Chapter 7 – Linkage and Chromosome Mapping
• Genetic Recombination results from physical exchanges between homologous chromosomes in
meiosis. A chiasma is the site of crossing-over where this reciprocal exchange takes place.
• Map distance between genes is measure in map units (mu). 1 mu is the interval in which 1
percent of crossing-over takes place.
• As the distance between genes increases, the incidence of multiple crossovers causes the
recombination frequency to be an underestimate of the true map distance.
• Genes that do not assort independently are usually located on the same chromosome and
exhibit what is called linkage and are called linked genes.
1. The parental classes are the most frequent phenotypes observed.
2. Approximately equal numbers of the two parental classes are observed. Similarly, equal
numbers of the two recombinant classes are also observed.
3. The closer two genes are on the chromosome, the less likely that there will be a recombination
event between them.
• A chiasma is the place on the homologous pair of chromosomes where the physical exchange
takes place…the site of crossing over.
• Crossing over is a reciprocal exchange (usually) at positions along the homologues. The
chromosomes are broken and rejoined in the process.
• Crossing over occurs at the four-chromatid stage in prophase I of meiosis. Each crossover
involves only two chromatids, although multiple crossovers can occur along the whole length of
• Chi-square analysis is used to analyze testcross data and determine whether a deviation is
– A null hypothesis (“the genes are NOT linked”) is used because it is not possible to
predict phenotype frequencies produced by linked genes.
• If two genes are not linked, a testcross should yield a 1:1 ratio of parentals:recombinants.
• = ∑ d /e (d=deviation value=(obs-exp))