3.An organism has alleles R1 and R2 on one pair of homologous chromosomes, and it has alleles T1 and T2 on another pair.
Diagram these pairs of homologs at the end of metaphase I, the end of telophase I, and the end of telophase II, and show
how meiosis in this organism produces gametes in expected Mendelian proportions. Assume no crossover between
homologous chromosomes.
5.In a breed of domestic cattle, horns can appear on males and on females. Males and females can also be hornless. The
following crosses are performed with parents from pure-breeding lines.
Cross I Cross II
Parents: horned male X hornless
female
Parents: hornless male X horned
female
F1: all males horned & all females
hornless
F1: all males horned & all females
hornless
F2: 3⁄4 males horned, 1⁄4 males hornless F2: 3⁄4 males horned, 1⁄4 males hornless
1⁄4 females horned, 3⁄4 females
hornless
1⁄4 females horned, 3⁄4 females
hornless
Explain the inheritance of this phenotype in cattle, and assign genotypes to all cattle in cross I.
6.A wild-type male and a wild-type female Drosophila with red eyes and full wings are crossed. Their progeny
are shown below.
a. Using clearly defined allele symbols of your choice, give the genotype of each parent.
b. What is/are the genotype(s) of females with purple eye? Of males with purple eye and miniature wing?
8.Fifteen bacterial colonies growing on a complete medium are replica-plated to a minimal medium. Twelve of
the colonies grow on minimal medium.
a. Using terminology from the chapter, characterize the 12 colonies that grow on minimal medium and
the 3 colonies that do not.
b. The three colonies that do not grow on minimal medium are replica-plated to minimal medium plus
the amino acid serine (min + Ser), and all three colonies grow. Characterize these three colonies.
c. The serine biosynthetic pathway is a three-step pathway in which each step is catalyzed by the
enzyme product of a different gene, identified as enzymes A, B, and C in the diagram below.
3-Phosphoglycerate 3-Phospho-hydroxypyruvate 3-Phosphoserine Serine
Mutant 1 grows only on min + Ser. In addition to growth on min + Ser, mutant 2 also grows on min + 3-PHP
and min + 3-PS. Mutant 3 grows on min + 3-PS and min + Ser. Identify the step of the serine biosynthesis
pathway at which each mutant is defective.
9..In rats, gene B produces black coat color if the genotype is B-, but black pigment is not produced if the genotype is bb.
At an independent locus, gene D produces yellow pigment if the genotype is D-, but no pigment is produced when the
genotype is dd. Production of both pigments results in brown coat color. If neither pigment is produced, coat color is
cream. Determine the genotypes of parents of litters with the following phenotype distributions.
10..In cattle, an autosomal mutation called Dexter produces calves with short stature and short limbs. Embryos that are
homozygous for the Dexter mutation have severely stunted development and either spontaneously abort or are stillborn.
What progeny phenotypes do you expect from the cross of two Dexter cows? What are the expected proportions of the
expected phenotypes?
11.Xeroderma pigmentosum (XP) is an autosomal recessive condition characterized by moderate to severe
sensitivity to ultraviolet (UV) light. Patients develop multiple skin lesions on UV-exposed skin, and skin cancers
often develop as a result. XP is caused by deficient repair of DNA damage from UV exposure.
a. Many genes are known to be involved in repair of UV-induced DNA damage, and several of these genes
are implicated in XP. What genetic phenomenon is illustrated by XP?
b. A series of 10 skin-cell lines was grown from different XP patients. Cells from these lines were fused,
and the heterokaryons were tested for genetic complementation by assaying their ability to repair DNA
damage caused by a moderate amount of UV exposure. In the table below, + indicates that the fusion
cell line performs normal DNA damage mutation repair, and – indicates defective DNA repair. Use this
information to determine how many DNA-repair genes are mutated in the 10 cell lines, and identify
which cell lines share the same mutated genes.
a. 4 brown, 4 black, 4 yellow, 4 cream
b. 3 brown, 3 yellow, 1 black, 1 cream
c. 9 black, 7 brown
6.Flower color in snapdragons results from the amount of the pigment anthocyanin in the petals. Red flowers are
produced by plants that have full anthocyanin production, and ivory-colored flowers are produced by plants that lack the
ability to produce anthocyanin. The allele An1 has full activity in anthocyanin production, and the allele An2 is a null
allele. Dr. Ara B. Dopsis, a famous genetic researcher, crosses pure-breeding red snapdragons to pure-breeding ivory
snapdragons and produces F1 progeny plants that have pink flowers. He proposes that this outcome is the result of
incomplete dominance, and he crosses the F1 to test his hypothesis. What phenotypes does Dr. Dopsis predict will be
found in the F2, and in what proportions?
7.Strains of petunias come in four pure-breeding colors: white, blue, red, and purple. White petunias are produced when
plants synthesize no flower pigment. Blue petunias and red petunias are produced when plants synthesize blue or red
pigment only. Purple petunias are produced in plants that synthesize both red and blue pigment. The mixture of red and
blue makes purple. Flower-color pigments are synthesized by gene action in two separate pigment-producing biochemical
pathways. Pathway I contains gene A that produces an enzyme to catalyze conversion of a colorless pigment designated
white1 to blue pigment. In Pathway II, the enzymatic product of gene B converts the colorless pigment designated white2
to red pigment. The two genes assort independently.
gene A
Pathway I: White1 → Blue
+ = Purple
Pathway II: White2 → Red
gene B
a. What are the possible genotype(s) for pure-breeding red petunias?
b. What are the possible genotype(s) for true-breeding blue petunias?
c. True-breeding red petunias are crossed to pure-breeding blue petunias, and all the F1 progeny have
purple flowers. If the F1 are allowed to self-fertilize and produce the F2, what is the expected
phenotypic distribution of the F2 progeny? Show your work.
8.Blue flower color is produced in a species of morning glories when dominant alleles are present at two gene loci, A and
B. (Plants with the genotype A- B- have blue flowers.) Purple flowers result when a dominant allele is present at only one
of the two gene loci, A or B. (Plants with the genotypes A- bb and aa B- are purple.) Flowers are red when the plant is
homozygous recessive for each gene (i.e., aabb).
a. Two pure-breeding purple strains are crossed, and all the F1 plants have blue flowers. What are the
genotypes of the parental plants?
b. If two F1 plants are crossed, what are the expected phenotypes and frequencies in the F2?
c. If an F1 plant is backcrossed to one of the pure-breeding parental plants, what is the expected ratio of
phenotypes among progeny?
d. Why is the phenotype ratio the same regardless of which parental strain is selected for the backcross?
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