3.2.1Unit 3: Practical Skills in Biology I
Gene Locus — IAL Edexcel Linear Revision Notes & Topical Questions
5 min read 5 parts Lesson 11 of 27 Unit quiz
Section 1 of 5Genes, Alleles and Loci
Each gene has a specific position, or locus, on a chromosome.
Gene
- A length of DNA that codes for a single polypeptide or protein, and each chromosome carries hundreds or thousands of them.
Locus
- The fixed position of a gene on a chromosome, so the gene for a trait is always found at the same locus.
Allele
- One of the different forms of a gene, which has a slightly different nucleotide sequence but occupies the same locus.
Homologous pair
- Two chromosomes of the same size and shape that carry the same genes at the same loci, one from each parent.
Genetic maps
- Experiments and genetic mapping have made it possible to work out the positions of genes on chromosomes.
Linked loci
- Gene loci are said to be linked if they lie on the same chromosome.
Section 2 of 5Types of Linkage
Types of Linkage
Autosomal linkage
- Genes on the same autosome do not assort independently and tend to be inherited together as a group.
Effect of linkage
- Linked alleles stay together in their original parental combinations unless they are separated by crossing over.
Sex linkage
- Genes located on a sex chromosome, usually the X chromosome, show a different pattern in males and females.
Sex chromosomes
- Females have two X chromosomes (XX) whereas males have one X and one Y chromosome (XY).
| Feature | Autosomal linkage | Sex linkage |
|---|---|---|
| Chromosome | Autosome (not X or Y) | X or Y |
| Inheritance | Genes inherited together | Different in males and females |
| Independent assortment | No | Not for the genes on the same chromosome |
Section 3 of 5Consequences for Inheritance
1
In a dihybrid cross with genes on different chromosomes, alleles assort independently and four types of gamete are formed.
2
If the genes are on the same chromosome, the alleles on each chromosome travel together into the same gamete.
3
A heterozygous parent with alleles AB on one chromosome and ab on the other would mainly produce gametes AB and ab.
4
Crossing over in prophase I can break up linked alleles, producing a small number of recombinant gametes.
5
Linkage therefore produces ratios in the offspring that differ from the ones expected for independent assortment.
Section 4 of 5Worked Example
Question
- A plant heterozygous for two linked genes has alleles A and B together on one chromosome and a and b together on the homologous chromosome.
- It is crossed with a plant aabb.
- Predict the offspring if no crossing over occurs.
- The heterozygous parent can make only two gamete types if no crossing over occurs: AB and ab.
- The aabb parent makes only ab gametes.
- The offspring are AaBb and aabb, in equal numbers.
Answer Offspring are AaBb and aabb in a 1:1 ratio, not the 1:1:1:1 expected with independent assortment.
Section 5 of 5Exam Checklist & Tips
3.2.1 – Key Points for the Exam
- Gene A length of DNA that codes for one polypeptide.
- Locus The position of a gene on a chromosome.
- Allele An alternative form of a gene.
- Linkage Inheritance together of genes that are located on the same chromosome.
- Autosome A chromosome that is not a sex chromosome.
Common Mistakes
- Using the word gene when allele is meant, such as saying that two genes are on the same locus.
- Saying linked genes are always inherited together, when crossing over can separate them.
- Thinking that sex-linked genes are found only in sex cells, when they are in all cells.
- Saying that the locus is the allele; the locus is the position.
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