotypic ratios.

• However, the laws of genetics still apply and particular traits can be traced through several generations of a family by constructing a family tree.

• A family tree is a diagram that shows how members of human family are related to one another.

Family trees

• Males and females are given different symbols. eg. males may be represented by a square and females by a circle.

• Symbols are then shaded or left unshaded depending on the phenotype of the individual.

• Individuals that produce offspring together are joined with a horizontal line.

• A vertical line joins offspring with their parents.

• It is possible to work out the genotypes of certain individuals in a family tree by analysing those of their parents and siblings.

Say hello……to the Reebops

Making Reebops

• In this activity you will make “Reebops” to help you understand how the inheritance of different alleles leads to variation in offspring

• Reebops are imaginary animals, made from play dough, pom poms and pipe cleaners

• They have 16 chromosomes (in 8 pairs) in their body cells

• Follow the instructions to find out

what happens when these

organisms breed!

Characteristic Genotype / phenotype code

antennae AA = 2 antennae Aa = 2 antennae aa = no antennae

body segments BB = 3 body segments

Bb = 3 body segments

bb = 2 body segments

tail TT = curly tail Tt = curly tail tt = straight tail

nose NN = red nose Nn = red nose nn = yellow nose

legs LL = green legs Ll = green legs ll = yellow legs

sex XX = female XY = male

eyes EE = 2 eyes Ee = 2 eyes ee = one eye

humps HH = 1 hump Hh = 1 hump hh = 3 humps

Genotype decoder key

A

a

B

b

E

e H

h

N

n

T

t

L

l

X

X

A

a

B

b

E

e H

h

N

n

T

t

L

l

X

Y

Method 1 : Making gametes 1) Find your envelope containing Mum Reebop‟s chromosomes and your Dad Reebop‟s chromosomes. 2) Open each envelope, take out the cards, keeping Mum‟s (pink) and Dad‟s (blue) separate. 3) Sort each set of chromosomes into pairs of the same length. Dad will have two unmatched chromosomes! 4) Now turn the cards over and randomly take one chromosome from each pair of Mum‟s chromosomes and place in a pile called „female gamete‟. 5) Randomly take one chromosome from each pair of Dad‟s chromosomes and place in a pile called „male gamete‟. Making the gametes like this is a model of inheritance – which halves the number of chromosomes in the gamete (haploid number), so that when gametes combine at fertilisation, the new individual has the correct number (diploid number).

6) Fertilise the female gamete with the male gamete by mixing together the female and male gamete piles. This is now your selection of „baby genes‟ (or your zygote chromosome set). 7) Put the chromosomes you haven‟t used back into their original envelopes. 8) Sort out the chromosomes of your new individual into pairs. You have now mixed a random selection of half the chromosomes from one parent with a random selection of half the chromosomes from the other parent to make a new combination. Each parent donated the haploid number of chromosomes (8 chromosomes) to make the diploid number (16 – now in 8 pairs again).

Method 2 : Fertilisation

11) Write down the letters you have obtained in the „genotype and phenotype table‟ for your „baby Reebop‟. For example, if you have one card with the letter A and another card with the letter a, your genotype is Aa. 12) Use the „decoder key‟ to decide what the characteristics (phenotype) of your baby Reebop will be based on your genotype description. For example, if is genotype is BB, it will have 3 body segments. 13) Collect all the materials you need for your baby Reebop and build it with the characteristics that its genes determine. 14) Put your baby in the Reebop nursery with the other newborns!

Method 3 : Development – from genotype to phenotype

Materials

• Green straws – Antennae

• Plasticine – Body segments

• Pipe cleaners – Tail

• Large pompoms – Nose

• Straws – Legs

• Small pompoms - Humps

Discussion questions

• What do you notice about the features of the babies?

• Are there any babies that are identical?

• How much genetic material does each parent provide?

• Where is this genetic material in the parent?

• Explain how sexual reproduction introduces genetic variation into offspring.

Genetics

Learning Intentions: •To learn about genetics crosses.

Success criteria: • Define the use of fruit flies in genetics experiments.

• Define family trees.