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 autosomes and 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 or XY)  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 error can 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 syndrome is 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 syndrome is 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