Ch. 15: Chromosomes and Genetics

1. Ch. 15: Chromosomes and Genetics • 1860’s: Mendel’s genetic experiments • 1875: Process of mitosis discovered by cytologists • 1890: Process of meiosis discovered by cytologists • 1900: Three botanists independently rediscovered Mendel’s principles of segregation and independent assortment • 1902: Cytology and genetics converge as scientists notice parallels between the behavior of Mendel’s factors and the behavior of chromosomes.

2. chromosomal theory of inheritance  Mendelian factors or genes are located on chromosomes which segregate and independently assort.

3. Thomas Hunt Morgan Columbia University early 1900’s selected the fruit fly, Drosophila melanogaster easily cultured, short generation time • Fruit flies have three pairs of autosomesand one pair of sex chromosomes.

4. Wild type – normal or most frequently observed phenotype • Mutant phenotype – phenotypes due to mutations in the wild-type gene  A gene’s symbol is based on the first mutant ( non-wild) If the mutant is recessive, the first letter is lower case (e.g. w = white eye allele in Drosophila) If the mutant is dominant, the first letter is capitalized Wild type traits are designated by a superscript +.

5. Linked Genes Genes found on the same chromosome Using recombination frequencies, distances between linked genes can be determined Recombination frequencies of 50% indicate unlinked genes or linked genes located so far apart that crossing over makes them appear unlinked

6. X-Y mechanism that determines sex at fertilization. When a sperm cell + ovum = zygote ( XX orXY)  A single gene, Sry, on the Y chromosome leads to normal male development. Sry probably codes for a protein that regulates other genes.

7. Fathers pass X-linked alleles to only and all of their daughters.  Males receive their X chromosome only from their mothers. Therefore, fathers cannot pass sex-linked traits to their sons. Mothers can pass sex-linked alleles to both sons and daughters.  Females receive two X chromosomes, one from each parent. Mothers pass on one X chromosome (either maternal or paternal) to each daughter and son Some examples of sex-linked traits in humans are color blindness, muscular dystrophy, and hemophilia.

8. Sex-linked disorders in humans: The human X-chromosome is much larger than the Y. Thus, there are more X-linked than Y-linked traits.  Most X-linked genes have no homologous loci on the Y chromosome.  Most genes on the Y chromosome not only have no X- counterparts, but they encode traits found only in males (e.g. testis-determining factor).

9. Nondisjunction – meiotic or mitotic error during which certain homologous chromosomes or sister chromatids fail to separate.

10. Aneuploidy – a condition where there is an abnormal number of certain chromosomes. An example of this is Down’s syndrome, which results from trisomy of chromosome 21. When an aneuploid zygote divides by mitosis, it transmits the chromosomal anomaly to all subsequent embryonic cells.

11. Chromosome breakage can alter chromosome structure in four ways: 1. Deletion – complete loss of a fragment of a chromosome lacking a centromere.2. Duplication– the lost fragment can join a homologous chromosome. 3. Translocation – the lost fragment can join to a nonhomologous chromosome.4. Inversion – the lost fragment reattaches to the original chromosome in reverse order.

12. Crossing-over errorcan also be a source of deletions and duplications

13. Aneuploidy Disorders: Down syndrome– 1 out of 700 U.S. childrentrisomy 21 Includes characteristic facial features, short stature, heart defects, mental retardation, susceptibility to respiratory infections, and a proneness to develop leukemia and Alzheimer’s disease. “most” are sexually underdeveloped/sterile

14. The incidence of Down syndrome offspring correlates with maternal age. WHY? think of how meiosis occurs in females

15. Sex chromosome aneuploidies in males: 1. Klinefelter Syndrome  genotype: XXY phenotype: Male sex organs with small testes; sterile; feminine body and perhaps breast enlargement; usually of normal intelligence

16. 2. Extra Y genotype: XYY phenotype: normal male; usually taller than average; normal intelligence and fertility

17. Sex chromosome aneuploidies in females: 1. Triple-X Syndrome genotype: XXX phenotype: usually fertile; can show a normal phenotype

18. 2.Turner Syndrome genotype: XO phenotype: short in stature; secondary sexual characteristics fail to develop; internal sex organs do not mature; sterile

19. Genomic imprinting causes certain genes to be expressed differently in the offspring depending upon whether the alleles were inherited from the ovum or from the sperm cell.

20. Prader-Willi syndromeis cause by a deletion from the paternal version of chromosome 15. characterized by mental retardation, obesity, short stature, and unusually small hands and feet.

21. Angelman syndromeis caused by a deletion from the maternal version of chromosome 15. characterized uncontrollable spontaneous laughter , jerky movements, and various motor and mental symptoms.

22. Fragile X syndrome - symptoms more common when inherited from maternal ovum.

23. X- Inactivation Barr body= inactive X in each cell of female Results in calico cats, sweat gland development