vendredi, avril 21, 2006
i wished they wouldn't simplify things when they teach. this is so much more interesting than those assertions that there's an equal chance of getting a boy or a girl.
as usual, during exam time, when i actually start reading textbooks, when i am bored enough to read the details, i find bits of information:
During meiosis in the male, the X and Y chromosomes separate from each other, producing two kinds of sperm, X-bearing and Y-bearing; the frequencies of the two types are approximately equal. XX females produce only one kind of egg, which is X-bearing. If fertilization were to occur randomly, approximately half the zygotes would be XX and the other half would be XY, leading to a 1:1 sex ratio at conception. However, in human beings, Y-bearing sperm have a fertilization advantage, and the zygotic sex ratio is about 1.3:1. During development, the excess of males is diminished by differential viability of XX and XY embryos, and at birth, males are only slightly more numerous than females (sex ratio 1.07:1). By the age of reproduction, the excess of males is essentially eliminated and the sex ratio is very close to 1:1.
Snustad and Simmons. Principles of Genetics. 3rd Ed. John Wiley & Sons, Inc.
i wished they wouldn't simplify things when they teach. this is so much more interesting than those assertions that there's an equal chance of getting a boy or a girl.
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