Genetics Problems
All problems are to be completed on a separate sheet of paper. Be sure to label each heading. Answer each question and show work (Punnett squares) fully.
Example Problems: Monohybrid
1. An allele for brown eyes B is dominant over that for blue eyes b. A blue-eyed man, both of whose parents
were brown-eyed, marries a brown-eyed woman. They have one child who is blue-eyed. What are the
genotypes of all the individuals mentioned?
2 The ability to taste the chemical PTC is determined by a single gene in humans with the ability to taste given by the dominant allele T and inability to taste by the recessive allele t. Suppose two heterozygous tasters (Tt) have a large family.
a. Predict the proportion of their children who will be tasters and nontasters. Use a Punnett square to illustrate how you make these predictions.
b. What is the likelihood that their first child will be a taster? What is the likelihood that their fourth child will be a taster?
c. What is the likelihood that the first three children of this couple will be nontasters?
Set 1: Mendel’s Monohybrid Cross. Use the information from the chart to solve the following
problems
Trait / Dominant Allele / Recessive allelePod shape / Smooth (S) / Constricted (s)
Pod color / Green (G) / Yellow (g)
Flower position / Axial-on side (A) / Terminal- on ends (a)
Plant height / Tall (T) / Short (t)
Flower color / Purple (P) / White (p)
Complete a punnett square and determine the appropriate genotype and phenotype percent as well as the correct terms to describe the genotypes
1.Example: Aa X aa
A / a / Genotype: 50% Aa; 50% aaa / Aa / aa / Genotype terms: 50% Heterozygous; 50% homozygous recessive
a / Aa / aa / Phenotype: 50% Axial; 50% terminal
2. Tt X Tt
3. Two heterozygous are crossed for green pods.
4. Hybrid for axial flowers is crossed with a plant that has pure-bred terminal flowering plant.
5. Homozygous tall with a short plant.
6. Heterozygous smooth pods crossed with constricted pods.
7. When a tall plant is crossed with a short plant, some of the offspring are short. What are the genotypes of the
parents and the offspring? What is the phenotypic ratio in the offspring?
8. Three out of 4 plants produced by a cross between two unknown pea plants have axial flowers and 1 out of 4
have terminal flowers. What are the genotypes of the parent plants?
9. A student wants to figure out if the tall plant he bought was a pure bred. What plant would he cross with the
tall plant to determine if he bought a pure bred? What is this called?
10. What cross would result in ½ offspring having green pods and ½ offspring having yellow pods?
Set 2: Monohybrid Practice (dominant/recessive)
1.A green pea plant (Gg) is crossed with a yellow pea plant (gg).
2. A true breeding tall plant is crossed with a hybrid tall plant.
3. A heterozygous tall plant is crossed with a short plant.
4. A hybrid red flower (Rr) is crossed with a white flower (rr).
5. If red is dominant (R) and white is recessive (r) what are the parents if 100% of the offspring are white? .
6. If black (B) is dominant and brown is recessive (b) what are the parents if all the chicks are black colored?
7.A homozygous dominant brown mouse is crossed with a heterozygous brown mouse (tan is the recessive
color). What are the phenotypic ratios of the offspring?
8.Two heterozygous white (brown fur is recessive) rabbits are crossed. What are the ratios for all possible
genotypes & phenotypes?
9.Two heterozygous red flowers (white flowers are recessive) are crossed. Is it possible to get all red flowers?
Is it probable to get all red flowers?
10.A cross between two plants yields 68 tall plants and 36 short plants. What are the probable genotypes of the
parents? Why?
11.A heterozygous white rabbit is crossed with a homozygous black rabbit. What are the genotypic and
phenotypic types and ratios?
Set 3: Probability Practice
1.In humans, curly hair (H) is dominant over straight hair. A heterozygous woman for hair curl marries a man with straight hair and they have children.
a. What is the genotype of the parents and what gametes can they produce?
b. What is the probability that the first child will have curly hair? The second child?
c. What is the probability that three children in a row will have curly hair?
