Principles of Heredity

1. Principles of Heredity Chapter 10

2. Gregor Mendel • Generally considered the ‘Father of Modern Genetics’ • Worked with pea plants, keeping careful records of his experiments • Used statistical analysis to establish several important genetic principles

3. Mendelian Genetics • Things he knew before he started: • How to control pea plant reproduction • That he had true-breeding plant strains • That hybrids between these strains did NOT breed true

4. Mendelian Genetics… • Things he didn’t know: • What DNA or chromosomes were • Anything about mitosis or meiosis • That traits in hybrids did not always ‘blend’ as was the idea of the time

5. Mendelian Genetics… • What he figured out: • Blending of two distinct traits in the parents did not always occur in the hybrid offspring • Each kind of inherited feature in an organism is controlled by 2 factors that behave like distinct particles • That some of these factors can mask others (that is, some are dominant while others are recessive) • The Principle of Segregation and The Principle of Independent Assortment

6. The Principle of Segregation • Alleles – alternate gene forms – are located on corresponding loci on homologous chromosomes • During gamete formation in meiosis, homologous chromosomes separate (when?) • During sexual reproduction, offspring receive one of these homologous chromosomes from each parent

7. Terms to review • Dominant / Recessive • Gene / Allele • Monohybrid cross / Dihybrid cross • Homozygous / Heterozygous • Phenotype / Genotype • Punnett square / Probability • Test cross

8. Monohybrid cross practice 1.) A TT (tall) pea plant is crossed with a tt (short) pea plant

9. Monohybrid cross practice… 2.) A Tt pea plant is crossed with a Tt pea plant.

10. Monohybrid cross practice… 3.) A heterozygous round seeded pea plant (Rr) is crossed with a homozygous round seeded pea plant (RR).

11. Monohybrid cross practice… 4.) A homozygous round seeded pea plant is crossed with a homozygous wrinkled pea seeded plant.

12. Monohybrid cross practice… 5.) In pea plants purple flowers are dominant to white flowers. Cross two white flowered plants.

13. Monohybrid cross practice… 6.) A white flowered pea plant is crossed with a pea plant that is heterozygous for the trait.

14. Monohybrid cross practice… 7.) Two pea plants, both heterozygous for the gene that controls flower color, are crossed.

15. Monohybrid cross practice… 8.) In guinea pigs, short hair is dominant over long hair. Show the cross for a pure breeding short haired guinea pig and a long haired guinea pig.

16. Monohybrid cross practice… 9.) Show the cross for two heterozygous guinea pigs.What percentage of the offspring will have short hair? ________What percentage of the offspring will have long hair? _______

17. Monohybrid cross practice… 10.) Two short haired guinea pigs are mated several times. Out of 100 offspring, 25 of them have long hair. What are the probable genotypes of the parents? Show the cross to prove it!

18. Dihybrid cross practice IN PEAS: R = round T = tall Y = yellow peas P = purple flowers r = wrinkled t = short y = green peas p = white flowers 1.) Homozygous tall, round parent X pure short, wrinkled parent

19. Dihybrid cross practice… IN PEAS: R = round T = tall Y = yellow peas P = purple flowers r = wrinkled t = short y = green peas p = white flowers 2.) Heterozygous for both height and flower color parent X short, white flowers parent

20. Dihybrid cross practice… IN PEAS: R = round T = tall Y = yellow peas P = purple flowers r = wrinkled t = short y = green peas p = white flowers 3.) Green peas, short plant X Heterozygous for yellow peas, homozygous for tall parent

21. Dihybrid cross practice… IN PEAS: R = round T = tall Y = yellow peas P = purple flowers r = wrinkled t = short y = green peas p = white flowers 4.) Heterozygous round, green peas X wrinkled peas, Heterozygous yellow peas

22. Dihybrid cross practice… IN PEAS: R = round T = tall Y = yellow peas P = purple flowers r = wrinkled t = short y = green peas p = white flowers 5.) Both parents heterozygous for height and flower color

23. The Principle of Independent Assortment • Another of Mendel’s ideas • Explains the results of these types of dihybrid crosses • Each different trait is inherited independently from the other • Now we know that this is due to meiosis – homologous chromosomes separate independently (again, when?)

