The terms gene, allele, dominant, recessive, homozygous, heterozygous, genotype and phenotype, using Punnett squares for a monohybrid cross, how the X and Y chromosomes determine sex, and how a genetic disorder such as cystic fibrosis is inherited.

What this dot point is asking

CCEA Double Award wants the genetics vocabulary, how to do a monohybrid cross with a Punnett square, how the X and Y chromosomes determine sex, and how a recessive genetic disorder like cystic fibrosis is inherited. Get the definitions exact, because the cross depends on using them correctly.

The genetics terms

Monohybrid crosses with a Punnett square

A monohybrid cross follows one characteristic controlled by one gene. A Punnett square shows the possible combinations of alleles in the offspring. List the alleles each parent can pass on along the top and side, then fill in the boxes.

Sex determination

Humans have one pair of sex chromosomes: females are XX and males are XY. The egg always carries an X. Half the sperm carry an X and half carry a Y. If an X sperm fertilises the egg, the baby is XX (female); if a Y sperm fertilises it, the baby is XY (male). So there is a 50 percent chance of each sex, which a Punnett square of XX crossed with XY confirms.

Inheriting a genetic disorder

Many genetic disorders are caused by a recessive allele. Cystic fibrosis is an example: a person needs two recessive alleles (one from each parent) to have the condition. A person with one recessive allele is a healthy carrier. If two carriers (both heterozygous) have a child, the Punnett square gives a 1 in 4 chance the child has the disorder, a 1 in 2 chance the child is a carrier, and a 1 in 4 chance the child has two normal alleles. Because carriers show no symptoms, a recessive disorder can stay hidden in a family for generations until two carriers have children together.

Examples in context

Example 1. Why two brown-eyed parents can have a blue-eyed child. If both parents are heterozygous (Bb), each can pass on the recessive b allele. A Punnett square shows a 1 in 4 chance of a bb (blue-eyed) child, even though both parents have brown eyes. This is the hidden recessive allele appearing.

Example 2. Genetic counselling for cystic fibrosis. If both parents are carriers of the cystic fibrosis allele, they can be told there is a 1 in 4 chance of an affected child. A Punnett square makes the risk clear, which is the basis of genetic counselling.

Try this

Q1. What is the genotype of a male in terms of sex chromosomes? [1 mark]

• Cue. XY.

Q2. What is a heterozygous genotype? [1 mark]

• Cue. Two different alleles for a gene (e.g. Bb).