Linkage and Crossing Over in Genetics
Linkage and crossing over are two important concepts in genetics that explain how genes are inherited and how genetic variation arises. Let’s break them down:
1. Linkage
Linkage refers to the phenomenon where genes that are located close together on the same chromosome tend to be inherited together. This happens because genes on the same chromosome do not assort independently during meiosis (cell division for gamete formation).
Why Does Linkage Occur?
- In Mendel’s Law of Independent Assortment, genes located on different chromosomes assort independently of one another during meiosis.
- However, genes that are physically close to each other on the same chromosome do not follow this rule. Instead, they are more likely to be inherited together as a linked group.
- The closer two genes are to each other on the chromosome, the less likely it is that they will be separated by crossing over during meiosis. As a result, they are more likely to be inherited together.
Example of Linkage
Imagine two genes:
- Gene A for flower color: A (dominant red), a (recessive white).
- Gene B for plant height: B (dominant tall), b (recessive short).
If Gene A and Gene B are located on the same chromosome and are close to each other, they are said to be linked. In this case, the inheritance of A and B together (and a and b together) is more likely than if they were located on different chromosomes.
2. Crossing Over
Crossing over is the process during meiosis where homologous chromosomes exchange segments of their chromatids. This occurs during prophase I of meiosis and leads to genetic recombination. Crossing over increases genetic diversity by producing new combinations of alleles that were not present in the parents.
How Crossing Over Works
- Homologous chromosomes pair up during meiosis and form structures called tetrads.
- At this point, the chromatids of the homologous chromosomes can physically overlap and exchange corresponding segments. This is known as crossing over.
- The result of crossing over is that new combinations of alleles are formed on the chromosomes, leading to genetic variation in the offspring.
Example of Crossing Over
If we have two homologous chromosomes in a tetrad:
- Chromosome 1 (from the mother): AB (where A = red flower color, B = tall plant)
- Chromosome 2 (from the father): ab (where a = white flower color, b = short plant)
After crossing over, the chromatids might exchange parts, creating new combinations of alleles, such as:
- Ab (from the mother’s chromosome 1)
- aB (from the father’s chromosome 2)
These new combinations of alleles (Ab and aB) are different from the original combinations (AB and ab). This recombination increases genetic diversity in the offspring.
Linkage and Crossing Over Together
Linkage and crossing over are related concepts:
- Linkage refers to genes that are inherited together because they are on the same chromosome.
- Crossing over can separate linked genes if they are far enough apart on the chromosome. This results in the creation of new combinations of alleles.
- The further apart two genes are on a chromosome, the more likely they are to be separated by crossing over, which reduces the degree of linkage. Conversely, genes that are close together have a higher chance of being inherited together.
Genetic Mapping Using Linkage and Crossing Over
- Geneticists can use the frequency of crossing over between genes to estimate the distance between them on the chromosome.
- If genes are close together, crossing over happens less frequently between them, and the genes are considered tightly linked.
- If genes are far apart, crossing over happens more frequently, and the genes are considered loosely linked.
Example of Linkage and Crossing Over in Fruit Flies (Drosophila)
In fruit flies, the genes for body color (B for black, b for gray) and wing shape (V for normal wings, v for vestigial wings) are linked on the same chromosome.
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Parental Genotypes:
- One parent has the genotype BBVV (black body, normal wings).
- The other parent has the genotype bbvv (gray body, vestigial wings).
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F1 Generation:
- All F1 offspring would have the genotype BbVv (black body, normal wings), as these alleles are inherited from both parents.
Now, during meiosis in the F1 generation, crossing over may occur between the linked genes for body color and wing shape. As a result, new allele combinations such as BV and bv might form. Some of the offspring might have the original parental combinations (BV and bv), but others might have recombinant combinations due to crossing over.
- Recombinant Offspring:
- Bv (black body, vestigial wings)
- bV (gray body, normal wings)
The frequency of these recombinant offspring depends on the distance between the linked genes. The greater the distance between two genes on the chromosome, the higher the chance they will be separated by crossing over, leading to a higher frequency of recombination.