Amacher Lecture 5 (9/12/08)
LECTURE 5: LINKAGE AND GENETIC MAPPING
Reading for this and previous lecture: Ch. 5, p 123-141
Problems for this and previous lecture: Ch. 5, solved problems I, II; #2 – 5, 7 – 9, 12, 14, 15,
20, 21, 23, 24 and 27
Recombination results when crossing over during meiosis separates linked genes
Reciprocal exchanges between homologous chromosomes are the physical basis of
recombination. Using chromosomes that had cytologically visible abnormalities, Creighton and
McClintock working with maize, and Stern, working with Drosophila, showed that
recombination depends upon the physical exchange of equal parts between maternal and paternal
chromosomes during meiosis. Both groups followed chromosomes that were physically marked
with cytologically visible abnormalities, so that a maternally-derived homolog could be easily
distinguished from the paternal one. The marked chromosomes also carried mutations in order to
monitor which progeny were the result of recombination.
Your book covers Stern’s experiments; we’ll review Creighton and McClintock’s study. They
used two different forms of Chromosome 9 in their experiment. One form was normal, and the
other had two cytological abnormalities, a heterochromatic knob at one end and a piece of
chromosome 8 fused at the other end (a translocation, which you’ll hear more about later in the
course). In addition to these physical differences, these two chromosomes were genetically
marked to detect recombination events. One marker gene controlled kernel color (C, colored; c,
colorless) and the other controlled kernal starch metabolism (Wx, starchy; wx, waxy). They set
up the following cross (* = knob, ~ = translocation):
* C Wx ~ / c wx x c wx/ c wx
All the parental types resulting from this cross retained the parental type chromosomal cytology.
The chromosomes of the recombinant progeny (C wx and c Wx) were then examined. Creighton
and McClintock showed that each recombinant carried only one of the cytological abnormalities
( * C wx and c Wx ~), indicating that physical exchange between the cytologically marked and
normal chromosome 9 homologs had occurred in the previous generation. Creighton and
McClintock’s paper ended with the statement “The foregoing evidence points to the fact that
cytological crossing-over occurs and is accompanied by the expected types of genetic cross-over.
Recombination frequencies for pairs of genes reflect the distances between them along a
chromosome.
Since genes are arranged linearly on chromosomes, Morgan proposed that different gene pairs
exhibited different linkage rates because the closer together two genes are the less likely they
will be separated by a recombination event. That is the probability of a crossover occurring
between two genes increases with the distance separating them. Sturtevant, an undergraduate in
Morgan’s lab, suggested that recombination frequency could be used to gauge the physical
distance between two genes: 1% RF = 1 cM = 1 map unit. Recombination frequency = #
recombinants/total progeny x 100.
