Heredity

Genetics

Heredity – traits passed from parents to offspring◦ex. Hair color, eye color, eye shape, height,

build, etc.We use heredity to alter offspring

◦Crops, pets, some people even try to find a mate to make genetically advantageous children.

Mendel

Gregor Mendel – an Austrian monk credited with discovering the basics of genetics.◦Genetics – study of heredity

Mendel studied garden peas

Mendel

He looked at 7 traits◦Flower color – purple and white◦Seed color – yellow and green◦Seed shape – smooth and wrinkled◦Pod color – green and yellow◦Pod shape – smooth or bumpy◦Flower position – middle of stem or tip of stem◦Plant height – tall or short

Mendel

Mendel

He was able to control pollination in pea plants because both male and female parts are enclosed in same flower.◦Self-fertilization – plant fertilized by itself,

creates a genetic copy◦Cross-pollination – plant is fertilized by pollen

from another plant. Sexual reproduction Different genetic information.

Mendel

Noticed that traits are expressed in simple ratios.◦What is the whole-number ratio of each of the

traits on slide 5?3 steps

◦Garden peas have traits that expressed in 2, clearly different forms.

◦Mating of peas is very easily controlled◦Peas are small, grow easily and quickly, and

produce many offspring.

Mendel

Mating can be easily controlled b/c◦Male and female parts are on the same plant◦They have the ability to self-pollinate or self-

fertilize. Self-fertilization – creates a genetic copy

◦Or they can cross-pollinate Cross-pollination – sexual reproduction where

one plant fertilizes another

Mendel

Mendel

3 Steps◦1. Produced a true-breeding P-generation

(Parental generation) by allowing each type to self-pollinate. True-breeding – all offspring display only one

trait◦2. Cross pollinated the 2 P-groups that had

contrasting forms of a trait to create an F1 generation. Record the number expressing each trait.

◦3. Self-pollinate the F1 group, creating and F2 generation and record the traits that appear.

Mendel

Mendel

Mendel’s results◦The F1 generation showed only one trait

◦When the F1 generation self-pollinated, the missing trait reappeared.

◦He found that in the F2 generation, there were 705 plants with purple flowers and 224 plants with white flowers. The ratio was 705 to 224(705:224 or 705/224). What would be the simplest whole number ratio to make analysis easier?

Mendel

Mendel’s results◦He noticed in all 7 traits the ratio in the F1

generation was 4:0◦In the F2 generation the ratio was 3:1

Review Question.◦Be able to explain Mendel’s experiment and the

results.

Mendel

Prior to Mendel, characteristics were thought to be a blend of parental characteristics◦For example if mom was short and dad was tall,

the offspring would be medium height◦Mendel’s experiment did not support the

blending idea◦Each parent has 2 “hereditable factors” or

traits that they can share. During Meiosis each gamete would receive 1 of the 2. So each parent can pass on 1 trait.

Mendel

Mendel’s hypothesis◦1. For each trait, an individual has 2 copies of

a gene – one from each parent◦2. There are alternative versions of genes –

flower color (purple or white). Allele◦3. When 2 different alleles occur together, one

of them may be completely expressed, while the other may have no observable effect on the organism Dominant – expressed form of a trait Recessive – trait not expressed unless no

dominant trait is present

Mendel

Mendel’s hypothesis◦4. When gametes are formed, the alleles for

each gene in an individual separate independently of one another, thus gametes carry only one allele for each inherited trait.

Studying Heredity

Many people depend on being able to predict hereditary outcomes for their livelihood. ◦Animal breeders, horticulturists, etc.

How do they predict the expected results?◦Punnett Square – diagram that predicts the

expected outcome of a genetic cross by considering all possible combinations of gametes in a cross.

Studying Heredity

If Y = yellow pea, and y = green pea. Where Y is dominant over y.◦ Y Y◦ y

◦ y

◦All combinations are Yy, so what color will all the peas be?

Yy Yy

Yy Yy

Studying Heredity

If Y = yellow pea, and y = green pea. Where Y is dominant over y.◦ Y y◦ y

◦ y

◦All combinations are Yy, yy, so what color will all the peas be and in what ratio?

Yy yy

Yy yy

Studying Heredity

If Y = yellow pea, and y = green pea. Where Y is dominant over y.◦ Y y

◦ Y

◦ y

◦All combinations are YY, Yy, yy, so what color will all the peas be and in what ratio?

YY Yy

Yy yy

Studying Heredity

Alleles – different versions of a gene.◦A single trait from one parent

Homozygous – 2 alleles of a particular gene present in an individual. (YY or yy)

Heterozygous – alleles of a particular gene in an individual are different. (Yy).

In slides 18-20, what are the ratios of possible combinations? Are they homozygous or heterozygous?

Studying Heredity

If freckles are dominant and a person has freckles, what combination of alleles do they have?

If they do not have freckles, what combination of alleles do they have?

Genotype – a set of alleles that an individual has

Phenotype – physical appearance of a trait.

Studying Heredity

If T = Tall, and t = short. Where T is dominant over t.◦ T t

◦ T

◦ t

◦What are the possible Genotypes?◦What the possible Phenotypes?

