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Chapter 15~ The Chromosomal Basis of Inheritance ________ The Chromosomal Theory of Inheritance Genes have specific loci on chromosomes and chromosomes undergo segregation and independent assortment Chromosomal Linkage Thomas Hunt Morgan 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 Sex-linkage: genes located on a sex chromosome More common in men, due to XY chromosomes, inability to masked faulty X conditions Sex Linkage inheritance 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 If Father has an X-linked condition – His sons can only inherited Ychromosome – His daughters will inherited the faulty X chromosome, 100% chance of being at least carrier How to get a female with X-linked condition? – Need mother who is a carrierXcX, and father with condition XcY – 50/50 chance of female with condition XcXc Human sex-linkage Sex-Linked Disorders – Color-blindness – Duchenne muscular dystropy (MD) – hemophilia X-inactivation: 2nd X chromosome in females condenses into a Barr body (e.g., tortoiseshell gene in calico cats) Genetic recombination 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 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 Linkage maps – Genetic map based on recombination frequencies Chromosomal errors, I Nondisjunction: – Homologous chromosomes or sister chromatids do not separate properly during meiosis I or meiosis I Aneuploidy: chromosome number is abnormal – Monosomy~ missing chromosome – Trisomy~ extra chromosome – Polyploidy~ extra sets of chromosomes Chromosomal errors, II Alterations of chromosomal structure: Deletion: removal of a chromosomal segment Duplication: repeats a chromosomal segment Inversion: segment reversal in a chromosome Translocation: movement of a chromosomal segment to another