Other Allelic Relationships for Genetics Help

EXAMPLE 2.16 One type of polydactyly (extra fingers and/or toes) in humans can be produced by a dominant gene (P). The wild-type condition with five digits on each limb is produced by the recessive genotype (pp). However, some heterozygous individuals (Pp) are not polydactylous. If 20% of Pp individuals do not show polydactyly (i.e., are wild type), the gene has a penetrance of 80%.

EXAMPLE 2.17 The polydactylous condition may be penetrant in the left hand (six digits) and not in the right (five digits), or it may be penetrant in the feet but not in the hands

EXAMPLE 2.18 The color of Drosophila eyes is governed by a series of alleles that cause the hue to vary from red or wild type (w⁺ or W) through coral (w^co), blood (w^bl), eosin (w^e), cherry (w^ch), apricot (w^a), honey (w^h), buff (w^bf), tinged (w^t), pearl (w^p), and ivory (wⁱ) to white (w). Each allele in the system except w can be considered to produce pigment, but successively less is produced by alleles as we proceed down the hierarchy: w⁺ > w^co > w^bl > w^e > w^ch > w^a > w^h > w^bf > w^t > w^p > wⁱ > w. The wild-type allele (w⁺) is completely dominant and w is completely recessive to all other alleles in the series. Compounds are heterozygotes that contain unlike members of a multiple allelic series. The compounds of this series that involve alleles other than w⁺ tend to be phenotypically intermediate between the eye colors of the parental homozygotes.

EXAMPLE 2.19 A classical example of multiple alleles is found in the ABO blood group system of humans, where the allele I^A for the A antigen is codominant with the allele I^B for the B antigen. Both I^A and I^Bare completely dominant to the allele i, which fails to specify any detectable antigenic structure. The hierarchy of dominance relationships is symbolized as (I^A = I^B) > i. Two antisera (anti-A and anti-B) are required for the detection of four phenotypes.

EXAMPLE 2.20 A slightly different kind of multiple allelic system is encountered in the coat colors of rabbits: C allows full color to be produced (typical gray rabbit); c^ch, when homozygous, removes yellow pigment from the fur, making a silver-gray color called chinchilla; c^ch, when heterozygous with alleles lower in the dominance hierarchy, produces light-gray fur; c^h produces a white rabbit with black extremities, called ''Himalayan''; c fails to produce pigment, resulting in albino. The dominance hierarchy may be symbolized as follows: C > c^ch > c^h > c.