Essential Cell Biology 5th edition

668 CHAPTER 19 Sexual Reproduction and Genetics
Figure 19−25 Recessive alleles all follow
the same Mendelian laws of inheritance.
Here we trace the inheritance of Type II
albinism, a recessive trait that is associated
with a single gene in humans. Note that
normally pigmented individuals can be
either homozygous (AA) or heterozygous
(Aa) for the dominant allele A.
albino man
(aa)
normally pigmented
woman (AA)
normally pigmented
man (AA)
albino woman
(aa)
all normally pigmented
offspring (Aa)
all normally pigmented
offspring (Aa)
75% normally pigmented
(AA or Aa)
25% albino
(aa)
retina of the eye. Because the recessive allele codes for a version of this
enzyme that is only weakly active or completely inactive, albinos have
white hair, white skin, and pupils that look pink because a lack of melanin
in the eye allows the red color of the hemoglobin in blood vessels in
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the retina to be visible.
The trait for albinism is inherited in the same manner as any other recessive
trait, including Mendel’s green peas. If a man who is homozygous
for the recessive albinism allele (genotype aa) has children with a woman
who has the same genotype, all of their children will be albino (aa).
However, if a homozygous nonalbino man (AA) marries and has children
with an albino woman (aa), their children will all be heterozygous (Aa)
and normally pigmented (Figure 19−25). If two nonalbino individuals
with an Aa genotype start a family, each of their children would have a
25% chance of being an albino (aa).
Of course, humans generally don’t have families large enough to guarantee
perfect Mendelian ratios. (Mendel arrived at his ratios by breeding and
counting thousands of pea plants for most of his crosses.) Geneticists that
follow the inheritance of specific traits in humans get around this problem
by working with large numbers of families—or with several generations
of a few large families—and preparing pedigrees that show the phenotype
of each family member for the relevant trait. Figure 19−26 shows
the pedigree for a family that harbors a recessive allele for deafness. It
also illustrates an important practical consequence of Mendel’s laws:
marriages between related individuals—called consanguineous (from the
Latin sanguis, “blood”)—create a greatly increased risk of producing children
that are homozygous for a deleterious recessive mutation.
Alleles for Different Traits Segregate Independently
Mendel deliberately simplified the problem of heredity by starting with
breeding experiments that focused on the inheritance of one trait at a time,
called monohybrid crosses. He then turned his attention to multihybrid