Page 87 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

Problems 79

h. codominance 8. a trait produced by the interaction b. Animals with the trident marking mate among
of alleles of at least two genes or themselves. Of 300 progeny, how many would be
from interactions between gene expected to have a trident, how many ebony bod-
and environment ies, and how many honey-colored bodies?
i. mutation 9. individuals with the same genotype
have related phenotypes that vary 6. A cross between two plants that both have yellow
in intensity flowers produces 80 offspring plants, of which 38
j. pleiotropy 10. a genotype that is lethal in some have yellow flowers, 22 have red flowers, and 20
situations (for example, high have white flowers. If one assumes that this varia-
temperature) but viable in others tion in color is due to inheritance at a single locus,
k. variable expressivity 11. the heterozygote resembles what is the genotype associated with each flower
neither homozygote color, and how can you describe the inheritance of
flower color?
Section 3.1 7. In radishes, color and shape are each controlled by a
2. In four-o’clocks, the allele for red flowers is incom- single locus with two incompletely dominant alleles.
Color may be red (RR), purple (Rr), or white (rr) and
pletely dominant to the allele for white flowers, so shape can be long (LL), oval (Ll), or round (ll). What
heterozygotes have pink flowers. What ratios of phenotypic classes and proportions would you expect
flower colors would you expect among the offspring among the offspring of a cross between two plants
of the following crosses: (a) pink × pink, (b) white × heterozygous at both loci?
pink, (c) red × red, (d) red × pink, (e) white × white,
and (f) red × white? If you specifically wanted to pro- 8. A wild legume with white flowers and long pods is
duce pink flowers, which of these crosses would be crossed to one with purple flowers and short pods.
most efficient? The F 1 offspring are allowed to self-fertilize, and the
3. The Aa heterozygous snapdragons in Fig. 3.3 are F 2 generation has 301 long purple, 99 short purple,
612 long pink, 195 short pink, 295 long white, and 98
pink, while AA homozygotes are red. However, short white. How are these traits being inherited?
Mendel’s Pp heterozygous pea flowers were every bit
as purple as those of PP homozygotes (Fig. 2.8). 9. Assuming no involvement of the Bombay phenotype
Assuming that the A allele and the P allele specify (in case you’ve already read ahead to Section 3.2):
functional enzymes, and the a and p alleles a. If a girl has blood type O, what could be the geno-
specify no protein at all, explain why the alleles types and corresponding phenotypes of her parents?
of gene A and the alleles of gene P interact so b. If a girl has blood type B and her mother has blood
differently. type A, what genotype(s) and corresponding
4. Recall from Chapter 2 (Fig. 2.20) that Mendel’s R phenotype(s) could the other parent have?
gene specifies an enzyme called Sbe1 that forms c. If a girl has blood type AB and her mother is also
branched starches. The dominant allele (R) makes AB, what are the genotype(s) and corresponding
protein, and the recessive allele (r) is nonfunctional. phenotype(s) of any male who could not be the
When considering the phenotype of round or wrin- girl’s father?
kled peas, R is completely dominant to r: RR and Rr 10. Several genes in humans in addition to the ABO gene
peas are both equally round and rr peas are wrinkled. (I) give rise to recognizable antigens on the surface of
Imagine that the phenotype described is instead the red blood cells. The MN and Rh genes are two exam-
average number of Sbe1 protein molecules in a pea. ples. The Rh locus can contain either a positive or a
How would you describe the dominance relation be- negative allele, with positive being dominant to nega-
tween R and r in this case? tive. M and N are codominant alleles of the MN gene.
5. In the fruit fly Drosophila melanogaster, very dark The following chart shows several mothers and their
(ebony) body color is determined by the e allele. The children. For each mother-child pair, choose the father
+
e allele produces the normal wild-type, honey- of the child from among the males in the right col-
colored body. In heterozygotes for the two alleles (but umn, assuming one child per male.
+ +
not in e e homozygotes), a dark marking called the
trident can be seen on the thorax, but otherwise the
+
body is honey-colored. The e snd e alleles are thus Mother Child Males
considered to be incompletely dominant. a. O M Rh(pos) B MN Rh(neg) O M Rh(neg)
+
+ +
a. When female e e flies are crossed to male e e b. B MN Rh(neg) O N Rh(neg) A M Rh(pos)
flies, what is the probability that progeny will have c. O M Rh(pos) A M Rh(neg) O MN Rh(pos)
the dark trident marking? d. AB N Rh(neg) B MN Rh(neg) B MN Rh(pos)