How is variation classified, and how are characteristics passed from parents to offspring?
Discrete and continuous variation and their genetic basis, the genetics terms phenotype, genotype, dominant and recessive, homozygous and heterozygous, and the use of Punnett squares to predict the outcomes of monohybrid crosses.
An SQA National 5 Biology answer on variation and inheritance, covering discrete and continuous variation and polygenic inheritance, the genetics terms phenotype, genotype, dominant, recessive, homozygous and heterozygous, and the use of Punnett squares to predict the outcomes of monohybrid crosses.
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What this dot point is asking
The SQA wants you to distinguish discrete from continuous variation and link each to its genetic basis, define the key genetics terms (phenotype, genotype, dominant, recessive, homozygous, heterozygous), and use a Punnett square to predict the genotypes and phenotypes from a monohybrid cross.
Two kinds of variation
So a single gene tends to give clear-cut groups, while many genes acting together (plus the environment) give a smooth range.
The language of genetics
A heterozygous organism (Bb) shows the dominant phenotype, because the dominant allele B is expressed even when only one copy is present.
Monohybrid crosses and Punnett squares
A monohybrid cross follows a single gene from parents to offspring. A Punnett square is the tool for predicting the result: you write the gametes of one parent across the top and the other down the side, then combine them in the boxes.
A cross between two heterozygous parents (Bb x Bb) gives a 3:1 ratio of dominant to recessive phenotype, the classic monohybrid result.
Examples in context
Example 1. Cystic fibrosis. Cystic fibrosis is caused by a recessive allele. Two healthy carrier parents (each heterozygous) can have an affected child, because a Punnett square of the cross gives a 1 in 4 chance of the homozygous recessive genotype, the only one that shows the condition.
Example 2. Human height as polygenic. Height does not come in a few fixed groups; it ranges smoothly from short to tall. This is because it is controlled by many genes and is also affected by diet and the environment, the hallmark of continuous, polygenic variation.
Try this
Q1. State what the phenotype of an organism means. [1 mark]
- Cue. Its observable characteristics (what you can see).
Q2. A cross of Bb x bb is made, with B dominant. What fraction of offspring show the recessive phenotype? [1 mark]
- Cue. Half (the bb offspring), a 1:1 ratio.
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA N5 style4 marksA homozygous brown-eyed parent (BB) is crossed with a blue-eyed parent (bb). Brown (B) is dominant. Use a Punnett square to give the genotypes and phenotypes of the offspring.Show worked answer →
A 4-mark cross needs the gametes, the Punnett square, the genotypes and the phenotypes.
The BB parent makes only B gametes. The bb parent makes only b gametes.
Combining them, every offspring is Bb.
Genotype: all offspring are Bb (heterozygous).
Phenotype: because B (brown) is dominant, all offspring have brown eyes.
Markers reward (1) correct gametes B and b, (2) the cross giving Bb, (3) all genotypes Bb, and (4) all phenotypes brown. A clear Punnett square earns the working marks.
SQA N5 style2 marksDistinguish between discrete and continuous variation, giving an example of each.Show worked answer →
Two contrasting points are needed, each with an example.
Discrete variation falls into distinct, separate groups with nothing in between, such as blood group (A, B, AB or O). It is usually controlled by a single gene.
Continuous variation shows a range of values between two extremes, such as human height. It is usually controlled by many genes (polygenic) and influenced by the environment.
Markers reward (1) discrete being distinct groups with an example and (2) continuous being a range with an example.
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Sources & how we know this
- SQA National 5 Biology Course Specification — SQA (2019)