Page 59 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

3.1 Extensions to Mendel for Single-Gene Inheritance 51

different alleles, that is, some form of variation, to “see” the Figure 3.7 The mouse agouti gene: One wild-type
transmission of a gene. Thus, in segregation studies, geneti- allele, many mutant alleles. (a) Black-backed, yellow-bellied
cists can analyze only genes with variants; they have no way (top left); black (top right); and agouti (bottom) mice. (b) Genotypes
of following a gene that comes in only one form. If all peas and corresponding phenotypes for alleles of the agouti gene.
were yellow, Mendel would not have been able to decipher (c) Crosses between pure-breeding lines reveal a dominance series.
Interbreeding of the F 1 hybrids (not shown) yields 3:1 phenotypic
the transmission patterns of the gene for the seed color trait. ratios of F 2 progeny, indicating that A, a , and a are in fact alleles of
t
We discuss mutations in greater detail in Chapter 7. one gene.
a (top left): © McGraw-Hill Education. Jill Birschbach, photographer. Arranged
by Alexandra Dove, McArdle Laboratory, University of Wisconsin-Madison; a (top
Allele frequencies and monomorphic genes right, bottom): © Charles River Laboratories
(a) Mus musculus (house mouse) coat colors
Because each organism carries two copies of every gene,
you can calculate the number of copies of a gene in a given
population by multiplying the number of individuals by 2.
Each allele of the gene accounts for a percentage of the total
number of gene copies, and that percentage is known as the
allele frequency. The most common alleles in a population
t t
are usually called the wild-type alleles, often designated by a a aa
+
a superscript plus sign ( ). An allele is considered wild-type
if it is present in the population at a frequency greater than
1%. A rare allele in the same population is considered a (b) Alleles of the agouti gene
mutant allele. (Note that the definitions of wild-type ver- Genotype Phenotype
sus mutant alleles are not static. A newly induced mutation
generates a mutant allele whose frequency can increase and A– agouti
t t
over time, the allele can become wild-type.) a a black/yellow
In mice, for example, one of the main genes determin- aa black
ing coat color is the agouti gene. The wild-type allele (A) A–
t
produces fur with each hair having yellow and black bands a a black/yellow
that blend together from a distance to give the appearance of
dark gray, or agouti. Researchers have identified in the labo- (c) Evidence for a dominance series
ratory 14 distinguishable mutant alleles for the agouti gene.
One of these (a ) is recessive to the wild type and gives rise
t
to a black coat on the back and a yellow coat on the belly;
another (a) is also recessive to A and produces a pure black
coat (Fig. 3.7). In nature, wild-type agoutis (AA) survive to agouti black back/yellow belly agouti
t t
reproduce, while very few black-backed or pure black mu- AA a a Aa t
t t
tants (a a or aa) do so because their dark coat makes it hard
for them to evade the eyes of predators. As a result, A is
present at a frequency of much more than 99% and is thus
the only wild-type allele in mice for the agouti gene. A gene agouti black agouti
with only one common, wild-type allele is monomorphic. AA aa Aa

Allele frequencies and polymorphic genes
In contrast, some genes have more than one common allele,
which makes them polymorphic. For example, in the ABO
B
blood type system, all three alleles—I , I , and i—have ap- black back/yellow belly black black back/yellow belly
A
t
t t
a a
a a
aa
preciable frequencies in most human populations. Although
t
all three of these alleles can be considered to be wild-type, Dominance series: A > a > a
geneticists instead usually refer to the high-frequency al-
leles of a polymorphic gene as common variants. Certain viruses. Some scientists think that evolution favors the
rare genes are so polymorphic that hundreds of allelic vari- emergence of new HLA gene alleles to ensure that no single
ants can be found in populations. We have already discussed pathogen among the many to which we are exposed in the
the case of the HLA histocompatibility genes in humans, environment could destroy the entire human population.
which encode cell surface proteins that help the immune That is, at least a few individuals with particular HLA gene
system deal with pathogenic invaders such as bacteria and alleles would be protected from any given pathogen.