Linkage
Biology: Cambridge International A Level, topic 16.2.3
Autosomal Linkage
Linkage is when genes that are close to one another on a chromosome are likely to be inherited together. Linkage shows that some allele combinations are not inherited independently of each other.
Mendel's law
- Mendel's law of independent assortment states that genes do not influence the sorting of alleles into gametes. This is not always the case.
- Some allele combinations are not inherited independently of each other.
- Genes that are located close to each other on the same chromosome are more likely to be inherited as a pair.
- This is called linkage.
Autosomes
- Autosomes are all the chromosomes except the sex chromosomes (X and Y).
- Autosomes are arranged in pairs called homologous chromosomes (one from the father and one from the mother).
- Homologous chromosomes consist of the same genes in the same order along the chromosome.
- There is some variation if the chromosomes have different alleles but the genes are the same.
Recombination
- When gametes are produced by meiosis, multiple sections of homologous chromosomes are exchanged in a process called recombination.
- If two genes are located in close proximity on the same chromosome, they are more likely to be exchanged together and not separated in recombination.
- The genes are more likely to be transmitted to a gamete together.
Linkage
- Genes that are close together on the same autosome are more likely to be transmitted to a gamete together.
- The genes are linked. This is autosomal linkage.
Sex Linkage
Linkage is where genes that are close to one another on a chromosome are likely to be inherited together. Sex linkage is different from autosomal linkage because it takes place in the sex chromosomes.
Sex chromosomes
- In many organisms, the sex chromosomes (X and Y) determine the sex of an individual.
- Sex chromosomes differ from autosomes because they are non-homologous.
- Non-homologous chromosomes do not consist of the same genes in the same order along the chromosome.
Y chromosome
- Human females have a homologous pair of X chromosomes (XX) and human males are heterozygous (XY).
- X and Y chromosomes contain a small region of similarity.
- The Y chromosome is considerably shorter than the X chromosome and contains fewer genes.
- If a gene is only found on the X chromosome, it is X-linked.
X-linked genes
- Males only have one copy of an allele for X-linked genes.
- There is no allele for the same gene on the Y chromosome. This is called hemizygosity.
- Hemizygosity means that there is no role of dominance and recessiveness.
- If a recessive gene is inherited on the X chromosome, it will always be present in the phenotype as it is the only allele present.
Haemophilia
- Haemophilia is a blood disorder that is X-linked.
- Humans males only need to inherit one recessive mutant X allele to be affected by the disorder but females must inherit two copies of the recessive allele.
- This means haemophilia is more common in males.
- Females can be carriers for the disorder when they are heterozygous.
Test yourself on Linkage
Which of these are NOT true for autosomal linkage?
Which of the following statements about linkage are true?
How are sex chromosomes different from autosomes?
Which of the following statements are true for X-linked genes?
What is the term given to the fact there is no role of dominance and recessiveness in X-linked genes?