Inheritance
Inheritance is how living things pass features from parent to offspring. It is the reason you might have your mother's eyes, your father's smile, your grandmother's laugh and your great-aunt's curly hair. Inheritance works through genes: every parent passes on half of their genes to each child, and those genes carry the instructions for the child's features. The rules of inheritance were first worked out by an Austrian monk called Gregor Mendel in the 1860s, by carefully growing thousands of pea plants in his monastery garden.
- Genes from each parentapprox. 50%Half of every child's genes come from each parent
- Versions of a geneAllelesYou have two of each, one from each parent
- Dominant traitWins outShows up even if you have only one copy
- Recessive traitHiddenOnly shows if you have two copies
- Rules first worked out1865By Gregor Mendel, using pea plants
- Eye colourMany genesNot just one, more complex than Mendel thought
How inheritance works
Most of your cells contain two copies of every gene: one from your mother and one from your father. The two copies might be slightly different versions of the same gene, called alleles. For example, the gene that helps decide your eye colour comes in several alleles: one for brown eyes, one for blue eyes, and several others.
When your body uses a gene, it usually uses both copies. Sometimes one allele's effect overrides the other. That is the difference between dominant and recessive traits.
Dominant and recessive traits
Mendel discovered that some alleles are dominant: their effect shows up even if you only have one copy. Other alleles are recessive: they only show up if you have two copies of them.
Take eye colour as a simplified example (the real biology is more complicated, but it works as an illustration):
- The brown-eye allele is dominant.
- The blue-eye allele is recessive.
If you inherit one brown-eye allele and one blue-eye allele, the brown allele wins and you will have brown eyes. If you inherit two brown-eye alleles you will also have brown eyes. Only if you inherit two blue-eye alleles will you have blue eyes. That is why two brown-eyed parents can sometimes have a blue-eyed child: both parents must be carrying a hidden blue-eye allele alongside their brown one.
Punnett squares
Geneticists predict the results of crosses with a simple grid called a Punnett square. Imagine two parents who each carry one brown-eye allele (B) and one blue-eye allele (b). When they have a child, the child gets one allele from each parent. The Punnett square shows all four possible combinations.
Mother's alleles across the top, father's down the side:
| B | b | |
| B | BB (brown) | Bb (brown) |
| b | Bb (brown) | bb (blue) |
So 3 out of 4 children will have brown eyes (BB, Bb or Bb), but 1 out of 4 will have blue (bb). This 3:1 ratio is exactly what Mendel found in his pea plant experiments.
Most traits are not so simple
Mendel's neat dominant-and-recessive rules work for some traits, but most real human features are more complicated.
- Multiple genes: human eye colour is actually controlled by more than 15 different genes acting together. That is why eye colour is not just blue or brown; it can be hazel, green, grey, amber and many shades in between.
- Co-dominance: some alleles do not override each other but both show up. Your blood type (A, B, AB or O) works this way.
- Incomplete dominance: sometimes the two alleles blend, giving a halfway result.
- Environment: many traits also depend on what happens to you (height, for example, depends on both genes and diet).
Sex-linked inheritance
Some traits and diseases are inherited differently because the genes responsible sit on the X or Y sex chromosome. Conditions like colour blindness and haemophilia are caused by recessive alleles on the X chromosome. Because males have only one X (paired with a Y), one bad copy is enough to cause the condition in males. Females have two X chromosomes, so they usually need two bad copies to be affected, which is much rarer. This is why colour blindness and haemophilia are much more common in boys than girls.
Deeper dive: epigenetics and the limits of "nature vs nurture"
For a long time biologists assumed that your DNA was your destiny: the genes you inherited at birth would essentially shape your features for life. The story now turns out to be more complicated.
Different cells in your body all contain the same DNA, yet they look and behave completely differently: a muscle cell does not look like a brain cell, a liver cell does not behave like a skin cell. The reason is that different genes are switched on (or off) in each cell type, by chemical tags attached to the DNA. The study of these on-off switches is called epigenetics (literally "above genetics").
Even more interesting is that epigenetic tags can change during your life in response to your environment. Stress, diet, exercise, exposure to toxins and even traumatic experiences can leave epigenetic marks on your DNA that affect which genes are active. Some of these marks can even be passed down to the next generation in limited ways, which means your grandchildren's biology may be partly shaped by what you eat or how you live today.
Epigenetics has helped resolve the old "nature versus nurture" debate. The answer is "both, all the time, interacting in subtle ways". Your genes set up the basic blueprint, but your life experiences keep fine-tuning which parts of that blueprint actually get used.
For the scientist who discovered these rules, see Gregor Mendel and heredity. For DNA changes that drive evolution, see mutations.