Incomplete Dominance.

In incomplete dominance, a heterozygote shows a phenotype that is intermediate between the two homozygous phenotypes. Neither allele is dominant over the other.

An example of incomplete dominance is flower color in four o'clock plant, like those shown in Figure 1. When a pure red-flowered four o'clock plant is crossed with a pure white-flowered four o'clock plant, the offspring will produce neither red nor white flowers. Instead, all flowers will be pink.

• Do you think the alleles blended to make pink?

In incomplete dominance, it is only the phenotype that is intermediate. The red and white alleles remain separate and distinct. Half the gametes of the pink four o'clock carry the allele for red and half carry the allele for white. Therefore, the genotypic ratio also becomes the phenotypic ratio.

Key Concepts

Incomplete dominance is a form of intermediate inheritance in which one allele for a specific trait is not completely dominant over the other allele. This results in a third phenotype in which the expressed physical trait is a combination of the dominant and recessive phenotypes.

Codominance

Another pattern of inheritance is codominance. This results when one allele is not dominant over the other. The resulting heterozygotes exhibit the traits of both parents. One example of codominance is the MN blood typing in humans. On the surface of our red blood cells are proteins bound to sugar molecules, forming complexes called antigens. One group of antigens are controlled by a pair of alleles, LM and LN. The pairing of these alleles will determine the blood type of an individual, and there are three: M, MN and N. Table 1 summarizes the genotypes and phenotypes of the MN blood typing in humans.

In codominance, both alleles are expressed equally in the phenotype of the heterozygote. For example, red cows crossed with white cows will have offspring that are roan cows. Roan refers to cows with red hair and white blotches.

Multiple Alleles

Sometimes, even if only two alleles control a trait, there may actually be more than two types of alleles available. This will also lead to more than two phenotypes expressed. Another blood group system in humans, the ABO system, is an example of a character governed by multiple alleles. Three alleles are responsible for this blood system:,, and The ABO blood type is determined by the presence or absence of two antigens, A and B. Allele i does not code for an antigen.

Sex Chromosomes and Sex Determination

Humans have 46 chromosomes in each cell. Observation of the human body cells shows 23 pairs of chromosomes for both males and females. Twenty-two pairs are somatic chromosomes. The 23rd pair consists of sex chromosomes. Human males and some other male organisms, such as other mammals and fruit flies, have non-identical sex chromosomes (XY). Females have identical (XX) sex chromosomes. • How is sex determined and inherited?

Let us study gamete formation based on the sex chromosomes. You will observe in Figure 3 that all egg cells receive an X chromosome, while half of the sperm cells receive X chromosomes and the other half receive Y chromosomes.