By the end of this lesson
- Define allele accurately.
- Relate alleles to loci on homologous chromosomes.
- Explain how allele differences may affect phenotype.
BIG QUESTION
How can the same gene contribute to different inherited traits?
Small sequence differences between alleles can change a gene product or its expression.
Alleles are versions of one gene
An alleleOne of the different forms of the same gene, found at the same locus on a chromosome. is an alternative form of a gene. Alleles occupy the same locus on homologous chromosomes but differ in DNA sequence.
A diploid individual usually carries two alleles at an autosomal locus, one inherited from each parent. A population can contain more than two allele versions.
Sequence differences can alter products
An allele may change the amount, structure or activity of a gene product. Some allele differences have no detectable phenotypic effect.
Alleles are not separate genes for opposite characteristics; they are versions of the same gene.
Allele combinations form a genotype
The allele combination at a locus is the genotypeThe alleles an organism has for a particular characteristic, such as Bb.. Its effect interacts with other genes and the environment to contribute to phenotype.
Notation such as A and a is a model; letter case indicates a stated dominance relationship, not biological strength or value.
Gene, allele and genotype
These terms answer different questions.
| Term | Meaning | Example model |
|---|---|---|
| Gene | DNA sequence with a functional product. | Pigment-production gene. |
| Allele | Sequence version of that gene. | A or a. |
| Locus | Chromosome position of the gene. | Corresponding site on homologues. |
| Genotype | Allele combination in an individual. | AA, Aa or aa. |
| Phenotype | Observable/measurable outcome. | Pigment amount. |
Language that earns precision
- allele
- Alternative DNA-sequence form of a gene.
- autosomal
- Located on a chromosome other than a sex chromosome.
- genotype
- Allele combination of an organism at one or more loci.
- phenotype
- Observable or measurable characteristics produced by genotype and environment.
Common misconceptions
Claim: An individual always has every allele in the population.
Correction: A diploid individual normally has two alleles at one autosomal locus.
Claim: Dominant means common, stronger or better.
Correction: It only describes expression in a heterozygote.
Claim: Alleles are found at different loci.
Correction: Alleles of the same gene occupy the same locus on homologues.
Match the answer to the command word
Write “alternative form of the same gene”.
Link sequence to product, then product to characteristic.
Recall, apply, then justify
Define an allele and explain where two alleles are found in a diploid cell.
Hint
Version, locus and homologues.
Suggested marking points
- An allele is an alternative form of a gene.
- The two alleles occupy the same locus.
- They are carried on a pair of homologous chromosomes.
Explain how two alleles can produce different phenotypes.
Hint
DNA sequence → product → trait.
Suggested marking points
- The alleles have different DNA base sequences.
- This may alter a gene product’s structure, amount or activity.
- The altered product can change the observable characteristic.
Explain why an allele can be present without producing an obvious phenotypic difference.
Planning hint
Code redundancy, dominance, expression and environment.
What a strong design includes
- The sequence difference may be outside a critical region.
- A coding change may be synonymous.
- The alternative product may retain similar function.
- A dominant allele may mask a recessive effect in a heterozygote.
- The gene may not be expressed in the observed tissue or condition.
- Environmental conditions may prevent a difference becoming apparent.