Mendelian Genetics

We know what genotype and phenotype are

We know what genes areWhat do genes do?

Genes provide the instructions for an organism’s potential development

Why potential ?

What affects the phenotype?

Mendelian Genetics

Phenotype is affected by genotype, environment chemicals & other genes

Example: 1 Why is this horse black & white?His genotype is either EE or Ee for black colorWhy the white markings?

Mendelian Genetics

Remember the phenotype is affected by other genes as well

The “other” gene is called the Tobiano gene, which overrides and cancels any color production at all in those locations

Why the white markings?

The gene for black hair production has been turned “off” or overridden at those locations

Mendelian Genetics

Example: 2 – Your heightThe genes you received from mom & dad determine your genotype

However, environmental factors, diet and hormone involvement affect the phenotype

Can different genotypes result in the same phenotype?

Can the same genotypes result in different phenotypes?

Mendelian Genetics

So someone who is “supposed” to be 6’3” but doesn’t get the nutrition probably won’t reach that heightThe opposite could be true for someone who is “supposed” to be 5’8” and gets too much hormone, they could be taller

Gregor Mendel (1822-1884)1843 – admitted into Augustinian Monastery1854 – began series of breeding experiments with

pea plants- no knowledge of mitosis or meiosis

Mendelian Genetics

1865 – reports conclusions of experimentsKEY POINTS: - began studying the inheritance of only 1 trait at

a time - controlled matings - kept accurate records of outcomesWhy pea plants?Easy to handle, produce lots of offspring, short life cycle, variation existed

Mendelian Genetics

Self-pollinate (selfing) – pollen fertilizes an egg from the same flower

Mendel allowed the plants he had to

There were 7 traits he studied in his experiments1. Flower & seed coat color

For many generations to attain true-breeders

2. Seed color3. Seed shape4. Pod color

5. Pod shape6. Stem height7. Flower position

Mendelian Genetics

Mendelian Genetics

Self-pollinateMendel had to be sure these plants didn’t

To manage this he removed the male parts (anthers) and used them where he desired to

Cross-pollinate – pollen fertilizes an egg from a different flower

Phenotypes of the resulting seeds (peas) were analyzed and then planted to produce the next gen

Mendelian Genetics

P-generationP X P = F1

F1 X F1 = F2 (through self-fertilization)

Ex. Wrinkled female x smooth male AND Smooth female x wrinkled male

If the results are the same…

Monohybrid crosses

Reciprocal crosses – done in both directions

If the results are different…

Mendelian Genetics

Mendel reasoned that there were “factors” (genes) that were passed from parent to offspring

F1 generation always showed traits of one parent, not both (dominant vs recessive)

Homozygous / heterozygous

Since the 2 traits he was examining replaced each other they were assumed to be alternative forms of the same trait (alleles)

Punnett squares

F2 generation showed traits of both parents (3:1) (1:2:1)

Mendelian Genetics

This means that all offspring carry one allele from each parent – the combination of alleles in the offspring is completely random

Medel’s principle of segregation -

T1T3 / T1T4 / T2T3 / T2T4

T1T2 x T3T4

Branch or Fork Diagram

The two members of a gene pair (alleles) segregate from each other during gamete formation

Mendelian GeneticsBranch or Fork Diagram

Mendelian Genetics

Possible outcomes, not actual – the percentages are for each offspring produced

Wild-type allele – the allele of a gene that is present in the highest frequency in a wild population

What do punnett squares or the branch diagram actually show us?

*mutations to these genes could produce nonfunctional, partially functional or totally absent proteins

Mendelian Genetics*If the function of the protein is lost due to the mutation it is called a loss-of-function mutation (usually recessive)

Mendel’s Principle of Independent Assortment – genes on different chromosomes behave

independently in gamete production

This means that the passing of one gene has no correlation with the passing of a second gene (TtGg) – the passing of the ‘T’ has no correlation with the passing of the ‘G’.

Complete a punnett square for the cross TtGg x ttGg

Mendelian Genetics

TG Tg tG tg

TG Tg tG tg

tG

tg

tG

tg

TG Tg tG tg

tG TtGG TtGg ttGG ttGg

tg

tG

tg

TG Tg tG tg

tG TtGG TtGg ttGG ttGg

tgTtGg Ttgg ttGg ttgg

tG

tg

TG Tg tG tg

tG TtGG TtGg ttGG ttGg

tgTtGg Ttgg ttGg ttgg

tGTtGG TtGg ttGG ttGg

tg

TG Tg tG tg

tG TtGG TtGg ttGG ttGg

tg TtGg Ttgg ttGg ttgg

tG TtGG TtGg ttGG ttGg

tg TtGg Ttgg ttGg ttgg

Dihybrid cross – cross between 2 individuals that are ‘dihybrid’, meaning they are both hybrid for 2 traits (TtGg x TtGg – 9:3:3:1)If you were to test 2 traits at the same time…

P generation: TTGG x ttgg both are ‘true-breeders’ therefore the F1 would be TtGg, completely hybrid

Mendelian Genetics

Trihybrid cross – cross between 2 individuals that are hybrid for 3 traitsP generation: TTGGBB x ttggbb both are ‘true-breeders’ therefore the F1 would be TtGgBb, completely hybrid

Monohybrid cross produces ____ phenotypes

Mendelian Genetics

Dihybrid cross produces ____ phenotypesTrihybrid cross produces ____ phenotypes

248

Can you come up with a mathematical formula to be able to determine the number of phenotypes produced in a genetic cross?

