Evolution__3rd_Edition

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(a) (b) (c)
CHAPTER 2 / Molecular and Mendelian Genetics 35
Male Female Male Female Male Female
Parents AA X AA Parents AA X Aa Parents Aa X Aa
Gametes
Offspring
Mendelian
ratio
A A Gametes A A a
AA Offspring AA
Gametes
Offspring
100% AA Mendelian 50% AA 50% Aa Mendelian 25% AA 50% Aa 25% aa
ratio
ratio
Figure 2.8
Mendelian ratios for: (a) an AA × AA cross, (b) an AA × Aa cross, and (c) an Aa × Aa cross.
The Mendelian ratios for
combinations of genes depend on
whether the genes are linked or
unlinked
Aa
examples of Mendelian ratios. They were discovered by Gregor Mendel in about
1856–63. Mendel was a monk, later Abbot, in St Thomas’s Augustinian monastery in
what was then Brünn in Austro-Hungary and is now Brno in the Czech Republic.
Mendelian ratios can also be for more than one genetic locus. If the alleles at one
locus are A and a, and at a second B and b, then an individual will have a double genotype,
such as Ab/Ab (double homozygote) or Ab/ab (single heterozygote). It has a
double set of genes at each locus, one set from each parent. The segregation ratios now
depend on whether the genetic loci are on the same or different chromosomes. Recall
that an individual human has a haploid number of 23 chromosomes and about 30,000
genes. That means there must be on average about 1,300 genes per chromosome.
Different genes on the same chromosome are described as being linked. Genes that are
very close together are tightly linked, those further apart are loosely linked. Genes that
are not on the same chromosome are unlinked.
The easy case is for two unlinked loci; the genes at the two loci then segregate independently.
Imagine first a cross in which only one of the loci is heterozygous, such as a
cross between an Ab/Ab male and an Ab/ab female. All the genes at the B locus are the
same, while at the A locus the male is AA and the female is Aa. The ratio of offspring will
be 50% AAbb and 50% Aabb, a simple extension of the one-locus case.
A more complicated cross is for a male AB/Ab and a female AB/ab. Both parents are
heterozygous for at least one locus. Again, the ratios of B locus genotypes associated
with each A locus genotype are those predicted by applying Mendel’s principles independently
to each locus. A cross between two Bb heterozygotes produces a ratio of offspring
of 25% BB : 50% Bb : 25% bb, and this ratio will be the same within each A locus
genotype. Thus, in the cross between a male AB/Ab and a female AB/ab, there will be
50% AA and 50% Aa offspring. Of the half which are AA, 25% are AB/AB, 50% are
AB/Ab, and 25% are Ab/Ab. Likewise for the 50% Aa genotypes. Add the two A genotypes
and the total offspring ratios are:
AB/AB AB/Ab Ab/Ab AB/aB AB/ab Ab/ab
1/8 1/4 1/8 1/8 1/4 1/8
A a A a
AA
Aa
aa