Chapter 15~ The Chromosomal Basis of Inheritance ________

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

Chromosomal Linkage 4 Thomas Hunt Morgan 4 Drosophilia melanogaster –Observed white eye mutation in males only –Red eyes are more common phenotype, known as “wild type” –Concluded that eye color gene was located on X chromosome 4 Sex-linkage: genes located on a sex chromosome 4 More common in men, due to XY chromosomes, inability to masked faulty X conditions

Sex Linkage inheritance 4 If mother is a carrier of X linked condition –Her son(s) will have a 50/50 chance of having the condition –Her daughters will have 50/50 chance of being carriers 4 If Father has an X-linked condition –His sons can only inherited Y- chromosome –His daughters will inherited the faulty X chromosome, 100% chance of being at least carrier 4 How to get a female with X-linked condition? –Need mother who is a carrierX c X, and father with condition X c Y –50/50 chance of female with condition X c X c

Human sex-linkage 4 Sex-Linked Disorders –Color-blindness –Duchenne muscular dystropy (MD) –hemophilia 4 X-inactivation: 2nd X chromosome in females condenses into a Barr body (e.g., tortoiseshell gene in calico cats)

Genetic recombination 4 Linked genes –Genes that are located very close on a chromosome and will be inherited together. –Only way to “unlink” them is due to random chance of crossing over 4 Genetic maps –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

Chromosomal errors, I 4 Nondisjunction: –Homologous chromosomes or sister chromatids do not separate properly during meiosis I or meiosis I 4 Aneuploidy: chromosome number is abnormal –Monosomy~ missing chromosome –Trisomy~ extra chromosome –Polyploidy~ extra sets of chromosomes

Chromosomal errors, II 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