24. Probability Rules • The product rule: • Predicts the combined probability of 2 independent events • If two or more events are independent of each other, the probability of both occurring is the product of their individual probabilities • Example: coin toss – heads two times in a row • ½ X ½ = ¼ or one chance in 4 • Also: Bb X Bb parents – producing a bb child • ½ b X ½ b = ¼ or one chance in 4

25. Probability Rules… • The sum rule • Predicts the combined probabilities of mutually exclusive events • If there is more than one way to get a result, we combine the probabilities by summing • Example: Bb X Bb parents chance of Bb child • 2 possibilities: B egg + b sperm; b egg + B sperm • B egg (½) X b sperm (½) = ¼ • And b egg (½) X B sperm (½) = ¼ • Then … ¼ + ¼ = ½

26. Probability Rules… • Most important – • Chance has no memory! • If events are truly independent, past events have no influence on the probability of future events… • Even though we don’t like this idea – ‘my luck is bound to change…’

27. Things Mendel didn’t know • Linked genes - inherited together because they are located on the same chromosome • Linked genes do not undergo segregation or independent assortment • The rates of crossing over can be used to determine the relative positions of genes on a chromosome • Higher crossing over rates indicate greater separation of genes on a chromosome • Each %age of crossing over rate = one map unit

28. Things Mendel didn’t know… • Sex chromosomes: • Female = XX - Male = XY • Male produces the sex determining gamete • In humans, Y chromosome has the SRY gene (sex reversal gene on the Y) – this acts as a genetic switch to cause testes to develop • Developing testes produce testosterone which determines other sexual characteristics • Everyone has at least one X – female is the ‘default’ sex… need a Y to develop as a male

29. Things Mendel didn’t know… • Sex-linked genes: • Located on the X chromosome only • Include genes for color perception and blood clotting – things all humans need • Females get two copies – can be either homozygous or heterozygous • Males only get one copy – they are hemizygous • Defects in these traits arise more in males than in females

31. Things Mendel didn’t know… • Dosage compensation: • Makes equivalent the female’s 2 ‘doses’ of the genes on the X chromosome to the male’s 1 • One X chromosome in each female cell is inactivated – called a Barr body • Individuals with heterozygous X-linked genes will often have a variegated phenotype as random X chromosomes are inactivated in the body

32. Calico cats – an example of X chromosome inactivation in action

33. Things Mendel didn’t know… • Incomplete dominance • The phenotype of the heterozygous individual is a blending of the two genes

34. Things Mendel didn’t know… ABO Blood Types: A and B alleles are codominant to each other Both are dominant to type O allele • Codominance: • The phenotype of the heterozygous individual expresses both genes, but without blending

35. Things Mendel didn’t know… • Multiple alleles: • Three or more alleles exist in the population, even though each individual only has two • Examples: • ABO blood types in humans • Coat color in rabbits

36. Things Mendel didn’t know… • Pleiotrophy • One gene with many effects • Often found in genetic diseases • Example: • Cystic fibrosis in humans • Homozygous individuals produce abnormally thick mucus in many body systems

37. Things Mendel didn’t know… • Epistasis • The presence of one allele can prevent or mask the expression of a gene at another loci • Example: • Coat color in Labrador retrievers • Pigment gene is either B (black) or b (brown) • Recessive ee blocks the expression of either BlackBrownYellow BBEE bbEE BBee BbEE bbEe Bbee BBEe bbee BbEe

38. Things Mendel didn’t know… • Polygenic inheritance • Mulitple independent pairs of genes have similar and additive effects on the phenotype • The phenotypes in a population will generally show a normal distribution curve • Examples: • Human skin and eye color

39. Things Mendel didn’t know… • Environmental interaction • Genetically identical individuals show different phenotypes based on environmental factors • Example: • Human height and intelligence • Nature versus nurture questions • Problems with experimental methods to answer these questions in humans….