Exercises: Dominance and recessiveness

Define the terms gene, allele, genotype, and phenotype, and provide a simple example relating them (for instance, flower color).

Explain what it means for an allele to be dominant and for an allele to be recessive. Can a recessive trait be expressed in a heterozygous individual? Explain your reasoning.

The diagram shows a Punnett square for a cross between two heterozygous individuals (Aa × Aa). Complete the missing cell and, assuming allele A (dark color) is dominant over a (light color), state the genotypic and phenotypic ratios of the offspring.

Explain the difference between homozygous and heterozygous, and state which of the two can produce two different types of gametes for that trait and which produces only one.

For a gene with two alleles, A and a, state which gametes an AA individual produces, which ones an Aa individual produces, and which ones an aa individual produces. How many different types of gametes does each one form?

Mendel crossed a pure-breeding yellow-seeded pea plant (AA) with a green-seeded one (aa). State the genotype and the phenotype of the whole first generation (F1) and state in your own words Mendel's first law, or law of uniformity.

A mouse shows the recessive phenotype for coat color. What is its genotype with complete certainty? And if it showed the dominant phenotype? Explain why in one case we know it for sure and in the other we do not.

A homozygous dominant plant (AA) is crossed with a heterozygous one (Aa). Draw the corresponding Punnett square and state the genotypic and phenotypic ratios of the offspring.

In pea plants, the allele for yellow color (A) is dominant over green color (a). Cross two heterozygous plants and state the expected genotypic and phenotypic ratios in the offspring.

In dogs, black coat (N) is dominant over brown coat (n). A heterozygous male is crossed with a brown-coated female. Calculate the probability that a puppy will have a brown coat.

A couple, both with the dominant phenotype for a trait, have a child with the recessive phenotype. Deduce the genotypes of the parents and explain how it is possible for the child to display a trait that neither parent shows.

In mice, gray coat (G) is dominant over white (g). Two gray mice are crossed and in the litter, of eight pups, two are white. Deduce the genotypes of the parents and explain whether the actual litter ratio (6 gray and 2 white) fits the expected theoretical ratio.

The F1 plants from the cross between pure yellow and green peas are all yellow and heterozygous. If the F1 plants are allowed to self-pollinate, calculate the genotypic and phenotypic ratios of the F2 and state Mendel's second law, or law of segregation.

In tomatoes, red fruit (R) is dominant over yellow (r). Cross a heterozygous plant with a yellow-fruited plant and state the genotypic and phenotypic ratios of the offspring. What is this type of cross called?

A plant with the dominant phenotype is crossed with one with the recessive phenotype, and the offspring consists of half dominant-phenotype plants and half recessive-phenotype plants. Determine the genotype of the dominant-phenotype plant and explain your answer using a Punnett square.

In a certain plant, the purple-flower allele (P) is dominant over the white-flower allele (p). A gardener crosses two plants and all the offspring have purple flowers. State two combinations of parental genotypes that would make this result possible and one that would make it impossible.

In peas, yellow color ($A$) is dominant over green ($a$), and smooth texture ($L$) is dominant over wrinkled ($l$). After self-fertilizing dihybrid plants ($AaLl$), a farmer selects only plants with green, smooth seeds for commercial cultivation, but needs to ensure they are true-breeding ($aaLL$) so that wrinkled seeds do not appear in subsequent harvests.

1. a) Design a cross that allows this farmer to find out whether a green, smooth plant is homozygous or heterozygous for texture, indicating the genotype and phenotype of the plant used as a mate.
2. b) Predict with reasoning the phenotypic percentages that would be obtained in the offspring according to each of the possible genotypes, and conclude which outcome would confirm that the plant is true-breeding.

In rabbits, black fur ($N$) is dominant over white fur ($n$). A breeder crosses a black male of unknown genotype with a white female to perform a test cross. A litter of 6 kits is born, all with black fur. The breeder concludes: "It is 100% proven that the male is homozygous ($NN$), because if he were heterozygous, white rabbits would have been born."

