It means that the genes are linked, it seeks to determine the frequency of recombination for a pair of genes.
DNA mapping refers to the variety of different methods that can be used to describe the positions of genes. The maps DNA may show different levels of detail, similar to topological maps of a country or city, to indicate how far two genes are to each other.
At low resolution, DNA can be mapped to the level of banding patterns that roughly show the distance between two genes after a chromosome has been stained with dye. Genetic mapping or linkage mapping can be used to indicate the relative order of genes on a chromosome.
A restriction map is another type of DNA map that roughly describes the relative positions of genes by separating sections of DNA at places known as restriction sites.
A physical DNA map describes the absolute position of genes on a chromosome. Physical maps can be constructed by breaking a section of DNA, a chromosome, or an entire genome into smaller pieces.
These overlapping DNA fragments can be cloned or copied. Then, sections of the DNA can be used to obtain nucleotide sequences and establish the precise locations of the genes.
When genes are on different chromosomes or widely separated on the same chromosome, they are classified independently and are said to be unlinked.
When genes are close together on the same chromosome, they are said to be related. That means that alleles, or versions of genes, already put together on a chromosome will be inherited as a unit.
We can see if two genes are linked, and how strongly, by using data from genetic crossovers to calculate the frequency of recombination.
By finding recombination frequencies for many pairs of genes, we can make linkage maps that show the order and relative distances of genes on the chromosome.
In general, organisms have many more genes than chromosomes. For example, we humans have about 19,000 genes on 23 chromosomes (present in two sets).
Similarly, the humble fruit fly, a favorite subject of study for geneticists, has around 130,000 genes on 4 chromosomes (also present in two sets).
The consequence? Each gene is not going to get its own chromosome, in fact, not even close. Quite a few genes will line up in a row on each chromosome, and some of them will be squashed close together.
Does this affect the way genes are inherited?
In some cases, the answer is yes. Genes that are closely linked together on a chromosome will tend to “stick together,” and the versions (alleles) of those genes that are together on one chromosome will tend to be inherited as a pair most of the time.
This phenomenon is called genetic linkage. When genes are linked, genetic crosses involving those genes will result in ratios of gametes (eggs and sperm) and offspring types that are not what we would predict from Mendel’s law of independent assortment.
What is the genetic link?
When genes are on separate chromosomes, or widely separated on the same chromosomes, they are classified independently. That is, when genes go to gametes, the allele received by one gene does not affect the allele received by the other.
In a double heterozygous organism (AaBb), it results in the formation of all possible types of gametes with equal or 25% frequency.
Why is this the case? Genes on separate chromosomes are classified independently due to the random orientation of homologous chromosome pairs during meiosis.
Homologous chromosomes are pairs of chromosomes that carry the same genes, but may have different alleles for those genes. One member of each homologous pair comes from the mother of an organism and the other from its father.
The homologues of each pair separate in the first stage of meiosis. In this process, which side the “dad” and “mom” chromosomes of each pair go is random. When we follow two genes, this results in four types of gametes that occur with the same frequency.
When genes are on the same chromosome but widely separated, they are classified independently due to crossover (homologous recombination). This is a process that occurs early in meiosis, in which homologous chromosomes randomly exchange matching fragments.
Crossing can bring new alleles together in combination on the same chromosome, causing them to enter the same gamete. When the genes are widely separated, the crossing occurs frequently enough that all types of gametes occur 25% frequently.
When genes are close together on the same chromosome, they still cross, but the result (in terms of the types of gametes produced) is different.
Rather than classify independently, genes tend to “stick together” during meiosis. That is, the alleles of genes that are already together on a chromosome will tend to be passed as a unit to the gametes. In this case, the genes are linked.
Common types of gametes contain parental configurations of alleles, that is, those that were already together on the chromosome in the body before meiosis (that is, on the chromosome that it obtained from its parents).
Rare types of gametes contain recombinant allele configurations, that is, ones that can only form if a recombination event (crossover) occurs between genes.
Why are recombinant gamete types rare?
The basic reason is that crosses between two genes that are close together are not very common. Crosses during meiosis occur at more or less random positions along the chromosome, so the frequency of crossovers between two genes depends on the distance between them.
A very short distance is in fact a very small “target” for crossover events, meaning that few of these events will take place (compared to the number of events between two more widely separated genes).
Thanks to this relationship, we can use the frequency of recombination events between two genes (that is, their degree of genetic linkage) to estimate their relative separation distance on the chromosome.
Two genes that are very close together will have very few recombination events and will be tightly linked, while two genes that are slightly further apart will have more recombination events and will be less closely linked.