The genotype describes the genetic makeup or alleles of the individual, while the phenotype describes the physical characteristic of the individual.

The parent generation refers to the generation in which the two organisms are crossed; the F1 generation is the first filial generation, or the offspring produced from the cross; the F2 generation is the second filial generation, or the offspring produced from a cross between two F1 organisms.

Since B is completely dominant over b, and you obtained a 3:1 ratio of phenotypes, the parents must have been heterozygous, therefore you can represent their genotypes as Bb.

If either parent were BB, you would obtain only brown offspring. If either parent were bb, they would have been white (rather than bronw).

First, we need to find the genotype of the man. We know that his father has the disease, and that his mother is homozygous dominant. For his father to display the autosomal recessive trait, he must be homozygous recessive. We can thus write the man's parents' genotypes, using A as the dominant allele and a as the recessive allele.

Parents: AA (mother) x aa (father)

Offspring: All offspring will be Aa.

The man must inherit one allele from each parent, meaning he must be heterozygous.

This man, we are told, marries a heterozygous woman and they have a child. Again, we can write out this cross.

Man and woman: Aa (man) x Aa (woman)

Offspring: 1 AA (dominant), 2 Aa (dominant), 1 aa (recessive)

The possible offspring from this cross show a three-to-one ratio for displaying the dominant phenotype. Only one of the four possible offspring will be homozygous recessive and display the recessive phenotype. This one-in-four chance correspond to a 25% chance.

We know that the allele is autosomal, the father is heterozygous and black, and the mother is homozygous and white. From this information, we can determine that black must be dominant to white since the heterozygote shows the black phenotype. We will use B to represent the dominant black allele and b to represent the recessive white allele.

Cross: Bb x bb

Offspring: Half Bb (black) and half bb (white)

We can see that roughly half of the offspring will show the white phenotype. If there are twelve offspring, six of them will be white.

Alleles are simply different versions of the same gene that encode for variations of the same characteristic. For example, different eye colors are encoded by different alleles of the same gene.

A genotype is the genetic makeup of alleles coding for a characteristic. In this example, a heterozygote has one recessive and one dominant allele, making the genotype Aa. The phenotype is the observable expression of the genotype. Since a heterozygote has one dominant allele and one recessive allele, the dominant allele is exhibited, thus the phenotype is brown hair.

Dominant alleles are neither better, nor worse than recessive alleles. They are simply expressed in the phenotype of a heterozygous pair. Several deleterious, and even deadly disorders like Huntington's disease, are inherited via an autosomal dominant pattern.

Note that the frequency of an allele or genotype in a population must be determined experimentally, and can be linked to environmental influences, autosomal versus sex-linked patterns, and numerous other factors.

The phenotype is the observable characteristic of an individual. In this example, a homozygous recessive plant will have a white flower phenotype and a genotype of aa.

The principle of dominance in genetics states that some alleles are dominant and are expressed instead of the recessive allele. Hybrids are crosses between parents with different traits, and genotypes for different eye colors are different.