The world is rich in examples of recessive phenotypes. Some are unremarkable, such as blue eye color, while others are unusual, such as the genetic disease hemophilia. Organisms have many physical and behavioral traits. If you imagine these traits to be variables, then phenotypes are the values that the variables can assume. For example, your hair color trait might be a phenotype of brown, black, blonde, red, gray, or white.
Your genotypes reside within the DNA chromosomes that you inherit from your parents. As with all sexually reproducing organisms, you receive a set of chromosomes from each parent. Humans have 23 pairs of chromosomes, of which one pair determines your gender. One marvelous feature of life is the mechanism that expresses the information encoded on chromosomes as proteins that are responsible for phenotypes. In humans, only about 2 percent of chromosomal real estate codes for proteins. These small stretches of DNA are called protein-coding genes.
Since you have two copies of each chromosome, you have a two versions, or alleles, of each gene. The one possible exception involves the X and Y sex chromosomes. Females have two copies of X and thus have two alleles for all the X genes. If you are male, you have an X and a Y, and therefore have only one copy of the genes that are unique to either chromosome. Scientists have found 2,000 genes on the X chromosome but only 78 on the Y. Homozygous alleles are identical, whereas heterozygous alleles have differing genetic information. Genes on the sex chromosomes are called allosomal and give rise to sex-linked phenotypes, such as color blindness and hemophilia.
Frequently, one allele dominates over the other allele, which is said to be recessive. The dominant allele masks the expression of its recessive partner and gives rise to dominant phenotypes, such as brown eyes. You’ll have brown eyes as long as you have one allele for brown eyes. To inherit the recessive phenotype of blue eyes, both your eye-color alleles must code for blue. In some cases, alleles are equally dominant and offspring express both phenotypes. For example, if you cross a red-flowered plant with a white-flowered one, a codominant expression gives you offspring having red and white spotted flowers. On the other hand, if the alleles were incompletely dominant, the offspring might have a blended phenotype of pink flowers.
In the 1860s, Gregor Mendel, the father of classical genetics, crossed pea plants having a variety of different phenotypes, including pea shape. When he crossed a round-pea plant, denoted R, with a wrinkled-pea, or W, variety, 75 percent of the offspring had round peas. Mendel reasoned that offspring had a 25 percent chance of inheriting identical RR alleles and a similar chance of receiving the WW alleles, although Mendel referred to alleles as factors. This meant that half the offspring were RW. Since 75 percent had round-peas, Mendel reasoned that R dominated W and that either the RR or the RW genotype produces the round pea phenotype. Wrinkled peas, having a WW genotype, are an example of a recessive phenotype.
