Page 126 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

118 Chapter 4 The Chromosome Theory of Inheritance

Bridges next predicted that fertilization of these four 4.7 Sex-Linked and Sexually
kinds of eggs from an XXY female by normal sperm would
generate an array of sex chromosome karyotypes associ- Dimorphic Traits in Humans
ated with specific eye colors in the progeny. Bridges veri-
fied all his predictions when he analyzed the eye colors and
sex chromosomes of a large number of offspring. For in- learning objectives
stance, he showed cytologically that all of the white-eyed
females emerging from the cross in Fig. 4.21b had two X 1. Determine from pedigree analysis whether human traits
chromosomes and one Y chromosome, while one-half of are X-linked or autosomal.
the white-eyed males had a single X chromosome and two 2. Explain how human cells compensate for the X-linked
Y chromosomes. Bridges’ painstaking observations pro- gene dosage difference in XX and XY nuclei.
vided compelling evidence that specific genes do in fact
reside on specific chromosomes.
A person unable to tell red from green would find it nearly
impossible to distinguish the rose, scarlet, and magenta in
The Chromosome Theory Integrates the flowers of a garden bouquet from the delicately varie-
Many Aspects of Gene Behavior gated greens in their foliage, or to complete a complex
electrical circuit by fastening red-clad metallic wires to red
Mendel had assumed that genes are located in cells. The ones and green to green. Such a person has most likely in-
chromosome theory assigned the genes to a specific kind of herited some form of red-green color blindness, a recessive
structure within cells and explained alternative alleles as condition that runs in families and affects mostly males.
physically matching parts of homologous chromosomes. In Among Caucasians in North America and Europe, 8%
so doing, the theory provided an explanation of Mendel’s of men but only 0.44% of women have this vision defect.
laws. The mechanism of meiosis ensures that the matching Figure 4.22 suggests to readers with normal color vision
parts of homologous chromosomes will segregate to differ- what people with red-green color blindness actually see.
ent gametes (except in rare instances of nondisjunction), In 1911, E. B. Wilson, a contributor to the chromo-
accounting for the segregation of alleles predicted by Men- some theory of inheritance, combined family studies of the
del’s first law. Because each homologous chromosome pair inheritance of color blindness with recent knowledge of the
aligns independently of all others at meiosis I, genes car- roles of the X and Y chromosomes in sex determination to
ried on different chromosomes will assort independently, make the first assignment of a human gene to a particular
as predicted by Mendel’s second law. chromosome. The gene for red-green color blindness, he
The chromosome theory is also able to explain the cre- said, lies on the X because the condition usually passes
ation of new alleles through mutation, a spontaneous from a maternal grandfather through an unaffected carrier
change in a particular gene (that is, in a particular part of a mother to roughly 50% of the grandsons.
chromosome). If a mutation occurs in the germ line, it can Several years after Wilson made this gene assignment,
be transmitted to subsequent generations. pedigree analysis established that various forms of hemo-
Finally, through mitotic cell divisions in the embryo philia, or bleeders disease (in which the blood fails to clot
and after birth, each cell in a multicellular organism re- properly), also result from mutations on the X chromosome
ceives the same chromosomes—and thus the same mater- that give rise to a relatively rare, recessive trait. In this con-
nal and paternal alleles of each gene—as the zygote text, rare means infrequent in the population. The family
received from the egg and sperm at fertilization. In this histories under review, including one following the de-
way, an individual’s genome—the chromosomes and genes scendants of Queen Victoria of England (Fig. 4.23a),
he or she carries—remains constant throughout life. showed that relatively rare X-linked traits appear more of-
ten in males than in females and often skip generations.
essential concepts The clues that suggest X-linked recessive inheritance in a
pedigree are summarized in Table 4.5.
• Segregation of homologous chromosomes into daughter Unlike color blindness and hemophilia, some—although
cells at meiosis I explains Mendel’s first law. very few—of the known rare mutations on the X chromo-
• Independent alignment of homologs with respect to each some are dominant to the wild-type allele. With such
other and crossing-over of nonsister chromatids during dominant X-linked mutations, more females than males show
meiosis I explain Mendel’s second law. the aberrant phenotype. This phenomenon occurs because all
• In organisms with XX/XY sex determination, males are the daughters of an affected male but none of the sons will
hemizygous for X-linked genes, while females have two have the condition, while one-half the sons and one-half the
copies. daughters of an affected female will receive the dominant
allele and therefore show the phenotype (see Table 4.5).