How traits are inherited

Two traits at once. Mendel crossed plants that differed in two ways. In the book's figure, a plant with round, green seeds (RRyy) is crossed with one with wrinkled, yellow seeds (rrYY).

• All F1 seeds were round and yellow (RrYy): round and yellow are dominant. Notice that neither parent was round and yellow: shape came from one parent and colour from the other.
• In F2, Mendel counted 315 round yellow, 108 round green, 101 wrinkled yellow, 32 wrinkled green: about 9 : 3 : 3 : 1.

Round-green and wrinkled-yellow were new combinations in F2. So the gene for seed shape and the gene for seed colour are passed on independently: one doesn't care what the other does.

"How does a gene actually make a plant tall?" A gene is a section of DNA with the instructions for one protein. Plant height depends on how much growth hormone the plant makes, and that depends on an enzyme (a protein). If the enzyme works well, plenty of hormone is made and the plant is tall. If the gene has a change that makes the enzyme less efficient, less hormone is made and the plant is short. So genes control traits through proteins.

Genes are carried on separate pieces of DNA called chromosomes. Each body cell has two of each chromosome, one from each parent. A germ-cell gets just one from each pair. When an egg and a sperm fuse, the pairs are complete again. That is why both parents contribute equally, and why shape and colour genes, on different chromosomes, can be inherited independently.

"What decides whether a baby is a boy or a girl?" It differs between animals. In some reptiles, the temperature at which the egg is kept decides it. Snails can even change sex.

In humans, 22 pairs of chromosomes are matched pairs. The last pair, the sex chromosomes, is different:

• women have a matched pair, XX;
• men have a mismatched pair, XY.

The mother always passes on an X. The father passes on either X or Y:

So the chance of a boy or a girl is always half and half, however many girls or boys a family already has.

Try the book's own question: a man with blood group A marries a woman with blood group O, and their daughter has blood group O. With what you now know about dominant and recessive, can you tell from this alone which of the two is dominant?