Page 50 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

42 Chapter 2 Mendel’s Principles of Heredity

For each of the four following crosses, give the f. The Sgr enzyme is not needed for the survival of a
genotypes of each of the parents. pea plant, but the genomes of organisms contain
many so-called essential genes needed for an indi-
Male Female vidual’s survival. For such genes, heterozygotes for
Wings Eyes Wings Eyes Offspring the normal allele and the null allele survive, but in-
1 tiny oval × tiny oval 78 tiny wings, oval eyes dividuals homozygous for the null allele die soon
24 tiny wings, narrow eyes after the male and female gametes, each with a null
2 normal narrow × tiny oval 45 normal wings, oval eyes allele, come together at fertilization. In light of
40 normal wings, narrow eyes your answer to part (e), what does this fact tell you
38 tiny wings, oval eyes
44 tiny wings, narrow eyes about the advantage to an organism of having two
3 normal narrow × normal oval 35 normal wings, oval eyes copies of their genes?
29 normal wings, narrow eyes g. Do you think that a single pea pod could contain
10 tiny wings, oval eyes peas with different phenotypes? Explain.
11 tiny wings, narrow eyes
4 normal narrow × normal oval 62 normal wings, oval eyes h. Do you think that a pea pod could be of one color
19 tiny wings, oval eyes (say, green) while the peas within the pod could be
of a different color (say, yellow)? Explain.
30. Based on the information you discovered in the previ-
ous problem, answer the following: 32. What would have been the outcome (the genotypic
a. A female fruit fly with genotype Tt nn is mated to and phenotypic ratios) in the F 2 of Mendel’s dihybrid
cross shown in Fig. 2.15 if the alleles of the pea color
a male of genotype Tt Nn. What is the probability gene (Y, y) and the pea shape gene (R, r) did not as-
that any one of their offspring will have normal sort independently and instead the alleles inherited
phenotypes for both characters? from a parent always stayed together as a unit?
b. What phenotypes would you expect among the off- 33. Recall that Mendel obtained pure-breeding plants with
spring of this cross? If you obtained 200 progeny, how either long or short stems and that hybrids had long
many of each phenotypic class would you expect? stems (Fig. 2.8). Monohybrid crosses produced an F 2
31. Considering the yellow and green pea color pheno- generation with a 3:1 ratio of long stems to short stems,
types studied by Gregor Mendel: indicating that this difference in stem length is gov-
a. What is the biochemical function of the protein erned by a single gene. The gene that likely controlled
that is specified by the gene responsible for the this trait in Mendel’s plants has been discovered, and it
pea color phenotype? specifies an enzyme called G3βH, which catalyzes the
b. A null allele of a gene is an allele that does not reaction shown in the accompanying figure. The prod-
specify any of the biochemical function that the uct of the reaction, gibberellin, is a growth hormone
gene normally provides. Of the two alleles Y and y, that makes plants grow tall. What is the most likely hy-
which is more likely to be a null allele? pothesis to explain the difference between the domi-
c. In terms of the underlying biochemistry, why is the nant allele (L) and the recessive allele (l)?
Y allele dominant to the y allele?
d. Why are peas that are yy homozygotes green? O H G3 H O H
e. The amount of the protein specified by a gene is CO
roughly proportional to the number of functional H CO H HO H CO H
2
2
copies of the gene carried by a cell or individual.
What do the phenotypes of YY homozygotes, Yy Precursor Gibberellin
heterozygotes, and yy homozygotes tell us about 34. The gene that likely controlled flower color (purple
the amount of the Sgr enzyme (the product of the or white) in Mendel’s pea plants has also been
pea color gene) needed to produce a yellow color? identified. The flower color gene specifies a protein

bHLH

OH
OH OH OH
HO O
HO O DFR HO O ANS 3GT HO O
+
OH
OH OH O-Glc
OH O OH OH OH OH
Colorless Colorless Colorless Anthocyanin