6. Genotype predictions and linkage maps are derived from knowing allele configurations (coupling or repulsion) and crossover frequencies, which are directly proportional to distances between genes.
11.3 How Chromosomes Determine Sex
7. Sex determination mechanisms set development on a course toward maleness or femaleness.
8. Sex determination mechanisms are diverse. In chromosomal strategies, the sex with two different sex chromosomes is heterogametic, and the sex with two of the same type of sex chromosome is homogametic.
9. In humans, the male is heterogametic, and the female is homogametic. The SRY gene on the Y chromosome controls other genes that stimulate development of male structures and suppress development of female structures.
11. X inactivation shuts off one X chromosome in the cells of female mammals, equalizing the number of active X-linked genes in each sex. Early in development, each female cell inactivates one X chromosome. A female is mosaic for X-linked heterozygous genes on the X chromosome because the X chromosomes are inactivated at random with respect to their parental origin.
12. Polyploid cells have extra full chromosome sets, and aneuploids have extra or missing individual chromosomes. A trisomy (one extra chromosome) is less harmful than a monosomy (one absent chromosome). Sex chromosome aneuploidy is less severe than autosomal aneuploidy. Nondisjunction, an uneven division of chromosomes in meiosis, causes aneuploidy.
13. Chromosomal rearrangements disrupt meiotic pairing, which can delete or duplicate genes. An inversion flips gene order, affecting the phenotype if it disrupts a vital gene. A Robertsonian translocation fuses the long arms of two nonhomologs. In a reciprocal translocation two nonhomologs exchange parts.