• Question: How do our facial and bodily features occur when our parents look totally different?

    Asked by Mia to Chris, Josh, Rebecca, Rob, Susan on 16 Jun 2015.
    • Photo: Josh Meyers

      Josh Meyers answered on 16 Jun 2015:

      Our appearances are determined by both genetic and environmental factors. The genetic factor is encoded by our DNA.

      Our parents’ DNA is stored in 46 chromosomes arranged in 23 pairs.
      The haploid sex cells (the sperm and the egg) contain 23 chromosomes (one from each pair) and once these fuse we will have aspects from both our mother and our father’s side.

      It may be that the genes that encoded for your fathers large nose were on the other strand of the chromosome (so they were never in the sperm cell). Or your mother may have a dominant gene which suppresses some aspect of appearance.

      Our parents may look different from one another, but you can’t escape their genes!

    • Photo: Susan Cartwright

      Susan Cartwright answered on 16 Jun 2015:

      This is because genetics is complicated. Essentially, you carry two copies of every gene (except if you are male, in which case there are genes on the X chromosome that you only have one copy of, and a few on the Y chromosome that you also only have one copy of). One of those copies came from your mum, and one from your dad.

      If the two copies of the gene are different, often one dominates the other. This means that genes can be passed down from generation to generation without ever being visible in the appearance of the person. An example is red hair: you can have red hair, even though neither of your parents did, because the gene for red hair is recessive: if you have one gene for red hair, and one for not-red, your hair won’t be red. So a red-hair gene can be passed down through generations of brown-haired people, and suddenly surface when one brown-haired person with a hidden red-hair gene marries another, and the two red-hair genes get passed to the same child.

      It’s the same deal with blue eyes (recessive to brown, so two brown-eyed parents can have a blue-eyed child) and straight hair (recessive to curly).

      Some genes have equal dominance, and you wind up with a mixture. An example is A and B blood groups: if you get an A gene from your mum, and a B gene from your dad, you have blood group AB.

      Sometimes there are spontaneous mutations. Several descendants of Queen Victoria had a serious blood disease called haemophilia, which is caused by a mutated gene on the X chromosome (so it almost always affects boys: it’s recessive, and girls who carry it are OK because of their second, healthy, gene on the other X chromosome, but boys only have their mother’s X chromosome, so no protection). The common factor is Victoria herself, but no ancestors of hers had it: the mutation must have occurred first in her (more precisely, probably in her father’s sperm). But more usually, the features that you have that don’t seem to come from your parents are the result of hidden recessive genes making themselves known.

      (Environment can also play a role. My dad was 6 foot 4 in (1.93 m), but both his parents were tiny. That’s almost certainly because they grew up in the late 19th century, when most working-class people didn’t have a very good diet – they did not reach their full growth potential. I am tall for a woman, at 1.76 m, and my brother is 1.95 m – clearly my dad’s genes specify height, but you have to get enough to eat to grow properly.)

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