Figure 6.8: Polygenic traits tend to result in a distribution that resembles a bell-shaped
curve, with few at the extremes and most in the middle. There may be 4 or 6 or more
alleles involved in the phenotype. At the left extreme, individuals are completely dominant
for all alleles, and at the other extreme, individuals are completely recessive for all alleles.
Individuals in the middle have various combinations of recessive and dominant alleles. ( 7 )
Linkage
Linkagerefers to particular genetic position orloci, or alleles inherited together, suggesting
thattheyarephysicallyonthesamechromosome, andlocatedclosetogetheronthatchromo-
some. A crossing-over event during prophase I of meiosis is rare between linked loci. Alleles
for genes on different chromosomes are not linked; they sort independently (independent
assortment) of each other during meiosis.
Ageneisalsosaidtobelinkedtoachromosomeifitisphysicallylocatedonthatchromosome.
For example, a gene (or loci) is said to be linked to the X-chromosome if it is physically
located on the X-chromosome chromosome.
Linkage Maps
The frequency of recombination refers to the rate of crossing-over (recombination) events
between two loci. This frequency can be used to estimate genetic distances between the two
loci, and create alinkage map. In other words, the frequency can be used to estimate how
close or how far apart the two loci are on the chromosome.
In the early 20thcentury, Thomas Hunt Morgan demonstrated that the amount of crossing
over between linked genes differs. This led to the idea that the frequency of crossover events
would indicate the distance separating genes on a chromosome. Morgan’s student, Alfred
Sturtevant, developed the first genetic map, also called a linkage map.