BSCI 222 Lecture Notes - Lecture 4: Punnett Square, Y Chromosome, Nondisjunction
16 Sep 2015
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Bsci 222 - lecture 4 - sex determination and sex-linked characteristics. Eye color: wild type = red, recessive = white. Results of reciprocal crosses (punnett squares) are not the same. Different phenotypic ratios in the two sexes. : sex chromosomes are not properly split during meiosis i. Both homologs end up in same daughter cell. Very rare; proper separation of x"s happens 90% of the time, and nondisjunction of x"s happens 10% of the time. May cause gametes with two x chromosomes, or an x and y chromosome. Few white-eyed females were observed where the expectation was all wild type females. Hypothesis: exceptional white-eyed females had two x chromosomes and a y chromosome. Nondisjunction creates flies with unusual sex chromosomes. Sex in drosophila determined by ratio of x:autosomes. X:a ratio of 1 produces normal organism. : all chromosomes that are not sex chromosomes. Ex: two x chromosomes : two autosomes = normal female.
