How are features passed from parents to offspring, and how can we predict them?
Explain the basics of inheritance using genes and alleles, and work out simple genetic crosses with a Punnett square
A scaffolded answer to the N(A)-Level Biology outcome on inheritance. What genes and alleles are, dominant and recessive alleles, and how to work out a simple monohybrid cross with a Punnett square.
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What this dot point is asking
This outcome wants you to explain the basics of inheritance: what genes and alleles are, the difference between dominant and recessive alleles, and how to work out a simple genetic cross using a Punnett square. You should be able to use letters (capital for dominant, lower case for recessive) and read off the ratio of offspring. The marks reward correct sex cells, a correctly filled Punnett square, and the right final ratio with reasoning.
The answer
Genes and alleles
A gene is a section of instructions that controls one feature, such as eye colour. Genes are carried on chromosomes inside the nucleus of a cell. Most features come in different versions, and a version of a gene is called an allele. You inherit one allele of each gene from each parent, so you have two alleles for each feature.
Dominant and recessive
The two alleles you have may be different. One can be dominant and the other recessive:
- A dominant allele shows its effect even if only one copy is present. We write it with a capital letter (for example B).
- A recessive allele only shows its effect when two copies are present. We write it with a lower-case letter (for example b).
So with brown (B) dominant and blue (b) recessive, a person who is BB or Bb has brown eyes, and only a person who is bb has blue eyes.
Genotype and phenotype
- The genotype is the alleles a person has, for example Bb.
- The phenotype is the feature you can see, for example brown eyes.
A genotype with two the same (BB or bb) is homozygous; one with two different (Bb) is heterozygous.
Working out a cross with a Punnett square
A Punnett square is a simple grid that shows the possible offspring from two parents. You put the sex cells (each carrying one allele) along the top and the side, then fill in the combinations. The grid then shows the genotypes of the offspring and lets you read off the ratio.
Examples in context
Example 1. Why two brown-eyed parents can have a blue-eyed child. If both parents are Bb (brown, but carrying a recessive blue allele), a Punnett square shows that one in four children is expected to be bb, with blue eyes. It explains how a recessive feature can reappear even when neither parent shows it.
Example 2. Why farmers cross plants carefully. By choosing which plants to cross, a farmer can make a wanted feature, such as large fruit, more common in the offspring. Using Punnett-square thinking, they predict the likely results of a cross. It shows inheritance used on purpose to improve crops.
Try this
Q1. State what an allele is. [1 mark]
- Cue. A version (or form) of a gene.
Q2. A plant is Tt, with T (tall) dominant over t (short). State its phenotype and give a reason. [2 marks]
- Cue. It is tall, because the dominant allele T shows its effect even though only one copy is present.
Q3. Two Tt plants are crossed. State the ratio of tall to short offspring expected. [2 marks]
- Cue. 3 tall to 1 short (3:1): the offspring are TT, Tt, Tt (all tall) and tt (short).
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original3 marksExplain what is meant by a dominant allele and a recessive allele, using an example.Show worked answer →
An allele is one version of a gene. A dominant allele shows its effect even if only one copy is present. A recessive allele only shows its effect when two copies are present (one from each parent).
For example, if the allele for brown eyes (B) is dominant and the allele for blue eyes (b) is recessive, then a person with Bb has brown eyes, because the dominant brown allele shows. A person only has blue eyes if they are bb, with two recessive alleles.
What markers reward: a dominant allele showing with one copy, a recessive allele needing two copies to show, and a clear example using letters (capital for dominant, lower case for recessive). Saying dominant means stronger without explaining the one-copy idea scores only partly.
Original5 marksA plant with the genotype Tt (tall) is crossed with another Tt plant, where T (tall) is dominant over t (short). Use a Punnett square to find the ratio of tall to short offspring expected.Show worked answer →
Each Tt parent makes two kinds of sex cell: T and t. The Punnett square combines them:
Parents: Tt and Tt. Sex cells T, t crossed with T, t.
The four combinations are: TT, Tt, Tt, tt.
TT, Tt and Tt are all tall (they have at least one T, the dominant allele). Only tt is short. So the ratio is 3 tall to 1 short (3:1).
What markers reward: the correct sex cells (T and t from each parent), the four offspring (TT, Tt, Tt, tt), identifying which are tall (any with a T) and which are short (tt), and the final 3:1 ratio. Forgetting that Tt is tall (because T is dominant) is the most common error.
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