2.List all the gametes that are possible with each of the following genotypes.
a) Aabb b) AaBB c)AaBb d)AABb
3.What is the probability of getting the gamete [st] from each of the following parents?
a) Sstt b) ssTt c) SsTt d) SStt
4.In a certain strain of mice, black coat (B) is dominant over white coat (b). Describe what you would do to determine the genotype of a male with a black coat and how would this enable you to choose between the genotypes BB or Bb.
5. What is the probability of each of the following sets of parents producing the given genotypes in their offspring?
Parents Genotype / Offspring Genotypea) / Tt x Tt / Tt
b) / Tt x tt / Tt
c) / SsTt x SsTT / SSTT
d) / SsTt x SSTt / sstt
6. In humans, the allele for albinism (lack of pigment) is recessive to the allele for normal skin pigmentation(N).
a) If two heterozygous parents have children what is the chance the child will be albino?
b) If the child is normal, what is the chance that it is a carrier (heterozygous for the albino allele?
c) If normal parents have an albino child, what is the probability that their next child will be normal for pigment?
Set 4: Calculations based on Generations
1. In tomatoes the texture of the skin may be smooth or peach (hairy). The Ponderosa variety has fruits with
smooth texture. The red peach variety has fruits with peach texture. Crosses between the two varieties produce
all smooth fruits. Crosses between these smooth fruited F1 plants produced 174 peach textured fruits and 520
smooth textured fruits. How are these skin textures inherited?
2. A brown mouse is mated is mated with two female black mice. When each female has produced several litters
of young, the first female has had 48 black and the second female has had 14 black and 11 brown young.
Deduce the pattern of inheritance of coat color and the genotypes of all of the parents.
*3. Two varieties of wild daisies are ordered by a florist. The first variety is white and has thin petals. The
second variety is yellow and has broad petals. Both varieties are true-breeding. The florist plants each
variety on opposite ends of a garden plot in front of her store. The following year she has mostly white
daisies with broad petals in the plot. Explain what most likely occurred. (assume both traits are
dominant/recessive). Could she get the original flower types back with the current white, broad petal daisies
– explain in terms of probabilities?
Example Problem: Dihybrid Cross
In pepper plants, green (G) fruit color is dominant to red (g) and round (R) fruit shape is dominant to square (r) fruit shape. These two genes are located on different chromosomes.
a. What gamete types will be produced by a heterozygous green, round plant?
b. If two such heterozygous plants are crossed, what genotypes and phenotypes will be seen in the offspring and in what proportions?
Set 5: Dihybrid Crosses
1.Two hybrid (heterozygous) pea plants for pod color and pod shape are crossed (see page one chart).
What are the genotypic and phenotypic ratios in the offspring? What is the phenotype of the parents
in this cross?
2.Mice running normally is dominant (R) while mice which run in circles (called waltzing) is
recessive. Mice which have black hair are dominant (B) while having brown hair is recessive. If a
homozygous running, black mouse is crossed with a homozygous waltzing, brown mouse – what are
the phenotypic and genotypic ratios of the offspring? What are the genotypes of the parents?
3.In rabbits, gray fur (G) is dominant to white fur. Also in rabbits, black eyes (B) are dominant to red
eyes. A male rabbit that is homozygous grey hair, homozygous red eyed is crossed with a female
rabbit that has white fur with heterozygous black eyes.
*4. A male and female rabbit (same traits and alleles as in #3) have 3 separate litters. Determine average the ratio of the 4 possible phenotypes for each litter based on the given table.
Litter number / Gray fur, black eyed / Gray fur, red eyed / White fur, black eyes / White fur, red eyed1 / 10 / 4 / 3 / 1
2 / 14 / 5 / 5 / 2
3 / 5 / 2 / 2 / 0
Ratio
Based on the calculated ratio – what would the genotypes of the parents be? Explain.
5. An aquatic arthropod called a Cyclops has antennae that are either smooth or barbed. The allele for
barbs (B) is dominant. In the same organism, resistance to pesticides is a recessive trait (P). A
Cyclops that is resistant to pesticides and has smooth antennae is crossed with one that is
heterozygous for both traits. What is the percent of the offspring will be resistant to pesticides?
