Page 92 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

84 Chapter 3 Extensions to Mendel’s Laws

34. A student whose hobby was fishing pulled a very un- or protein B is sufficient to prevent blue pigment
usual carp out of Cayuga Lake: It had no scales on its production from precursor 2. The recessive mutant
body. She decided to investigate whether this strange alleles a and b specify no protein. Two different
nude phenotype had a genetic basis. She therefore ob- pure-breeding mutant strains with white flowers
tained some inbred carp that were pure-breeding for were crossed and complementation was observed so
the wild-type scale phenotype (body covered with that all the F 1 were blue.
scales in a regular pattern) and crossed them with her Colorless A B
nude fish. To her surprise, the F 1 progeny consisted precursor 1 Blue
of a 1:1 ratio of wild-type fish and fish with a single
linear row of scales on each side. Colorless Blue
precursor 2
a. Can a single gene with two alleles account for this A B
result? Why or why not?
b. To follow up on the first cross, the student allowed a. What are the genotypes of each white mutant strain
and the F 1 ?
the linear fish from the F 1 generation to mate with
each other. The progeny of this cross consisted of b. If the F 1 are selfed, what would be the phenotypic
fish with four phenotypes: linear, wild type, nude, ratio of the F 2 ?
and scattered (the latter had a few scales scattered ir- 36. This problem examines possible biochemical explana-
regularly on the body). The ratio of these phenotypes tions for variations of Mendel’s 9:3:3:1 ratio. Except
was 6:3:2:1, respectively. How many genes appear to where indicated, compounds 1, 2, 3, and 4 have dif-
be involved in determining these phenotypes? ferent colors, as do mixtures of these compounds. A
c. In parallel, the student allowed the phenotypically and B are enzymes that catalyze the indicated steps of
wild-type fish from the F 1 generation to mate with the pathway. Alleles A and B specify functional en-
each other and observed, among their progeny, wild- zymes A and B, respectively; these are completely
type and scattered carp in a ratio of 3:1. How many dominant to alleles a and b, which do not specify any
genes with how many alleles appear to determine of the corresponding enzyme. If functional enzyme is
the difference between wild-type and scattered carp? present, assume that the compound to the left of the
d. The student confirmed the conclusions of (c) by arrow is converted completely to the compound to the
right of the arrow. For each pathway, what phenotypic
crossing those scattered carp with her pure- ratios would you expect among the progeny of a dihy-
breeding wild-type stock. Diagram the genotypes brid cross of the form Aa Bb × Aa Bb?
and phenotypes of the parental, F 1 , and F 2 genera-
tions for this cross and indicate the ratios observed. a. Independent pathways
e. The student attempted to generate a true-breeding Compound 1 Enz A Compound 2
nude stock of fish by inbreeding. However, she
found that this was impossible. Every time she Compound 3 Enz B Compound 4
crossed two nude fish, she found nude and scat-
tered fish in the progeny, in a 2:1 ratio. (The scat- b. Redundant pathways
tered fish from these crosses bred true.) Diagram Enz A
the phenotypes and genotypes of this gene in a Compound 1 Compound 2
nude × nude cross and explain the altered Enz B
Mendelian ratio. c. Sequential pathway
f. The student now felt she could explain all of her Compound 1 Enz A Compound 2 Enz B Compound 3
results. Diagram the genotypes in the linear × lin-
ear cross performed by the student in (b). Show the d. Enzymes A and B both needed to catalyze the re-
genotypes of the four phenotypes observed among action indicated.
the progeny and explain the 6:3:2:1 ratio. Compound 1 Enz A + Enz B Compound 2
35. Suppose that blue flower color in a plant species is e. Branched pathways (assume enough of compound
controlled by two genes, A and B. The dominant al- 1 for both pathways)
leles A and B specify proteins that function in the
pathways shown below. The A and B proteins are Compound 1 Enz A Compound 2
both required to make blue pigment from a colorless
precursor. A and B proteins also independently inhibit Enz B
the production of blue pigment from a different color-
less precursor; that is, the presence of either protein A Compound 3