1) Genetic linkage refers to genes located near each other on a chromosome being inherited together during meiosis. Closely located genes are less likely to separate onto different chromatids during crossover. 2) Thomas Hunt Morgan expanded understanding of genetic linkage by observing that crossover frequency between linked genes could indicate their distance on a chromosome. 3) Alfred Sturtevant developed the first genetic maps by using crossover frequency to determine the distance between linked genes in centimorgans.

1. CRAMMER
TALK - 1
gene mapping

2. Genetic linkage is the tendency of genes
that are located proximal to each other on
a chromosome to be inherited together
during meiosis.
Genes whose loci are nearer to each other
are less likely to be separated onto
different chromatids during chromosomal
crossover, and are therefore said to be
genetically linked.

3.  Genetic linkage was first
discovered by the British geneticists
William Bateson, Edith Rebecca
Saunders and Reginald Punnett
shortly after Mendel's laws were
rediscovered .

4. The understanding of genetic linkage
was expanded by the work of Thomas
Hunt Morgan. Morgan's observation that
the amount of crossing over between
linked genes differs led to the idea that
crossover frequency might indicate the
distance separating genes on the
chromosome.

5.  Alfred Sturtevant, a student of Morgan's,
first developed genetic maps, also known as
linkage maps.
Sturtevant proposed that the greater the
distance between linked genes, the greater
the chance that non-sister chromatids would
cross over in the region between the genes.

6.  By working out the number of
recombinants it is possible to obtain a
measure for the distance between the
genes. This distance is expressed in terms
of a genetic map unit (m.u.), or a
centimorgan and is defined as the
distance between genes for which one
product of meiosis in 100 is recombinant

7. Alfred Henry Sturtevant was an
American geneticist. Sturtevant
constructed the first genetic map of
a chromosome in 1913. Throughout
his career he worked on the
organism Drosophila melanogaster
with Thomas Hunt Morgan.

8. His main contributions to science include
his analysis of genetic “linkage groups,”
which became classical method of
chromosome mapping that we still use
today. In 1913, he determined that
genes were arranged on chromosomes
in a linear fashion, like beads on a
necklace. He also showed that the gene
for any specific trait was in a fixed
location (locus).

9. Sturtevant determined that genes of
Drosophila are arranged in linear order.
One of Sturtevant’s principle
contributions was his introduction to
the concept that the frequency of
crossing-over between two genes could
help determine their proximity on a
linear genetic map.

10. As we cannot see genes to measure intergenic
distances in customary units hence, we use an
arbitrary unit of measurement called Map unit to
describe the distances between linked genes.
One map unit represents the linear distances within
which one percent crossing over takes place.i.e.
1 map unit = 1% crossing over.
One map unit is now referred to as cM (centiMorgan)
In the honour of Morgan’s contribution.

11. PROBLEM 1
In a cross in Drosophila the heterozygous
animal with grey body and long wing with
black body and vestigial wings, the progeny
has the animal in the following ratio : grey
vestigial 24: grey long 126 :
Black long 26 : black vestigial 124.
What is the frequency of recombinants in
the population ?

12. PROBLEM 2
Maize has 10 pairs of chromosomes.
How many linkage groups will be
present if all the genes are mapped ?
(a) 15 (b) 10 (c) 20 (d) 40

13. PROBLEM : 3
A scientist is studying two traits in a type of beetle: body
texture and color. Smooth body (B) is dominant to bumpy
body (b), and gray (G) is dominant to white (g).
A BbGg and a bbgg beetle mate and produce the following
offspring:
Phenotype Number of Offspring
Smooth, gray 345
Bumpy, white 355
Smooth, white 148
Bumpy, gray 152
What is the percent recombination frequency for this
cross? (Round to nearest whole number)