Recessive Trait Definition
A recessive trait is a trait that is expressed when an organism has two recessive alleles, or forms of a gene. Traits are characteristics of organisms that can be observed; this includes physical characteristics such as hair and eye color, and also characteristics that may not be readily apparent, e.g. shape of blood cells. Every organism that organizes its DNA into chromosomes has two alleles for a trait, one from their mother and one from their father. Alleles can be dominant or recessive. Dominant alleles mask the effects of recessive alleles, so a recessive trait is only expressed when an organism has two recessive alleles for a gene.
Gregor Mendel was an Austrian monk who did research on pea plants in the 19th century. He found that when he crossed purple-flowered pea plants with white-flowered pea plants, all of their offspring were purple. When he then crossed this new purple generation with each other, 75% of the offspring were purple and 25% were white. Where did the white color come from, and why did it skip a generation? In this case, white is a recessive trait. The allele for white color was in the first offspring generation, but it was masked by the dominant purple allele. Then, when that generation bred, some offspring received both recessive alleles, and were white as a result. When a trait is controlled for by one gene that has dominant and recessive alleles, this is called simple Mendelian inheritance.
Alleles of traits are represented by any pair of capital or lowercase letters, with the dominant allele being capital and the recessive allele being lowercase. For example, we could designate P as representing the purple allele in pea plants and p representing the white allele. Individual pea plants are either PP, Pp, or pp for the trait of flower color. PP and Pp individuals are purple, while pp individuals are white. The alleles an organism has makes up its genotype, while the organism’s physical appearance resulting from its alleles is called its phenotype. Only pp pea plants would show the white phenotype, for example.
PP and pp individuals are called homozygotes because both of their alleles are the same form, with PP individuals having two dominant alleles and pp individuals having two recessive alleles. Pp individuals, like that first generation of purple pea plants bred from the purple and white pea plants, are called heterozygous because they have two different forms of alleles for one gene. These individuals show the dominant trait because the dominant allele masks the recessive allele.
This diagram, called a Punnett square, shows what happened when Mendel crossed his purple pea plants. Since the parents had the alleles for the dominant and the recessive traits, approximately one-fourth of their offspring showed the recessive white flower trait.
Examples of Recessive Traits
Many traits we observe in the people around us are examples of dominant and recessive traits. For example, having a straight hairline is recessive, while having a widow’s peak (a V-shaped hairline near the forehead) is dominant. Cleft chin, dimples, and freckles are similar examples; individuals with recessive alleles for a cleft chin, dimples, or freckles do not have these traits. Having round (as opposed to almond-shaped) eyes is recessive, along with inability to roll one’s tongue. Attached earlobes (as opposed to free) is also a recessive trait. Having blue eyes is recessive to brown eyes, but eye color is an example of a polygenic trait, a trait that is affected by more than one gene, so it cannot be explained via simple Mendelian inheritance. (Eye color being polygenic is why green and hazel eyes exist; a person with green or hazel eyes has some genes for brown eyes and some for blue eyes.)
Some disorders are autosomal recessive, such as cystic fibrosis, Tay-Sachs disease, and sickle cell anemia. Autosomal means that they are caused by a recessive gene found in one of the chromosomes that is not a sex chromosome (i.e., not found on the X or Y chromosomes). Certain other disorders are X-linked recessive. They are found on the X chromosome and are more common in males, since males have only one X chromosome. Colorblindness, hemophilia, and Duchenne muscular dystrophy are examples of recessive X-linked disorders.
In Other Animals
There are many examples of recessive traits in non-human animals as well. In dogs, traits like yellow fur, white spots, and smooth hair are recessive. In cats, white fur, brown (as opposed to black) fur, and long hair are recessive traits. In sheep, black wool and blue eyes are recessive. In pigs, drooping ears and cloven hooves are recessive traits. In order to breed animals with certain traits, people who breed animals must understand dominant and recessive traits and use selective breeding to select for the traits they want in these animals. They must also be careful to avoid inbreeding, which occurs when closely related individuals mate. Inbreeding can cause harmful effects because it is more likely that closely related individuals will have the same recessive alleles.