1. Mendelian Genetics We know what genotype and phenotype are
We know what genes are
What do genes do?
Genes provide the instructions for an organism’s potential development
Why potential ?
What affects the phenotype?

2. 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 color
Why the white markings?

3. 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

4. Mendelian Genetics Example: 2 – Your height
The 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?

5. Mendelian Genetics Gregor Mendel (1822-1884)
So someone who is “supposed” to be 6’3” but doesn’t get the nutrition probably won’t reach that height
The opposite could be true for someone who is “supposed” to be 5’8” and gets too much hormone, they could be taller
Gregor Mendel ( )
1843 – admitted into Augustinian Monastery
1854 – began series of breeding experiments with pea plants
- no knowledge of mitosis or meiosis

6. Mendelian Genetics 1865 – reports conclusions of experiments
KEY POINTS:
- began studying the inheritance of only 1 trait at a time
- controlled matings
- kept accurate records of outcomes
Why pea plants?
Easy to handle, produce lots of offspring, short life cycle, variation existed

7. Mendelian Genetics Mendel allowed the plants he had to
Self-pollinate (selfing) – pollen fertilizes an egg from the same flower
For many generations to attain true-breeders
There were 7 traits he studied in his experiments
1. Flower & seed coat color
2. Seed color
5. Pod shape
3. Seed shape
6. Stem height
4. Pod color
7. Flower position

8. Mendelian Genetics

9. Mendelian Genetics Mendel had to be sure these plants didn’t
Self-pollinate
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

10. Mendelian Genetics P-generation
P X P = F F1 X F1 = F2 (through self-fertilization)
Monohybrid crosses
Reciprocal crosses – done in both directions
Ex. Wrinkled female x smooth male AND Smooth female x wrinkled male
If the results are the same…
If the results are different…

11. Mendelian Genetics
F1 generation always showed traits of one parent, not both (dominant vs recessive)
F2 generation showed traits of both parents (3:1) (1:2:1)
Mendel reasoned that there were “factors” (genes) that were passed from parent to offspring
Since the 2 traits he was examining replaced each other they were assumed to be alternative forms of the same trait (alleles)
Homozygous / heterozygous
Punnett squares

12. Mendelian Genetics Medel’s principle of segregation -
The two members of a gene pair (alleles) segregate from each other during gamete formation
This means that all offspring carry one allele from each parent – the combination of alleles in the offspring is completely random
T1T2 x T3T4
T1T3 / T1T4 / T2T3 / T2T4
Branch or Fork Diagram

13. Mendelian Genetics
Branch or Fork Diagram

14. Mendelian Genetics
What do punnett squares or the branch diagram actually show us?
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
*mutations to these genes could produce nonfunctional, partially functional or totally absent proteins

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

16. Mendelian Genetics
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 16

17. TG Tg tG tg 17

18. TG Tg tG tg 18

19. TG Tg tG tg TtGG TtGg ttGG ttGg19

20. TG Tg tG tg TtGG TtGg ttGG ttGg Ttgg ttgg20

21. TG Tg tG tg TtGG TtGg ttGG ttGg Ttgg ttgg21

22. TG Tg tG tg TtGG TtGg ttGG ttGg Ttgg ttgg22

23. Mendelian Genetics
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
Trihybrid cross – cross between 2 individuals that are hybrid for 3 traits
P generation: TTGGBB x ttggbb both are ‘true-breeders’ therefore the F1 would be TtGgBb, completely hybrid 23

24. Monohybrid cross produces ____ phenotypes
Mendelian Genetics
Monohybrid cross produces ____ phenotypes 2
Dihybrid cross produces ____ phenotypes 4
Trihybrid cross produces ____ phenotypes 8
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 24

25. Monohybrid cross produces ____ genotypes
Mendelian Genetics
Monohybrid cross produces ____ genotypes 3
Dihybrid cross produces ____ genotypes 9
Trihybrid cross produces ____ genotypes 27
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 25

26. Pedigree Analysis
Genetic evaluation of human inheritance is difficult because it is not ethically possible to control the matings
Therefore 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 generations
Proband – affected individual in which the pedigree is discovered

27. Pedigree Symbols
Male
Affected / Unaffected
Female
Carrier / Heterozygous
Mating
Carrier of sex-linked
Parents with 1 boy and 1 girl (birth order)
Stillbirth
Twins
Marriage of blood relatives

28. Pedigree Analysis
Generations – numbered with Roman numerals (II)
Individuals – numbered with Arabic numerals (2)
Refer to a particular person as II - 2
*If affected individual is born to unaffected parents:
probably caused by recessive trait
*If affected individual is born to affected parents:
May be caused by dominant or recessive trait
Recessive Traits
Require homozygosity
May have originated from a mutation

32. Recessive Traits
Ex. albinism
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
Of those affected by rare recessive traits…
1. Most have “normal” parents (heterozygous)
2. Matings between heterozygous individuals should produce a 3:1 ratio of “normal” progeny
3. When both parents are affected, homozygous, their offspring will usually exhibit the trait

33. Dominant Traits
Expressed when heterozygous or homozygous
Dominant mutant alleles produce phenotypes due to gain-of-function mutations they produce new genes with new functions
Ex. Woolly hair, Achondroplasi,
Brachydactyly, Marfan syndrome
Because mutant dominant alleles are rare it is rare to find an individual homozygous for the mutant dominant allele

34. Dominant Traits
Characteristics of dominant inheritance
1. An affected individual must have an affected parent
2. Usually does not skip generations
3. Heterozygous individual will transmit the mutant gene to half their progeny

35. Mendelian Genetics
How can one determine the genotype of an individual exhibiting the dominant phenotype?
Test cross – cross of an individual of unknown genotype, usually dominant, with a homozygous recessive individual to determine the unknown
Data resulting from genetic crosses rarely match the “expected” ratios
It is the job of the geneticist to do statistical analysis to understand the significance of the deviation from the predicted results 35

36. 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

37. 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

38. 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

39. 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?

41. 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

42. 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?

43. 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?

44. 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?

45. a. What is the genotype of the mother?
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?

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