1. 4 Chapter 15~ The Chromosomal Basis of Inheritance

2. The Chromosomal Theory of Inheritance 4 Genes have specific loci on chromosomes and chromosomes undergo segregation and independent assortment

3. Chromosomal Linkage 4 Thomas Hunt Morgan – work proved that genes are on specific chromosomes 4 Worked with Drosophilia melanogaster (fruit fly) 4 Crossed a red eyed female w/ a white eyed male –All F1 were red –F2 = 3red:1white –Found certain traits seemed to appear only in males 4 XX (female) vs. XY (male) 4 Sex-linkage: genes located on a sex chromosome 4 Linked genes: genes located on the same chromosome that tend to be inherited together

4. Genetic recombination 4 Crossing over –Genes that DO NOT assort independently of each other –Material is exchanged before assortment happens 4 Genetic maps –Process discovered by Sturtevant (student of Morgans) –Ordered list of genetic loci along a particular chromosome –The further apart 2 genes are, the higher the probability that a crossover will occur between them and therefore the higher the recombination frequency 4 Linkage maps –Genetic map based on recombination frequencies

5. Human sex-linkage 4 SRY gene: gene on Y chromosome that triggers the development of testes 4 Fathers= pass X-linked alleles to all daughters only (but not to sons) 4 Mothers= pass X-linked alleles to both sons & daughters 4 Sex-Linked Disorders: Color-blindness; Duchenne muscular dystropy (MD); hemophilia 4 X-inactivation: 2nd X chromosome in females condenses into a Barr body (randomly) – genes are only expressed from one 4 Females may be a mosaic 4 Barr body is reactivated in ovaries 4 Ex. Tortoiseshell cats & Calico cats (white determined by another gene)

6. Chromosomal errors, I 4 Nondisjunction: members of a pair of homologous chromosomes do not separate properly during meiosis I or sister chromatids fail to separate during meiosis II 4 Aneuploidy: chromosome number is abnormal –Monosomy~ missing chromosome–Turner syndrome –Trisomy ~ extra chromosome - Down syndrome (pair 21), Kleinfelter syndrome –Polyploidy~ extra sets of chromosomes – common in plants

7. Chromosomal errors, II 4 Alterations of chromosomal structure: 4 Deletion: removal of a chromosomal segment 4 Duplication: repeats a chromosomal segment 4 Inversion: segment reversal in a chromosome 4 Translocation: movement of a chromosomal segment to another

8. Genomic imprinting 4 Def: a parental effect on gene expression 4 Identical alleles may have different effects on offspring, depending on whether they arrive in the zygote via the ovum or via the sperm. 4 Fragile X syndrome: higher prevalence of disorder and retardation in males