Concepts Summary

• Linked genes do not assort independently. In a testcross for two completely linked genes (no crossing over), only nonrecombinant progeny are produced. When two genes assort independently, recombinant progeny and nonrecombinant progeny are produced in equal proportions. When two genes are linked with some crossing over between them, more nonrecombinant progeny than recombinant progeny are produced.
• Recombination frequency is calculated by summing the number of recombinant progeny, dividing by the total number of progeny produced in the cross, and multiplying by 100%. The recombination frequency is half the frequency of crossing over, and the maximum frequency of recombinant gametes is 50%.
• Coupling and repulsion refer to the arrangement of alleles on a chromosome. Whether genes are in coupling or in repulsion determines which combination of phenotypes will be most frequent in the progeny of a testcross.
• Interchromosomal recombination takes place among genes located on different chromosomes through the random segregation of chromosomes in meiosis. Intrachromosomal recombination takes place among genes located on the same chromosome through crossing over.
• A chi-square test of independence can be used to determine whether genes are linked.
• Recombination rates can be used to determine the relative order of genes and distances between them on a chromosome. One percent recombination equals one map unit. Maps based on recombination rates are called genetic maps; maps based on physical distances are called physical maps.
• Genetic maps can be constructed by examining recombination rates from a series of two-point crosses or by examining the progeny of a three-point testcross.
• Some multiple crossovers go undetected; thus, genetic maps based on recombination rates underestimate the true physical distances between genes.
• Human genes can be mapped by examining the cosegregation of traits in pedigrees.
• A lod score is the logarithm of the ratio of the probability of obtaining the observed progeny with the assumption of linkage to the probability of obtaining the observed progeny with the assumption of independent assortment. A lod score of 3 or higher is usually considered evidence for linkage.
• Molecular techniques that allow the detection of variable differences in DNA sequence have greatly facilitated gene mapping.
• Genomewide association studies locate genes that affect particular traits by examining the nonrandom association of a trait with genetic markers from across the genome.
• Nucleotide sequencing is another method of physically mapping genes.
• Rates of recombination vary widely, differing among species, among and along chromosomes within a single species, and even between males and females of the same species.