Linked Genes Definition
The genes that are likely to be inherited together are called linked genes. These genes are physically close to one another and located on the same chromosome thus they inherit together in the offspring.
The chromosomes are recombined during meiosis that results in gene swaps between homologous chromosomes. The chances of being recombines are higher, if the genes are close together. The linked genes cannot be located in different chromosomes.
Homologous Recombination
During meiosis, the chromosomes are recombined in which the chromosomes can be cut at random points and combined with another copy of a homologous chromosome, which also has cut at the same point. Thus the DNA comprises another homologous chromosome at the end.
A chromosome can contain hundreds or thousands of genes that code for different traits. All organism that reproduce by sexual methods have two copy of each chromosome, in which both the copies have same genes for the same traits, such as hair color.
Different alleles are present in each copy of chromosomes for a particular trait. For example: an allele may code for black hair and other copy for brown hair. The homologous chromosomes are the chromosomes that have same genes but may have different alleles.
Gametes including sperm and egg have only one copy of each chromosome. The cell undergo meiosis to produce haploid gametes that results in production of daughter cells with only one copy of chromosomes.
The cell then makes an identical copy of each copy and form two pairs of homologous chromosomes. The sister chromatids are the identical copies of chromosomes thus total four pairs of sister chromatids are present.
All the pairs of sister chromatids or homologous chromosomes aligns in a row during recombination. Then the recombination takes place, in which the DNA are cut and joined.
The swapping of a chunk of DNA between homologous chromosomes results in a formation of new combination of alleles. The DNA cut at random sites and form new gametes every time when meiosis occurs. The random cuts leads to numerous possible combination.
The cell divides into two in the next step and each daughter cell receives two copies of each chromosome carrying different alleles and also the recombinant DNA. The last step includes the division of cells without replication that results in forming four daughter cells, each with one copy of chromosome.
Recombination of Linked Genes
To describe linked genes the description of homologous recombination is very important. Now we know that recombination is a process of cut a DNA at random site and join with another homologous chromosome. Now, we learn about inheritance of linked genes.
Let’s understand it with the example of freckles and red hair. Many peoples have red and freckles hairs. The condition occur more often that it would by chance therefore many red-haired people have freckles.
The reason behind is close location of genes coding for freckles and red hair is the same chromosome. During homologous recombination, there are a few chances that the DNA will cut in between the two genes. Thus they do not separate and inherited together. The low chances for the split up of these two genes increase the probability of their inheritance together most of the time.
Gene Maps
Based on the frequencies of recombination, researchers have develop various methods to find the location of genes. To find this, the offspring of an organism named fruit fly or Drosophila melanogaster is analysed. The results show that the offspring will inherit the four alleles in equal percentage (25%) if two genes are located on different chromosomes.
In this condition, allele A of chromosome 1 and allele A of chromosome 2 will be inherited in 25%, and allele B of chromosome 1 and allele A of chromosome 2 will be inherited in 25%, other 25% will inherit allele A of chromosome 1 and allele B of chromosome 2, and 25% will inherit allele B of chromosome 1 and allele B of chromosome 2.
It is calculated in terms of the alleles shared with the parent organism. As result, 50% would inherit the parental genes and other 50% would inherit a combination of the parental genes. On the other hand, the offspring inherit four alleles in different percentages if two different genes sit on the same chromosomes.
In this condition the more than 50% offspring inherit the parental genes, whereas less than 50% offspring inherit combination of the parental genes. The genes are on the same chromosomes but far apart, if the percentage is close to expected 50%.
However, it is more likely that the genes will separate at the time of recombination. The genes sit very close together on the chromosome, if the percentage of the offspring having combined parental genes is very low and here, there chances of separation during recombination are also very low.
Linked Genes Citations
- X- and Y-Linked Chromatin-Modifying Genes as Regulators of Sex-Specific Cancer Incidence and Prognosis. Clin Cancer Res . 2020 Nov 1;26(21):5567-5578.
- X-linked diseases: susceptible females. Genet Med . 2020 Jul;22(7):1156-1174.
- The X-linked retinopathies: Physiological insights, pathogenic mechanisms, phenotypic features and novel therapies. Prog Retin Eye Res . 2021 May;82:100898.
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