What number can you see?
Red-green colorblindness is a common inherited trait in humans. About 1 in 10 men have some form of color blindness, however, very few women are color blind. Why?
Mendelian Inheritance in Humans
Characteristics that are encoded in DNA are called genetic traits. Different types of human traits are inherited in different ways. Some human traits have simple inheritance patterns like the traits that Gregor Mendel studied in pea plants. Other human traits have more complex inheritance patterns.
Mendelian inheritance refers to the inheritance of traits controlled by a single gene with two alleles, one of which may be dominant to the other. Not many human traits are controlled by a single gene with two alleles, but they are a good starting point for understanding human heredity. How Mendelian traits are inherited depends on whether the traits are controlled by genes on autosomes or the X chromosome.
Autosomal traits are controlled by genes on one of the 22 human autosomes. Consider earlobe attachment. A single autosomal gene with two alleles determines whether you have attached earlobes or free-hanging earlobes. The allele for free-hanging earlobes (F) is dominant to the allele for attached earlobes (f). Other single-gene autosomal traits include widow’s peak and hitchhiker’s thumb. The dominant and recessive forms of these traits are shown in the figure below. Which form of these traits do you have? What are your possible genotypes for the traits?
The chart in the figure below is called a pedigree. It shows how the earlobe trait was passed from generation to generation within a family. Pedigrees are useful tools for studying inheritance patterns.
Other single-gene autosomal traits include widow’s peak and hitchhiker’s thumb. The dominant and recessive forms of these traits are shown in the figure below. Which form of these traits do you have? What are your possible genotypes for the traits?
Traits controlled by genes on the sex chromosomes are called sex-linked traits, or X-linked traits in the case of the X chromosome. Single-gene X-linked traits have a different pattern of inheritance than single-gene autosomal traits. Do you know why? It’s because males have just one X chromosome. In addition, they always inherit their X chromosome from their mother, and they pass it on to all their daughters but none of their sons. This is illustrated in the figure below.
Because males have just one X chromosome, they have only one allele for any X-linked trait. Therefore, a recessive X-linked allele is always expressed in males. Because females have two X chromosomes, they have two alleles for any X-linked trait. Therefore, they must inherit two copies of the recessive allele to express the recessive trait. This explains why X-linked recessive traits are less common in females than males. An example of a recessive X-linked trait is red-green color blindness. People with this trait cannot distinguish between the colors red and green. More than one recessive gene on the X chromosome codes for this trait, which is fairly common in males but relatively rare in females (the figure below).
- A minority of human traits are controlled by single genes with two alleles.
- They have different inheritance patterns depending on whether they are controlled by autosomal or X-linked genes.