1. a) Refute the breeder's conclusion by calculating the probability that a litter of 6 kits is entirely black even if the father is heterozygous ($Nn$).
2. b) Explain what minimum outcome in the litter would have been sufficient to determine the male's genotype beyond any doubt, and justify why.

Albinism is an autosomal recessive condition ($a$) compared to normal pigmentation ($A$). A couple in which both have normal pigmentation have a firstborn child with albinism. Facing a new pregnancy, the father-to-be comments: "Since the probability of albinism is 1 in 4 and the first child was already born with albinism, the next three will definitely be born with normal pigmentation."

1. a) Refute the father's statement by calculating the actual probability that the second child is born with albinism and the probability that the next three are all born with normal pigmentation.
2. b) If a daughter with normal pigmentation is eventually born, calculate with justification the probability that she is a carrier of the albinism allele.

In pea plants, the allele for yellow color ($A$) is dominant over green ($a$), and the allele for round texture ($L$) is dominant over wrinkled ($l$). At an agricultural station, a plant with a dominant phenotype (yellow, round seeds) is crossed with a plant with a double recessive phenotype (green, wrinkled seeds). The offspring consists of exactly 248 plants with yellow, round seeds and 252 plants with green, round seeds, with no plants with wrinkled seeds appearing.

1. a) Reasonably deduce the complete genotype of the parent plant with yellow, round seeds.
2. b) If two plants from the offspring that are green and round are crossed with each other, predict the expected genotypes and phenotypic ratios in the new offspring, justifying your answer with the corresponding cross.

Two pure-breeding pea lines are crossed: one with yellow, round seeds ($AALL$) and another with green, wrinkled seeds ($aall$). All resulting F1 plants have yellow, round seeds. Next, the F1 is self-pollinated, yielding 800 plants in the F2.

1. a) Among the F2 plants with yellow, round seeds, a student claims that half of them have exactly the same genotype as the F1 ($AaLl$). Demonstrate through genetic analysis whether the student's claim is correct or erroneous by calculating the fraction and the exact number of dihybrid plants within that phenotype.
2. b) How many of the 800 F2 plants are expected to be homozygous for both traits at the same time? State which phenotypes they will display and in what amounts.

Cystic fibrosis is a severe autosomal recessive hereditary disease ($a$). A healthy couple visits a genetic counseling clinic after discovering that both are heterozygous carriers ($Aa$). They wish to have two children.

1. a) Reasonably calculate the probability that at least one of the two children is born affected by the disease, and compare it with the probability that both are born healthy.
2. b) After their first child is born completely healthy, they say with relief: "Since the first one was spared, the risk for the second one is lower." Refute their belief by explaining what the actual genetic risk is for the second child, and calculate the probability that this first healthy child is a carrier of the causative allele.

Two healthy parents have three children: a healthy son, a healthy daughter, and another daughter affected by an autosomal recessive metabolic disease. Years later, the healthy son plans to have children with a healthy woman whose family has no history of the disease (so she is assumed to be homozygous healthy).

1. a) Deduce the genotypes of the parents and the affected daughter, and justify why the probability that the healthy brother is a carrier is 2/3 and not 1/2.
2. b) Calculate the final probability that a future child of this new couple will be a carrier of the recessive disease allele. Justify the steps of your probabilistic deduction.

In the tomato plant, tall stem ($A$) is dominant over dwarf stem ($a$), and red fruit ($R$) is dominant over yellow fruit ($r$). A team crosses two dihybrid plants ($AaRr \times AaRr$) and obtains 160 offspring with the following counts: 92 tall with red fruit, 28 tall with yellow fruit, 31 dwarf with red fruit, and 9 dwarf with yellow fruit.

1. a) Interpret these results by comparing them with the exact theoretical counts that Mendel's law of independent assortment would predict for a sample of 160 individuals.
2. b) One of the 92 plants with tall stems and red fruit is selected at random. Design a cross that allows you to confirm whether this plant is heterozygous for both genes ($AaRr$), stating which phenotype and genotype you should cross it with, and which phenotypic ratios you would expect if the hypothesis were correct.