Most characteristics are influenced by multiple genes, but some are determined by just one.
An individual will inherit two alleles for each gene, the alleles for a gene may be dominant or recessive.
A Punnett square models inheritance of alleles showing possible genotypes of offspring as homozygous or heterozygous.
Punnett squares are used to predict proportion of genotypes/phenotypes, or probability of occurrence in the offspring.
Each instance of sexual reproduction is independent, so does not affect the probability for subsequent offspring.

Keywords

Punnett square - A model used to show the inheritance of alleles from parents to offspring.
Gamete - A sex cell (i.e. sperm and egg) that carries half the genetic information required for an individual organism.
Genotype - The combination of alleles that an individual has for each gene.
Ratio - Shows hows how much of one value there is compared to another.
Probability - A measure of the chance or likelihood of an event occurring.

Common misconception

A gamete has two alleles, Punnett square has too many alleles for gametes. Also that only four offspring are produced and therefore each will definitely have the genotype and phenotype shown.

Model the alleles within gametes so it is clear. Refer to half the gametes having each version, moving away from the idea of divisions of 4.

Use tossing a coin as an example of 50% probability of heads or tails each time, each throw being independent of the last as a model for probability of genotypes in offspring.

Teacher tip

Licence

This content is © Oak National Academy Limited (2024), licensed on Open Government Licence version 3.0 except where otherwise stated. See Oak's terms & conditions (Collection 2).

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Starter quiz

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6 Questions

Q1.

Which is the genetic code?

The sequence of amino acids in DNA.

The sequence of amino acids in a protein.

Correct answer: The sequence of nucleotide bases in DNA.

The sequence of nucleotides bases in protein.

Q2.

Which statement best describes the relationship between phenotype and genotype?

Genotype only depends on phenotype.

Genotype at least partly depends on phenotype.

Phenotype only depends on genotype.

Correct answer: Phenotype at least partly depends on genotype.

Q3.

Horse body cells have 64 chromosomes. How many chromosomes will there be in a horse's unfertilised egg cell?

Correct Answer: 32, Thirty two

Q4.

True or false? Offspring inherit different genes from each parent.

true

Correct answer: false

Q5.

In humans the CF protein affects the stickiness of mucus. There are two alleles F and f. Individuals with the genotype ff have very sticky mucus. Which allele is dominant?

Correct answer: F

Q6.

Plants produce either red pigmented flowers or white flowers with no pigment. The RR or Rr genotypes have red flowers. Plants with rr genotype have white flowers. Order the statements to explain how.

1 - The petal cells contain a colourless chemical.

2 - The R allele codes for an enzyme.

3 - The enzyme catalyses a chemical reaction.

4 - The colourless chemical is converted to a red pigment.

5 - The plant has red flowers.

Exit quiz

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6 Questions

Q1.

Match the word to its meaning.

Correct Answer:heterozygous ,has two different alleles for a gene

has two different alleles for a gene

Correct Answer:homozygous ,has two identical alleles for a gene

has two identical alleles for a gene

Correct Answer:probability ,a measure of the chance of an event happening

a measure of the chance of an event happening

Correct Answer:ratio ,shows how much of one value there is compared to another

shows how much of one value there is compared to another

Q2.

The nucleus of a chimpanzee body cell contains 48 chromosomes. How many chromosomes will there be in the nucleus of a chimpanzee zygote?

Correct answer: 48

Q3.

A woman is heterozygous (Ff) for the gene that codes for a protein. What genotypes can her eggs have?

They will all be F.

They will all be f.

Correct answer: Some will be F and some will be f.

They will all be Ff.

Q4.

Two heterozygous (Rr) red flowered plants cross pollinate each other. Use the Punnett square to work out the ratio of red : white flowering offspring.

Correct Answer: 3:1, Three to one, 3 to 1, 3 : 1

Q5.

A couple who are both heterozygous (Ff) for the A gene want to have a child. Use the incomplete Punnett square to work out the probability that the child will be homozygous recessive.

Correct Answer: 0.25, 25%, one in four, 1 in 4

Q6.

Flowers can be red (RR or Rr) or white (rr). A single plant with red flowers pollinates itself. Some of the offspring plants have white flowers. What was the genotype of the parent plant?

Correct answer: Rr