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.

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 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.

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 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.

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.