Chapter 14 Chromosomes and Human Inheritance

1. Chapter 14Chromosomes and Human Inheritance

2. 14.1 Shades of Skin • Variations in skin color may have evolved as a balance between vitamin D production and UV protection • More than 100 gene products are involved in the synthesis of melanin, and the formation and deposition of melanosomes • Mutations in some of these genes may have contributed to regional variations in human skin color

3. Variation in Human Skin Color

4. 14.1 Human Chromosomes • Geneticists study inheritance patterns in humans by tracking genetic disorders and abnormalities through families • Charting genetic connections with pedigrees reveals inheritance patterns of certain traits

5. Pedigrees • Inheritance patterns in humans are typically studied by tracking observable traits in families over generations • A standardized chart of genetic connections (pedigree) is used to determine the probability that future offspring will be affected by a genetic abnormality or disorder • Pedigree analyses also reveals whether a trait is associated with a dominant or recessive allele, and whether the allele is on an autosome or a sex chromosome

6. Standard Symbols Used in Pedigrees male female marriage/mating offspring individual showing trait being studied sex not specified generation

7. Polydactyly

8. A Pedigree for Polydactyly

9. A Pedigree for Huntington’s Disease

10. Genetic Abnormalities and Disorders • A genetic abnormality is an uncommon version of a trait that is not inherently life-threatening, • A genetic disorder causes medical problems that may be severe • A genetic disorder is often characterized by a specific set of symptoms (a syndrome)

11. Types of Genetic Variation • Single genes on autosomes or sex chromosomes govern more than 6,000 genetic abnormalities • Most human traits, including skin color, are polygenic (influenced by multiple genes) and some have epigenetic contributions or causes

12. Patterns of Inheritance • Based on variations in single genes (Mendelian patterns) • Autosomal dominant • Autosomal recessive • X-linked recessive • X-linked dominant • Based on variations in whole chromosomes • Changes in chromosome number • Changes in chromosome structure

13. Table 14-1 p221

14. Table 14-1 p221

15. Table 14-1 p221

16. Table 14-1 p221

17. Recurring Genetic Disorders • Mutations that cause genetic disorders are rare and put their bearers at risk • Such mutations survive in populations for several reasons • Reintroduction by new mutations • Recessive alleles are masked in heterozygotes • Heterozygotes may have an advantage in a specific environment

18. Take-Home Message: How do we study inheritance patterns in humans? • Inheritance patterns in humans are often studied by tracking traits through generations of families • A genetic abnormality is a rare version of an inherited trait; a genetic disorder is an inherited condition that causes medical problems • Some human genetic traits are governed by a single gene and inherited in a Mendelian fashion; many others are influenced by multiple genes and epigenetics

19. ANIMATED FIGURE: Pedigree diagrams To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

20. 14.3 Autosomal Inheritance Patterns • An allele is inherited in an autosomal dominant pattern if the trait it specifies appears in heterozygous people • An allele is inherited in an autosomal recessive pattern if the trait it specifies appears only in homozygous people

21. Autosomal Dominant Inheritance • A dominant autosomal allele is expressed in homozygotes and heterozygotes • Tends to appear in every generation • With one homozygous recessive and one heterozygous parent, children have a 50% chance of inheriting and displaying the trait • Examples: • Achondroplasia • Huntington’s disease • Hutchinson–Gilford progeria

22. normal mother affected father meiosis and gamete formation affected child normal child disorder-causing allele (dominant) Stepped Art Figure 14-3a p222

23. Figure 14-3b p222

24. Figure 14-3c p222

25. Autosomal Recessive Inheritance • Autosomal recessive alleles are expressed only in homozygotes • Heterozygotes are carriers and do not have the trait • A child of two carriers has a 25% chance of expressing the trait • Examples: • Albinism • Tay-Sachs didease

