Dominance and recessiveness

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

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? Reason your answer.

The diagram shows a Punnett square for a cross between two heterozygous individuals (Aa × Aa). Complete the missing cell and, assuming allele A (dark colour) is dominant over a (light colour), 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.

In pea plants, the allele for yellow colour (A) is dominant over green colour (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 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 mice, grey coat (G) is dominant over white (g). Two grey 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 grey and 2 white) fits the expected theoretical ratio.

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.