Gene interactions.docx

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Biological Sciences
Rahul Warrior

Gene interactions Independent assortment of alleles is illustrated by behavior of two pairs of homologues during meiosis For an organism with genotype AaBb, two equally likely arrangements of paired homologs can occur One yields gametes AB and ab, whereas the other produces gametes Ab and aB; these four gamete combinations occur with equal likelihood 1. Non-disjunction occurs in meiosis 1 at 1/2000 2. Segregation in meiosis 2 results in a 1:1 ratio of XX and 0 gametes 3. 1/2 the progeny are XXX or Y0, both of which are lethal 4. The other 1/2 of progeny have the “exceptional” phenotypes 1. The w gene was always inherited with the X chromosome 2. Males have an X but no Y and are hemizygous 3. Thus Bridges (and Morgan) could argue that genes were on chromosomes 4. Drosophila X/O is male, XXY is female • In placental mammals males are XY, Females are XX. • The Y chromosome contains very few genes – thus males are hemizygous • X-linked recessive mutations – phenotype is visible in female homozygotes and • X-linked dominant mutations – heterozygous/homozygous females and hemizygous males are affected 1. Many more males than females have the trait due to hemizygosity 2. Arecessive male mated to a homozygous dominant female produces all offspring with the dominant phenotype, and all female offspring are carriers 3. Matings of recessive males with carrier females give half dominant and half recessive offspring of both sexes 4. Matings of homozygo
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