26. carrier father carrier mother meiosis and gamete formation affected child carrier child normal child disorder-causing allele (recessive) Stepped Art Figure 14-4a p223

27. Figure 14-4b p223

28. INTERACTION: Autosomal-dominant inheritance To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

29. INTERACTION: Autosomal-recessive inheritance To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

30. Take-Home Message: How do we know a trait is associated with an allele on an autosome? • With an autosomal dominant inheritance pattern, persons heterozygous for an allele have the associated trait; the trait appears in every generation • With an autosomal recessive inheritance pattern, only persons who are homozygous for an allele have the associated trait, which can skip generations

31. 12.4 Examples of X-Linked Inheritance • Traits associated with recessive alleles on the X chromosome appear more frequently in men than in women • A man cannot pass an X chromosome allele to a son • Mutated alleles on the X chromosome cause or contribute to over 300 genetic disorders

32. X-Linked Recessive Pattern • More males than females have X-linked recessive genetic disorders • Males have only one X chromosome and can express a single recessive allele • A female heterozygote has two X chromosomes and may not show symptoms • Males transmit an X only to their daughters, not to their sons

33. carrier mother normal father meiosis and gamete formation normal daughter or son carrier daughter affected son recessive allele on X chromosome Stepped Art Figure 14-6a1 p224

34. Some X-Linked Recessive Disorders • Red-green color blindness • Inability to distinguish certain colors caused by altered photoreceptors in the eyes • Duchenne muscular dystrophy • Degeneration of muscles caused by lack of the structural protein dystrophin • Hemophilia A • Bleeding caused by lack of blood-clotting protein

35. Figure 14-6b1 p224

36. You may have one form of red–green color blindness if you see a 7 in this circle instead of a 29. You may have another form of red–green color blindness if you see a 3 instead of an 8 in this circle. Figure 14-6c1 p224

37. INTERACTION: X-linked inheritance To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

38. Hemophilia A in Descendents of Queen Victoria of England

39. Take-Home Message: Is a trait associated with an allele on an X chromosome? • Men who have an X-linked recessive allele have the trait associated with the allele; heterozygous women do not, they have a normal allele on their second X chromosome – the trait appears more often in men • Men transmit an X-linked allele to their daughters, but not to their sons

40. 14.5 Heritable Changes in Chromosome Structure • On rare occasions, a chromosome’s structure changes; such changes are usually harmful or lethal, rarely neutral or beneficial • A segment of a chromosome may be duplicated, deleted, inverted, or translocated

41. Duplication • DNA sequences that are repeated two or more times • Duplication may be caused by unequal crossovers in prophase

42. Deletion • Loss of some portion of a chromosome • Usually causes serious or lethal disorders • Example: Cri-du-chat

43. Inversion • Part of the sequence of DNA becomes oriented in the reverse direction, with no molecular loss

44. Translocation • If a chromosome breaks, the broken part may get attached to a different chromosome, or to a different part of the same one • Most translocations are reciprocal, or balanced, which means that two chromosomes exchange broken parts • A reciprocal translocation between chromosomes 8 and 14 is the usual cause of Burkitt’s lymphoma

45. Translocation D With a translocation, a broken piece of a chromosome gets reattached in the wrong place. This example shows a reciprocal translocation, in which two chromosomes exchange chunks.

46. Chromosome Changes in Evolution • Changes in chromosome structure can reduce fertility in heterozygotes; but accumulation of multiple changes in homozygotes may result in new species • Certain duplications may allow one copy of a gene to mutate while the other carries out its original function • Example: X and Y chromosomes were once homologous autosomes in reptile-like ancestors of mammals

47. Evolution of X and Y Chromosomes from Homologous Autosomes

48. (autosome pair) Ancestral reptiles >350 mya Figure 14-9a p227

49. Y X SRY Ancestral reptiles 350 mya Figure 14-9b p227

50. Y X area that cannot cross over Monotremes 320–240 mya Figure 14-9c p227