Chapter 11 Complex Inheritance and Human Heredity

1. Chapter 11 Complex Inheritance and Human Heredity 11.1 Basic Patterns of Human Inheritance

2. Recessive Genetic Disorders • Mendel’s work went unnoticed by the scientific community for about 30 years then it was rediscovered in the early 1900s. • At that time many scientists were interested in the cause of diseases and noticed that some diseases “ran in families”. • Alkaptonuria was the first identified (recessive) genetic disorder. Alkaptonuria, from an enzyme deficiency, causes black acidic urine and later in life affects bones and joints.

3. Recessive Genetic Disorders • A recessive trait is expressed when the individual is homozygous recessive for the trait. • Both parents would need to have at least one recessive allele. • Usually the parents are heterozygous (carriers) for the disorder.

5. Cystic Fibrosis • Affects the mucus-producing glands, digestive enzymes, and sweat glands • Chloride ions are not absorbed into the cells of a person with cystic fibrosis but are excreted in the sweat. • Without sufficient chloride ions in the cells, a thick mucus is secreted

6. Cystic Fibrosis

7. Cystic Fibrosis

8. Albinism • Caused by altered genes, resulting in the absence of the skin pigment melanin in hair and eyes • White hair. • Very pale skin • Pink pupils

9. Albinism

10. Tay-Sachs Disease • Caused by the absence of the enzymes responsible for breaking down fatty acids called gangliosides • Gangliosides accumulate in the brain, inflating brain nerve cells and causing mental deterioration. • Death by age 2

11. Dominant Genetic Disorders 99.9% of population is homozygous recessive for achondroplasia

12. Huntington’s Disease • Affects the nervous system • Latent disorder affects age 30 to 50 • Gradual loss of brain function (“holes” in brain) • Genetic test available • Result of allele mutation at tip of chromosome #4

13. Achondroplasia • Most common form of dwarfism • 75% of individuals born to parents of average size, result of new mutation • Lethal spontaneous abortion in homozygous dominant genotype

14. Pedigree Analysis

15. Pedigree Analysis • Are females • Are males • Shaded in circles and squares are affected individuals • Roman Numerals (I – IV) are generations • Lines across represent mating • Lines down represent offspring

16. Pedigree Analysis • Count the number of affected males and affected females. If most males and few or no females most likely sex linked trait. • Look at the affected individuals. If every individual with the trait has a parent with the trait then this trait is dominant. If non-affected parents produce an offspring with the trait then it is recessive. • Determine the phenotype and genotype of every individual

17. Pedigree Analysis • Count the number of affected males and affected females. If most males and few or no females most likely sex linked trait. • Look at the affected individuals. If every individual with the trait has a parent with the trait then this trait is dominant. If non-affected parents produce an offspring with the trait then it is recessive. • Determine the phenotype and genotype of every individual

18. Affected males: 1 Affected females: 2 (not sex linked) No affected individual has parent with the trait, means recessive All affected individuals would be homozygous recessive, aa All parents of affected individuals would be heterozygous, Aa Siblings of affected individuals would be heterozygous (Aa) or homozygous dominant (AA) Pedigree Analysis

19. Pedigree Analysis

20. Chapter 11 Complex Inheritance and Human Heredity 11.2 Complex Patterns of Inheritance

21. Incomplete Dominance • Many scientists repeated Mendel’s work expecting to get similar results with different species. Carl Correns crossed white snapdragons with red snapdragons • According to Mendelian genetics, results would have yielded 100% dominant phenotype (red or white) • Correns found 100% pink

22. Incomplete Dominance • Incomplete Dominance is the appearance of an intermediate phenotype • Both alleles are expressed producing both the red and white proteins which gives the appearance of pink

23. Codominance • Both phenotypes of the two homozygotes are expressed • See red and white (not pink)

24. Both phenotypes of the two homozygotes are expressed See red hairs and white hairs (not pink) Codominance Roan Cattle

25. Codominance • Sickle cell disease most common genetic disorder in African Americans (1 in 400). • In heterozygous condition both normal (round) and sickle red blood cells are produced • Few disease symptoms in heterozygous individual

26. Codominance • Sickle cells do not effectively carry oxygen, cause blockage in blood vessels, pain and fatigue. • Heterozygous condition protects against malaria (see p.303)

