Linkage & Gene Mapping

Eukaryotic genomes consist of many genes, typically hundreds to thousands. However, most species have only a few dozen chromosomes.
As a result, each chromosome is likely to carry hundreds to thousands of different genes.

The arrangement of genes on chromosomes raises a question regarding Mendel’s law of independent assortment:
- Independent Assortment refers to the alleles of two different traits assorting into gametes and being inherited independently.
Examples to illustrate independent assortment:
- Example A:
- Gametes from the P generation seeds RRYY and rryy result in F1 generation seeds RrYy.
- Resulting haploid gametes: 1/4 RY, 1/4 Ry, 1/4 rY, 1/4 ry.
- Example B:
- Combining RRYY and rryy leads to RrYy offspring; the F2 generation includes various haploid gametes.

Independent Assortment: Alleles of different characters assort independently into gametes
- Example scenario with ratios (for simplicity, R = round, r = wrinkled, Y = yellow, y = green):
- Gametes: 1/4 RY, 1/4 Ry, 1/4 rY, 1/4 ry.

Linked Assortment: Alleles of different characters are inherited together (e.g., if R and Y are linked, they remain together in gametes).
Predictions under linked assortment scenario; offspring results show higher frequencies of parental types than recombinants.

Chi-Square tests can determine if genes are linked or assort independently:
- Correct Statements:
- An independent assortment hypothesis is not proposed as data suggest linkage.
- A large chi-square value indicates poor agreement between observed and expected data, leading to the rejection of the null hypothesis.

William Bateson and Reginald Punnett studied two traits: flower color and pollen shape, expecting simple Mendelian inheritance.
Results from their F2 generation show deviations from expected ratios, suggesting linked inheritance; e.g., observed ratios:
- Purple flowers, long pollen: 296 (expected 240)
- Much lower observed for other combinations (e.g., purple flowers, round pollen).

Crossing Over is the process that allows for independent assortment even when genes are on the same chromosome:
- Produces both non-recombinant and recombinant gametes, resulting in combinations not seen in parent chromosomes.
- Non-recombinant offspring inherit the same combination of alleles as parents; recombinant offspring arise from crossing over.

Completely Linked Genes: Expect no recombination, yielding all offspring resembling the parental phenotypes.
Probabilities of Crossing Over:
- Understand how crossing over probability declines as genes are physically closer on the same chromosome.

Thomas Hunt Morgan provides early evidence of gene linkage through studies on traits in Drosophila, focusing on body color, eye color, and wing length:
- Non-recombinant offspring appear more frequently than expected, indicating linkage.
- Some crossing over does occur, producing recombinant offspring, though frequencies differ (some recombinations are more common based on gene distance).

Recombination frequency reflects the genetic distance between linked genes, calculated with the formula:
- ext{Distance} ext{(cM)} = rac{ ext{Number of recombinant offspring}}{ ext{Total number of offspring}} imes 100
Example Calculation:
- If observing 150 recombinant offspring out of 500 total, distance = (150/500) imes 100 = 30 ext{cM} .

The maximum proportion of recombinant offspring for linked genes is 50%. Double crossovers may go undetected, leading to estimation errors in distance.
Accurate mapping requires recognizing double crossovers and their impact on counts.
Gene order and mapping can be determined through tri-hybrid crosses and phenotypic analysis of F2 generation outcomes;
- Common phenotypes suggest linkage, while rare phenotypes may indicate double crossover events.

Chromosome pairs may undergo crossing in several locations, complicating inheritance patterns.
Inheritance patterns can guide researchers in deducing the physical arrangement of linked genes along chromosomes, exceeding basic Mendelian ratios.