L

Fertilization

Fj generation

VaYY

Yellow

Nonyellow

Yellow

Nonyellow

'Ayy

Conclusion: YY mice die, and so

23 of progeny are Yy, yellow /3 of progeny are yy, nonyellow

I 5.3 A 2 : J ratio among the progeny of a cross results from the segregation of a lethal allele.

discussion about Cuenot's results among his colleagues, but it was eventually realized that the yellow allele must be lethal when homozygous ( FIGURE 5.3). A lethal allele is one that causes death at an early stage of development— often before birth — and so a some genetypes may not appear among the progeny.

Cuenot originally crossed two mice heterozygous for yellow: Yy X Yy. Normally, this cross would be expected to produce 1/4 YY, 1/2 Yy, and 1/4 yy (see Figure 5.3). The homozygous YY mice are conceived but never complete development, which leaves a 2:1 ratio of Yy (yellow) to yy (nonyellow) in the observed offspring; all yellow mice are heterozygous (Yy).

Another example of a lethal allele, originally described by Erwin Baur in 1907, is found in snapdragons. The aurea strain in these plants has yellow leaves. When two plants with yellow leaves are crossed, 2/3 of the progeny have yellow leaves and 1/3 have green leaves. When green is crossed with green, all the progeny have green leaves; however, when yellow is crossed with green, 1/2 of the progeny are green and

1/2 are yellow, confirming that all yellow-leaved snapdragons are heterozygous. A 2 : 1 ratio is almost always produced by a recessive lethal allele; so observing this ratio among the progeny of a cross between individuals with the same phenotype is a strong clue that one of the alleles is lethal.

In both of these examples, the lethal alleles are recessive because they cause death only in homozygotes. Unlike its effect on survival, the effect of the allele on color is dominant; in both mice and snapdragons, a single copy of the allele in the heterozygote produces a yellow color. Lethal alleles also can be dominant; in this case, homozygotes and heterozygotes for the allele die. Truly dominant lethal alleles cannot be transmitted unless they are expressed after the onset of reproduction, as in Huntington disease.

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