TT Tt

Tt tt

Studying Heredity

Law of Segregation◦2 alleles for a trait segregate (separate) when

gametes are formed◦Each gamete receives 1 of the alleles

Law of independent assortment – alleles of different genes separate independently of one another.◦Each gamete is going to have a completely

different combination of alleles than another.◦No 2 gametes are going to carry the exact

same traits.

Review Questions

Differentiate between alleles and genes.Apply the terms homozygous,

heterozygous, dominant, or recessive to describe the following 2 genotypes of a pea plant; Pp and pp.

Identify the phenotype of a rabbit with the genotype Bb, where B = black fur and b = brown fur.

Is the rabbit in #3 heterozygous or homozygous?

Review Questions

Fill in two Punnett squares and describe the possible offspring. G = green, g = yellow pea color

G g G G

g g

g g

Probability

Probability – likelihood that a specific event will occur.◦can be written as decimals, percentages, or

fractions.◦P = number of one kind of possible outcomes /

total number of all possible outcomes◦Flipping a coin: heads or tails

Probability of landing on heads◦½◦50%◦0.5

Probability

What is the probability of flipping 2 coins and landing on heads twice?

heads tails

heads

tails

Heads-heads = ¼ = 25% Heads-tails = ¼ + ¼ = 50% Tails-tails = ¼ = 25%

HeadsHeads 1/4

HeadsTails1/4

HeadsTails1/4

TailsTails1/4

Pedigree

Pedigree – family history showing how traits are inherited over several generations◦Very similar to a family tree◦Usually shown as a pictorial graph.

Probability

Pedigree

Pedigree

Sex-linked trait – trait whose allele is located on the X chromosome.◦Most are recessive◦Because males have only one X, a male who

carries a recessive allele on the X chromosome will exhibit the sex-linked trait. Muscular Dystrophy

◦A female who carries the recessive allele on on X chromosome will not exhibit the condition if there is a dominant allele on her other X.

◦Usually only found in males

Pedigree

If a person is homozygous, they will show the trait they were given whether dominant or recessive.

If heterozygous, the dominant trait will show.

Review Questions

Predict the expected phenotype and genotype ratios among the offspring of two individuals who are heterozygous for freckles (Ff) by using a Punnett square. Freckles (F) are dominant.

Calculate the probability that an individual heterozygous for cleft chin (Cc) and an individual homozygous for cleft chin will produce offspring that are homozygous recessive for a cleft chin (cc).

Patterns and complexity

Unfortunately, genetics are not so black and white. Many traits are influenced by multiple genes

Polygenic Traits – several genes influence a single trait.◦May be on the same chromosome or on

different chromosomesIncomplete Dominance – individuals

that display intermediate traits from both parents. ◦Combined traits.


Incomplete Dominance◦R=Red, W=white, neither is dominant◦ R W

◦ R

◦ W

◦RR = Red, WW = White, RW = Pink

RR RW

RW WW


Incomplete Dominance◦S=straight hair, C=curly hair◦ S C

◦ S

◦ C

◦SS = Straight, CC = Curly, SC = Wavy

SS SC

SC CC


Codominance – when two dominant alleles are expressed at the same time◦Horse and cow hair color.◦If a Red horse and a white horse mate, the

offspring will have red and white hair in the approximate ratio of alleles.


Multiple alleles – genes with 3 or more possible alleles

Blood type◦What blood type are you?◦A, B, AB, O?◦O is the most common – 45% of population◦A – 40% of population◦B – 10% of population◦AB – 5 % of population

O is recessive, both A and B are dominant


Blood type◦ IA IB i

◦ IA

◦ IB

◦ i

IAIA IAIB IAi

IBIA IBIB IBi

IAi IBi ii


Multiple traits – peas, R=round, r=wrinkled, Y=yellow, y=green◦ RY rY Ry ry

◦ RY

◦ rY

◦ Ry

◦ ry

RRYY RrYY RRYy RrYy

RrYY rrYY RrYy rrYy

RRYy RrYy RRyy Rryy

RrYr rrYy Rryy rryy


Environmental influence on expression of traits.◦Some vary due to environmental factors◦Arctic fox has fur that varies based on

temperature In winter, it is white In summer, it is dark

◦Hydrangea flowers Color varies based on acidity of soil


Some traits are caused by mutations and are call genetic disorders.◦Sickle cell anemia◦Cystic fibrosis◦Hemophilia – on X chromosome

Usually shows up in males◦Muscular dystrophy – on X chromosome

Usually only in males Loss of muscle, fatal

◦Huntington’s Chorea – dominant Accompanied by loss of muscular control and

emotional problems, eventually death.

Practice Problems

Differentiate between incomplete dominance and co-dominance.

Identify 2 examples of traits that are influenced by environment. (Do not use the 2 in the notes, find new ones)

Describe how males inherit hemophilia and why it almost always shows up in males.

Practice Problems

What are the Genotypes of the parents in the following

Yy Yy

Yy Yy

YY Yy

YY Yy

Yy yy

Yy yy

YY Yy

Yy yy

Heredity

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