Genetics

What is Genetics?

• The branch of biology that seeks to explain biological variation

• Heredity: Transmission of characteristics (traits) from parent to offspring

Vocab You Should Recall:

• Diploid-– a cell with 2 sets of homologous

chromosomes (2n)

• Haploid-– a cell with 1 set of

chromosomes (n) without a homologous pair

• Gamete- – a male or female sex cell (n)

• Zygote- – a fertilized egg cell (2n)

More to Recall:

• Chromosome – a long piece of DNA

containing many alleles• Autosome

– All chromosomes not involved in determining sex

• Gene– The length of DNA that

codes for a trait.– Genes come in pairs that

separate in the formation of gametes.

New Vocab

• Genotype– An organisms

genetic make-up (allele combination)

•Phenotype- The physical appearance of a gene (visible trait)

What is the relationship between genes (genotype) and observable characteristics (phenotype)?

Phenotype = Genotype + Environment.

Genetically identical hydrangeas growing in soils of different acidity (different environments).

The phenotype = genotype + environment principle applies equally to human traits.

Different Genotypes Can Produce the Same Phenotype

• Allele– Alternative versions

of a gene (one from each parent 2 alleles = one gene)

• Homozygous– Having a pair of

identical alleles for a characteristic (pure)

• Heterozygous– Having 2 different

alleles for a characteristic (hybrid)

For example, the gene for seed shape in pea plants exists in

two forms:

•one form or allele for round seed shape (R)

•the other for wrinkled seed shape (r).

•A homozygous plant would contain the following alleles for seed shape:

•RR or rr.

•A heterozygous plant would contain the alleles Rr

Classroom GeneticsCharacteristic: capital=dominant / lower

case=recessivePersonal Phenotype and

GenotypeNumber w/trait % of Class with Trait

Hairline widow's peak (H) continuous hairline (h)

Earlobes unattached (E) attached (e)

Hitchhiker's thumb top segment of thumb straight (T) top segment of thumb can be bent back (t)

U-shaped tongue able to roll tongue in trough-like shape (R) unable to roll tongue (r)

• Dominant-– In heterozygote, the allele that is

expressed in phenotype• Recessive

– In heterozygote, the allele that is completely masked in the phenotype

Earlobes:

Free ear lobes (dominant trait)

Attached ear lobes (recessive trait) Dimples:

Dimples (dominant trait)

No dimples (recessive trait)

Tongue-Rolling:

Rolling up edges (dominant trait)

not rolling (recessive)

• Monohybrid Cross– a genetic cross

between individuals differing in one trait

•Dihybrid Cross- a cross between individuals differing in two traits

• F1 Generation

– The first generation of hybrid offspring in a genetic cross

• F2 Generation

– Offspring resulting from interbreeding of the hybrid F1 generation.

• Test Cross– Breeding of recessive

homozygote with dominant phenotype, but unknown genotype

• Punnett square-– diagram used by

biologists to predict the possible outcome of a genetic cross

Mendelian Genetics

• Gregor Mendel (1822- 1884)– Known as the “Father of

Modern Genetics”– Austrian Monk (and HS

science teacher) who wondered how plants obtain atypical characteristics

– Wrote “Experiments with Hybrid Plants”

Mendel’s Experiments…..

Gregor Mendel’s Rules of Inheritance• Rule of Dominance and Recessiveness:

– The allele that expresses itself in the phenotype when a gene is heterozygous (hybrid) is the dominant allele. The allele that is masked is the recessive allele. Ex. Tongue rolling.

• Rule of Incomplete Dominance: – When a gene is heterozygous, incomplete dominance (or co-

dominance) results when the phenotype is a mixture of the two genotypes. Ex. Red, pink and white snapdragons.

• Rule of Segregation: – During Meiosis, two alleles of a gene separate during the formation of

gametes (egg and sperm).

• Rule of Independent Assortment: – Alleles of one gene separate independently of the alleles of any other

gene. In other words, the way in which one pair of alleles segregates has nothing to do with the way any other pair of alleles segregate.

Nondisjunction

• Failure of homologous chromosomes to separate in meiosis

• Web Karyotyping activity

Punnett Square:• Predicts offspring genotype from parental

gamates in a visual form:

Tongue Rolling• Dolly cannot roll her tongue, so we know

she is homozygous recessive for that trait.– Phenotype: non roller– Genotype: tt

• Dolly’s dad cannot roll his tongue either– Phenotype: non roller– Genotype: tt

• Dolly’s mom can roll her tongue.– Phenotype: roller– Genotype: TT or Tt

• In order to have a daughter who cannot roll her tongue, what does Dolly’s mom genotype have to be?

t t

T

?

Tt Tt

-t -t

In order for Dolly to be a non-tongue roller, her mom has to be heterozygous for tongue rolling

tt ttt

Incomplete Dominance

•gene expression in which the phenotype of a heterozygous individual is intermediate between those of the parents.

