Reproduction and heredity
Heredity
Work out the ratios of a Mendelian cross, tell a dominant trait from a recessive one, and explain how chromosomes let two traits be inherited independently and how they settle a child's sex.
Mendel was not the first person to breed peas and watch what came out. What he did that nobody had done was count. Round against wrinkled, tall against short, violet against white — he kept a tally of how many plants of each kind appeared in each generation, and the numbers turned out to fall into simple ratios. Those ratios are the evidence that inheritance follows rules rather than blending like paint.
Two copies, one of them showing
A sexually reproducing organism carries two copies of the factor — the gene — for a trait, one from each parent. The two copies may be the same or different. A dominant trait shows when a single copy of it is present; a recessive trait shows when both copies are of that kind. That is why crossing a tall pea with a short one gives a first generation that is tall throughout, with no plants of medium height.
Worked example
Cross a tall round-seeded pea with a short wrinkled-seeded one. The first generation is tall with round seeds. Why does the second generation contain tall plants with wrinkled seeds?
Tallness and round seeds are the dominant traits, so the first generation shows both of them and hides the other two.
The recessive factors have not gone anywhere — they are carried along unseen, which is why they can reappear.
Try it together
Mendel self-pollinates his first-generation tall plants and raises 100 second-generation plants.
Answer with a bare number each time.
1.One quarter of the second generation comes out short. Out of the 100 plants, how many would you expect to be short?
Have a go
Have a go on your own. A pea plant is short. What can you say about its two copies of the height factor?
Ready to practise?
Eight questions on what you have just read. Nothing is timed, and you can play as many times as you like.