The Chromosomal Basis of Inheritance in Depth

1. Chapter 15 The Chromosomal Basis of Interhitance

2. The Chromosome Theory Thomas Hunt Morgan worked with Drosophila (dew loving) and showed a connection between chromosomes and inheritance

3. Female Male

4. Chromosome theory The unifying theory stating that inheritance patterns may be generally explained by assuming that genes are located in specific sites on chromosomes, and found in a linear sequence.

5. Figure 15.2 Morgan’s first mutant

6. Chromosome Map of Drosophila (Fruit Flies)

7. Fruit Fly Inheritance

8. Recombination due to Crossing Over

9. Sex linked traits occur because… they are determined by genes located on (linked) either the X or Y chromosome, but not on both.

11. Sex linked traits… Hemophilia: an X-linked genetically inherited recessive disease in which one or more of the normal blood clotting factors is not produced. Hemophilia most often afflicts males.

12. Other X - Linked traits • Hemophilia • Red-green color blindness • Duchene muscular dystrophy • Fragile X-syndrome

13. Colorblind persons can see the circle but not the star

14. Y - linked trait… This trait always occurs when the gene is present…but we can’t know if it is dominant. Why not?

15. X Inactivation in Female Mammals • During fetal development, one female X chromosome become inactive and lies along the inside of the nuclear membrane • Which X chromosome inactivates is random and varies from cell to cell, however during mitotic division the same X chromosome remains inactive • The inactive X chromosome is called a Barr body

16. Only occurs in female mammals since they have 2 X chromosomes, unlike men Females consist of a mosaic of two types of cells; those with active and those with inactive X chromosomes BAR BODIES

17. X Inactivation in Female Mammals Tortoiseshell cats show mosaicism with patches of orange and black fur. Therefore, all tortoiseshell cats are female.

18. Alterations of Chromosome Number • Nondisjunction – members of a pair of homologous chromosomes fail to move apart properly during meiosis I or sister chromosomes do not separate during meiosis II. • One gamete receives two of the same type of chromosome and the other receives no copy.

19. Nondisjunction

20. Alterations of Chromosome Number • Aneuploidy – an abnormal number of chromosomes • Monosomic – a chromosome is missing (2n - 1) • Trisomic – a chromosome is present in triplicate (2n + 1) • Polyploidy – an organism with two or more complete chromosome sets • Triploidy – 3n • Tetraploidy – 4n

21. Alterations of Chromosome Structure Caused by a breakage of a chromosome

22. Translocation

23. Human Disorders Due to Chromosomal Alterations • Down’s syndrome affects 1 out of 700 births in the US • Results from a trisomy of chromosome 21 (2n+1) • Characteristic facial features, short stature, heart defects, susceptibility to respiratory infection, and mental retardation • Individuals are also more prone to develop leukemia and Alzheimer's disease; these diseases are located on chromosome 21 • Risk – 0.04% for women under 30 1.25% for women over 30

24. Human Disorders Due to Chromosomal Alterations Down’s syndrome

25. Human Disorders Due to Chromosomal Alterations Klinefelter’s Syndrome • XXY sex chromosomes • Individual usually sterile • Have both male and female secondary sexual characteristics • Occurs in offspring of mother’s over 40 years old.

26. Human Disorders Due to Chromosomal Alterations XYY Condition • Males who have an extra Y chromosome • Tend to be very tall and have subnormal intelligence • 1 in 1000 births

27. Human Disorders Due to Chromosomal Alterations Turner’s Syndrome • Monosomy X chromosome; individual is female • 1 in 5000 births • Usually sterile

28. Human Disorders Due to Chromosomal Alterations Triple X Syndrome • Trisomy – XXX chromosomes • 1 in 1000 births • Usually a healthy female

29. Human Disorders Due to Chromosomal Alterations • Trisomy chromosome 8 • Mental retardation, skeletal deformities, internal organs malformed • 1 in 3,000 births • Patau Syndrome • Trisomy 13 • Malformations of major organs • Usually die within 1 year • 1 in 7,500 – 10,000 births • 88% maternal; 12% paternal

30. Patau’s Syndrome – Trisomy 13

31. Human Disorders Due to Chromosomal Alterations • Edward Syndrome • Trisomy 18 • Thin, frail children with short eyelids, low set ears, webbed fingers & toes, and skin folds. • In males, the testes do not descend • 20-30% die in the first month • 90% die by the age of 1 • 1 in 5,000 births • Affects girls more than boys

32. Human Disorders Due to Chromosomal Alterations Cri du Chat • Deletion of the short arm of chromosome 5 • High pitched cry, low birth weight • Difficulties sucking and swallowing • Mental retardation • Most die in infancy – those that live are severely delayed in development • 1 in 50,000 live births

33. Constructing a Pedigree • Use the appropriate symbols: Unaffected Male Unaffected Female Affected Male Affected Female Male carrier of trait Mating of Offspring 2. Label each generation down the left hand side of your pedigree. 3. Label each individual in your pedigree with his or her name. 4.  It is easiest to construct your pedigree working from the most recent generation backwards.