1. The Chromosomal Basis of Interhitance
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. BAR BODIES
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

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.