Page 140 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

132 Chapter 4 The Chromosome Theory of Inheritance

d. Is the penetrance of the cancer phenotype complete 50. In marsupials like the opposum or kangaroo, X inactiva-
or incomplete? tion selectively inactivates the paternal X chromosome.
e. Is the expressivity of the cancer phenotype unvary- a. Predict the possible coat colors of the progeny of
ing or variable? both sexes if a female marsupial homozygous for a
f. Are any of the cancer phenotypes associated with mutant allele of an X-linked coat color gene was
the BRCA2 mutation sex-limited or sex-influenced? mated with a male hemizygous for the alternative
g. How can you explain the absence of individuals wild-type alleles of this gene.
diagnosed with cancer in generations I and II? b. Predict the possible coat colors of progeny of both
48. In 1995, doctors reported a Chinese family in which sexes if a male marsupial hemizygous for an allele
of an X-linked coat color gene was mated with a
retinitis pigmentosa (progressive degeneration of the female homozygous for the alternative wild-type
retina leading to blindness) affected only males. All allele of this gene.
six sons of affected males were affected, but all of the
five daughters of affected males (and all of the c. Why are the terms recessive and dominant not use-
children of these daughters) were unaffected. ful in describing the alleles of X-linked coat color
a. What is the likelihood that this form of retinitis genes in marsupials?
pigmentosa is due to an autosomal mutation d. Why would marsupials heterozygous for two
showing complete dominance? alleles of an X-linked coat color gene not have
b. What other possibilities could explain the inheri- patches of fur of two different colors as did the tor-
toiseshell cats described in the previous problem?
tance of retinitis pigmentosa in this family? Which
of these possibilities do you think is most likely? 51. The pedigree diagram below shows a family in which
49. In cats, the dominant O allele of the X-linked orange many individuals are affected by a disease called Leri-
Weill dyschondrosteosis (LWD). People with LWD
gene is required to produce orange fur; the recessive o are short in stature due to leg bone deformities; arm
allele of this gene yields black fur. Tortoiseshell cats bones are also malformed in some individuals. The
have coats with patches of orange fur alternating mutant gene responsible for LWD was identified in
with patterns of black fur. Approximately 90% of all 1998 as SHOX, a gene located in a pseudoautosomal
tortoiseshell cats are females. region (PAR1) of the X and Y chromosomes.
a. Explain why tortoiseshell cats are nearly always a. Is the SHOX allele that causes LWD dominant or
female. recessive? Explain. (Note: Sex reversal is not involved.)
b. What types of crosses would be expected to b. Even though SHOX is located on the X chromo-
produce female tortoiseshell cats? some, the pedigree is atypical for an X-linked
c. Suggest a hypothesis to explain the origin of male allele. What features of the pedigree are incompatible
tortoiseshell cats. with X-linkage?
d. Calico cats (most of which are females) have c. For each affected individual in the pedigree, determine
patches of white, orange, and black fur. Suggest a whether the SHOX disease allele is on the X or the Y.
hypothesis for the origin of calico cats.
d. Explain the inheritance pattern of the SHOX disease
+
allele and the SHOX normal allele in pedigree.
e. Diagram the crossover event that generated the Y chro-
mosome in individual III-5. Your diagram should indi-
+
cate the positions of the SHOX (disease) and SHOX
(normal) alleles on the X and Y chromosomes in the
+
germ-line cells of individual II-3, and SRY on Y.

I
1 2
II
1 2 3 4 5
III
1 2 3 4 5 6 7
Tortoiseshell Calico IV
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