Incomplete Dominance is Neither Dominant nor Recessive

Sometimes the two forms of a gene that determine a trait are neither dominant nor recessive. Imagine that you could cross a purebred red snapdragon with a purebred white snapdragon. You might expect the offspring be either red or white, depending on which was the dominant form. However, this is not always the case.

The first Punnett square (Chart 2.4) shows you that all the offspring would be pink, for neither gene is dominant. As you see, the possible genes from the purebred red flower are R or R. The possible genes from the purebred white flower are W or W. The hybrid offspring have the genotype RW. Since neither gene is dominant, the phenotype is pink, a blend of red and white. This is an example of incomplete dominance.

Chart 2.4

	R	R
W	RW	RW
W	RW	RW

In the 2nd Punnett square (Chart 2.5), you see what happens when 2 hybrid pink snapdragons are crosses. The genotypes of these offspring would be RR, RW, and RW & WW. You knew from looking at the Punnett square that the phenotypes would be 25% red, 25% white and 50% pink.

Chart 2.5

	R	W
R	RR	RW
W	RW	WW

X Chromosomes & y Chromosomes

Two chromosomes called the sex chromosomes determine your sex. You receive one sex chromosome from your mother’s egg cell & one sex chromosome from your father’s sperm cell. Sex chromosomes are named the X chromosome & the Y chromosome.

Egg cells contain 1 X chromosome. Males, however, make two kinds of sperm. Half of the sperm contains one X chromosome. The other half of the sperm contains one Y chromosome. If a sperm containing an X chromosome unites with an egg, the genotype of the fertilized egg will be XX. A female will result. Thus the sex of the offspring depends on which kind of sperm unites with the egg.

Since both kinds of sperm have nearly an equal chance of fertilizing an egg, there is nearly an equal chance that offspring will be male or female.

Sex Cells Form by Meiosis

Sperm cells and egg cells are quite different from your body cells. Body cells form by mitosis. All cells produced through mitosis have the same number of chromosomes as their parent cells. As you see, the chromosomes of body cell duplicate and separate. Two new body cells form from one original parent cell. Consider what would happen if two cells formed through mitosis combined in sexual reproduction. The offspring would have twice as many chromosomes as its parents. As this process continued, each succeeding generation would have double the chromosome number of its parents. Sexual reproduction does not increase the number of chromosomes, because gametes have only half the number of chromosomes found in somatic cells.

Sex cells do not form by mitosis. They form by especial kind of cell division called meiosis (two successive nuclear divisions in which the chromosome number is reduced from diploid (2n) to haploid (n) and segregation and reassortment of the genes occur). Meiosis is often called “reduction division”, because it forms sex cells that are reduced to half the number of chromosomes as body cells. Your sex cells have 23 single chromosomes. The chromosomes in somatic cells occur in pairs called homologous chromosomes. The chromosomes in a pair are alike in appearance of in the type of genetic information they carry. The 46 chromosomes in human cells form 23 homologous pairs. Cells that have homologous chromosomes are said to have the diploid number of chromosomes. Gametes have a haploid number of chromosomes. Gametes are formed by a type of nuclear division called meioses. In humans, meiosis occurs in the male testes and the female ovaries.

Meiosis (Fig. 2.5, 2.6) begins with an original parent cell that has 23 pairs of chromosomes. In the first division of meiosis, the DNA has already replicated forming two twin strands. The two twin stranded chromosomes come together in a pair. Often at this point some of the DNA of one chromosome break off and crosses over to another chromosome. Crossing over the DNA helps to explain the difference between parents and their young. The chromosomes then line up, separate, and move to opposite ends of the cell. Two new cells form.

In the second division, the chromosomes line up in the center of each cell. This time, the chromosomes don’t duplicate. The strands merely separate. Each strand moves to opposite sides of the cells and becomes a chromosome. The cells then divide. Four new cells are the sex cells. They have half the number of chromosomes as the original parent cells. When