Some general rules for genetics problems:• Two alleles are necessary for a trait• The genes are symbolized by the first

letter of the dominant gene.• The letter for the dominant gene is

always capitalized.• The letter for the recessive trait is

always lower case (make sure you can tell the difference between the two)

• Wild Type is the typical form of the organism, strain, or gene

• Pure traits are those with identical genes (homozygous).

• Hybrids have mixed genes for the same trait (heterozygous).

• Gametes only carry one allele for each trait (they are haploid)

Probability• Mathematic tool used for predicting the

likelihood of events

• Equal to the expected frequency of a particular event when an experiment is repeated an infinite number of times

• Probabilities in genetics are often predicted based on a hypothesis, which is tested with real data

• Mendel predicted outcomes of pea plant crosses and then tested them thousands of times over 8 years!

Probability = # times event is expected to happen# opportunities (trials)

• usually expressed as a fraction (or %).–Ex.

• The chance of a coin landing heads up is one out of two or ½ (50%)

• The chance of drawing an ace out of a deck of cards is 4 out of 52 or 1/13 (7.7%)

• Why can the principles of probability be used to predict the outcomes of genetic crosses?

Segregation of alleles in meiosis & uniting of a certain egg and sperm is random, like a coin flip.

• Cystic fibrosis is an autosomal recessive disorder. – To have the disease, an individual has to be

homozygous recessive (ff)

• A man and woman are both carriers for cystic fibrosis (Ff). What are the chances of them having a child with the disorder?

F f

F

f

FFFf

Ff ff

First Law of Probability

• the results of one chance event have no effect on the results of subsequent chance events.

• ex. My cousin already has 3 boys. His wife is pregnant. What is the probability of his fourth child being a boy?

½

If my friend Victoria and her husband Peter are carriers of cystic fibrosis and have already had a son with the disorder, what are their chances of having another child with the disorder?

¼ or 25%

Two Other Rules of Probability you’ll need to understand

• Rule of Multiplication- – the probability of a compound event is equal to the

PRODUCT of the separate probabilities of the independent single events

– prob(a and b) = p(a)p(b)

Ex. The probability of flipping a coin and getting 2 tails in a row:

The probability of flipping tails on the 1st penny= ½

x

The probability of flipping tails on the 2nd penny= ½

=

The probability of getting tails on both

pennies= ¼

Rule of Addition-

• The probability of an event that can occur in two or more alternative ways is the SUM of the separate probabilities of the different ways.

Ex. The probability of flipping 1 head and 1 tail in a toss with 2 coins:

The probability of flipping tails on the 1st penny and heads on the 2nd penny = (½)(½) = ¼

+

The probability of flipping heads on the 1st penny and tails on the 2nd penny= (½)(½) = ¼

=

The probability of getting one tail and one head in a toss with 2 coins = ½

Practice Problems

• What is the probability of any couple having 4 boys in a row?

½ x ½ x ½ x ½ = 1/16 or 6.25%

What is the probability of my friend Victoria and her husband having two kids with cystic fibrosis?

¼ x ¼ = 1/16

Is the following sentence true or false?

False

The past outcomes of coin flips greatly affect the outcomes of future coin flips.

How can you be sure of getting the expected 50 : 50 ratio from flipping a coin?

You must flip the coin many times.

Therefore, the _____ the number of offspring from a genetic cross, the closer the resulting numbers will get to expected values.

GREATER

How to determine the expected outcome of a coin toss

• a = heads. On a coin, there is a ½ probability of tossing heads

• b = tails. On a coin, there is a ½ probability of tossing tails

• When tossing coins simultaneously, use binomial expansion to determine all the possible outcomes/frequencies.

Chi Square (X2)

• How well do the observed results fit with the expected?

• X2 indicates the degree of deviation, tells you the % of cases in which such a deviation might be expected by chance.

• The higher the X2 Probability (P value from table), the more likely that the results are “true.” The lower the X2 P, the more likely there’s something askew (coin weighted differently on each side, sticky side, etc)

Degrees of Freedom

• # of possibilities – 1

For a coin, you can toss heads or tails• 2 possibilities• Degrees of freedom would be 2-1= 1

For a die, the degrees of freedom would be

6 – 1= 5

Dihybrid Crosses• In this type of cross, each trait is considered separately

(just as in a monohybrid cross)• How many traits are involved in a dihybrid cross?

2

• How many pairs of genes are involved?

2

• How many total genes are involved?

4

• How many genes for each trait are found in gametes? 1

• How many total genes are involved in each gamete? 2

Practice #1• A tall green plant is crossed with a short yellow

plant. All of the offspring are tall and green.– Genotypes of the parents:

– Genotypes of the gametes:

Practice #2• A green wrinkled pea plant is crossed with a

yellow round pea plant. All of the offspring have green round peas. Show how genes will arrange to do this:

– Genotypes of the parents:

– Genotypes of the gametes:

Genotype and phenotype of offspring:

• genotype

• Phenotype

Practice #3• If a plant that is hybrid for green and pure for wrinkled

peas is crossed with a yellow wrinkled pea plant, predict the genotypes and phenotypes of the offspring.

