Alleles, dominant and recessive, genotype and phenotype, homozygous and heterozygous, monohybrid crosses and Punnett squares, the inheritance of sex by the X and Y chromosomes, and single-gene inherited disorders such as cystic fibrosis and polydactyly.

What this dot point is asking

OCR wants you to define alleles, genotype, phenotype, homozygous and heterozygous, complete monohybrid crosses using Punnett squares to predict genotypes and ratios, explain how the X and Y chromosomes determine sex, and explain single-gene inherited disorders.

The language of inheritance

Two more terms you must use precisely:

• Homozygous: the two alleles are the same (for example BB or bb).
• Heterozygous: the two alleles are different (for example Bb).

By convention a dominant allele is written as a capital letter and the matching recessive allele as the same letter in lower case (B and b). An organism inherits one allele of each gene from each parent, one on each chromosome of a pair.

Monohybrid crosses and Punnett squares

A monohybrid cross follows the inheritance of a single gene. A Punnett square is the tool that predicts the genotypes and the ratio of offspring.

To use one: write one parent's possible gametes along the top, the other parent's down the side, and fill each box by combining the alleles.

Ratios are predictions of probability, not guarantees. A 3:1 ratio means each offspring has a $\dfrac{3}{4}$ chance of the dominant phenotype, not that exactly three in every four will show it. With small numbers the actual results often differ from the predicted ratio.

Inheritance of sex

The sex chromosomes are the 23rd pair in humans. Females are XX and males are XY.

• All egg cells carry an X chromosome.
• Sperm carry either an X or a Y, in roughly equal numbers.

So the sperm determines the sex of the baby: an X sperm gives XX (female), a Y sperm gives XY (male). A Punnett square of XX $\times$ XY gives XX, XX, XY, XY, a 1 : 1 ratio, which is why about half of babies are each sex.

Single-gene inherited disorders

Some disorders are caused by a fault in a single gene, and Punnett squares are used to work out the chance of inheriting them.

• Cystic fibrosis is caused by a recessive allele, so a person needs two recessive alleles (ff) to have the disorder. Heterozygous people (Ff) are healthy carriers who can still pass the allele on.
• Polydactyly (extra fingers or toes) is caused by a dominant allele, so just one copy is enough to cause it, and an affected person usually has an affected parent.

Knowing whether the allele is dominant or recessive tells you how the disorder runs through a family tree and lets you predict the probability for future children.