3A3

1. 3A3 Transmission of Genes from Parent to offspring

2. Mystery of heredity • Before the 20th century, 2 concepts were the basis for ideas about heredity • Heredity occurs within species • Traits are transmitted directly from parent to offspring • Thought traits were borne through fluid and blended in offspring • Paradox – if blending occurs why don’t all individuals look alike?

3. Early work • Josef Kolreuter – 1760 – crossed tobacco strains to produce hybrids that differed from both parents • Additional variation observed in 2nd generation offspring contradicts direct transmission • T.A. Knight – 1823 – crossed 2 varieties of garden pea, Pisum sativa • Crossed 2 true-breeding strains • 1st generation resembled only 1 parent strain • 2nd generation resembled both

4. Gregor Mendel • Chose to study pea plants because: • Other research showed that pea hybrids could be produced • Many pea varieties were available • Peas are small plants and easy to grow • Peas can self-fertilize or be cross-fertilized

6. Mendel’s experimental method • Usually 3 stages • Produce true-breeding strains for each trait he was studying • Cross-fertilize true-breeding strains having alternate forms of a trait • Also perform reciprocal crosses • Allow the hybrid offspring to self-fertilize for several generations and count the number of offspring showing each form of the trait

8. Monohybrid crosses • Cross to study only 2 variations of a single trait • Mendel produced true-breeding pea strains for 7 different traits • Each trait had 2 variants

9. F1 generation • First filial generation • Offspring produced by crossing 2 true-breeding strains • For every trait Mendel studied, all F1 plants resembled only 1 parent • Referred to this trait as dominant • Alternative trait was recessive • No plants with characteristics intermediate between the 2 parents were produced

10. F2 generation • Second filial generation • Offspring resulting from the self-fertilization of F1 plants • Although hidden in the F1 generation, the recessive trait had reappeared among some F2 individuals • Counted proportions of traits • Always found about 3:1 ratio

12. 3:1 is 1:2:1 • F2 plants • ¾ plants with the dominant form • ¼ plants with the recessive form • The dominant to recessive ratio was 3:1 • Mendel discovered the ratio is actually: • 1 true-breeding dominant plant • 2 not-true-breeding dominant plants • 1 true-breeding recessive plant

14. Conclusions • His plants did not show intermediate traits • Each trait is intact, discrete • For each pair, one trait was dominant, the other recessive • Pairs of alternative traits examined were segregated among the progeny of a particular cross • Alternative traits were expressed in the F2 generation in the ratio of ¾ dominant to ¼ recessive

15. 5 element model • Parents transmit discrete factors (genes) • Each individual receives one copy of a gene from each parent • Not all copies of a gene are identical • Allele – alternative form of a gene • Homozygous – 2 of the same allele • Heterozygous – different alleles

16. Alleles remain discrete – no blending • Presence of allele does not guarantee expression • Dominant allele – expressed • Recessive allele – hidden by dominant allele • Genotype – total set of alleles an individual contains • Phenotype – physical appearance

17. Principle of Segregation • Two alleles for a gene segregate during gamete formation and are rejoined at random, one from each parent, during fertilization • Physical basis for allele segregation is the behavior of chromosomes during meiosis • Mendel had no knowledge of chromosomes or meiosis – had not yet been described

18. Punnett square • Cross purple-flowered plant with white-flowered plant • P is dominant allele – purple flowers • p is recessive allele – white flowers • True-breeding white-flowered plant is pp • Homozygous recessive • True-breeding purple-flowered plant is PP • Homozygous dominant • Pp is heterozygote purple-flowered plant

21. Human traits • Some human traits are controlled by a single gene • Some of these exhibit dominant and recessive inheritance • Pedigree analysis is used to track inheritance patterns in families • Dominant pedigree – juvenile glaucoma • Disease causes degeneration of optic nerve leading to blindness • Dominant trait appears in every generation

24. Recessive pedigree – albinism • Condition in which the pigment melanin is not produced • Pedigree for form of albinism due to a nonfunctional allele of the enzyme tyrosinase • Males and females affected equally • Most affected individuals have unaffected parents

26. Dihybrid crosses • Examination of 2 separate traits in a single cross • Produced true-breeding lines for 2 traits • RR YY x rryy • The F1 generation of a dihybrid cross (RrYy) shows only the dominant phenotypes for each trait • Allow F1 to self-fertilize to produce F2

27. F1 self-fertilizes • RrYy x RrYy • The F2 generation shows all four possible phenotypes in a set ratio • 9:3:3:1 • R_Y_:R_yy:rrY_:rryy • Round yellow:round green:wrinkled yellow:wrinkled green

30. Principle of independent assortment • In a dihybrid cross, the alleles of each gene assort independently • The segregation of different allele pairs is independent • Independent alignment of different homologous chromosome pairs during metaphase I leads to the independent segregation of the different allele pairs

31. Probability • Rule of addition • Probability of 2 mutually exclusive events occurring simultaneously is the sum of their individual probabilities • When crossing Pp x Pp, the probability of producing Pp offspring is • probability of obtaining Pp (1/4), PLUS probability of obtaining pP (1/4) • ¼ + ¼ = ½

32. Rule of multiplication • Probability of 2 independent events occurring simultaneously is the product of their individual probabilities • When crossing Pp x Pp, the probability of obtaining pp offspring is • Probability of obtaining p from father = ½ • Probability of obtaining p from mother = ½ • Probability of pp= ½ x ½ = ¼

33. Testcross • Cross used to determine the genotype of an individual with dominant phenotype • Cross the individual with unknown genotype (e.g. P_) with a homozygous recessive (pp) • Phenotypic ratios among offspring are different, depending on the genotype of the unknown parent

35. Extensions to Mendel • Mendel’s model of inheritance assumes that • Each trait is controlled by a single gene • Each gene has only 2 alleles • There is a clear dominant-recessive relationship between the alleles • Most genes do not meet these criteria

36. Polygenic inheritance • Occurs when multiple genes are involved in controlling the phenotype of a trait • The phenotype is an accumulation of contributions by multiple genes • These traits show continuous variation and are referred to as quantitative traits • For example – human height • Histogram shows normal distribution

38. Pleiotropy • Refers to an allele which has more than one effect on the phenotype • Pleiotropic effects are difficult to predict, because a gene that affects one trait often performs other, unknown functions • This can be seen in human diseases such as cystic fibrosis or sickle cell anemia • Multiple symptoms can be traced back to one defective allele

39. Multiple alleles • May be more than 2 alleles for a gene in a population • ABO blood types in humans • 3 alleles • Each individual can only have 2 alleles • Number of alleles possible for any gene is constrained, but usually more than two alleles exist for any gene in an outbreeding population

40. Incomplete dominance • Heterozygote is intermediate in phenotype between the 2 homozygotes • Red flowers x white flowers = pink flowers • Codominance • Heterozygote shows some aspect of the phenotypes of both homozygotes • Type AB blood

42. Human ABO blood group • The system demonstrates both • Multiple alleles • 3 alleles of the I gene (IA, IB, and i) • Codominance • IA and IBare dominant to i but codominant to each other

44. Environmental influence • Coat color in Himalayan rabbits and Siamese cats • Allele produces an enzyme that allows pigment production only at temperatures below 30oC

45. Epistasis • Behavior of gene products can change the ratio expected by independent assortment, even if the genes are on different chromosomes that do exhibit independent assortment • R.A. Emerson crossed 2 white varieties of corn • F1 was all purple • F2 was 9 purple:7 white – not expected

47. Sickle Cell Anemia

48. Tay-Sachs Disease

49. Huntington’s Disease

50. X-linked Color Blindness