• Genotypes of the parents:

• Genotypes of the gametes:

Genotype and phenotype of offspring: genotype

Phenotype

Rule of Independent assortment

• The random selection of one trait will not determine the random selection of another

• In other words, the genes for your eyes are transmitted independently of the genes for your height

See the dihybrid example to the right:

• As you can see, there are 4 possible outcomes.

•One letter does not affect the selection of the other.

What Works for Peas Also Works for Humans

Consider a cross between parents heterozygous for both deafness

and albinism.

This is the same 9:3:3:1 ratio seen for Mendel’s cross involving pea color and shape.

Non-Mendelian Genetics

• i.e. exceptions to Mendel’s Rules

• Covers traits that have a range of phenotypes, not just 2 possibilities

Incomplete Dominance

•pattern of gene expression in which the phenotype of a heterozygous individual is intermediate between those of the parents.

INCOMPLETE DOMINANCE cont’d:

• Yellow coat color in guinea pigs is produced by the homozygous genotype YY

• Cream color by the heterozygous genotype Yy

• White by the homozygous genotype yy.

What genotypic and phenotypic ratios are matings between cream-colored individuals likely to produce?

1:2:1yellow: cream: white

Codominance

• the condition in which both alleles in a heterozygous organism are expressed.

• Ex. Roan horses or cows

CO-DOMINANCE cont’d:• In cattle, reddish coat color is

not completely dominant to white coat color. Heterozygous individuals have coats that are roan colored (ie. reddish, but with spots of white hairs).

• What would be the results of the following crosses:

a. red x roan:b. white x roan:c. red x white:d. roan x roan:

Genotype phenotype 1:1 RR:Rr 1:1 Red: Roan1:1 Rr:rr 1:1 Roan:White

All Rr all Roan1:2:1 RR:Rr:rr 1:2:1 Red:Roan:White

Multiple Alleles

• genes with three or more alleles

• Ex. Blood types- there is an A, B, and O allele (IA, IB, i)

• (More on this when we discuss blood type genetics)

Polygenic Trait-• when several genes influence one trait.

• Ex. Eye color, height, hair, and skin color

Height is a polygenic trait

Pleiotropy

• one gene that affects more than one seemingly unrelated phenotype

Anemia, infections, weakness, impaired growth, liver and spleen failure, death.

Traits (phenotypes) associated with the sickle cell allele.

Epistasis

• one gene masks the expression of a different gene for a different trait

• Ex. The gene for albinism masks the effects of genes for skin, hair, and eye color.

Epistasis cont’d• In Labs, black is

dominant to chocolate (B or b). Yellow is recessive epistatic (when present, it blocks the expression of the black and chocolate alleles) E or e.

• Determine the number of chocolate labs produced from a black female and a yellow male (BbEe x bbee)

Phenotype Possible Genotypes

BBEE BbEE BBEe BbEe

bbEEbbEe

BBeeBbeebbee

Influence of Environment

• phenotype depends on conditions in the environment.

• Ex. Arctic fox, Siamese cats, height, and skin color

Temperature sensitive – The cold extremities (ears, nose, tail, and feet) express pigmentation while the warm body does not)

Height in humans is polygenic. Each upper case letter adds 3”. Males (aabbcc) are 5’ and females (aabbcc) are 4’ 7”.

How tall would a male with the genotype AaBbCc be?5’9”

A female? 5’4”

Probability that mating with this male & female result in a child that is homozygous recessive for all three traits?(1/4)3 = 1/64

How tall would that child be if it were a girl?4’7”

BbEe x bbee

BE Be bE be

be BbEe Bbee bbEe bbee

Color Black Yellow Brown Yellow

Pea Plants• In sweet peas, purple flower color (P) is dominant over white

(p), but there is also a control gene such that if the plant has a “C”, the purple has “permission” to express itself. – If the plant is “cc”, the purple does not “have permission” to

express itself and the flower will be white anyway. – If a plant with homozygous purple, controlled flowers(CC)

is crossed with a plant with white, non-controlled(cc) flowers, diagram the Punnett square for the F1 and F2

generations and calculate the phenotype ratios.• First, what are the genotypes of the parents in the first generation?

PPCC and ppcc• What are the genotypes of their offspring?

PpCc

• What is always the phenotypic ratio for a dihybrid cross:

genotype

P-C-P-ccppC-ppcc

9: Dominant for both traits

3: Dominant for first trait and recessive for second

3: Recessive for first trait and dominant for second

1: Recessive for both traits

What are the phenotypes?P-C- purpleP-cc whiteppC- whitePpcc white