BISC105 Lecture Notes - Lecture 3: Sex Organ, Barr Body, Dihydrotestosterone

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Maleness/femaleness is determined beginning at fertilization: sex chromosomes carried by gametes combine into a zygote. Outcome depends on distribution and expression of genes on x and y chromosomes and interactions between genes on chromosomes with different environmental factors. Sex ratio: proportion of males to females in a population: primary ratio (after fertilization) is not 1:1. Many more males are conceived: between birth and age 20 the ratio gets closer to 1:1, after age 20, females start to outnumber males exponentially. Genetic and environmental factors are responsible for the higher death rate among males. Accidents are leading cause of death in males between 15-35 years old. 7-6: defining sex in stages: chromosomes, gonads, and hormones. Chromosomal sex of an individual may differ from phenotypic sex. Sex is formed in stages: chromosomal sex occurs at fertilization with formation of a zygote with xx or. Xy: sex organs don"t develop until seventh or eighth week of pregnancy.

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