Page 86 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

78 Chapter 3 Extensions to Mendel’s Laws

useful and less ambiguous. First, it is easier to identify a Consider now the possibility that two genes are
mouse with the bb genotype because a brown mouse must involved. From a cross between plants heterozygous
have this homozygous recessive genotype. Second, the re- for two genes (W and P), the F 2 generation would
sults are completely different for each of the three possible contain a 9:3:3:1 ratio of the genotypes W– P–, W–
genotypes when you use the bb test mouse. (In contrast, a pp, ww P–, and ww pp (where the dash indicates that
Bb test mouse would yield both black and brown progeny the allele could be either a dominant or a recessive
whether the albino mouse was Bb or bb; the only distin- form). Would any combinations of the 9:3:3:1 ratio be
guishing feature is the ratio.) To determine the full geno- close to that seen in the F 2 generation in this example?
type of the albino mouse, you should cross it to a brown The numbers appear to fit best with a 9:4:3 ratio.
mouse (which could be CC bb or Cc bb). What hypothesis would support combining two of the
II. In a particular kind of ornamental flower, the wild- classes (3 + 1)? If w is epistatic to the P gene, then
type flower color is deep purple, and the plants are the ww P– and ww pp genotypic classes would have
true-breeding. In one true-breeding mutant stock, the the same white phenotype. With this explanation, 1/3
flowers have a reduced pigmentation, resulting in a of the F 2 lavender plants would be WW pp, and the
lavender color. In a different true-breeding mutant remaining 2/3 would be Ww pp.
stock, the flowers have no pigmentation and are thus b. Upon self-fertilization, WW pp plants would produce
white. When a lavender-flowered plant from the first only lavender (WW pp) progeny, while Ww pp plants
mutant stock was crossed to a white-flowered plant would produce a 3:1 ratio of lavender (W– pp) and
from the second mutant stock, all the F 1 plants had white (ww pp) progeny.
purple flowers. The F 1 plants were then allowed to
self-fertilize to produce an F 2 generation. The 277 F 2 III. Huntington disease is a rare dominant condition in hu-
plants were 157 purple : 71 white : 49 lavender. mans that results in a slow but inexorable deterioration
a. Explain how flower color is inherited. Is this trait of the nervous system. The disease shows what might
be called age-dependent penetrance, which is to say
controlled by the alleles of a single gene? that the probability that a person with the Huntington
b. What kinds of progeny would be produced if lav- genotype will express the phenotype varies with age.
ender F 2 plants were allowed to self-fertilize? Assume that 50% of those inheriting the HD allele will
express the symptoms by age 40. Susan is a 35-year-
Answer old woman whose father has Huntington disease. She
a. Are any modes of single-gene inheritance compatible currently shows no symptoms. What is the probability
with the data? The observations that the F 1 plants that Susan will show symptoms in five years?
look different from either of their parents and that the
F 2 generation is composed of plants with three differ- Answer
ent phenotypes exclude complete dominance. The ra- This problem involves probability and penetrance. Two condi-
tio of the three phenotypes in the F 2 plants has some tions are necessary for Susan to show symptoms of the dis-
resemblance to the 1:2:1 ratio expected from codomi- ease. A 1/2 (50%) chance exists that she inherited the mutant
nance or incomplete dominance, but the results would allele from her father, and if she does inherit the disease allele,
then imply that purple plants must be heterozygotes. a 1/2 (50%) chance exists that she will express the phenotype
This conflicts with the information provided that pur- by age 40. Because these are independent events, the probabil-
ple plants are true-breeding. ity is the product of the individual probabilities, or 1/4.

PROBLEMS

Vocabulary d. permissive condition 4. a heritable change in a gene
1. For each of the terms in the left column, choose the e. reduced penetrance 5. genes whose alleles alter
phenotypes produced by the
best matching phrase in the right column. action of other genes
a. epistasis 1. one gene affecting more than one f. multifactorial trait 6. less than 100% of the individuals
phenotype possessing a particular genotype
b. modifier genes 2. the alleles of one gene mask the express it in their phenotype
effects of alleles of another gene g. incomplete 7. environmental condition that
c. conditional lethal 3. both parental phenotypes are dominance allows conditional lethals to
expressed in the F 1 hybrids live