the same mutation. The inheritance patterns shown by dominant/recessive
alleles that are associated with a clinical condition follow the Mendelian rules
explained above. If only a single member of a homologous pair of chromosomes
carries the mutation, it can, however, be passed on to the next generation and
the parent is described as a carrier (Figure 15.11 (A) and (B)). If one parent is
homozygous for the normal gene (MM) and the other parent heterozygous for
the mutated allele (Mm) then 50% of the offspring will be homozygous for the
normal gene (MM) and the other 50% heterozygous (Mm) and carriers. None
of the children will be affected. If both parents are heterozygotes (Mm) then
25% of offspring will be dominant homozygotes (MM), 50% of offspring will
be heterozygotes and carriers (Mm) but the remaining 25% will be recessive
homozygotes (mm) and express the condition (Figure 15.11 (B)).
INHERITANCE AND MUTATIONSCZhhVg6]bZY!BVjgZZc9Vlhdc!8]g^hHb^i]:YLddY )'(
Figure 15.11 The inheritance patterns shown for a recessive allele of a gene between (A) dominant homozygous
and heterozygous parents and (B) two heterozygous parents.
Heterozygosity means that autosomal recessive mutations, even one resulting
in a lethal allele, may go unnoticed and be maintained in the population for
many generations, until the resultant allele has become widespread in the
population. The new allele will become evident only when a chance mating
brings two copies of it together in the homozygous condition.
Sex-linked Genetic Diseases
A number of genetic diseases are caused by defective alleles of genes of the
sex (X and Y) chromosomes. The X chromosome contains many more genes
than the Y, although they do have some genes in common, thus any defective
(mutated) gene on the X chromosome is likely to be expressed in males
(XY) but be masked in females (XX). Genetic diseases associated with the
X chromosome are commonest and they are often referred to as X- or sex-
linked genetic conditions. X-linked diseases can be recessive or dominant,
although the former, for example hemophilia described in Chapter 13, are
the better known (Table 15.1). Given that females have two X chromosomes
but males only one together with a Y chromosome, then the expression of
sex-linked genes differs between females and males because many genes
on the Y chromosome lack a corresponding allele on the X chromosome.
Thus X-linked recessive genes are only expressed in females if there are two
copies of the gene; one on each of the X chromosomes. However, for males,