Recessive Gene Definition
A recessive gene is a gene whose effects are masked in the presence of a dominant gene. Every organism that has DNA packed into chromosomes has two alleles, or forms of a gene, for each gene: one inherited from their mother, and one inherited from their father. A recessive gene is only expressed when an organism has two recessive alleles for that gene. This is also known as being homozygous recessive. If an organism has one dominant and one recessive allele, it will show the dominant trait.
Gregor Mendel, an Austrian monk who lived during the 19th Century, is considered the father of genetics for his experiments on pea plants. In one experiment, he crossed a purple-flowering pea plant with a white-flowering pea plant, and all the offspring were purple. Then, when he crossed the purple offspring with each other, 75 percent of their offspring had purple flowers and 25 percent had white flowers. This experiment shows the principle behind simple dominant and recessive inheritance, which came to be known as Mendelian inheritance. White flower color was recessive, and purple flower color was dominant.
In genetics, alleles are represented by letters. For example, we can have the capital letter P represent the purple allele, and the lowercase letter p represent the white allele. Dominant alleles are always represented by capital letters, and recessive alleles are always represented by lowercase letters. Different alleles for the same gene are represented by the same letter.
In Mendel’s experiment, the purple pea plant he started with was PP, and the white pea plant he started with was pp. Their offspring were all Pp. They inherited one allele from each parent, so they had one allele for purple and one for white. Then, when Pp x Pp offspring were crossed, roughly 1/4 of the offspring were PP, 2/4 were Pp, and 1/4 were pp. Since 3/4 of the offspring had at least one dominant allele P, 3/4 were purple. Only 1/4 were pp, so 1/4 were white. Even though a total of 3/4 of the offspring had the recessive p allele, the white flower trait was hidden in 2/4 of offspring, masked by the dominant P allele. Recessive genes are only expressed if both of an organism’s alleles for that gene are recessive.
The alleles an organism has are collectively known as their genotype, while their physical appearance and traits are their phenotype. PP and Pp pea plants had different genotypes, but they both had the purple phenotype. Organisms with two copies of the same allele for a gene, for instance PP or pp pea plants, are said to be homozygous for that gene. Organisms that have one dominant and one recessive allele, such as the Pp pea plants, are called heterozygous.
Examples of Recessive Genes
Many traits are controlled by a single gene and are either dominant or recessive. Dimples, freckles, cleft chins, and a widow’s peak are all dominant traits, so not having these traits is recessive. A person will not have these traits if they have two recessive alleles. Other traits expressed due to recessive genes include attached earlobes, inability to roll one’s tongue, and round (versus almond-shaped) eyes.
Autosomal Recessive Disorders
Specific recessive genes are more often mentioned in the context of disorders. Some disorders, such as Tay-Sachs disease, sickle-cell anemia, and phenylketonuria (PKU) are caused by recessive genes in autosomal chromosomes (chromosomes that are not sex chromosomes). Tay-Sachs is a disorder that leads to the destruction of nerve cells, and there is no known cure for it; individuals with the disease often die by age four. Sickle-cell anemia is a disorder that results in sickle-shaped red blood cells that cannot carry oxygen efficiently; it can result in pain, anemia, and bacterial infections, and can be managed with folic acid, penicillin, and blood transfusions. PKU causes too much phenylalanine to build up within the body, and can cause intellectual disability.
X-Linked Recessive Disorders
In addition, some disorders are caused by recessive genes on the X chromosome and are more likely to affect males, since males have only one X chromosome and therefore do not have a dominant copy of the allele. X-linked recessive disorders include red-green colorblindness, hemophilia, a disorder that makes it harder for blood to clot and close wounds during injury, and Duchenne muscular dystrophy, which causes muscle degeneration.
Inbreeding, which is when closely related organisms produce offspring, can lead to an increase in detrimental recessive genes being expressed. This is because closely related individuals are more likely to have the same recessive genes. When offspring in a population are less healthy and have less of a chance of surviving and reproducing due to recessive genes inherited through inbreeding, this is called inbreeding depression.
One example of the effects of inbreeding was seen in Charles II of Spain, who lived during the 17th Century. He was the last Habsburg to rule Spain. Inbreeding was widespread among royalty in Spain and other countries of Europe in order to preserve “royal blood”; for instance, first cousins would marry, or uncles would marry nieces. Charles II was the result of generations of inbreeding, and had physical and mental disabilities. He never produced an heir, and was most likely impotent.
Another example of an unusual characteristic that came about through inbreeding could be seen in the Fugate family, members of which lived in rural Kentucky in the 19th and 20th Centuries. In the remote area in which they lived, many families intermarried, and in this small gene pool, some inbreeding occurred. As a result, many of the Fugates’ descendants were born with methemoglobinemia, a recessive disorder that causes blue-tinged skin.