BIOLOGY 160
GENETICS PROBLEMS
INSTRUCTIONS
WINTER 2011
Show all of your work on each question to receive full credit. Showing your work for full credit
includes all of the following in every problem:
1.
2.
3.
4.
5.
6.
The cross (written out as a cross/mating), the genotypes of both parents
The gametes each parent can produce and their proportions
Punnett Square (if required: more than one genotypic or phenotypic class of progeny)
Genotypic and Phenotypic ratios of the offspring
Answers to the questions succinctly stated in a simple declarative sentence
Stating the mode (modes: some of the problems may have more that one!) of
inheritance in a simple declarative sentence
PLEASE DO NOT CHANGE ALLELIC DESIGNATIONS!
Monohybrid Crosses
1) In tomatoes, tall stature (T) is dominant over dwarf stature (t). If you cross a homozygous tall plant with a homozygous dwarf
plant in the parental generation, what would the phenotypic and genotypic ratios of the F 1 progeny (offspring) be?
If the F1s were crossed, what would the genotypic and phenotypic ratios of the progeny be? What would this generation
be called?
2) What genotype would you cross the tall plants obtained in the F1 generation with in order to determine if they were
homozygous or heterozygous? What is this type of cross called? Show your work to gain a better understanding of this
concept.
3) In guinea pigs black coat (B) is dominant over white coat (b). In a cross between two black coated guinea pigs, the following
progeny were obtained:
8 black coated guinea pigs
3 white coated guinea pigs
From these data, determine the genotypes of the parents of the guinea pigs produced from this cross.
4) One day while perusing the grapes at my local Safeway store, I spied a mutant fruit fly that had no wings. This trait is called
vestigial (vg) by fruit fly geneticists and is recessive to the wild type allele for normal wings (+). So, I captured the little devil
and decided to cross this vestigial fly with one of my prized normal winged flies. All of the progeny from this cross have
normal wings. What is the genotype of my prized normal winged fly? If I were to cross two of the F 1s, what would the
expected genotypic and phenotypic ratios of the F2 generation be?
5) The ability to taste a bitter chemical, phenylthiocarbamide (PTC), is due to a dominant allele in humans. In terms of tasting
ability, what are all of the possible phenotypes of a man both of whose parents were tasters? What are his possible
genotypes? Use T for the designation of the dominant allele and t as the designation of the recessive allele.
6) All of the progeny produced in a cross between a spotted rabbit and a solid-colored rabbit are spotted. When these F1 rabbits
are crossed among themselves, they produced 23 spotted rabbits and 8 solid-colored rabbits. a) Which of the alleles in rabbits
is dominant? b) About how many of the 23 spotted rabbits in the F2 will be homozygous? c) How many solid colored
progeny would be homozygous?
7) In snapdragons, the alleles for flower color exhibit an incomplete dominance between the two alleles (red, Cr, and white, Cw).
The floral color of a heterozygous plant is pink as these flowers have less red pigment than homozygous red plants. What
offspring are produced from a cross of a heterozygous plant and a homozygous white flowered plant?
8) The palomino horse is golden in color. Unfortunately, for horse fanciers, palominos do not breed true. Use the following
allelic designations for this trait: Ccr = cream and Cch = chestnut. In a series of matings between palominos, the following
offspring were obtained:
65 palominos, 32 cream-colored horses, 34 chestnut (reddish-brown) horses
What is the probable mode of inheritance of palomino coloration?
9) The ABO blood system of human blood typed are determined by 2 of 3 alleles (IA, IB, i). The A and B alleles are codominant
and both are completely dominant over the recessive o allele. What are the potential blood types of the offspring produced by
a female with blood type A (genotype IA i) and a male with blood type B (genotype IB i)?
Dihybrid Crosses
10) For the following parental genotypes indicate the potential gamete types.
Genotype
Gamete types
AABB
AaBb
Aabb
AABb
aaBb
11) In tomato plants, tall plant alleles (D) are dominant over dwarf plant alleles (d), and red fruited alleles (Y) are dominant over
yellow fruited alleles (y). A homozygous tall, red-fruited plant is crossed with a homozygous dwarf, yellow-fruited plant.