27. Multiple Alleles • More than two alleles • Human blood groups have three alleles: ABO • Blood Types: Genotypes A: IA IA orIAi B: IB IB orIBi AB: IA IB O: ii Both IA and IB are dominant to i and are codominant to each other

28. Multiple Alleles • Multiple alleles can demonstrate a hierarchy of dominance. • In rabbits, four alleles code for coat color: C, cch, ch,and c. • C is dominant to cch, ch,and c. • cch is dominant to chand c. • ch is dominant to c. • c is recessive to all

29. Multiple Alleles Chinchilla ? Albino Light gray Dark gray Himalayan

30. Epistasis • Variety is the result of one allele hiding the effects of another allele. eebb eeB_ E_bb E_B_ No dark pigment present in fur Dark pigment present in fur

31. Sex Determination • Sex chromosomesdetermine an individual’s gender. • Human males are XY; females XX • Autosomes are the nonsex chromosomes • Humans have 22 pairs of autosomes

32. Dosage Compensation • The X chromosome is more than three times larger than the Y chromosome; has more than three times more information • Females (XX) have two copies of this information while males (XY) have only one copy of the X chromosome • In females one of the X chromosomes stops working; which X is a random event and becomes a Barr Body

33. Dosage Compensation • Barr bodies form from the inactivated “extra” X chromosome in females • Colors of the calico cat are caused by the random inactivation of a particular X chromosome (orange on one X and black on the other X chromosome)

34. Sex-Linked Traits • The X and Y chromosomes are not homologous (do not pair: carry different information) • Males only have one X chromosome; only one allele for each trait; only need one recessive to show the trait • Males show the sex linked traits much more often than females

35. Sex-Linked Traits Red Green Color Blindness • 8% of United States males affected • Red and green colors look like shades of brown • Are you color blind?

36. Hemophilia is characterized by delayed blood clotting. Common in Royal Families of Europe due to intermarriage Sex-Linked Traits

37. Polygenic Traits • Poly = many; genic = genes • Many genes needed to determine a trait • Most human traits are polygenic

38. Environmental Influences • Environment has an influence on phenotype • Sunlight: without enough sunlight most flowering plants do not bear flowers • Water: many plants lose their leaves in response to lack of water. • others: diet, exercise

39. Environmental Influences • Temperature: Siamese cat’s tail, feet, ears and nose are dark in response to cooler temperatures; the black pigment production is responsive to temperature

40. Twin Studies • Helps scientists separate genetic contributions from environmental contributions • Traits that appear frequently in identical twins are at least partially controlled by heredity. • Traits expressed differently in identical twins are strongly influenced by environment. • Interesting information from identical twins separated at birth in Minnesota Twin Studies

41. Chapter 11 Complex Inheritance and Human Heredity 11.3 Chromosomes and Human Heredity

42. Karyotype Studies • Karyotype is a micrograph (picture) of chromosomes at metaphase arranged by size in homologous pairs. • Twenty-two pairs of autosomes are the same for males and females. • Males have one X chromosome and one Y chromosome (not paired) and females have two X chromosomes (paired). • Karyotypes are useful for detecting chromosome abnormalities

43. Karyotype Studies Chromosome smear Karyotype

44. Telomeres • Telomeres protective end caps on the chromosomes made of DNA and proteins. • Short repetitive nonessential DNA sequences • Prevent one chromosome form binding to another chromosome (not sticky)

45. Telomeres • Absolutely essential for chromosome function • Telomeres decrease in size (length) as an organism ages and may play a role in aging and cancer. • Cancer cells have increased length of telomeres compared to adult cells. • Telomere function is an intense field of research

46. Nondisjunction • Failure of chromosomes to separate during cell division • Results in one cell getting too many chromosomes and one cell not getting enough. • If nondisjunction occurs during meiosis the resulting gametes could form an organism with every cell having an incorrect chromosome number.

48. Down Syndrome • Results of parental gamete nondisjunction resulting in trisomy (three chromosomes ) of chromosome #21. • Symptoms include: mental retardation, distinctive facial features, short stature, heart defects, sexually underdeveloped, sterile, more likely to develop leukemia and Alzheimer's, shorter life span

49. Down Syndrome

50. Down Syndrome • Incidence of Down Syndrome increases with increasing age of the mother • Risk increases to 6% for mothers over age 45 • Recommended fetal testing for mother’s age 35 and older