Gregor Mendel was a 19th century monk who discovered how genetic information is passed from parents to children. For each trait, such as eye colour, blood type, height and personality traits, each child inherits some genetic information from their mother, and some from their father. The information from one parent may dominate over that of another parent. To really understand this we need to look a bit closer at the experiments Mendel conducted.
Mendel did much of his research with pea plants. He noticed that:
- Some plants had green peas, even when they were bred from plants with yellow peas
- Some plants with yellow peas only ever produced offspring with yellow peas, although there would sometimes be plants with green peas in future generations
- Breeding from plants with green peas never produced plants with yellow peas
He concluded that pea colour (yellow or green) was determined by just one point of information in DNA, with each new plant getting one instruction for pea colour from their father, and another instruction from their mother. If both parents passed down information to make yellow peas then the offspring would have yellow peas. If one parent passed down information to make yellow peas, and the other provided information to make green peas, the plant would make yellow peas but be a carrier for green peas (meaning that it might then have offspring of its own with green peas, if bred with another plant that either had green peas or was also a carrier). Only if a plant received instructions to make green peas from both parents, would it make green peas.
All this is explained really well in this video.
Rather than the information from parents being mixed together, Mendel concluded that one set of instructions took dominance over the other set. In the case of Mendel’s peas, yellow is dominant and green is recessive.
In simple organisms, like pea plants, many traits are influenced in this way. In more complex organisms, like humans, most traits (except for some rare diseases) aren’t influenced by just one piece of DNA information, but by many different pieces of information.
Let’s think about eye colour in humans. If eye colour was influenced by just one piece of genetic information we would expect everybody in the world to have one of two eye colours (let’s say blue and brown). We would also expect to see more of the dominant eye colour (let’s say brown), than of the recessive colour (blue). Two blue eyed parents would only ever have blue eyed babies. Brown eyed parents would have mostly brown eyed babies, but might occasionally have a baby with blue eyes -if they were both carriers for blue eyes.
But eye colour in humans is much more varied than just blue and brown. There are different shades of blue and brown. Some brown eyes are so dark they appear black, some blue are so light they almost seem white. There are also green and grey eyes, and any combination of these colours. All with different patterns. At the best current estimate, eye colour is influenced by as many as 16 different bits of information from parental DNA.
Ironically, eye colour is often chosen as a way of explaining Mendel’s principles in high school science lessons by talking only about blue and brown eyes. Research has suggested that understanding genetic influences on traits in such a binary, on/off, dominant/recessive way can lead people to misunderstand and over estimate the influence of genes in complex traits.