Genetics Chapter 1: Gene Linkage and Recombination

Gene Linkage and Recombination

• Genes linked together on the same chromosome usually assort together, but can become separated by recombination.
• The probability of recombination increases as the distance between two genes on the same chromosome increases.
• Recombination data can be used to generate maps of the relative locations of genes on chromosomes.

Detecting Linkage

• Syntenic genes are genes located on the same chromosome.
• Analyzing the progeny of dihybrid crosses can detect linkage between genes.
• X-linked genes in Drosophila, such as w+ (red eyes) and w (white eyes), and y+ (brown body) and y (yellow body), can be used to detect linkage.
• F1 males get their only X chromosome from their mothers, while F1 females are dihybrids.

Detecting Linkage (cont)

• Comparing allele configurations in F2 to P generation can detect linkage.
• Deviation from 1:1:1:1 segregation in F2 indicates that the genes are linked.
• Only the F2 male progeny are counted in this cross involving X-linked genes.

Designation of "Parental" and "Recombinant"

• The designation of "parental" and "recombinant" relates to past history.

Autosomal Genes and Linkage

• Autosomal genes can also exhibit linkage.
• Detecting linkage involves generating a double heterozygote and crossing to homozygous recessive (testcross).

Chi Square Test

• The Chi square test measures the probability that ratios are evidence of linkage.
• Deviations from 1:1:1:1 ratios can represent chance events or linkage.

Recombination during Meiosis I

• Recombination during meiosis I can be visualized by light microscopy.
• Prophase I involves the movement of chiasmata, and anaphase I involves the separation of chromosomes after chiasmata reach the telomeres.
• Two recombinant and two parental gametes are produced.

Properties of Linked versus Unlinked Genes

• Linked genes are located on the same chromosome, while unlinked genes are located on different chromosomes.

Genetic Maps

• Recombination frequencies (RF) can be used as a measure of physical distance between two linked genes.
• 1 percent recombination = 1 RF = 1 map unit (m.u.) = 1 centiMorgan (cM).

Mapping Genes by Comparisons of Two-Point Crosses

• Actual distance between genes may not add up, and the most accurate maps are obtained by summing many small intervening distances.
• It is difficult to determine gene order if two genes are close together.

Three-Point Crosses

• Three-point crosses provide faster and more accurate mapping.
• A testcross of triply-heterozygous F1 involves normal wings (vg+), vestigial wings (vg), black body (b+), gray body (b), red eyes (pr+), and purple eyes (pr).

Analyzing the Results of a Three-Point Cross

• Testcross progeny have four sets of pairs of genotypes.
• The most frequent pair has a parental configuration of alleles.
• The least frequent pair results from double crossovers.
• Examination of the double crossover class reveals which gene is in the middle.

Inferring the Location of Crossover Event

• Examine the numbers of progeny to infer the location of crossover event.
• Compare the configuration of alleles at two genes at a time to the parental configuration.

Inferring the Location of Crossover Events (cont)

• The location of crossover event can be inferred by analyzing the data.

Genetic Map Deduced from Three-Point Cross

• RF = Recombinants/Total offspring x 100.
• Correction for double crossovers is necessary for accurate calculation.

Correction for Double Crossovers

• The calculation that accounts for double crossovers is more accurate than the simple calculation.

Test your understanding of gene linkage, recombination, and gene mapping on chromosomes. Learn how genes are transmitted and how to analyze progeny to detect linkage. Explore the principles of genetics and how they apply to gene mapping.