Human Inheritance

1. Human Inheritance Chapter 14

2. Human Genetic Analysis • Inheritance patterns in humans are typically studied by tracking observable traits that crop up in families over many generations • Data is organized in pedigree charts

3. Human Genetic Analysis • Pedigree chart symbols

4. Human Genetic Analysis

5. Human genetic analysis • Some easily observed human traits follow Mendelian inheritance patterns • Controlled by a single gene • Two alleles, one dominant and one recessive

6. Human Genetic Analysis • Mid-digital hair • Dominant condition (MM, Mm) • Presence of hair between the two top joints of your fingers • Even the slightest amount of hair indicates the dominant phenotype • Recessive condition (mm) • Complete absence of hair

7. Human Genetic Analysis • Tongue rolling • Dominant condition (TT, Tt) • Ability to roll one’s tongue • Recessive condition (tt) • Inability to roll the tongue

8. Human Genetic Analysis • Widow’s peak • Dominant condition (WW, Ww) • A distinct downward point in the frontal hairline • Recessive condition (ww) • A continuous hairline

9. Human Genetic Analysis • Earlobe attachment: • Dominant condition (EE, Ee) • Detached or free earlobes • Recessive condition (ee) • Earlobes attached directly to the head

10. Human Genetic Analysis • Hitchhiker’s thumb • Dominant condition (HH, Hh) • Cannot extend the thumbs backward to approximately 45° • Recessive condition (hh) • Can bend the thumbs at least 45°, if not further

11. Autosomal Inheritance Patterns • Dominant Patterns • Dominant alleles are expressed in both homozygotes and heterozygotes • The trait specified tends to appear in every generation

12. Autosomal Inheritance Patterns • Dominant Patterns • Examples • Achondroplasia • The allele interferes with formation of the embryonic skeleton • Adults average about 4’4” and have short limbs • Huntington’s disease • Nervous system slowly deteriorates • Involuntary muscle movements increase

13. Autosomal Inheritance Patterns • Recessive Patterns • Expressed only in homozygous people • Traits may skip generations • Heterozygotes are carriers of the allele, but do not express the trait

14. Autosomal Inheritance Patterns • Recessive Patterns • Examples • Albinism • Lack of melanin • Tay-Sachs disease • Malfunction of a lysosomal enzyme that breaks down gangliosides • Lipids accumulate to toxic levels in nerve cells

15. X-linked Inheritance Patterns • The X and Y chromosomes carry different genes • Recessive alleles on the X chromosome create a unique pattern of inheritance

16. X-linked Inheritance Patterns • More males affected • Each male receives one X and one Y chromosome • Two possible genotypes: XAY, XaY • Females receive two X chromosomes • Three possible genotypes: XAXA, XAXa , XaXa • Thus heterozygous males are affected while heterozygous females are not

17. X-linked Inheritance Patterns • Affected fathers cannot pass X-linked recessive alleles to a son • All children who inherit their father’s X chromosome are female XA XA Xa XAXa XAXa Daughters Y XAY XAY Sons

18. X-linked Inheritance Patterns • Affected fathers cannot pass X-linked recessive alleles to a son • Heterozygous females are the bridge between an affected male and his affected grandson Heterozygote daughter XA XA XA Xa Xa XAXa XAXa XA XAXA XAXa Affected father Y XAY XAY Y XAY XaY Affected grandson

19. X-linked Inheritance Patterns • Examples • Red-Green color blindness • Hemophilia A • Duchenne Muscular Dystrophy

21. Fig. 14.8, p. 209

22. X-linked Inheritance Patterns • X-linked dominant alleles that cause disorders are rarer because they tend to be lethal in male embryos • X-linked hypophosphatemic rickets • Rett syndrome • Aicardi syndrome

23. Heritable Changes in Chromosome Structure • Changes in chromosome structure • Are rare • Usually cause drastic health effects • Responsible for ~1/2 of miscarriages • Genetic disorders • Sometimes evolutionarily important • Occur spontaneously or induced by exposure to certain chemicals or radiation

24. Heritable Changes in Chromosome Structure • Changes in chromosome structure • Duplication • Happens during prophase I of meiosis • Crossing over occurs unequally between homologs • One chromosome will have a deleted segment • The other will have the duplication • Huntington’s

25. Heritable Changes in Chromosome Structure • Changes in chromosome structure • Deletion • The loss of some portion of a chromosome • Cause serious disorders and are often lethal in mammals • Cri-du-chat • Deletion in chromosome 5 • Causes mental impairment and an abnormally shaped larynx

26. Heritable Changes in Chromosome Structure • Changes in chromosome structure • Inversion • A segment of DNA is flips in the reverse direction • Usually no genes are lost • Causes infertility • Inverted segments cause homologs to mispair during meiosis

27. Heritable Changes in Chromosome Structure • Changes in chromosome structure • Translocation • A piece of one chromosome may break and attach • to a different chromosome or • to a different part of the same chromosome • Burkitt’s lymphoma: reciprocal translocation between chromosomes 8 and 14 • Can cause infertility (mis-pairing during meiosis)

28. Heritable Changes in Chromosome Structure • Chromosome changes in Evolution • Large-scale changes in chromosomes can lead to speciation • Most are usually lethal or cause genetic disorders and infertility • In a very few instances chromosome changes can be adaptive • Multiple globin chain genes possibly arose due to duplications • An individual homozygous for an inversion could become the founder of a new species

29. Heritable Changes in Chromosome Number • During meiosis homologs may fail to separate • Referred to as nondisjunction • Affects the chromosome number at fertilization • Normal gamete (n) + nondisjunction gamete (n + 1) = zygote (2n +1) • Trisomic zygote will have three of one type of chromsome and two of every other type • Normal gamete (n) + nondisjunction gamete (n - 1) = zygote (2n -1) • monosomic zygote will have one of one type of chromsome and two of every other type

30. Metaphase I Anaphase I Telophase I Metaphase II Anaphase II Telophase II Stepped Art Fig. 14.12, p. 212

31. Heritable Changes in Chromosome Number • Trisomy and monosomy • Trisomy (polyploid) is common in some plants, insects, and fish • Usually fatal in humans • Trisomy 21 individuals will survive infancy • Down syndrome

32. Fig. 14.13, p. 213

33. Heritable Changes in Chromosome Number • Changes in Sex Chromosome Number • Turner syndrome (XO) • Inherit an unstable Y from the dad • Female, short • XXX syndrome • Usually does not result in physical or medical problems • Klinefelter syndrome (XXY) • Overweight, tall, normal intelligence, estrogen > testosterone • XYY syndrome • Taller than average, mild mental impairment • NOT predisposed to a life of crime

34. Summary • Human genetic analysis • Pedigree charts • Autosomal inheritance patterns • Dominant and recessive • X-linked inheritance patterns • More common in males • Chromosome structure changes • Duplications • Deletions • Inversions • Translocations • Chromosome number changes • Trisomy • Sex chromosome numbers

35. Fig. 14.17, p. 217

36. p. 217