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.
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