Page 95 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

Problems 87

just answered in part (c)? You should specify the normal have a 25 times higher frequency of heart
lowest numerical likelihood that could possibly be attacks than unaffected individuals. People with se-
consistent with the data. rum cholesterol levels three or more times higher than
46. This problem illustrates why classical geneticists in normal have severely blocked arteries and almost al-
the days before DNA analysis usually needed to ways die before they reach the age of 20. The follow-
work with traits showing complete penetrance. ing pedigrees show the occurrence of FH in four
Consider the sweet peas shown in Fig. 3.15, where Japanese families:
the A– B– genotypic class normally produces purple a. What is the most likely mode of inheritance of
flowers and all other genotypic classes have white FH based on these data? Do any individuals in
flowers. any of these pedigrees not fit your hypothesis?
a. If the parental generation is AA bb × aa BB, what What special conditions might account for such
phenotypic ratio do you expect in the F 2 genera- individuals?
tion, assuming complete penetrance? b. Why do individuals in the same phenotypic class
b. Suppose now that only 75% of A– B– individuals (unfilled, yellow, or red symbols) show such
have purple flowers (that is, the penetrance of this variation in their levels of serum cholesterol?
trait is 75%). What phenotypic ratio do you now
expect among the F 2 plants? Key to serum cholesterol levels:
Family 1
c. In doing these types of crosses, what kinds of re- < 250 mg/dl
sults (other than an unexpected F 2 ratio) might sug-
gest that penetrance of the purple phenotype is 250–500 mg/dl
incomplete?
> 500 mg/dl
47. Spherocytosis is an inherited blood disease in which Family 2
the erythrocytes (red blood cells) are spherical instead
of biconcave. This condition can be inherited in a
dominant fashion, with ANK1 (the nonfunctional mu-
+
tant allele) dominant to ANK1 . In people with sphe-
rocytosis, the spleen recognizes the spherical red
blood cells as defective and removes them from the
bloodstream, leading to anemia. The spleen in differ- Family 3
ent people removes the spherical erythrocytes with
different efficiencies. Some people with spherical
erythrocytes suffer severe anemia and some mild ane-
mia, yet others have spleens that function so poorly no
symptoms of anemia exist at all. When 2400 people Family 4
+
with the genotype ANK1 ANK1 were examined, it
was found that all of them had spherical erythrocytes,
2250 had anemia of varying severity, and 150 had no
symptoms. (Assume that ANK1 ANK1 homozygotes
do not exist.)
a. Does this description of people with spherocytosis
represent incomplete penetrance, variable expres- 49. You have come into contact with two unrelated
sivity, or both? Explain your answer. Can you de- patients who express what you think is a rare
rive any values from the numerical data to measure phenotype—a dark spot on the bottom of the foot.
penetrance or expressivity? According to a medical source, this phenotype is seen
b. Suggest a treatment for spherocytosis and describe in 1 in every 100,000 people in the population. The
how the incomplete penetrance and/or variable two patients give their family histories to you, and
expressivity of the condition might affect this you generate the pedigrees that follow.
treatment. a. Given that this trait is rare, do you think the
48. Familial hypercholesterolemia (FH) is an inherited inheritance is dominant or recessive? Are there
trait in humans that results in higher-than-normal se- any special conditions that appear to apply to the
rum cholesterol levels [measured in milligrams of inheritance?
cholesterol per deciliter of blood (mg/dl)]. People b. Which nonexpressing members of these families
with serum cholesterol levels that are roughly twice must carry the mutant allele?