The Genetics of Some Common Conditions

 I asked some friends recently what they'd like to read about in this blog. One of them answered that she'd like to learn about common genetic conditions like color blindness and heterochromia. Perhaps you've been wondering about these too.

Color Blindness

Red/green color blindness is by far the most common type, with an incidence of 1 in 12 males and 1 in 200 females in those of Northern European ancestry. These individuals have trouble distinguishing between shades of red, yellow, and green. The gene for color blindness is carried on the X chromosome, which explains why many more men are affected than women. We each have two copies of every gene, one from each parent. The only exception to this is for those genes on the sex chromosomes, which are designated X and Y. A female (with some exceptions) has two Xs and a male (typically) has one X and one Y. So women have two copies of any gene on the X chromosome, whereas men only have one. Any gene variant or abnormality on the X chromosome will be expressed in a man, whereas a woman will have a second copy to compensate, and she will be an unaffected carrier. On occasion, women are affected too. This may be due to an affected male having a daughter with a carrier female, and both pass their altered copies of the gene on to her. In some rarer situations, the condition can be the result of long-standing diseases such as diabetes, multiple sclerosis, some liver diseases and almost all eye diseases.

Heterochromia

Eye color is determined primarily by the concentration and distribution of melanin, or pigment. Heterochromia is a condition where a person has two different color eyes, and it may either be inherited or caused by disease or injury. It may be complete, in which one iris is a different color from the other, or sectoral, in which part of one iris is a different color from the rest. And then there's central heterochromia, which involves a ring around the pupil or even spikes of different colors radiating from the pupil. The affected eye may be hyperpigmented or hypopigmented, due to an alteration of the genes that determine melanin distribution. While due to inbreeding it is common in some breeds of cats, dogs, cattle and horses, it is uncommon in humans. The incidence is fewer than 200,000 people in the U.S.

Any gene variant or abnormality on the X chromosome will be expressed in a man, whereas a woman will have a second copy to compensate, and she will be an unaffected carrier.

Janice Berliner, 2020