Linked Genes
Biology: IB Diploma Higher Level, topic 15.1.2
Linked Genes
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.
- 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.
Linked genes - example
- Imagine a dihybrid cross involving flower color and plant height in which the genes are next to each other on the chromosome.
- If one homologous chromosome has alleles for tall plants and red flowers, and the other chromosome has genes for short plants and yellow flowers, then when the gametes are formed:
- The tall and red alleles will go together into a gamete.
- The short and yellow alleles will go into other gametes.
- These are called the parental genotypes because they have been inherited intact from the parents of the individual producing gametes.
Linked genes - example 2
- However, unlike if the genes were on different chromosomes, there will be no gametes with tall and yellow alleles and no gametes with short and red alleles.
- If you create the Punnett square with these gametes, you will see that the Mendelian prediction of a 9:3:3:1 outcome of a dihybrid cross would not apply.
- As the distance between two genes increases, the probability of one or more crossovers between them increases, and the genes behave more like they are on separate chromosomes.
Linked genes and distances
- Geneticists have used the proportion of recombinant gametes (the ones not like the parents) as a measure of how far apart genes are on a chromosome.
- Using this information, they have constructed elaborate maps of genes on chromosomes for well-studied organisms, including humans.
Test yourself on Linked Genes
Which of these are NOT true for autosomal linkage?
Which of the following statements about linkage are true?