Inheritance of Single-Gene Differences – discovered by Gregor Mendel!!!

1. Inheritance of Single-Gene Differences – discovered by Gregor Mendel!!! • Mendel: father of genetics • Quick review of terminology • Mendel’s Empirical approach • Monohybrid cross • Mendel’s Postulates to explain his data • Mendel’s First “law” equal segregation • Punnent Square • Dihybrid cross • Mendel’s Second “law” independent assortment • The branch diagram & probabilities • Using the testcross

2. I. Gregor Johann Mendel Who was this “Father of Genetics”?

3. Transmission genetics – link between meiosis & Mendel’s postulates • Mendel determined the transmission of discrete units (geneslocated on chromosomes) from parent to offspring, predicting the formation of gametes • Future cytological studies suggested a correlation exists between the behavior of chromosomes during meiosis and the transmission of traits

4. A. Terminology review • Genes come in different forms = ALLELES • i.e. there may be a single gene for flower color but several alleles, each producing a different color • Each individual has 2 alleles per gene (1 derived from mother, 1 from father) • Phenotype = expressed form of a character (what an individual looks like) • Genotype = specific set of alleles carried by an individual (the actual genetic composition) • Homozygous = the alleles of a gene are identical (AA) • Heterozygous = the alleles of a gene are different (Aa) • Dominant allele = an allele that expresses its phenotypic effect even when heterozygous… therefore AA and Aa have the same phenotype • Recessive allele = An allele whose phenotypic effect is not expressed in a heterozygote… therefore (a) can only be expressed when the individual is homozygous – (aa).

5. Terminology cont. - Genetic Crosses • Controlled mating of two specific organisms • Self Cross= cross to oneself (plants, fungi) • Haploid Cross= simplest, each gene present in 1 copy only (fungi) • Diploid Cross= each gene present in 2 copies • Homozygote cross (AA x AA), aka pure-breeding • Heterozygote cross (Aa x Aa) • Testcross = cross with a known homozygote recessive • Backcross = hybrid offspring are crossed with one of the parents • Reciprocal Cross = in an initial cross, if the female parent has the mutant condition & the male parent has the wild type condition - The reciprocal cross is the reverse (female is wild type & male is mutant)

6. B. Mendel’s success with the empirical approach • Came up with an elegant model of experimental design • chose a good “model” organism: Pisum sativum • restricted his examination to one or very few pairs of contrasting traits in each experiment • took meticulous notes with accurate quantitative records

8. Mendel’s Empirical approach By using controlled crosses, Mendel designed experiments to determine the quantitative relationships from which laws could be discovered

9. Looked at contrasting characteristics of the garden pea -seed coat, seed color, petal color, pod shape, pod color, stem size, axial/terminal flowers.

10. II. The Monohybrid cross • Hybridization = when two plants of the same species but with different characteristics are crossed (mated) to each other. • Mono = dealing with one pair of contrasting characteristics • P – parental generation • F1 – First filial generation • F2 – Second filial generation

11. Mendel’s results from the monohybrid crosses

12. A. Mendel’s Postulates to explain his data • the existence of unit “factors” – particulate theory of inheritance • Traits inherited as discrete units that remain unchanged as they pass from parent to offspring • genes are in pairs, thus 2 phenotypes must be determined by 2 different alleles of 1 gene • When two unlike unit factors responsible for a single character are present in a single individual, one unit factor is dominant to the other (Dominanance/Recessiveness) • the principle of segregation, genetic units segregate from each other

13. Points 4 & 5: 4) gametic content – the F2 3:1 ratio is based on a 1:1 segregation in a heterozygote 5) random fertilization – gametes are brought together for fertilization in a random manner

15. B.Mendel’s Law of equal segregation: • Equal Segregation = The two members of a gene pair segregate from each other into the gametes; so half the gametes carry one member of the pair and the other half of the gametes carry the other member of the pair.

19. C. Using Punnett Squaresin Genetic Crosses Punnett squares used for monohybrid crosses • Considers only genes of interest • List sperm genotypes across top • List egg genotypes down side • Fill in boxes with zygote genotypes

20. P p p PP P p p Genotypes 2 Frequencies Phenotypes Frequencies Making a Punnett Square:Heterozygous X Heterozygous P p Eggs of Heterozygous Plant P P P P p Pollen ofHeterozygous Plant p P p p p 1 1 White Purple 3 (75%) 1(25%)

21. D. Using the testcross to determine if the parent is heterozygous • The organism of the dominant phenotype is crossed to a known homozygous recessive individual

22. III. Mendel’s Dihybrid Cross • Follows the inheritance of two different traits within the same individual. • i.e. Yellow, Round x Green Wrinkled

24. F1 cross: GgWw x GgWw

25. A. Mendel’s (postulate) Second Law of independent assortment: • Independent Assortment = two different genes on different chromosomes will randomly assort their alleles during gamete formation The alleles assort independently Possible gametes produced from meiosis

26. (Hair color) & (Hair length) Black/Brown Short/Long P: Black, short x Brown, long F1: all black, short F2: Black, short x Black, short: BbSs x BbSs

28. Review Mendel’s Laws

29. B. Probability and statistics in genetics • How can we calculate the expected ratios of the phenotypes/genotypes for progeny? • How can we determine if our results are significantly different from what we would expect under Mendelian principles? • Pr(A) = Probability of an event A, number between 0 and 1 that measures the likelihood that A will occur when the experiment is performed • Accuracy of prediction depends on sample size

30. Probability Rules • Product rule: the probability of independent events occurring together is the product of the probabilities of the individual events • Pr(A) x Pr(B) = Pr(A and B) • Sum rule: probability of either of two mutually exclusive events occurring is the sum of their individual probabilities. • Pr(A) + Pr(B) = Pr(A or B) • Conditional probability: the probability that one outcome will occur, given the specific condition upon which the outcome is dependent • Prc= Pr(a)/Pr(b)

31. e.g. Product rule in practice If you self cross an F1 dihybrid yellow, round pea plant- What proportion of offspring will be yellow and round? Probability of producing yellow peas: ¾ (Y/Y or Y/y) Probability of producing round peas: ¾ (R/R or R/r) Therefore, Yellow-Round offspring: ¾ x ¾ = 9/16 What if you crossed pure breeding tall plants with purple flowers that make yellow round peas with short plants with pure breeding white flowers that make green wrinkled peas?