Example Problem: Incomplete Dominance
A hybrid pink CRCW snapdragon was crossed with a pure white one CWCW. Red flower color is incompletely dominant.
a.Make a Punnett square and list the genotypic and phenotypic ratios of the F1 generation
Set 6: Incomplete Dominance
1.Suppose you have two rose plants, both with pink flowers. You cross the two plants and are surprised to find that, while most of the offspring are pink, some are red and some are white. You decide that you like the red flowers and would like to make more. What cross would you perform to produce the most red flowered plants? Your mother decides she would like some of the pink flowered roses. Which cross would give you the most pink flowered plants?
2. A naturalist visiting an island in the middle of a large lake observes a species of small bird with three distinct types of beaks. Those with short, crushing beaks (BB) consume hard shelled nuts, those with long, delicate beaks (bb) pick the seeds from pine cones, and those with intermediate beaks (Bb), consume both types of seeds though they are not as good at either. Which of the mated pairs will have the best adapted offspring in a year in which most of the food available is in the form of hard shelled nuts? Which of the mated pairs will have the best adapted offspring to a diverse food supply?
3. In northeast Kansas there is a creature know as a wildcat. It comes in three colors, black (B), gray, and white (W). This trait is controlled by a single gene with incomplete dominance. What would be the genotypes and phenotypes of the offspring if a black wildcat were crossed with a gray one?
4. Coat color in mice is incompletely dominant. Yellow and white-colored mice are homozygous, while cream-colored mice are heterozygous. If two cream-colored mice mate, what phenotypic ratio can we expect of their offspring?
5. In radishes, red and white are pure-breeding colors, while hybrids are purple. If a red radish is crossed with a white radish, what will be the phenotype of the F2 generation (assuming the F1 generation self-pollinates)? Show the Punnett Square.
Example Problem: Codominance / Multiple Alleles
A,B,O Blood system. A person's blood type with respect to the ABO blood system is a multiple allele system where the person's blood type is determined by which two autosomal alleles from the set IA, IB or i an individual has. Paul is blood type O. His father is blood type A and his mother is blood type B. What are the genotypes of his parents? What are the possible phenotypes Paul’s brother?
Set 7: Codominance / Multiple Alleles
Human blood types genotypes can be expressed the following ways.
Phenotype (Blood type) / GenotypeO – universal donor / ii
A / IAIA or IAi
B / IBIB or IBi
AB + universal receiver / IAIB
1.Suppose a father of blood type AB and a mother of blood type B have a child of type B. What blood types are possible in their subsequent children?
2.Suppose a child is of blood type A and the mother is of type O. What type or types may the father belong to?
3.Suppose a father and mother claim they have been given the wrong baby at the hospital. Both parents are blood type A. The baby they have been given is blood type O. What evidence bearing on this case does this fact have?
4. In some chickens, the gene for feather color is controlled by codominance. The allele for black is B and the allele for white is W. The heterozygous phenotype is known as erminette (black and white spotted). Two erminette chickens were crossed. What are the genotypes and phenotypes of the offspring?
5. In shorthorn cattle, when a red bull (RR) is crossed with a white cow (WW), all the offspring are roan—a spotted, red and white or milky red color. What offspring are expected from mating a roan bull and a roan cow?
Example Problem: Polygenic
Melanin is coded for when the gene L, M, or N is present. The more of these genes are present, then the darker the skin the individual. Rank the following genotypes in order of the darkest to lightest skin color phenotypes.
______LLMMNN / ______LXMXXX______LLMXNN / ______XXMMXX
______LXMMXX / ______LXMMNX
Set 8: Polygenic Inheritance
1.A farmer notices that the length of his watermelons ranges from very long (dominant-STUV) to very short (stuv-recessive) with many watermelons falling in-between at average lengths. The trait of watermelon length is based on polygenic inheritance. Watermelon length is controlled by 4 different genes