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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.


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
Left Handedness

There are several theories of how handedness develops in human beings. Interestingly, about 40% of infants and 97% of toddlers demonstrate a hand preference. Handedness displays a complex inheritance pattern. A growing body of evidence suggests that genetic variance in handedness cannot be explained by a single genetic locus. Handedness requires the contributions of many genes, at least 40 according to some studies. A large 2006 study of twins from over 25,000 families (Medland et al.) has indicated that the heritability of handedness is roughly 24%. That means that the proportion of observed variation in handedness that results from inherited genetic factors is about 24%, whereas the proportion resulting from environmental ones is about 76%.

Male Pattern Baldness

Approximately 30% of males experience some degree of hair loss by age 30, 50% by age 50, and 80% by age 70. The proportion of variance in male pattern baldness that is attributable to genetic factors has been estimated from twin studies to be approximately 80%. Studies have identified 8 - 15 independent genetic loci linked to baldness. While it is commonly believed that baldness is inherited from the maternal grandfather, it's actually a lot more complicated than that, though there is some genetic evidence behind it. Both men and women experience hair loss, but research usually focuses on men. Research has also shown that female pattern hair loss is not predicted by the same genetic markers as male pattern baldness. One of the well-known genes related to hair loss codes for an androgen receptor (AR) protein. The job of this protein is to help hair follicle cells detect androgen hormones like testosterone in the circulation. Androgens affect when, where, and how much the hair grows, and.because the AR gene is located on the X chromosome, males can only inherit it from their mothers. Certainly this supports the notion that baldness is inherited from the maternal grandfather, but again, research shows that baldness is a polygenic condition. In fact, many of the genetic variants associated with male pattern baldness are not located on sex chromosomes, and somehow seem to be more predictive of the development of baldness than variants that are located on sex chromosomes. Individually, each gene may slightly increase the likelihood of going bald, and each additional variant will increase the risks more.

If you found this interesting and have questions about more common conditions, let me know - I'll write about them!
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