The world is rich in examples of recessive phenotypes. Some recessive phenotype examples 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.
Phenotype and Genotype
Your genotypes reside within the DNA chromosomes that inherited from your parents. As with all sexually reproducing organisms, you receive one set of 23 chromosomes from each parent. Humans have 23 pairs of chromosomes, a total of 46 chromosomes. One pair of chromosomes determines gender.
One marvelous feature of life is the mechanism that expresses the information encoded on chromosomes as proteins that are then 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.
These genotype pairs determine the physical appearance or phenotype for each trait. When the both genes code for the same version of a trait the phenotype will match the genotype. For example, if both genes code for blue eyes, the offspring will have blue eyes.
If, however, the two genes carry different variations for the trait, the phenotype will be determined by the dominant gene. For example, the gene for brown eyes is dominant while the gene for blue eyes is recessive. Unless other factors influence the outcome, the genotype combination of a brown-eye gene from one parent and a blue-eye gene from the other parent will result in a brown-eyed child.
Genes and Alleles
Since you have two copies of each chromosome, you have 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 genes" and give rise to sex-linked phenotypes, such as color blindness and hemophilia.
Dominant and Recessive
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.
To define recessive traits requires determining relative dominance. The allele for green eyes is recessive to brown eyes but dominant over blue eyes. In addition, the interactions of genes influence phenotype expression. For example, at least eight different genes take part in the translation of the genotypes for eye color into the phenotype or physical appearance of eye color in the offspring.
In some cases, alleles are equally dominant and the offspring express both phenotypes. For example, if you cross a red-flowered plant with a white-flowered one, a co-dominant 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. Human A and B blood types are co-dominant while blood type O is another recessive phenotype example.
Doing the Math
In the 1860s, Gregor Mendel, the father of classical genetics, crossed pea plants having a variety of different phenotype examples, including pea shape. When he crossed a round-pea plant, denoted R, with a wrinkled-pea (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.
- Cornell University: A Quantitative Understanding of Human Sex Chromosomal Genes; Sarif Hassan et al, 2013
- Memorial University: Round or Wrinkled; Mendel's Peas Explained
- Palomar College: Basics Principles of Genetics: Exceptions to Simple Inheritance
- National Institutes of Health - National Library of Medicine: Basic Genetic Terms
- HudsonAlpha Institute for Biotechnology: Genetics of Eye Color
- Thinkstock/Stockbyte/Getty Images