Page 54 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

46 Chapter 3 Extensions to Mendel’s Laws

phenotype, allowing explanation of the observed deviations without chal-
lenging Mendel’s basic laws.
One general theme stands out from these breeding studies: To make
sense of the enormous phenotypic variation of the living world, geneticists
usually try to limit the number of variables under investigation at any one
time. Mendel did this by using pure-breeding, inbred strains of peas that dif-
fered from each other by one or a few traits, so that the action of single
genes could be detected. Similarly, twentieth-century geneticists used inbred
populations of fruit flies, mice, and other experimental organisms to study
specific traits. Of course, geneticists cannot study people in this way. Human
populations are typically far from inbred, and researchers cannot ethically
Figure 3.1 Some phenotypic variation perform breeding experiments on people. As a result, the genetic basis of
poses a challenge to Mendelian analysis.
Lentils show complex speckling patterns that are much human variation remained a mystery. The advent of molecular biol-
controlled by a gene that has more than two alleles. ogy in the 1970s provided new tools that geneticists now use to unravel the
© Jerry Marshall genetics of complex human traits, as will be described in later chapters.

3.1 Extensions to Mendel one parent for the trait under consideration, the allele car-
ried by that parent is deemed dominant to the allele carried
for Single-Gene Inheritance by the parent whose trait is not expressed in the hybrid. If,
for example, a mating between a pure-breeding white line
and a pure-breeding blue line produces F 1 hybrids that are
learning objectives white, the white allele of the gene for color is dominant to
the blue allele. If the F 1 hybrids are blue, the blue allele is
1. Categorize allele interactions as completely dominant, dominant to the white one (Fig. 3.2).
incompletely dominant, or codominant. Mendel described and relied on complete dominance in
2. Recognize progeny ratios that imply the existence of sorting out his ratios and laws, but it is not the only kind of
recessive lethal alleles. dominance he observed. Figure 3.2 diagrams two situations
3. Predict from the results of crosses whether a gene is in which neither allele of a gene is completely dominant. As
polymorphic or monomorphic in a population. the figure shows, crosses between true-breeding strains can
produce hybrids with phenotypes that differ from both par-
ents. We now explain how these phenotypes arise.
William Bateson was an early interpreter and defender of
Mendel. Bateson, who coined the terms genetics, allelo-
morph (later shortened to allele), homozygote, and hetero- Figure 3.2 Different dominance relationships. The
phenotype of the heterozygote defines the dominance relationship
zygote, entreated the audience at a 1908 lecture: “Treasure between two alleles of the same gene (here, A and A ). Dominance
2
1
your exceptions! … Keep them always uncovered and in is complete when the hybrid resembles one of the two pure-breeding
sight. Exceptions are like the rough brickwork of a growing parents. Dominance is incomplete when the hybrid resembles neither
building which tells that there is more to come and shows parent; its novel phenotype is usually intermediate. Codominance
where the next construction is to be.” Consistent exceptions occurs when the hybrid shows traits from both pure-breeding parents.
to simple Mendelian ratios revealed unexpected patterns of Type of A A A A A A hybrids
2 2
1 1
1 2
Dominance
single-gene inheritance. By distilling the significance of
1
these patterns, Bateson and other early geneticists extended Complete A is dominant to A 2 1
2
the scope of Mendelian analysis and obtained a deeper un- A is recessive to A
derstanding of the relationship between genotype and phe- 2 1
notype. We now look at the major extensions to Mendelian Complete A is dominant to A 2
1
A is recessive to A
analysis elucidated over the last century.
1
2
A and A are
Incomplete incompletely dominant
Dominance Is Not Always Complete relative to each other
A consistent working definition of dominance and reces- 1 2
siveness depends on the F 1 hybrids that arise from a mating Codominant A and A are
codominant relative
between two pure-breeding lines. If a hybrid is identical to to each other