Upload
others
View
4
Download
0
Embed Size (px)
Citation preview
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