BEYOND MENDEL

1. BEYOND MENDEL Chapter 14

2. Mendelian Inheritance in Humans

3. Incomplete Dominance • Heterozygotes have intermediate phenotype • i.e. Snapdragon flower color • Notation varies widely, so on the AP test use the words “incompletely dominant”

4. Figure 14.9 Incomplete dominance in snapdragon color

5. Figure 14.9x Incomplete dominance in carnations

6. Codominance • Two alleles affect phenotype in separate, distinguishable ways • Both show up if both are present • i.e. checkered chicken feathers, ABO blood types • Blood type • Alleles IA and IB specify different enzymes that attach different sugars • IAIB - both enzymes, both sugars = codominance • ii – no sugars, no enzymes

7. Multiple Alleles • Most genes have more than two allele forms • i.e. ABO blood types is also an example of a trait with 3 alleles for 4 different phenotypes • Alleles: IA, IB, i • Phenotypes: A, B, AB, or O blood types

8. Figure 14.10 Multiple alleles for the ABO blood groups

9. Figure 14.10x ABO blood types

11. Principles of Dominance and Recessiveness Relationships • Ranges from complete dominance and various degrees of incomplete and codominance • Alleles do not involve the ability of one allele to supress another at the level of the DNA • i.e. round vs. wrinkled pea shape • Dominant allele codes for synthesis of an enzyme that coverts sugar to starch. • Recessive wrinkled pea, starch is not made, the sugar causes osmotic uptake of water and when it dries it wrinkles • Dominance does not mean “common” in a population • i.e. polydactyly is dominant

12. Polydactyly is Dominant!!

13. Pleiotropy • One gene affects an organism in many ways • i.e Sickle-cell disease and Marfan Syndrome • Marfan Syndrome is a mutation in the gene for fibrillin, a protein that occurs in connective tissue. It affects 1 in every 10,000 people • Skeleton • Heart and blood vessels • Lungs • Nervous System • Eyes • Skin

14. Figure 14.15 Pleiotropic effects of the sickle-cell allele in a homozygote

15. Marfan Syndrome

16. Epistasis • Gene at one locus alters the phenotypic expression of a different gene at a second locus • i.e. Hair color in mammals: mice, labradors; comb shape in poultry • In mice, black (B) coat is dominant to brown coat color (b) • A second gene determines whether or not pigment will be deposited in the hair…dominant (C) for color results in deposition • If the mouse is cc, it will be albino regardless of the genotype at the black/brown locus.

17. Figure 14.11 An example of epistasis

20. Polygenes • Quantitative characteristics vary along a continuum in a population • Additive effect of two or more genes on a single phenotype. • This is the converse of pleiotropy! • i.e. Skin pigmentation is controlled by at least 3 separately inherited genes (A, B, C) that add together to determine the darkness of skin color • AaBbCc would have an intermediate skin color

21. Figure 14.12 A simplified model for polygenic inheritance of skin color

23. What would the genotype of a person with light skin color be? ____________ • What would the genotype of a person with very dark skin color be? _____________

24. Environmental Effects • Nature vs. Nurture • genetic and environmental factors affect phenotypes • i.e. hydrangea color – affected by soil (pH, water, temp, etc.)

26. Figure 14.14 Pedigree analysis • Reveals the patterns in human inheritance through generations in families

28. Recessively Inherited Genetic Disorders • Allele that causes a genetic disorder codes either for a malfunctioned protein or for no protein at all • Heterozygotes produce a sufficient amount of the specific protein • Carriers because they can transmit the recessive allele to their offspring • They do not have the disease • To have the disorder, one must be homozygous recessive

29. Recessively Inherited Genetic Disorders • Albinism • Cystic Fibrosis • Tay-Sachs • ALS (Lou Gehrig’s) • Sickle Cell Anemia • Phenylketonuria (PKU) • Alkaptonuria

30. Amyotropic Lateral Sclerosis (Lou Gerhig’s Disease) • Neurodegenerative – degradation of motor nerves that control voluntary • Leads to twitching, denervation, atrophy – loss of all movement • Mutation on chromosome 21 • 1-2 of 100,000 people/year

31. Tay-Sachs Disease Heterozygotes resistant to tuberculosis

32. Sickle Cell Anemia

33. Sickle cell patients resistant to malaria

34. Cystic Fibrosis Heterozygotes resistant to cholera

37. Dominantly Inherited Genetic Disorders • Offspring have a 50% chance of receiving the dominant allele. • Most lethal dominant alleles have survived in the population b/c late onset • Achondroplasia • Huntington’s disease • Progeria • Marfan syndrome

39. Multifactorial Disorders • Have a genetic component and a significant environmental influence • i.e. • Heart disease • Cancer • Diabetes • Alcoholism • Schizophrenia • Manic-depressive disorder

40. Genetic Testing and Counseling • Amniocentesis • 14th -16th week of pregnancy, needle inserted into uterus and extracts 10ml of amniotic fluid, fetal cells are cultured for several weeks then karyotyped • Chorionic Villus Sampling (CVS) • 8-10 weeks of pregnancy, narrow tube inserted vaginally that suctions out sample of fetal tissue from placenta; immediate karyotyping • Newborn Screening • used for many diseases such as PKU

41. PKU: Phenylketonuria

42. Figure 14.17 Testing a fetus for genetic disorders