Lesson 1 of 9 / Inheritance
One gene, different alleles
How can two chromosomes carry the same gene but different information?
I can distinguish a gene from an allele and locate alleles on homologous chromosomes.
I can distinguish genes and alleles.Syllabus
Syllabus K327 / K328, Topic 12(a). Distinguish genes and alleles.
About 4 min
Make a guess. You are not marked.
Your two copies of chromosome 7 came from different parents. Do they carry the same genes?
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Yes, the same genes, but possibly different alleles.
Homologous chromosomes carry the same genes at the same positions. The versions, the alleles, can differ.
The key idea
A gene is a sequence of DNA; an allele is an alternative form of that gene. Homologous chromosomes carry the same genes at corresponding positions.
This lesson uses one gene on a pair of homologous chromosomes. One chromosome came from each parent.
R and r are alternative forms of one gene. The genotype is heterozygous Rr, even though the chromosomes carry corresponding genes.
Blue/purple: homologous chromosomes from different parents. Gold bands: one gene position. Letters: alleles. Original schematic; chromosome size and gene length are not to scale.
Explanation
A gene is a sequence of DNA that carries information used to make a polypeptide, at the level of this syllabus. An allele is an alternative form of a particular gene. Alleles can have different DNA base sequences and can lead to different versions of a characteristic.
In a diploid cell, homologous chromosomes carry corresponding genes at the same positions. One member of each pair is inherited from each parent. The two alleles at a given position may be the same or different.
For a stated simple inheritance model, we can represent two alleles using the same letter in different forms, such as R and r. The shared letter identifies the gene being followed. The notation must be defined in the question; a capital letter alone does not tell you what characteristic it represents.
A chromosome contains many genes, and a characteristic can depend on more than one gene and on the environment. A one-gene diagram deliberately follows one feature. Do not generalise it to mean that every human characteristic is controlled by one dominant and one recessive allele.
Step by step
- 1
Identify the gene
Name the inherited feature or gene specified in the model.
- 2
Define its alleles
State what each symbol represents before using a genetic diagram.
- 3
Locate the pair
A diploid genotype records the alleles at corresponding positions on homologous chromosomes.
Worked example
Same position, different sequence
Two homologous chromosomes carry different versions of a seed-shape gene at the same position. What are the versions called?
One way to explain it
They are alleles of the same gene. Their positions correspond, but their DNA sequences may differ.
Why this answer works
- The gene is the shared unit being followed.
- Different versions are alleles, not necessarily different genes.
Watch out for this
A student says: "An allele is another name for a whole chromosome." What is wrong with this?
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A chromosome carries many genes. An allele is a version of one gene at a particular position.
Check your understanding
Which description correctly defines an allele?
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An alternative form of the same gene
Two homologous chromosomes can carry different alleles at corresponding positions.
Quick recall
How can two chromosomes carry the same gene but different information?