The dominant a allele of the agouti gene encodes a

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The dominant A allele of the agouti gene encodes a protein that regulates hair color such that the pigmentation shifts from black (eumelanin) at the tips to orange (phaeomelanin) near the roots, producing the agouti coloring. The recessive allele, a , inhibits the shift to orange pigmentation and thereby results in black pigment production throughout the entire hair (when an animal is aa ). The C gene allows pigmentation to occur. If an individual is homozygous recessive for the C gene ( cc ), pigmentation will not occur and it will be albino regardless of the genotype of the A gene. Experiment 4A. Bridges observed an 8:4:3:1 ratio because the cream-eye gene can modify the X-linked eosin-eye allele, but not the red or white alleles. First described by Morgan and Bridges using the eosin allele of Drosophila . In addition to the X-linked red-eye allele ( w + ) and white-eye allele ( w ), Morgan and Bridges found another allele they called eosin ( w-e ), which results in eyes that are a pale orange color. The red allele is dominant to the eosin allele, and expression of the eosin allele depends on the number of copies of the allele (homozygous females have eosin eyes while hetereozygous females with one eosin allele and one white allele have light-eosin eyes. Hemizygous male flies have light-eosin eyes. Within true-breeding cultures of flies with eosin eyes, there was occasionally a fly that had cream-colored eyes. The hypothesis Cream-colored eyes are due to the effect of an allele that is in the same gene as the eosin allele or in a second gene that modifies the expression of the eosin allele.
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11 Testing the hypothesis (Figure 4.22) Cross males with cream-colored eyes to wild-type females. Observe the F 1 offspring and then allow the offspring to mate with each other. Observe and record the eye color and sex of the F 2 generation. The data and interpreting the data Cream-eyed male x wild-type female F 1 : all red eyes F 1 male x F 1 female F 2 : 104 females with red eyes 47 males with red eyes 44 males with light-eosin eyes 14 males with cream eyes The F 2 generation indicates that the cream allele is not in the same gene as the eosin allele. Had it been, none of the F 2 males would have had eosin eyes; instead, there would have been a 1:1 ratio of red-eyed males and cream-eyed males. The actual results were consistent with the idea that the male flies of the parental generation had both the eosin allele and the cream allele. Bridges concluded that the cream allele was an allele of a different gene. One possibility is that the cream allele is an autosomal recessive. If so, we can let C represent the dominant allele (which does not modify the eosin phenotype) and c a represent the cream allele (which modifies the eosin color to cream).
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