Page 63 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

3.2 Extensions to Mendel for Two-Gene Inheritance 55

Recessive lethality relatively immune to both conditions, so high frequencies of
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People who are homozygous for the recessive Hbβ allele both alleles persist in tropical environments where malaria is
often develop heart failure because of stress on the circula- found. We explore this phenomenon in more quantitative
tory system. Many sickle-cell sufferers die in childhood, detail in Chapter 21 on population genetics.
adolescence, or early adulthood.
essential concepts

• Two alleles of a single gene may exhibit complete
Different dominance relations dominance, in which heterozygotes resemble the
Comparisons of heterozygous carriers of the sickle-cell homozygous dominant parent; incomplete dominance, in
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allele—individuals whose cells contain one Hbβ and which heterozygotes have an intermediate phenotype;
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one Hbβ allele—with homozygous Hbβ Hbβ (normal) and codominance, in which heterozygotes display
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and homozygous Hbβ Hbβ (diseased) individuals make aspects of each homozygous phenotype.
it possible to distinguish different dominance relation- • New alleles of a gene arise by mutation. Alleles with a
ships for different phenotypic aspects of sickle-cell frequency greater than 1% in a population are wild-type;
anemia (Fig. 3.9b). alleles that are less frequent are mutant.
At the molecular level—the production of β-globin • When two or more wild-type alleles (common variants)
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proteins—both alleles are expressed such that Hbβ and exist for a gene, the gene is polymorphic; a gene with
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Hbβ are codominant. At the cellular level, in their effect only one wild-type allele is monomorphic.
on red blood cell shape, the Hbβ and Hbβ alleles show • In pleiotropy, one gene contributes to multiple traits. The
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either complete dominance or codominance depending on dominance relationship between any two alleles can vary
altitude. Under normal oxygen conditions, the great major- depending on the trait.
ity of a heterozygote’s red blood cells have the normal bi- • Homozygotes for a recessive lethal allele that fails to
concave shape (Hbβ is dominant to Hbβ ). When oxygen provide an essential function will die. If a recessive lethal
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levels drop, however, sickling occurs in some Hbβ Hbβ cells allele has dominant effects on a visible trait, two-thirds of
(Hbβ and Hbβ are codominant). During World War II, the surviving progeny of a cross between heterozygotes
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soldiers who were heterozygous carriers and who were air- will display this trait.
lifted in transport planes to cross the Pacific experienced
sickling crises for this reason.
Considering the trait of resistance to malaria, the
Hbβ allele is dominant to the Hbβ allele. The reason is
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that infected Hbβ Hbβ cells are resistant to malaria be- 3.2 Extensions to Mendel for
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cause they break down before the malarial organism has a Two-Gene Inheritance
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chance to reproduce, just like the Hbβ Hbβ cells de-
scribed previously. But luckily for the heterozygote, for
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the phenotypes of anemia or death, Hbβ is recessive to learning objectives
Hbβ . A corollary of this observation is that in its effect on
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general health under normal environmental conditions and 1. Conclude from the results of crosses whether a single
its effect on red blood cell count, the Hbβ allele is domi- gene or two genes control a trait.
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nant to Hbβ . 2. Infer from the results of crosses the existence of
Thus, for the β-globin gene, as for other genes, domi- interactions between alleles of different genes
nance and recessiveness are not an inherent quality of al- including: additivity, epistasis, redundancy, and
leles in isolation; rather, they are specific to each pair of complementation.
alleles and to the level of physiology at which the pheno-
type is examined. When discussing dominance relation-
ships, it is therefore essential to define the particular Two genes can interact in several ways to determine a sin-
phenotype under analysis. gle trait, such as the color of a flower, a seed coat, a chick-
The complicated dominance relationships between the en’s feathers, a dog’s fur, or the shape of a plant’s leaves. In
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Hbβ and Hbβ alleles help explain the puzzling observation a dihybrid cross like Mendel’s, each type of interaction
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that the normally deleterious allele Hbβ is widespread in produces its own signature of phenotypic ratios. In the fol-
certain populations. In areas where malaria is endemic, het- lowing examples, the alternate alleles of each of the two
erozygotes are better able to survive and pass on their genes genes are completely dominant (such as A and B) and re-
than are either type of homozygote. Hbβ Hbβ individuals cessive (a and b). For simplicity, we sometimes refer to a
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often die of sickle-cell disease, while those with the genotype gene name using the symbol for the dominant allele,
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Hbβ Hbβ often die of malaria. Heterozygotes, however, are for example, gene A. In addition, we refer to the protein