Page 131 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

Solved Problems 123

XY individuals have testes that make the hormone testos- essential concepts
terone, but in the absence of the androgen receptor to
which it binds, the testosterone has no effect. Without the • Sex-linked (X-linked) traits show sex-specific inheritance
androgen receptor, these people cannot develop male gen- patterns because sons always inherit their father’s Y
italia (penis and scrotum) nor male internal duct systems chromosome, while daughters always inherit their father’s
(the vas deferens, seminal vesicles, and ejaculatory ducts); X chromosome.
instead, their external genitalia assume the default female • Random inactivation of either the maternal or paternal
state (labia and clitoris). However, the testes make another X chromosome in XX cells ensures that male and female
hormone that prevents the formation of female internal mammalian cells express equivalent amounts of the
duct systems (including the fallopian tubes, uterus, and proteins encoded by most X-linked genes.
vagina). The result is that persons with CAIS are exter- • Mutations of genes—whether autosomal or X-linked—can
nally female but sterile because they lack the internal duct have different effects in males and females.
systems of either sex.

WHAT’S NEXT

T. H. Morgan and his students, collectively known as the body yellow instead of brown, another shortened the
Drosophila group, acknowledged that Mendelian genetics wings, yet another made bent instead of straight body
could exist independently of chromosomes. “Why then, we bristles. These findings raised several compelling ques-
are often asked, do you drag in the chromosomes? Our an- tions. First, if the genes for all of these traits are physically
swer is that because the chromosomes furnish exactly the linked together on the X chromosome, does this linkage
kind of mechanism that Mendelian laws call for, and since affect their ability to assort independently, and if so, how?
there is an ever-increasing body of information that points Second, does each gene have an exact chromosomal
clearly to the chromosomes as the bearers of the Mendelian address, and if so, does this specific location in any way
factors, it would be folly to close one’s eyes to so patent a affect its transmission? In Chapter 5 we describe how the
relation. Moreover, as biologists, we are interested in hered- Drosophila group and others analyzed the transmission
ity not primarily as a mathematical formulation, but rather patterns of genes on the same chromosome in terms of
as a problem concerning the cell, the egg, and the sperm.” known chromosome movements during meiosis, and how
The Drosophila group went on to find several X- they then used the information obtained to localize genes
linked mutations in addition to white eyes. One made the to specific chromosomal positions.

SOLVED PROBLEMS

I. In humans, chromosome 16 sometimes has a heavily chromosome 16 homologs in the mother lacked blobs,
stained area in the long arm near the centromere. This but the father was heterozygous for blobs. Which par-
feature can be seen through the microscope but has no ent experienced nondisjunction, and in which meiotic
effect on the phenotype of the person carrying it. division did it occur?
When such a “blob” exists on a particular copy of
chromosome 16, it is a constant feature of that chromo-
some and is inherited. Answer
A couple conceived a child, but the fetus had This problem requires an understanding of nondisjunction
multiple abnormalities and was miscarried. When the during meiosis. When individual chromosomes contain
chromosomes of the fetus were studied, it was discov- some distinguishing feature that allows one homolog to be
ered that it had three copies of chromosome 16 (it was distinguished from another, it is possible to follow the path
trisomic for chromosome 16), and that two of the of the two homologs through meiosis. Because the fetus
three chromosome 16s had large blobs. Both had two chromosome 16s with the blob, we can conclude
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