Page 55 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

3.1 Extensions to Mendel for Single-Gene Inheritance 47

Incomplete dominance: The F 1 hybrid resembles with an enzymatic role in red pigment production. The
2
neither pure-breeding parent white allele (A ) does not give rise to a functional enzyme,
1
A cross between pure late-blooming and pure early-blooming but the red allele (A ) does. Thus, in snapdragons, two red
1 1
pea plants results in an F 1 generation that blooms in be- alleles per cell (A A ) produce a double dose of a red-
tween the two extremes. This is just one of many examples producing enzyme, which generates enough pigment to
1 2
of incomplete dominance, in which the hybrid does not make the flowers look fully red. In the heterozygote (A A ),
resemble either pure-breeding parent. F 1 hybrids that differ one copy of the red allele per cell results in only enough
from both parents often express a phenotype that is inter- pigment to make the flowers look pink. In the homozygote
2 2
mediate between those of the pure-breeding parents. Thus, for the white allele (A A ), where there is no functional
with incomplete dominance, neither parental allele is dom- enzyme and thus no red pigment, the flowers appear white.
inant or recessive to the other; both contribute to the F 1
phenotype. Mendel observed plants that bloomed midway
between two extremes when he cultivated various types of Codominance: The F 1 hybrid exhibits traits
pure-breeding peas for his hybridization studies, but he did of both parents
not pursue the implications. Blooming time was not one of A cross between pure-breeding spotted lentils and pure-
the seven characteristics he chose to analyze in detail, al- breeding dotted lentils produces heterozygotes that are both
most certainly because in peas, the time of bloom was not spotted and dotted (Fig. 3.4a). These F 1 hybrids illustrate a
as clear-cut as seed shape or flower color. second significant departure from complete dominance. The
In many plant species, flower color serves as a striking progeny look like both parents, which means that neither
example of incomplete dominance. With the floret clusters the spotted nor the dotted allele is dominant or recessive to
of snapdragons, for instance, a cross between pure-breeding the other. Because both traits show up equally in the hetero-
red-flowered parents and pure-breeding white yields hy- zygote’s phenotype, the alleles are termed codominant.
brids with pink blossoms, as if a painter had mixed red and Self-pollination of the spotted/dotted F 1 generation gener-
white pigments to get pink (Fig. 3.3a). If allowed to self- ates F 2 progeny in the ratio of 1 spotted : 2 spotted/dotted : 1
pollinate, the F 1 pink-blooming plants produce F 2 progeny dotted. The Mendelian 1:2:1 ratio among these F 2 progeny
bearing red, pink, and white flowers in a ratio of 1:2:1 establishes that the spotted and dotted traits are determined
(Fig. 3.3b). This is the familiar genotypic ratio of an ordi- by alternative alleles of a single gene. Once again, because
nary single-gene F 1 self-cross. What is new is that because the heterozygotes can be distinguished from both homozy-
the heterozygotes look unlike either homozygote, the phe- gotes, the phenotypic and genotypic ratios coincide.
notypic ratios are an exact reflection of the genotypic ratios. In humans, some of the complex membrane-anchored
The simplest biochemical explanation for this type of molecules that distinguish different types of red blood cells
incomplete dominance is that each allele of the gene under exhibit codominance. For example, one gene (I) with
B
A
analysis specifies an alternative form of a protein molecule alleles I and I controls the presence of a sugar polymer
Figure 3.3 Pink flowers are the result of incomplete dominance. (a) Color differences in these snapdragons reflect the activity of
one pair of alleles. (b) The F 1 hybrids from a cross of pure-breeding red and white strains of snapdragons have pink blossoms. Flower colors
in the F 2 appear in the ratio of 1 red : 2 pink : 1 white. This ratio signifies that the alleles of a single gene determine these three colors.
a: © Henry Hemming/Getty Images RF
(a) Antirrhinum majus (snapdragons) (b) A Punnett square for incomplete dominance

1 1
2 2
P A A A A
Gametes A 1 A 2

1 2
1 2
F (all identical) A A A A
1
F 2 A 1 A 2
A 1
1 1
1 2
A A A A
A 2
2 2
1 2
A A A A
2 2
1 2
1 1
1 A A (red) : 2 A A (pink) : 1 A A (white)