What are the potential types and proportions of offspring from this cross?
What is the outcome if two plants from the F1 generation are crossed?
12) How would you determine the genotype of a tall, red-fruited tomato plant?
What would be the results of the test-cross if the tall, red-fruited plant was:
homozygous for both genes?
heterozygous for only height?
heterozygous for only fruit color?
heterozygous for both genes?
13) In dogs, 1 gene determines whether a dog barks or is silent when trailing. The allele (B) for barking is dominant over the
allele (b) for silent trailers. Also, erect ears (E) are dominant over drooping ears (e).
What kinds of pups (and in what proportions) would be expected from a cross between a heterozygous, erect-eared
barker, and a droop-eared, silent trailer?
14) In humans, the allele for tongue rolling (T) is dominant for the non-tongue rolling allele (t), and the allele for non-ear
wiggling (W) is dominant over the ear wiggling allele (w). What are the expected results of a cross between an ear-wiggling,
heterozygous tongue roller, and an heterozygous non ear wiggling, homozygous tongue roller?
15) In humans, hair color is controlled by two interacting genes. The same pigment, melanin, is present in both brown-haired and
blond-haired people, but brown hair has much more of it. Brown hair (B) is dominant over blonde hair (b). Whether any
melanin can be synthesized depends on another gene. The dominant form (M) allows melanin synthesis; the recessive form
(m) prevents melanin synthesis. Homozygous recessive (mm) individuals are albino. What will be the expected genotypic
and phenotypic proportions in the children of the following parents: double heterozygous brown-haired mother and blondehaired heterozygous melanin production (Mm) father?
16) In some situations, a gene at one location affects the expression of a gene at another location. In poultry, the type of comb is
genetically determined by two genes (R or r, and P or p) in an epistatic condition. The allele for a rose comb (R) is dominant
over the allele a single comb (r) and the allele for a pea comb (P) is dominant over a single comb (p). When the poultry has at
least one dominant allele for both traits it has a walnut comb. The following table lists the phenotypes and the associated
genotypes of this epistasis.
Single comb
(rrpp)
Pea comb
(rrPP or rrPp)
Walnut Comb
(RRPP, RRPp, RrPP,
RrPp)
Rose Comb
(Rrpp or RRpp)
What would you predict about the types of combs among the offspring of two heterozygous walnut-combed fowl?
Sex-Linked Problems
17) One of the genes for baldness is located on the X chromosome and is recessive to the allele for normal hair at the same gene.
(XB = normal hair, Xb = bald, Y = male) If a bald male were to marry a carrier female, what would be the probability that the
first born son would be bald? What percentage of the females produced in this liaison would be carriers?
18) Dave is color blind. Color blindness is sex-linked and the allele which is responsible for this driving menace is recessive to
the allele for normal color vision. Judith, Dave’s wife, has normal vision. Dave and Judith had twelve offspring, all of which
had normal vision. Their children were equally distributed as to sex. What is Judith’s most likely genotype? Could she be a
carrier? Why or why not?
19) The gene for hemophilia, the inability of the blood to clot, is located on the X chromosome. The allele for normal clotting
(H) is dominant over the hemophiliac allele (h) and females who are heterozygous for this gene are said to be carriers. In a
cross between a phenotypically normal female and a normal male, who had a rather large family I might add, the following
offspring were observed:
5 normal females, 2 hemophiliac males, and 3 normal males
What was the genotype of the mother? What are the possible genotypes of each of the females produced through these
matings? What is the genotype of the normal males?
20) A carrier female and a normal male have 1 child. What is the probability that this child is a carrier of hemophilia? What is
the probability the child is a hemophiliac? What is the probability that the child is a male?
21) A) In humans, one of the genes determining color vision is located on the X chromosome. The dominant form (C) produces
normal color vision; red-green color blindness (c) is recessive. If a man with normal color vision has children a color-blind
woman, what is the probability of their having a color-blind son? What is the probability of their having a color-blind
daughter?
B) In the couple described in A) above, the woman gives birth to a color-blind daughter. The man sues for divorce claiming
the woman was unfaithful to him and committed adultery. Based on your answers above will his case stand up in court?
Explain your answer.