Genetics is the study of how certain traits or characteristics are inherited from parents to offspring. You’ve probably noticed that children often share similar features with their biological mom and dad. This is because information controlling things like eye color, height, and risk for certain illnesses is passed down through generations in the form of DNA. The sections of DNA that determine specific traits are called genes.
In the early 1900s, a scientist named Thomas Hunt Morgan was studying inheritance in fruit flies. He was looking at how different physical characteristics were passed on from parent flies to their babies. Morgan’s experiments showed that some traits tended to be inherited together. For example, flies with red eyes also frequently had short wings. This meant that the genes for these two traits were somehow connected.
Morgan realized that genes located close together on the same chromosome tend to stick together when cells divide. He called this “linkage.” Genes that are far apart on a chromosome or located on different chromosomes are not linked. They assort independently and randomly. Morgan’s findings helped overturn the idea that genes behave independently. It led to the concept of linkage groups, which are sets of genes on the same chromosome that are usually inherited as a unit.
Linkage maps (also called genetic maps) illustrate the position and relative distances between genes along a chromosome. They allow scientists to study inheritance patterns and track the locations of genes linked to diseases. The process of constructing these maps is done through analyzing the results of genetic crosses. Let’s break down the key steps:
- Study the inheritance of genes with known, visible traits like flower color or wing shape.
- Track which versions of the genes (known as alleles) are passed from parents to offspring.
- Calculate recombination frequencies – the percentage of offspring that are recombinants. Recombinants have a new combination of alleles compared to the parents.
- Use recombination frequencies to calculate map distances in units called centimorgans. One centimorgan equals a 1% chance that a marker will separate from a gene due to crossing over in cell division.
- Arrange the genes relative to each other based on lowest to highest map distances. Closer genes have lower recombination frequencies.
For example, let’s say Gene A and Gene B are located on the same chromosome. In one cross, 28% of offspring show recombination between these two genes. This means of the 100 flies in the study, 28 had a new combination of Gene A and Gene B compared to their parents. So the recombination frequency is 28%. To convert to map units, we calculate:
Map Distance = Recombination Frequency x 100 Map Distance between Gene A and Gene B = 28 x 100 = 28 centimorgans
This process is known as two-point mapping. It gives the distance between two specific genes. Three-point mapping adds a third gene to get a more complete picture. The recombination frequencies between each pair of genes is used to determine gene order and precise locations. Linkage maps created from many crosses become detailed representations of entire chromosomes.
Scientists use linkage maps to better understand inheritance and identify disease-causing genes in families. Breeders apply linkage data to track down genes involved in desirable traits. And linkage relationships provide a framework for assembling DNA sequence maps of whole genomes, including the human genome.
While constructing maps takes work, the principles of linkage and recombination are straightforward:
- Linked genes on the same chromosome tend to be inherited together. They remain connected unless crossover occurs.
- Recombination events (crossing over) can reshuffle genes, giving offspring new combinations of traits.
- The frequency of recombination between two genes provides a measure of the distance separating them.
- Multi-point gene mapping reveals precise locations and order of genes.
- Completed linkage maps provide a big picture view of all the genes on a chromosome.
Now let’s walk through some practice problems related to linkage mapping:
Practice Problem 1
In fruit flies, Gene A controls eye color and Gene B controls wing size. When a pure-breeding strain with normal eyes and long wings is crossed with a pure-breeding strain with mutant eyes and short wings, the following offspring resulted:
Normal eyes, long wings – 651 Mutant eyes, short wings – 108
Normal eyes, short wings – 140 Mutant eyes, long wings – 101
a) What is the recombination frequency between Gene A and Gene B? b) What is the map distance in centimorgans?
To solve: a) The two classes of offspring with recombined traits are those with normal eyes, short wings and mutant eyes, long wings. There are 140 + 101 = 241 recombinants. The total number of flies is 651 + 108 + 140 + 101 = 1000. So the recombination frequency is 241/1000 = 0.241.
b) Map Distance = Recombination Frequency x 100 Map Distance = 0.241 x 100 = 24.1 centimorgans
The map distance between Gene A and Gene B is 24.1 centimorgans.
Practice Problem 2
Three genes are located on the same chromosome in tomatoes. Gene C controls fruit color, Gene D controls plant height, and Gene E controls leaf shape. A study found the following recombination frequencies:
C and D – 11% C and E – 5% D and E – 17%
Create a linkage map showing the relative positions of these three genes.
First we need to use the recombination frequencies to determine gene order. The lowest frequency is between Genes C and E, so these must be closest together on the chromosome. The highest frequency is between Genes D and E, so these are furthest apart. Therefore, the gene order is C – E – D.
Next we convert recombination frequencies to map distances:
C and D = 11 centimorgans C and E = 5 centimorgans
D and E = 17 centimorgans
Then we can draw the linkage map:
C-------5 cM-------E-------12 cM--------D
In this map, there are 5 centimorgans between Genes C and E, and 12 centimorgans between Genes E and D, for a total distance of 17 centimorgans between Genes C and D.
Constructing accurate linkage maps requires analyzing multiple crosses and pedigree data. But these two examples demonstrate the basic process of using recombination frequencies to determine gene order and distances. Linkage mapping has been crucial for untangling inheritance patterns and connecting genes to traits. The ability to literally map out the locations of genes on chromosomes has revolutionized our understanding of genetics over the past century.