Special Genetics

Incomplete Dominance: When one homozygous dominant parent crosses with a homozygous recessive parent, the offspring would be only one type—heterozygous. However, sometimes heterozygous offspring has different trait with the dominant parent. For example, some heterozygous flowers have pink color when their parents have red and yellow flowers.

Multiple Alleles: In different blood types, there are three types of alleles--I^a, I^b, and i. The first two alleles are both dominant, and the last one is recessive. Blood type A can be produced by two I^a or one I^a and i. Similarly, blood type B can be formed by two I^b or one I^b and i. Blood type O can only formed by two recessive alleles. And Blood type AB can be formed by one I^a and one I^b.

Sex-linked Genes: Genes that locate on a sex chromosome are sex-linked genes. X chromosome carries more genes than Y chromosome. Males only have one allele for most X-linked genes. Females have two alleles for X-linked genes. In a Punnett square, when a male is crossed with a female, 50% of the offspring would be a boy because there are two alleles that have Y chromosome. Diseases such as hemophilia and color blindness can be passed on through sex-linked crosses.

Dihybrid Cross

The difference between a dihybrid cross and a monohybrid cross is that a dihybrid cross involves two types of alleles at the same time.

Usually, we can figure out the offspring of a dihybrid cross in two ways. The first way is to take one allele from each trait at a time from a single parent and combine every two to form a gamete. List the male gametes on one row of the Punnett square and female gametes on the column of the Punnett square. We can have 16 genotypes from the cross and then conclude the genotypic and phenotypic ratios.

The second way is to create two Punnett squares by separating the two traits. Put one allele at a time from each parent on the sides of the square. If there were two traits such as black/brown hair and brown/blue eyes, the first Punnett square can be allele combinations for hair color and the second square for eye color. Then, we can determine the genotypic and phenotypic ratios of offspring by combining the traits of two squares.