Heredity

How do variations build up over generations, and why is there more variation with sexual reproduction?

Each generation inherits the differences of the one before and adds newly created ones. Asexual reproduction adds only minor differences from small inaccuracies in DNA copying, while sexual reproduction generates far greater diversity; selection of variants by the environment forms the basis of evolution.

Heredity

The rules by which traits and characteristics are reliably passed from parents to offspring. Its most obvious outcome is that each new generation shares a similar basic design, even while showing subtle variations.

Why was Mendel able to work out the rules of inheritance when others studying peas had not?

He combined his science with mathematics, and he was the first to count how many individuals showed a particular trait in each generation, using contrasting visible characters such as tall and short plants.

F1 generation

The first generation of progeny from crossing two parents. When Mendel crossed tall and short pea plants, every F1 plant was tall, with no medium-height plants, so only one parental trait appeared rather than a mixture of the two.

What did self-pollinating Mendel's tall F1 plants reveal?

Their F2 progeny were not all tall: one quarter were short. The shortness trait had therefore been inherited by the F1 plants too, but only tallness was expressed, which led Mendel to propose that two copies of a factor controlling each trait are present.

In Mendel's cross of pure tall (TT) and pure short (tt) pea plants, what trait combinations appear in the F2 generation, and in what ratio?

TT, Tt and tt in the ratio 1:2:1. Both TT and Tt plants are tall and only tt plants are short, so three-quarters of the F2 plants are tall and one quarter are short.

Dominant trait

A trait expressed when even a single copy of its factor is present, like T for tallness: TT and Tt plants are both tall. A recessive trait, like t, appears only when both copies are recessive, as in short tt plants.

How did Mendel's crosses involving two characteristics show that traits are inherited independently?

Tall plants with round seeds crossed with short plants with wrinkled seeds gave an F1 that was all tall with round seeds. In F2, new combinations also appeared, tall plants with wrinkled seeds and short plants with round seeds, so the two traits had separated and recombined.

Gene

A section of DNA that provides the information for making one protein. Mendel called these units factors; sexually reproducing organisms carry two copies of each, one inherited from each parent.

How do genes control characteristics such as the height of a plant?

A gene carries the information for a protein, such as an enzyme. Plant height can depend on the amount of a growth hormone, which depends on how efficiently an enzyme makes it; if the gene is altered so the enzyme works less efficiently, less hormone is made and the plant is short.

Chromosome

One of the separate, independent pieces of DNA in which a cell's set of genes is carried, rather than as a single long thread. Each cell has two copies of every chromosome, one inherited from each parent.

Why must genes be carried on separate chromosomes for traits to be inherited independently?

If each parent passed on its whole gene set as one long piece of DNA, factors such as seed shape and seed colour would stay linked. Because each germ cell takes one chromosome from each pair, of either maternal or paternal origin, traits can recombine, and fusion restores the normal chromosome number.

Is the sex of an individual always determined genetically?

No. In some reptiles the incubation temperature of the fertilised eggs decides whether the young develop as male or female, and snails can change sex. In human beings, however, sex is largely determined genetically.

Sex chromosomes

The one pair of human chromosomes that is not always a perfect pair; the other 22 pairs have matching maternal and paternal copies. Women have two X chromosomes, while men have a normal-sized X and a short Y.

Sex determination in humans

Every child inherits an X chromosome from the mother, so the child's sex depends on what it inherits from the father. A child who receives an X from the father is a girl, and one who receives a Y is a boy, so half the children are expected to be boys.

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