Genetic linkage: Difference between revisions
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=== Principle === | === Principle === | ||
In backcross AaBb x aabb, if A and B genes segregate independently, The double heterozygote parent makes 4 gamete types AB, aB, Ab and ab, while the homozygous parent makes only ab gametes. Therefore offspring genotypes are AaBb, aaBb, Aabb and aabb. That makes 4 phenotypic groups in case of complete dominance, incomplete dominance and codominance, with equal probability 25% each, segregation ratio is 1:1:1:1. If A and B are localized on a single chromosome, and heterozygote parent AaBb is AB/ab (haplotypes “AB” and “ab”, cis-phase of linkage), then AaBb and aabb offspring groups will be more populated compared to Aabb and aaBb groups, which require a crossing over in AB/ab parent for their formation. These less common individuals are called recombinants. In trans-phase of linkage AaBb = Ab/aB and the situation is opposite compared to cis (AaBb and aabb are recombinants and they are less numerous). See Fig. 1). | In backcross AaBb x aabb, if A and B genes segregate independently, The double heterozygote parent makes 4 gamete types AB, aB, Ab and ab, while the homozygous parent makes only ab gametes. Therefore offspring genotypes are AaBb, aaBb, Aabb and aabb. That makes 4 phenotypic groups in case of complete dominance, incomplete dominance and codominance, with equal probability 25% each, segregation ratio is 1:1:1:1. If A and B are localized on a single chromosome, and heterozygote parent AaBb is AB/ab (haplotypes “AB” and “ab”, cis-phase of linkage), then AaBb and aabb offspring groups will be more populated compared to Aabb and aaBb groups, which require a crossing over in AB/ab parent for their formation. These less common individuals are called recombinants. In trans-phase of linkage AaBb = Ab/aB and the situation is opposite compared to cis (AaBb and aabb are recombinants and they are less numerous). See Fig. 1). | ||
[[File:Geneticlinkage.png|thumbnail|Figure 1. All possible outcomes of a dihybrid backross regarding the possible relationship between the two genes. Green bars under the chromosomes show percentage of each genotype/phenotypes (for incomplete linkage, the percentage can be anywhere between complete linkage and no linkage). Gray regions on the chromosomes stress that with the analysis of two genes, it is impossible to say where exactly the crossing over took place, or whether there was one or more crossovers. ]] | |||
Revision as of 08:53, 11 July 2017
Definition
Genetic linkage is a phenomenon where certain alleles of different genes do not conform to the Mendel´s law of independent segregation because they are localized on the same chromosome, not very far from each other.
History
Linkage was discovered by Bateson and Punnett in 19XX in sweet pea (Lathyrus odoratus) , but explained first by T. H. Morgan using fruitflies (see lecture el.lf1.cuni.cz/linkage).
Principle
In backcross AaBb x aabb, if A and B genes segregate independently, The double heterozygote parent makes 4 gamete types AB, aB, Ab and ab, while the homozygous parent makes only ab gametes. Therefore offspring genotypes are AaBb, aaBb, Aabb and aabb. That makes 4 phenotypic groups in case of complete dominance, incomplete dominance and codominance, with equal probability 25% each, segregation ratio is 1:1:1:1. If A and B are localized on a single chromosome, and heterozygote parent AaBb is AB/ab (haplotypes “AB” and “ab”, cis-phase of linkage), then AaBb and aabb offspring groups will be more populated compared to Aabb and aaBb groups, which require a crossing over in AB/ab parent for their formation. These less common individuals are called recombinants. In trans-phase of linkage AaBb = Ab/aB and the situation is opposite compared to cis (AaBb and aabb are recombinants and they are less numerous). See Fig. 1).
