Inheritance

In the mid-19th century Gregor Mendel carried out breeding experiments on plants. One of his observations was that the inheritance of each characteristic is determined by 'units' that are passed on to descendants unchanged.

Why Mendel's discovery was not recognised until after his death is a question in its own right. He was working before chromosomes had been seen, so there was nothing physical to attach his 'units' to; his work was published in an obscure journal and read by few scientists; and his mathematical, ratio-based approach was unlike the biology of the time, so its significance was not obvious. Only when chromosomes were observed behaving the same way did his results suddenly explain something.

This scientific work by many scientists led to the gene theory being developed.

Some characteristics are controlled by a single gene — fur colour in mice, and red-green colour blindness in humans. Each gene may have different forms called alleles. The alleles present, or genotype, operate at a molecular level to develop characteristics that can be expressed as a phenotype.

Most characteristics, though, are the result of multiple genes interacting rather than a single gene. That is worth remembering when a question asks whether a single-gene cross is the whole story: it is not, and saying so is often the last mark.

Express the outcome however the question asks, using direct proportion and simple ratios: as a ratio (3 : 1), as a fraction (¾), as a percentage (75%), or as a probability. All four say the same thing, and being able to move between them is examined directly. You should also be able to extract and interpret information from genetic crosses and family trees — in a family tree, work backwards from anyone showing a recessive phenotype, because you know their genotype for certain.

Family trees are read with two conventions and one deduction. The conventions: squares are male, circles female, and a shaded shape means affected. The deduction: start from anyone affected whose parents are not.

A condition that skips a generation like that must be recessive, because a dominant allele shows itself whenever it is present — so an affected child would need an affected parent. Once you know it is recessive, every affected person is homozygous recessive, and both of their parents must carry a copy. That single step usually unlocks the rest of the tree.

Ordinary human body cells contain 23 pairs of chromosomes. Twenty-two pairs control characteristics only, but one pair carries the genes that determine sex.

• In females the sex chromosomes are the same: XX. • In males they are different: XY.

Some disorders are inherited, caused by the inheritance of certain alleles. Two are named:

• Polydactyly — having extra fingers or toes — is caused by a dominant allele. • Cystic fibrosis — a disorder of cell membranes — is caused by a recessive allele.

The difference matters for the genetics. Polydactyly appears if a child inherits just one copy, so an affected child usually has an affected parent. Cystic fibrosis needs two copies, so two unaffected carriers can have an affected child — which is what makes it hard to predict.

Embryo screening tests embryos for these alleles, and you should be able to make informed judgements about the economic, social and ethical issues when you are given information.

In favour: it can prevent a child being born with a painful, life-shortening disorder; it reduces the lifelong cost of treatment and care; and parents who know they are carriers can have a child without that fear.

Against: screening implies that a life with a disorder is worth less, which many disabled people reject; it involves discarding embryos, which some people object to; it is expensive; and there is a concern about where the line is drawn once selection becomes possible at all.

Gene therapy raises the same tension — it may alleviate suffering, and it raises the same ethical questions. A good answer gives both sides and then reaches a judgement.

Most inherited characteristics you meet are controlled by two alleles, one dominant and one recessive. Blood group is controlled by three, and two of them are codominant — when both are present, both are expressed, and neither masks the other.

The three alleles are usually written as A, B and O. A and B are codominant with each other, and both are dominant over O. Because a person still inherits only two alleles, one from each parent, six genotypes are possible and they give four blood groups.

Sex-linked inheritance is different again. Some genes sit on the X chromosome and have no counterpart on the much smaller Y chromosome. A female has two X chromosomes, so she carries two copies of every gene on the X. A male has one X and one Y, so he carries only one copy — and whatever allele that single copy holds is expressed, dominant or not.

That is why sex-linked disorders such as haemophilia and red–green colour blindness are far more common in males. A female with one faulty allele and one normal one is a carrier: she does not have the condition, because her second X chromosome carries a working copy, but she can pass the faulty allele to a son who will.