2n n = number of independently assorting, heterozygous gene pairs

Monohybrid cross produces ____ genotypes

Mendelian Genetics

Dihybrid cross produces ____ genotypesTrihybrid cross produces ____ genotypes

3927

Can you come up with a mathematical formula to be able to determine the number of phenotypes produced in a genetic cross?

3n n = number of independently assorting, heterozygous gene pairs

Pedigree AnalysisGenetic evaluation of human inheritance is difficult because it is not ethically possible to control the matingsTherefore we often rely on pedigree analysis to determine patterns of inheritance (how it is passed from gen to gen)How do we know if genes are passed? We rely strictly on phenotypes over several generationsProband – affected individual in which the pedigree is discovered

Pedigree SymbolsMale Affected / Unaffected

Female

Mating

Parents with 1 boy and 1 girl (birth order)

Twins

Carrier / Heterozygous

Carrier of sex-linked

Stillbirth

Marriage of blood relatives

Pedigree AnalysisGenerations – numbered with Roman numerals (II)

Refer to a particular person as II - 2

Individuals – numbered with Arabic numerals (2)

*If affected individual is born to unaffected parents:

*If affected individual is born to affected parents:

probably caused by recessive trait

May be caused by dominant or recessive traitRecessive TraitsRequire homozygosityMay have originated from a mutation

Ex. albinism

Of those affected by rare recessive traits…1. Most have “normal” parents (heterozygous)

Recessive Traits

In U.S. – 1 in 17,000 of the white population

1 in 28,000 of the African American pop

1 in 10,000 of the Irish population

2. Matings between heterozygous individuals should produce a 3:1 ratio of “normal” progeny3. When both parents are affected, homozygous, their offspring will usually exhibit the trait

Dominant mutant alleles produce phenotypes due to gain-of-function mutations they produce new genes with new functions

Ex. Woolly hair, Achondroplasi,

Dominant Traits

Because mutant dominant alleles are rare it is rare to find an individual homozygous for the mutant dominant allele

Expressed when heterozygous or homozygous

Brachydactyly, Marfan syndrome

Dominant Traits

3. Heterozygous individual will transmit the mutant gene to half their progeny

1. An affected individual must have an affected parent2. Usually does not skip generations

Characteristics of dominant inheritance

Test cross – cross of an individual of unknown genotype, usually dominant, with a homozygous recessive individual to determine the unknown

Mendelian GeneticsHow can one determine the genotype of an individual exhibiting the dominant phenotype?

Data resulting from genetic crosses rarely match the “expected” ratiosIt is the job of the geneticist to do statistical analysis to understand the significance of the deviation from the predicted results

Questions

1. A purple-flowered pea plant is crossed with a white-flowered pea plant. All the F1 plants produce purple flowers. When the F1 plants are allowed to self-pollinate, 401 of the F2 have purple flowers and 131 have white flowers. What are the genotypes of the parental and F1 generation plants?ANSWER: P – PP x pp

F1 – Pp x Pp

F2 – probably deduce a 1:2:1ratio

Questions

2. Consider 3 gene pairs Aa, Bb, and Cc, each of which affects a different character. In each case the uppercase latter signifies the dominant allele and the lowercase letter the recessive allele. These 3 gene pairs assort independently of each other. Calculate the probability of obtaining a the following:

a. an AaBBCc zygote from a cross of individuals that are AaBbCc x AaBbCc

Questions

b. an AaBBcc zygote from a cross of individuals that are aaBBcc x AAbbCC

c. an A_B_C_ phenotype from a cross of individuals that are AaBbCC x

AaBbcc

d. an aabbcc phenotype from a cross of individuals that are AaBbCc x

aaBbcc

Questions

3. In chickens, the white plumage of the leghorn breed is dominant over colored plumage, feathered shanks are dominant over clean shanks, and pea comb is dominant over single comb. Each of the gene pairs segregates independently. If a homozygous white, feathered, pea-combed chicken is crossed with a homozygous colored, clean, single-combed chicken and the F1 are allowed to interbreed, what proportion of the birds in F2 the will produce only white, feathered, pea-combed progeny if mated to a colored, clean-shanked, single combed birds?

Questions

4. In tomatoes, red fruit color is dominant to yellow. Suppose a tomato plant homozygous for red is crossed with one homozygous for yellow. Determine the appearance of:

a. the F1 tomatoes

b. the F2 tomatoes

c. the offspring of a cross of the F1 tomatoes back to the red parent

d. the offspring of a cross of the F1 tomatoes back to the yellow parent

Questions

5. In maize, a dominant allele A is necessary for seed color, as opposed to colorless (a). Another gene has a recessive allele wx that results in waxy starch, as opposed to normal starch (Wx). The two genes segregate independently. An AaWxWx plant is testcrossed. What are the phenotypes and relative frequencies of offspring?

Questions

6. In guinea pigs, rough coat (R) is dominant over smooth coat (r). A rough-coated guinea pig is bred to a smooth one, giving eight rough and seven smooth progeny in the F1 generation.

a. What are the genotypes of the parents and their offspring?

b. If one of the rough F animals is mated to its rough parent, what progeny would you expect?

Questions

7. In cattle, the polled (hornless) condition (P) is dominant over the horned (p) phenotype. A particular polled bull is bred to three cows. Cow A, which is horned, produces a horned calf; polled cow B produces a horned calf; and horned cow C produces a polled calf. What are the genotypes of the bull and the three cows, and what phenotypic ratios do you expect in the offspring of these three matings?

Questions

8. Consider the following pedigree, in which the allele responsible for the trait (a) is recessive to the normal allele (A):

a. What is the genotype of the mother?

b. What is the genotype of the father?

c. What are the genotypes of the children?

d. Given the mechanism of inheritance involved, does the ratio of children with the trait to those without match what would be expected?