An individual inherits two copies of each gene in a pair of chromosomes.
Versions of genes are called alleles, they can be recessive or dominant, indicated with a capital or lowercase letter.
The combination of alleles that an organism has is called its genotype.
Different alleles of a gene are associated with different versions of the same characteristic called the phenotype.
Some phenotypes are determined only by genotype, others are also affected by the environment.

Common misconception

A dominant condition is 'stronger' and that only the strongest characteristic is inherited from one parent.

Two genes are inherited, so both alleles contribute to the genotype. In some cases one is dominant and therefore will always be expressed (through protein structure) in the phenotype, the recessive allele is still present.

Keywords

Allele - A genetic variant in a gene creates an allele (a different version of the gene), which produces a different phenotype.
Dominant - One or both dominant alleles in the genotype for the characteristic to be expressed in the phenotype.
Recessive - Two recessive alleles in the genotype for the characteristic to be expressed in the phenotype.
Genotype - The combination of alleles an organism has for a characteristic (i.e. phenotype).
Phenotype - Features that result, at least partly, from the genetic code of an organism’s genes.

Keep repeating the idea that alleles are different codes for the same protein, different proteins lead to different characteristics (phenotypes). This is important when they use the genotype code as the link between the genotype and protein formation can easily be forgotten.

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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6 Questions

Q1.

Phenotype refers to an organism’s observable characteristics. Which factors can affect an organism’s phenotype?

Its genetic code only.

Correct answer: Its genetic code and its environment.

Its environment only.

Q2.

Starting with the smallest, put these in size order.

1 - Nucleotide

2 - Gene

3 - Chromosome

4 - Nucleus

Q3.

What is the genome in a unicellular organism?

All the genes in the cell.

All the non-coding DNA in the cell.

Correct answer: All the DNA in the cell.

All the proteins in the cell.

All the enzymes in the cell.

Q4.

Which is the best description of a DNA molecule?

Correct answer: A polymer made of two nucleotide chains forming a double helix.

A monomer made of one nucleotide chain forming a helix.

A polymer made of one nucleotide chain forming a helix.

A monomer made of two nucleotide chains forming a double helix.

A polymer made of three nucleotide chains forming a triple helix.

Q5.

True or false? All genes code for enzymes.

True

Correct answer: False

Q6.

Put these in order to describe the production of a red pigment in a petal.

1 - The genetic code in a gene is copied.

2 - Amino acids are joined together in sequence to make a polypetide chain.

3 - The chain folds up to make an enzyme.

4 - The enzyme speeds up a chemical reaction.

5 - A red pigment is produced.

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6 Questions

Q1.

Adult body cells contain two copies of each chromosome. Gametes are sex cells. Which statement about gametes is true?

Gametes don’t contain chromosomes.

Correct answer: Gametes contain one copy of each chromosome.

Gametes contain two copies of each chromosome.

Gametes have four copies of each chromosome.

Q2.

The diagram shows a chromosome pair. Four genes are shown. One gene is labelled on the left chromosome. Which position (A, B, C or D) shows the location of this gene on the right chromosome?

Correct Answer: A

Q3.

Alleles are ...

different chromosomes.

different genes.

Correct answer: genetic variants of the same gene.

genetic variants of different genes.

Q4.

A plant’s flowers are either red or white. Plants with the genotypes RR and Rr have red flowers. Which allele is recessive?

Correct Answer: r

Q5.

Match each word to its correct meaning.

Correct Answer:Allele,A genetic variant of a gene coding for a different phenotype.

A genetic variant of a gene coding for a different phenotype.

Correct Answer:Dominant,Only one copy of this type of allele will cause a phenotypic feature.

Only one copy of this type of allele will cause a phenotypic feature.

Correct Answer:Genotype,The combination of alleles an organism has for a feature.

The combination of alleles an organism has for a feature.

Correct Answer:Phenotype ,Observable features that are at least partly caused by genes.

Observable features that are at least partly caused by genes.

Correct Answer:Recessive,Two copies of this allele are needed to cause a phenotypic feature.

Two copies of this allele are needed to cause a phenotypic feature.

Q6.

True or false? Different alleles have different genetic codes, so they produce a different amino acid sequence and so a protein with a different 3D shape.

Correct answer: True

False