Page 177 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

Solved Problems 169

b. The probability that he will have the antigen and ich- Picking out the parental classes is easy. If all the other
thyosis (genotype: STS XG / Y) is 5/100. classes are rare, the two most abundant categories are those
+
c. Two classes of gametes exist that contain the ichthyo- gene combinations that have not undergone recombination.
+
sis allele: STS XG (45%) and STS XG (5%). If the to- Next, two sets of two phenotypes should exist, one set cor-
tal number of gametes is 100, then 50 will have the responding to a single crossover between the first and sec-
STS allele. Of those gametes, 5 (or 10%) will have the ond genes and the other set to a single crossover between
+
XG allele. Therefore a 1/10 probability exists that a the second and third genes. Finally, there should be a pair
son with the STS allele will have the Xg antigen. of classes containing small numbers that result from dou-
ble crossovers. In this example, no flies are found in the
II. Drosophila females of wild-type appearance but hetero- double crossover classes, which would have been the two
zygous for three autosomal genes are mated with males missing phenotypic combinations: one is glassy eyes, and
showing the three corresponding autosomal recessive the other is coal body and striped thorax.
traits: glassy eyes, coal-colored bodies, and striped tho- Look at the most abundant classes to determine which
raxes. One thousand (1000) progeny of this cross are alleles were on each chromosome in the female heterozy-
distributed in the following phenotypic classes: gous parent. One parental class had the phenotype of coal
body (484 flies), so one chromosome in the female must
Wild type 27 + +
Striped thorax 11 have contained the t , g , and c alleles. (Notice that we can-
Coal body 484 not yet say in what order these alleles are located on the
Glassy eyes, coal body 8 chromosome.) The other parental class was glassy eyes and
Glassy eyes, striped thorax 441 striped thorax, corresponding to a chromosome with the t,
Glassy eyes, coal body, striped thorax 29
+
g, and c alleles.
a. Draw a genetic map based on these data. To determine the order of the genes, compare the
+
+
b. Show the arrangement of alleles on the two homol- t g c double crossover class (not seen in the data) with
+
ogous chromosomes in the parent females. the most similar parental class (t g c ). The alleles of g
+
c. Normal-appearing males containing the same and c retain their parental associations (g c ), while the t
chromosomes as the parent females in the preceding gene has recombined with respect to both other genes in
cross are mated with females showing glassy eyes, the double recombinant class. Thus, the t gene is between
coal-colored bodies, and striped thoraxes. Of 1000 g and c.
progeny produced, indicate the numbers of the In order to complete the map, calculate the recombina-
various phenotypic classes you would expect. tion frequencies between the center gene and each of the
genes on the ends. For g and t, the nonparental combinations
of alleles are in classes 2 and 4, so RF = (11 + 8)/1000 =
Answer 19/1000, or 1.9%. For t and c, classes 1 and 6 are nonparen-
A logical, methodical way to approach a three-point cross tal, so RF = (27 + 29)/1000 = 56/1000, or 5.6%.
is described here. The genetic map is
a. Designate the alleles: c + t + g +

+
t = wild-type thorax t = striped thorax 5.6 m.u. 1.9 m.u.
+
g = wild-type eyes g = glassy eyes
+
c = wild-type body c = coal-colored body b. The alleles on each chromosome were already deter-
+
+
+
mined (c, g , t and c , g, t). Now that the order of
In solving a three-point cross, designate the types of events loci has also been determined, the arrangement of the
that gave rise to each group of individuals and the genotypes alleles can be indicated as follows:
of the gametes obtained from their mother. [The paternal gam-
etes contain only the recessive alleles of these genes (t g c). c t + g +
These alleles from the father allow the traits associated with
the recessive maternal alleles to appear in the progeny.]
c + t g
Progeny Number Type of event Genotype c. Males of the same genotype as the starting female
+
+
+
+
+
1. wild type 27 single crossover t g c + (c t g / c t g) could produce only two types of gam-
+
+
+
+
2. striped thorax 11 single crossover t g c + etes: parental types c t g and c t g because no
+
+
3. coal body 484 parental t g c recombination occurs in male fruit flies. The progeny
+
4. glassy eyes, coal body 8 single crossover t g c expected from the mating with a homozygous reces-
5. glassy eyes, striped thorax 441 parental t g c +
6. glassy eyes, coal body, 29 single crossover t g c sive female are thus 500 coal body and 500 glassy-
striped thorax eyed, striped thorax flies.