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Gene Linking & Mapping: Definition & Example

Instructor: Emily Lockhart

Emily has taught science and has a master's degree in education.

This lesson explains gene linking. Gene linkage was first understood by Thomas Hunt Morgan. His experiments are used as examples to show how gene linkage can be used to understand gene mapping.

Gene Linking and Mapping

You have inherited your looks from a long and complicated lineage that tell the story of who you are and where you came from. Inheritance of all genes follows the principles of Mendelian genetics. It wasn't until certain genes began to disobey Mendel's rules, that the concept of linked genes was discovered. This lesson traces genes that are linked and explores mapping genes. In order to not get lost in the vocabulary of this lesson, it may benefit to brush up on the following topics. Having a solid background in Mendel's principles of segregation, and law of independent assortment will be necessary. It may also be a good idea to have a good foundation of chromosome structure and behavior, especially crossing over.

Genes That Don't Assort Independently

Without the knowledge of chromosomes, Mendel laid the groundwork for understanding inheritance of genes. Recall that genes are found on chromosomes, and homologous chromosomes segregate independently of one another during meiosis. Two different genes should end up in sex cells after meiosis in predicted ratios. However, sometimes certain genes end up together in higher frequencies. This phenomenon occurs when genes are close together, and on the same chromosome. Genes that are on the same chromosome are defined as linked.

All 23 pairs of chromosomes in humans. The dark bands represent different genes. Genes on the same chromosome are linked.
chromosomepairskaryotype

Linked genes travel together, and therefore end up together in the same sex cells after meiosis. It is similar to commuting together to work or school: you will arriving at the same time, at the same place. Genes are passengers, and if they are on the same bus, they will end up together.

Sometimes genes traveling on the same chromosome can end up switching to another homologous chromosome. Using the same illustration, it's similar to a passenger switching buses. This occurs due to the process of crossing over, and complicates linked inheritance. Crossing over results in recombination. Recombination means that the offspring have different combination of traits than their parents. Two genes on the same chromosome that have experienced crossing over will follow the laws of independent assortment again. It then must be stated that linked genes located closely together on chromosomes travel together. Linked genes farther apart on chromosomes may be subject to independently assort due to the phenomenon of crossing over.

While genes AB are initially linked together, after crossing over the two undergo recombination and will be inherited independently.
morgancrossingover

Recombinant and Nonrecombinant Offspring

Recombination of traits sets children apart from their parents. There will never be, nor has there ever been, another you. While the genes that make you do come from your parents, the traits are inherited in different combinations than your folks. Having new combinations of alleles in gametes is called recombinant gametes. Crossing over, and recombination of traits from two parents, leads to the many combinations of traits, making offspring unique from their parents. If progeny contains the same combination of alleles as the parent, it is called nonrecombinant.

Genes linked closely will not be mixed up, so they will be inherited exactly like the parent, and be nonrecombinant. By knowing the frequency of appearance of nonrecombinant and recombinant traits in offspring, the relative distance between genes on a chromosome can be calculated. Thomas Morgan first realized this in the fruit fly and mapped the genes on their chromosome II. Knowing the distance apart genes are on a chromosome allows scientist to sketch where genes are located, this is called gene mapping.

Gene Mapping

If genes separate with independent assortment you would expect to find a predicted ratio of offspring. If, say, an F1 dihybrid cross is bred with a double mutant, the results should be equal numbers of four different offspring.

The cross of independent assortment:

(F1 dihybrid) GgLl x ggll (double mutant)

  • In this lesson: Gray bodied (G) Black bodied (g) Long wing (L) short wing (l)

Offspring are 1GgLl: 1ggll: 1Ggll 1: ggLl.

Two offspring have the same identical genetics as the parents (GgLl and ggll). Two have unique combinations (Ggll and ggLl.). In other words, half the offspring are nonrecombinant and the other half is recombinant.

If the genes are very closely linked, and located close together, the ratios will not follow this test, but instead have all nonrecombinant offspring.

The cross if genes are very close together:

GgLl x ggll

Offspring are all GgLl or ggll

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