Page 88 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

80 Chapter 3 Extensions to Mendel’s Laws

11. Alleles of the gene that determines seed coat patterns an allele that determines the absence of a pattern is
in lentils can be organized in a dominance series: recessive to the other six alleles, each of which pro-
marbled > spotted = dotted (codominant alleles) > duces a different pattern. All heterozygous combina-
clear. A lentil plant homozygous for the marbled seed tions of alleles show complete dominance.
coat pattern allele was crossed to one homozygous for a. How many different kinds of leaf patterns (includ-
the spotted pattern allele. In another cross, a homozy- ing the absence of a pattern) are possible in a pop-
gous dotted lentil plant was crossed to one homozy- ulation of clover plants in which all seven alleles
gous for clear. An F 1 plant from the first cross was are represented?
then mated to an F 1 plant from the second cross. b. What is the largest number of different genotypes
a. What phenotypes in what proportions are expected that could be associated with any one phenotype?
from this mating between the two F 1 types? Is there any phenotype that could be represented by
b. What are the expected phenotypes of the F 1 plants only a single genotype?
from the two original parental crosses? c. In a particular field, you find that the large major-
12. One of your fellow students tells you that there is no ity of clover plants lack a pattern on their leaves,
way to know that the spotted and dotted patterns on the even though you can identify a few plants repre-
S
lentils in Fig. 3.4a are due to codominant alleles (C and sentative of all possible pattern types. Explain this
D
C ) of a single gene C. He claims that spotting could be finding.
controlled by gene S, with a completely dominant allele 15. Fruit flies with one allele for curly wings (Cy) and
S that directs spotting and a recessive allele s that di- one allele for normal wings (Cy ) have curly wings.
+
rects no spots. Likewise, he claims that dotting could be When two curly-winged flies were crossed, 203
controlled by a separate gene D, with a completely curly-winged and 98 normal-winged flies were
dominant allele D that directs dotting and a recessive al- obtained. In fact, all crosses between curly-winged
lele d that directs no dots. Is he correct, or does the in- flies produce nearly the same curly : normal ratio
formation in Fig. 3.4a argue against this idea? Explain. among the progeny.
13. In a population of rabbits, you find three different a. What is the approximate phenotypic ratio in these
coat color phenotypes: chinchilla (C), himalaya (H), offspring?
and albino (A). To understand the inheritance of coat
colors, you cross individual rabbits with each other b. Suggest an explanation for these data.
and note the results in the following table. c. If a curly-winged fly was mated to a normal-
winged fly, how many flies of each type would you
expect among 180 total offspring?
Cross number Parental phenotypes Phenotypes of progeny
16. In certain plant species such as tomatoes and petu-
1 H × H 3/4 H : 1/4 A nias, a highly polymorphic incompatibility gene S
2 H × A 1/2 H : 1/2 A with more than 100 known alleles prevents self-fer-
3 C × C 3/4 C : 1/4 H tilization and promotes outbreeding. In this form of
4 C × H all C incompatibility, a plant cannot accept sperm carry-
5 C × C 3/4 C : 1/4 A ing an allele identical to either of its own incompat-
6 H × A all H ibility alleles. If, for example, pollen carrying
7 C × A 1/2 C : 1/2 A sperm with allele S of the incompatibility gene
1
8 A × A all A lands onto the stigma (a female organ) of a plant
9 C × H 1/2 C : 1/2 H that also carries the S allele, the sperm cannot fer-
1
10 C × H 1/2 C : 1/4 H : 1/4 A
tilize any eggs in that plant. (This phenomenon
a. What can you conclude about the inheritance of occurs because the pollen grain on the stigma cannot
coat color in this population of rabbits? grow a pollen tube to allow the sperm to unite with
b. Ascribe genotypes to the parents in each of the the egg.)
10 crosses. For the following crosses, indicate whether any
progeny would be produced, and if so, list all possible
c. What kinds of progeny would you expect, and in genotypes of these progeny.
what proportions, if you crossed the chinchilla par- 1 2 1 2
ents in crosses 9 and 10? a. ♂ S S × ♀ S S 3
2
2
1
14. In clover plants, the pattern on the leaves is deter- b. ♂ S S × ♀ S S
3
1
2
mined by a single gene with multiple alleles that are c. ♂ S S × ♀ S S 4
related in a dominance series. The gene is not pleio- d. Explain how this mechanism of incompatibility
tropic. Seven different alleles of this gene are known; would prevent plant self-fertilization.