Linkage Part 1 (Medium)

Meiosis and Crossing Over

• Crossing over occurs during Prophase of Meiosis I, where non-sister chromatids of homologous chromosomes exchange DNA segments.
• Replicated sister chromatids (homologues) associate as bivalents.
• Crossing over can produce recombinant genotypes.

Recombinant Cells and Offspring

• Haploid cells containing a combination of alleles not found in the original chromosomes are called non-parental or recombinant cells.
• Recombinant offspring are produced by the exchange of DNA between two homologous chromosomes during meiosis in one or both parents.
• Recombinant offspring have a novel combination of genetic material due to a crossover event during gamete formation in (at least) one of their parents.

Morgan's Experiments

• Thomas Hunt Morgan's experiments provided the first direct evidence of linkage in Drosophila.
• He investigated three traits: body color, eye color, and wing length, which showed a non-parental combination of traits.
• Morgan observed a higher proportion of trait combinations found in the parental generation, suggesting that the genes are located on the X chromosome and tend to be transmitted together as a unit.

Linkage and Synteny

• Linkage refers to the phenomenon where genes close together on a chromosome tend to be transmitted as a unit, influencing inheritance patterns.
• Synteny means two or more genes are located on the same chromosome and are physically linked.
• Chromosomes are called linkage groups, containing a group of genes that are linked together.
• The number of linkage groups is equal to the number of types of chromosomes in a species.

Bateson and Punnett's Cross

• Bateson and Punnett's cross involving two traits (flower color and pollen shape) did not yield the expected 9:3:3:1 phenotypic ratio.
• This suggests that independent assortment does not always occur.

Altering Linkage

• Janssens proposed that crossing over involves a physical exchange between homologous chromosomes, which Morgan realized was consistent with his data.
• Morgan assumed crossing over did not occur between the X and Y chromosomes.
• The three genes studied by Morgan were not found on the Y chromosome.

Morgan's Hypotheses

• Morgan hypothesized that the genes for body color, eye color, and wing length are located on the X-chromosome.
• He proposed that alleles will tend to be inherited together due to crossing over.
• The likelihood of crossing over depends on the distance between the two genes, with crossing over more likely to occur between genes that are far apart.