Mapping the Distance Between Genes Using Recombination Data

(frequency of crossovers) *see section 15.3, pages 299 - 304

Linked genes tend to be inherited together because they are located near each other on the same chromosome:

•  Each chromosome has hundreds or thousands of genes (except the Y chromosome)

•  Genes located on the same chromosome that tend to be inherited together are called linked genes

Mapping the Distance Between Genes Using Recombination Data: •  Alfred Sturtevant constructed a genetic map, an ordered list of

the genetic loci along a particular chromosome (not precise gene location, but relative)

•  Sturtevant predicted that the farther apart two genes are, the higher the probability that a crossover will occur between them and therefore the higher the recombination frequency

•  A linkage map is a genetic map of a chromosome based on recombination frequencies (crossovers)

•  Distances between genes can be expressed as map units; one map unit, or (centimorgan), represents a 1% recombination frequency

•  Map units indicate relative distance and order, not precise locations of genes

•  Genes that are far apart on the same chromosome can have a recombination frequency near 50%

•  Such genes are physically linked, but genetically unlinked, and behave as if found on different chromosomes because they are so far apart

F1 Generation

Meiosis

Metaphase I

Anaphase I

Metaphase II

All F1 plants produce yellow-round seeds (YyRr).

SEGREGATION: The two alleles for each gene (homologous chromosomes) separate independently during Anaphase I. e.g. each gamete has a 50:50 chance of receiving one of two alleles.

INDEPENDENT ASSORTMENT: Alleles of genes on non-homologous chromosomes align independently during Metaphase I. Genes separate into gametes independently. e.g. YR = Yr = yR = yr

Y

R R

R R

y y

Y r r

r r

Y Y

Y Y

Y Y

y y

y y

y y

R R

R R

r r

r r

r r r r Y Y Y Y

R R R R

2

y y y

y

2

1 1

YR 1 4 yr 1 4 Yr 1 4 yR 1 4