Chapter summary

Everything in this chapter followed from one observation — that no two individuals are quite alike — and one method: counting.

The chapter in eight points

• Characteristics transmitted from generation to generation are inherited. Those produced by environment, food or exercise are not.
• Mendel showed that each characteristic is controlled by a pair of factors, one from each parent, and that one form can be dominant over the other, which is then recessive.
• Crossing two heterozygotes gives 3 : 1 by appearance and 1 : 2 : 1 by genotype, because gametes combine at random and there are two ways to make the heterozygote.
• A factor is a gene: a length of DNA on a chromosome. The two forms of a gene are alleles; a matching pair is homozygous and a differing pair heterozygous.
• Genotype is the alleles carried; phenotype is the characteristic as expressed. Only the genotype is inherited.
• A human body cell has 44 autosomes and 2 sex chromosomes. A female is XX and a male XY, so the father's gamete decides the sex of the child, with an even chance each time.
• Genes on the X chromosome are sex-linked. A male has only one, so a single recessive allele shows in him — which is why haemophilia and red-green colour blindness are commoner in men.
• Selective breeding improves crops and livestock using the alleles a species already has; genetic engineering transfers a gene between species, producing recombinant DNA.

Terms introduced in this chapter