Page 60 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

52 Chapter 3 Extensions to Mendel’s Laws

y
One Gene May Contribute Figure 3.8 A is a pleiotropic and recessive lethal allele.
to Several Characteristics (a) A cross between inbred agouti mice and yellow mice yields a
1:1 ratio of yellow to agouti progeny. The yellow mice are therefore
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Mendel derived his laws from studies in which one gene A A heterozygotes, and for the trait of coat color, A (for yellow) is
dominant to A (for agouti). (Note we assume that if they could
determined one trait, but, always the careful observer, he survive, A A mice would have the same coat color as A A mice.)
Y
Y Y
himself noted possible departures. In listing the traits se- (b) Yellow mice do not breed true. In a yellow × yellow cross, the
y
lected for his pea experiments, Mendel remarked that spe- 2:1 ratio of yellow to agouti progeny indicates that the A allele is a
cific seed coat colors are always associated with specific recessive lethal.
flower colors. (a) All yellow mice are heterozygotes.
The phenomenon of a single gene determining a num-
ber of distinct and seemingly unrelated characteristics is
known as pleiotropy. Because geneticists now know that
each gene determines a specific protein (or RNA) and that y
each gene product can have a cascade of effects on an P AA A A
organism, we can understand how pleiotropy arises. Among
the Maori people of New Zealand, for example, many men F 1 y
develop respiratory issues and are also sterile. These men A A
are said to exhibit a syndrome—a group of problems that A A A AA
y
are usually seen together. Researchers have found that the
fault lies with the recessive allele of a single gene. The y
gene’s normal dominant allele specifies a protein necessary (b) Two copies of A cause lethality.
for the action of cilia and flagella, both of which are hairlike
structures extending from the surfaces of some cells. In
men who are homozygous for the recessive allele, cilia that
normally clear the airways fail to work effectively, and
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flagella that normally propel sperm fail to do their job. P A A A A
Thus, one gene determines a protein that affects both
respiratory function and reproduction. Because most F 1
proteins act in a variety of tissues and influence multiple A y A
biochemical processes, mutations in almost any gene may
have pleiotropic effects. A y A A A A
y y
y
y
A A A AA
Recessive lethal alleles = not born
A significant variation of pleiotropy occurs in alleles that
not only produce a visible phenotype but also affect viabil-
ity. Mendel assumed that all genotypes are equally viable— Note again that dominance and recessiveness are de-
that is, they have the same likelihood of survival. If all fined in the context of each pair of alleles. Even though, as
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genotypes were not equally viable, and a large percentage previously mentioned, agouti (A) is dominant to the a and
of, say, homozygotes for a particular allele died before a mutations for black coat color, it can still be recessive to
germination or birth, you would not be able to count them. the yellow coat color allele. The yellow mice in the preced-
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This lethality would alter the 1:2:1 genotypic ratios and the ing cross are A A heterozygotes, and the agoutis, AA homo-
3:1 phenotypic ratios predicted for the F 2 generation. zygotes. So far, no surprises. But a mating of yellow to
Consider the inheritance of coat color in mice. As yellow produces a skewed phenotypic ratio of two yellow
mentioned earlier, wild-type agouti (AA) animals have mice to one agouti (Fig. 3.8b). Among these progeny, mat-
black hairs with a yellow stripe that appear dark gray to ings between agouti mice show that the agoutis are all
the eye. One of the 14 known mutant alleles of the agouti pure-breeding and therefore AA homozygotes as expected.
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gene, A , gives rise to mice with a much lighter, almost yel- However, no pure-breeding yellow mice appear among the
low color. When pure-breeding AA mice are mated to yel- progeny. When the yellow mice are mated to each other,
low mice, the offspring always emerge in a 1:1 ratio of the they unfailingly produce 2/3 yellow and 1/3 agouti off-
two coat colors (Fig. 3.8a). From this result, we can draw spring, a ratio of 2:1, so the yellow mice must be heterozy-
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three conclusions: (1) All yellow mice must carry the wild- gotes (A A). In short, one can never obtain pure-breeding
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type A allele even though they do not express the agouti yellow mice (A A ).
phenotype; (2) yellow is therefore dominant to agouti; and How can we explain this phenomenon? The Punnett
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(3) all yellow mice are A A heterozygotes. square in Fig. 3.8b suggests an answer. Two copies of the