How are genes and heredity involved in color vision?
You ask, “how is color blindness inherited?” But first we must separate color blindness into two groups: acquired and congenital. Acquired color blindness starts after birth and may worsen over time. Congenital color blindness is present from birth and does not worsen with time. This blog is about congenital color blindness and it is an inherited trait. It appears to have evolved to give a competitive advantage in the visual perception of the environment. For example, the ability to tell the difference between red and green allows a gatherer to find the red fruit among the green leaves of a plant more easily than a competitor who cannot see the color difference.
What are DNA, genes, and chromosomes?
Most cells in the body contain a nucleus that contain DNA, which codes for proteins. The DNA is organized into genes, which, in turn, are organized into large units called chromosomes. Each of us has 23 pairs of chromosomes (as in the Ancestry.com genetic test, “23 and me”). The pair consists of one set of chromosomes from the mother and one from the father. Collectively, they represent the genetic blueprint of the individual.
Are there different degrees of severity?
Color perception is complex. There are many genes that have an impact of the ability to see color. Therefore, there is wide variation in the ability to perceive colors among individuals. There are a few, well-recognized causes of major color deficiency in humans and the most common is red-green color deficiency. These individuals have difficulty seeing the difference between red and green. Despite this limitation, they function well. They have no trouble with the blue-yellow spectrum.
What is the most common form of color blindness?
Red-green color deficiency is the most common form of color blindness. The genes for red-green color deficiency are located on the X chromosome. The X chromosome is one of the two sex (as in gender) genes, the other being the Y chromosome. Females possesses two X chromosomes and males possess an X and a Y chromosome. A child receives one sex chromosome from each parent. Thus, females inherit one X (either of the two maternal X chromosomes) from her mother and one X (the only paternal X chromosome) from her father. Males inherit one X (of her two X’s) from his mother and the (only) Y from his father. I emphasize the number of chromosomal options because it has an impact on heredity of genetic traits, as we will see later.
Is red-green color deficiency a recessive trait?
The red-green color deficiency trait is considered a recessive trait by genetic specialists. That is, the trait is only (prominently) apparent if the affected gene is not paired with a normal gene (remember that each gene on each chromosome is one of a pair…one from each parent). Recessive genes on the X chromosome (X-linked recessive genes) are inherited in a peculiar fashion because, in males, there is no X pair. Instead, males have only one X chromosome. Therefore, an X-linked recessive gene manifests itself with symptoms in males, but not in females (with a normal fellow X chromosome). This explains the difference in the prevalence in color deficiency between males and females. Overall, color deficiency is present in about 8% of males compared with 0.5% of females.
What does this mean for me and my family?
The practical translation from the genetic explanation is somewhat simpler. The father with red-green color deficiency will not give any of his sons the gene for color deficiency. This is because the gene is located on his X chromosome and he gave his sons his Y chromosome. However, the father will pass on the X-linked recessive gene to ALL of his daughters. Because the trait is recessive, the daughters will not be red-green deficient. This assumes that their fellow (paired) X chromosome from their mother is normal. The color vision deficiency, however, will appear in 50% of the daughters’ male children. Likewise, half of their daughters will carry the trait asymptomatically.
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