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.

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.

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?

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.

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.

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.

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 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.

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.

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 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.

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.