32. P: T/T;P/P;Y/Y;R/R x t/t;p/p;y/y;r/r F1: T/t;P/p;Y/y;R/r F2? What is the probability of having tall plants with purple flowers that make yellow peas? (T/-;P/-;Y/-;R/-) ¾ x ¾ x ¾ x ¾ = 81/256

33. Properties of probabilities • The probability of an event always takes on a value between 0 and 1 • The probability of two events occurring together is equal to Pr(A) x Pr(B) • If two events A and B are mutually exclusive, then the probability the either A or B occurs is equal to Pr(A) + Pr(B)

34. A pure breeding black guinea pig is crossed with a pure breeding tan guinea pig. If black is dominant to tan, what will the genotype and phenotype of the F1 be? Give proportions. • For the above, what would the genotypes and phenotypes of offspring from an F1 x F1 cross be? Give proportions.

35. The forked-line method, or branch diagram • Calculate the probability of obtaining an aa; B-; C- zygote from the cross Aa; Bb; Cc X Aa; Bb; Cc. • Much simpler than using the Punnent square for looking at more than one trait • Genetic ratios – expressed as probabilities • Based on the product rule of probability • Pr(A) x Pr(B) = Pr(A and B)

36. Binomial expansion • Used to predict the probability of an unordered combination of events • each event possesses one of two mutually exclusive characteristics, eg. curly hair or straight hair • the outcome for any one event is independent of the outcome for any other event • Example: from a cross between two tall plants, Tt x Tt, what is the probability of having 2 dwarf plants out of five offspring?

37. Equation to determine the probability of unordered events from a cross between two tall plants, Tt x Tt, what is the probability of having 2 dwarf plants out of five offspring? Pr(x successes in n trials) = n! (n-x)! x! pxqn-x Step 1. calculate individual probabilities (p & q) p = ¼, q = ¼ Step 2. determine # of events in category x and the total # of events x = 2 n = 5 Step 3. substitute values for p, q, x in the equation 5! 3!2! 0.252 x 0.253 =120/12 x (0.0625)(0.015625) =0.0097 or 0.97%

38. The ability to taste phenylthiocarbamide is an autosomal dominant phenotype, and the inability to taste it is recessive. If a taster woman with a nontaster father marries a taster man who, in a previous marriage had a nontaster daughter, what is the probability: a. that their first child will be a nontaster girl b. that their first child will be a taster girl c. that two out of three children will be nontasters 1/8 (½ x ¼) (½ x ¾) 3/8 14%

39. Inheritance of Gene Differences – non-Mendelian geneic interactions, part 2 • Non-Mendelian ratios - Interactions between the alleles of one gene • Interactions between the alleles of more than one gene • Gene interaction • Epistasis

40. A. Incomplete Dominance Two alleles (heterozygote) produce an intermediate phenotype • At the molecular level, the mutant allele results in a reduced amount of functional protein • 2 doses = 100% • 1 dose = 50% • 0 dose = None

41. incomplete dominance • F1 hybrids have an appearance somewhere in between the phenotypes of the two parental varieties • F1 is pink, an intermediate color between white and red, F2 1:2:1

42. Example: Tay-Sachs disease – Homozygous recessive individuals are severely affected (death by age 3), Heterozygotes express only about 50% of hexosaminidase enzyme for lipid metabolism. Slightly affected. The closer we look, the more we find that heterozygotes are different from homozygous dominant individuals.

43. B. Multiple alleles • Some genes are found in three or more alleles that are different from each other • e.g. white clover, coat color in rabbits

44. C = full coat color; dominant to all other alleles cch = chinchilla coat, a partial defect in pigmentation; dominant to ch and c ch = himalayan coat, color in only certain parts of body; dominant to c c = albino, no color; recessive to all other alleles A rabbit with chinchilla fur is mated to a himalayan. Some of their F1 offspring have himalyan fur, some have chinchilla fur and some are albino. Name the genotypes of the parents and the genotypic ratios of the F1 offspring.

45. Codominance • Specific type of multiple alleles, when two alleles are equally expressed in the heterozygous individual • e.g. ABO blood group

46. C. Lethal alleles • Allele in an essential gene that has the potential of causing the death of an organism. • Age of onset • Conditional lethal alleles • Semilethal alleles

48. II. Interactions between the alleles of more than one gene Genes interact in concert with other genes and with the environment to influence a particular characteristic. Final product

49. I Interactions between genes produce many different phenotypes • Most traits can be affected by the contributions of two or more genes Examples: morphological characteristics - Height, weight, growth rate, pigmentation

50. One trait, involving between two genes • Bateson & Punnett 1906 studied comb morphology in chickens – found a departurefrom expected ratio for a single trait 1 trait: comb 9/16 Walnut 3/16 Rose 3/16 Pea 1/16 Single