The parental generation then would have been ccx w x

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The parental generation, then, would have been: CCX w+ X w+ female x c a c a X w-e Y male And the F 1 generation would have been all red-eyed with: Cc a X w+ X w-e females and Cc a X w+ Y males The predicted F 2 outcome is shown in the Punnett square on page 90: 8 red-eyed females : 4 red-eyed males : 3 light-eosin-eyed males : 1 cream- eyed male This phenotypic outcome proposes that the specific modifier allele, c a , can modify the phenotype of the eosin allele but not the red-eye allele, and that the eosin allele can be modified only when the c a allele is homozygous. The predicted 8:4:3:1 ratio agrees reasonably well with Bridge’s data.
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12 Due to gene redundancy, loss-of-function alleles may have no effect on phenotype. An organism that has had both copies of a gene altered to eliminate gene function is said to have undergone a gene knockout. Not all gene knockouts produce a phenotypic effect, because a second gene can sometimes compensate for the loss of function of the knocked out gene. This is called gene redundancy, and it would take two gene knockouts in the same organism to produce a phenotypic change (Figure 4.23). Gene duplication can lead to redundancy. Paralogs are previously identical genes created by gene duplication that are now different due to accumulated mutations. Gene redundancy can lead to offspring ratios that differ from the 9:3:3:1 patterns seen in simple Mendelian genetics (Figure 4.24). This experiment looked at capsule shape in a weed called shepherd’s purse. The common shape is triangular and the plants are dominant (functional) for two genes ( T_V_ ). Strains producing smaller ovate capsules have a double knockout ( ttvv ). When a true-breeding plant with rectangular capsules ( TTVV ) was cross with a plant having ovate capsules, all of the F 1 generation plants had triangular capsules. When the F 1 plants were crossed to one another, there was a 15:1 ratio of triangular to ovate in F 2 . Having one functional copy of either gene ( T or V ) is sufficient to produce the triangular phenotype. Only the ttvv double knockout will have smaller ovate capsules. The phenotypic effects of a mutation can be reversed by a suppressor mutation. A suppressor mutation is a second mutation that reverses the phenotypic effect of the first mutation. If the second mutation is in a different gene than the first, it is called an intergenic suppressor or an extragenic suppressor. The Hairless gene in Drosophila demonstrates intergenic suppression by specific mutations in the Suppressor of Hairless gene. (Figure 4.25). This indicates a physical interaction between Hairless and Suppressor of Hairless. Please see the Conceptual and Experimental Summaries for Chapter 4 on pages 93-94. This lecture outline was prepared from Genetics: Analysis and Principles, by Brooker, 2009 (3rd edition). It contains phrases and entire sentences taken verbatim from that source, and is in no way meant to represent original work by Mark Bierner.
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