The predicted ratio is due to the recessive lethality

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the predicted ratio is due to the recessive lethality of having two yellow Y alleles. 3 : 1 2 : 1 ratio of phenotypes if certain combinations of alleles were deadly to the newly formed embryo? Because each unlucky embryo receiving a lethal combination of alleles dies, that phenotype would not be represented in the next generation at all. Such a condition, called lethality , was first documented soon after Mendel’s groundbreaking paper was rediscovered in the early part of the 20th century. A dominant lethal allele is one that kills its recipient.A recessive lethal allele is only deadly when it is paired with another recessive lethal in a homozygote. In 1904, the French geneticist Lucien Cuenot was performing experiments on the inheritance of coat color in mice. He found that the yellow coat color was dominant to the wild-type brownish color called agouti (Figure 3-15).When heterozygous mice,yellow in color, but each carrying an allele for the agouti color, were bred, the phenotypic ratio of offspring was yellow/agouti, instead of the dominant/recessive predicted by Mendel’s laws. Furthermore, testcrosses between yellow mice and homozygous agouti mice proved that all yellow mice were heterozygous. (How can a testcross prove that individuals showing the dominant trait are heterozygous and not homozygous? Check the BioInquiry web site to remind yourself of how a testcross works.) Cuenot could not find a single mouse that was homozygous for the dominant allele. Where were all the homozygous yellow mice? Later experiments showed that a double dose of the yellow allele is a deadly combination. All such embryos die early in development, skewing the ratio of offspring away from dominant/recessive and toward the observed outcome (Figure 3-16). The yellow allele in mice,although dominant in its effect on coat color,is an example of an allele that is recessive in its lethality. It may seem that recessive lethals would be the victims of natural selection, quickly eliminated from populations by their deleterious effects. But because the carriers of many recessive lethals can survive and reproduce normally, such alleles can remain hidden within the genes of a population for many generations, only killing their carriers when, by random chance, they are paired with similar alleles in deadly combination. But what of dominant lethals, or alleles that kill when present in a single copy? Common sense might lead us to conclude that a dominant lethal allele could not possibly survive in a population.After all, any individual unfortunate enough to acquire even a single lethal dominant would surely die, eliminating the deadly allele from the popula- tion.That is certainly the case with many lethal dominants, but not all. Huntington’s dis- ease is an example of a deadly human disease caused by a dominant lethal allele. The disease is characterized by uncontrolled movements and mental deterioration, followed by death. Individuals need inherit only one copy of the Huntington’s gene to exhibit the lethal phenotype. Because the onset of the disease occurs late in life, usually between
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