Gene mapping | Biomolecules | MCAT | Khan Academy

Genetic Recombination Basics

• Genetic recombination allows determination of the distance between genes on chromosomes.

• Mapping chromosomes can pinpoint exact gene locations.

Chromosome Structure

• Homologous Chromosomes: A pair of chromosomes from different parents (e.g., orange/paternal and yellow/maternal).

• Sister Chromatids: Identical chromatids on the same chromosome, carrying identical genes.

• Chromosomes appear as X shapes following replication, consisting of two sister chromatids joined at a centromere.

• A chromosome is counted by the number of centromeres present.

Gene Representation

• Genes are represented by colored circles on the chromatids:

  • Green Genes: Represent complexion traits (e.g. dark vs. light complexion).

  • Purple Genes: Represent hair color traits.

  • Blue Genes: Represent eye color traits.

Genetic Recombination Process

• Genetic recombination typically occurs between homologous chromosomes—not sister chromatids (identical).

• Chromatids exchange segments of genetic material, leading to new allele combinations:

  • Example changes occur by swapping segments at various points (e.g., top, middle, bottom).

• The proximity of genes affects the likelihood of recombination.

Distance and Recombination Frequency

• Gene Proximity:

  • Greater distances between genes increase the likelihood of recombination.

  • Closer genes are less likely to recombine.

• Recombination is more probable between distant genes, such as purple and green genes, compared to closer genes like blue and purple genes.

Centimorgans (cM)

• Centimorgan (cM): A unit for measuring the distance between genes; also known as a map unit (m.u.).

• Defined as the distance over which there is a 1% chance that recombination occurs during meiosis.

Examples of Recombination

• Example 1:

  • If two genes are 25 map units apart, recombination occurs 25% of the time (1/4).

  • A visualization often includes identified chromatids swapping sections to illustrate recombination.

• Example 2:

  • If genes are 6 map units apart, recombination occurs 6% of the time (6/100).

Mapping Genes through Statistical Analysis

• Statistical analysis of recombination frequency can help map genes on a chromosome.

• This allows for a clearer understanding of gene locations and relationships on chromosomes.