So you’ve found a possible sibling or donor match, but you’re not quite sure you’re ready to take the plunge and submit to a DNA test. Here’s some of the most well known and oftentimes downright bizarre Mendelian traits in order to make a more informed decision to pursue a DNA test. Remember however, that even if you and your potential donor or sibling do not share a specific trait does not mean you are not related!!
1. Blood type – ABO blood types are based on the presence or absence of A or B antigens on the red blood cells. Someone who is blood type A has A-antigens present and Anti-B antibodies on their red blood cells. Someone who is blood type B has the reverse. Someone who is AB has both A- and B-antigens but no antibodies. Someone who is O has no antigens and both Anti-A and Anti-B antibodies.
A and B blood types are co-dominant (neither dominates over the other – A and B allele give AB blood type), and both are dominant over O blood type, which is recessive. Therefore to have O blood type you must have inherited an O allele from BOTH parents. If you inherit an O from one parent and a B from the other you are B blood type. Someone who is AB inherited one from each. A and B always trump O. If you are blood type B and your mother is also B your donor could have been any blood type (AO, B, O, or AB) and you would have had to inherit either a B or O allele. If you are blood type O and your mother is O, your donor could have been any blood type (AO, BO, or O) except AB. If you are A and your mother is O your donor MUST have been blood type A (or AB).
The positive or negative refers to the ‘Rh factor’, and it’s simply the presence or absence of the Rh antigen, where positive is dominant over negative.
2. Dimples – dominant trait. If you have dimples one of your parents MUST have had dimples. If you don’t have dimples, your donor may or may not have had dimples.
3. Cleft chin – dominant trait (same as dimples).
4. Earlobes – free hanging earlobes are dominant (AA or Aa) and attached earlobes are recessive (aa). If your mother has attached earlobes and you have free hanging earlobes, your donor must have had free hanging earlobes.
5. Freckles [that fade in winter (as opposed to sunspots that don’t)] – dominant presence of Mc1R gene (controls melanocytes = pigment). If you have freckles and your mother does not, your donor most likely had freckles (variable depending on UV exposure and skin complexion).
6. Hitchhiker’s thumb – recessive trait. You had to have inherited both recessive alleles from each parent, but both parents could have normal thumbs so difficult to determine.
7. Widow’s peak – dominant trait (same as dimples and cleft chin).
8. Ability to taste phenylthiocarbamide (PTC) – PTC is an organic compound that has either a very bitter taste (dominant) or no taste at all (recessive) depending on the person’s genetic make-up. It’s said that about 70% of the general population can taste PTC. Often this is a science experiment done in high school biology classes to teach Mendelian inheritance.
9. Eye color – while previously thought to be a single gene, now found to to be much more complex. In simplest terms, brown eyes are dominant over green and blue eyes, and green eyes are dominant over blue eyes. Two brown-eyed parents can have children with any eye color (depending on what their genotype is...BB x BB has only brown-eyed offspring, but Bb x Bb can have brown or blue-eyed children) but two blue-eyed parents can only have blue-eyed children since blue eyes are recessive (bb x bb).
There have been two genes found implicated in eye color (but it’s still more complicated than just this)…Bey2 (brown eye) and Gey (green eye). Bey2 has two alleles – brown and blue, and Gey has two alleles – green and blue. Every person has two alleles for each of these genes. Bey2 can have three different combinations…brown-brown, brown-blue, and blue-blue. The first two produce brown eyes and the latter produces blue eyes, since brown is dominant over blue. But there is a second gene, Gey which has two alleles as well, and here green is dominant over Gey-blue and also Bey2-blue. Therefore, if one parent is blue-eyed and the other is green-eyed, depending on what the genotype of the green-eyed parent (green-green or green-blue of the Gey gene) is, all their children could be green-eyed, or a 2:1 ratio of green:blue eyed children.
10. Hair type – incomplete dominance trait, where straight (HH), wavy (Hh) and curly (hh) follow a spectrum. Two straight-haired parents have all children with straight hair, but if one parent has straight and one has curly hair, instead of one being dominant over the other, the children all end up with a combination of the two, or wavy hair. If one parent has curly hair and the other has wavy hair 50% of the children will have curly hair and 50% will have wavy hair. Same if one parent has straight hair and the other has wavy hair – 50% of the children would have straight hair and 50% would have wavy hair.