How is genetic information stored and passed on, and how do we predict the offspring of a cross?
DNA, genes and chromosomes, the key genetic terms, dominant and recessive alleles, genotype and phenotype, monohybrid crosses with Punnett squares, continuous and discontinuous variation, and the causes of variation including mutation.
A focused CCEA GCSE Single Award Science answer on genetics, covering DNA, genes and chromosomes, dominant and recessive alleles, genotype and phenotype, monohybrid crosses with Punnett squares, continuous and discontinuous variation, and the causes of variation including mutation.
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What this dot point is asking
CCEA wants you to know how DNA, genes and chromosomes are related, define the key genetic terms, explain dominant and recessive alleles, distinguish genotype and phenotype, use Punnett squares for monohybrid crosses, and describe continuous and discontinuous variation and its causes including mutation.
DNA, genes and chromosomes
The key genetic terms
Monohybrid crosses
A Punnett square predicts the offspring of a cross. You write each parent's gametes along the top and side, then fill in the combinations.
A ratio is the expected average over many offspring; each individual birth is a separate probability.
Variation and its causes
Examples in context
Example 1. Why two brown-eyed parents can have a blue-eyed child. Brown eye colour (B) is dominant to blue (b). Two brown-eyed parents who are both heterozygous (Bb) can each pass on a b allele. A Punnett square of Bb x Bb gives a 1 in 4 chance of a bb child, who has blue eyes. This shows how a recessive characteristic can reappear when two carriers have children, a common CCEA cross.
Example 2. Genes and environment together. Two plants with the same alleles for height can still grow to different sizes if one is kept in the shade with poor soil. Their genotype is the same, but the environment changes the phenotype. Human height works the same way: genes set a potential, and diet and health decide how much of it is reached. Recognising that many features depend on both genes and environment is a key CCEA idea.
Try this
Q1. What does heterozygous mean? [1 mark]
- Cue. Having two different alleles for a gene, such as Tt.
Q2. Give one example of continuous variation and one of discontinuous variation. [2 marks]
- Cue. Continuous: height (or mass). Discontinuous: blood group (or tongue rolling).
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA SAS 20215 marksIn mice, black fur (B) is dominant to brown fur (b). Cross a heterozygous black mouse with a brown mouse and give the expected ratio of offspring.Show worked answer →
Five marks for the gametes, the Punnett square and the ratio.
The heterozygous black mouse is Bb, so its gametes are B or b.
The brown mouse is bb, so its gametes are all b.
Combining them in a Punnett square gives the offspring genotypes Bb, Bb, bb, bb.
Half are Bb, which is black, and half are bb, which is brown.
So the expected phenotype ratio is 1 black to 1 brown, a 50 percent chance of each. Markers reward the correct gametes, a completed Punnett square, the genotypes and the 1 to 1 ratio.
CCEA SAS 20194 marksExplain the difference between continuous and discontinuous variation, giving an example of each.Show worked answer →
Four marks: a definition and an example for each.
Continuous variation is where a feature has a range of values with no distinct groups, such as height or mass. It is shown on a histogram.
Discontinuous variation is where a feature falls into distinct categories with no in-between values, such as blood group (A, B, AB or O). It is shown on a bar chart.
Markers reward the idea of a range versus distinct categories, and a correct example for each. A common error is giving height as discontinuous.
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Sources & how we know this
- CCEA GCSE Science: Single Award specification — CCEA (2017)