Transcript Operating genetic material

Discovery
Genetic linkage was first discovered by the British geneticists William Bateson, Edith Rebecca
Saunders and Reginald Punnett shortly after Mendel's laws were rediscovered. The understanding
of genetic linkage was expanded by the work of Thomas Hunt Morgan.
Linkage
A linkage map is a genetic map of a species or experimental population that shows the position of
its known genes or genetic markers relative to each other in terms of recombination frequency,
rather than a specific physical distance along each chromosome.
Thomas Hunt Morgan's Drosophila
melanogaster genetic linkage map. This was the
first successful gene mapping work and
provides important evidence for the
chromosome theory of inheritance. The map
shows the relative positions of allelic
characteristics on the second Drosophila
chromosome. The distance between the genes
(map units) are equal to the percentage of
chromosomal crossover events that occurs
between different alleles.
Dihybrid cross is a cross between two pure lines (varieties,
strains) that differ in two observed traits. In Mendelian
sense, between the alleles of both these loci there is a
relationship of complete dominance - recessive. For
example: RRYY/rryy parents result in F1 offspring that are
heterozygous for both R and Y (RrYy).The rules of meiosis,
as they apply to the dihybrid, are codified in Mendel's first
law and Mendel's second law, which are also called the Law
of Segregation and the Law of Independent Assortment,
respectively.
Crossing over
Crossing over occurs between prophase 1 and metaphase 1 and is
the process where homologous chromosomes pair up with each
other and exchange different segments of their genetic material to
form recombinant chromosomes. It can also happen during mitotic
division, which may result in loss of heterozygosity. Crossing over
is essential for the normal segregation of chromosomes during
meiosis. Crossing over also accounts for genetic variation, because
due to the swapping of genetic material during crossing over, the
chromatids held together by the centromere are no longer identical.
So, when the chromosomes go on to meiosis II and separate, some
of the daughter cells receive daughter chromosomes with
recombined alleles. Due to this genetic recombination, the
offspring have a different set of alleles and genes than their parents
do. In the diagram, genes B and b are crossed over with each other,
making the resulting recombinants after meiosis Ab, AB, ab, and
aB
Recombination
A current model of meiotic recombination, initiated by a
double-strand break or gap, followed by pairing with an
homologous chromosome and strand invasion to initiate the
recombination repair process. Repair of the gap can lead to
crossover (CO) or non-crossover (NCO) of the flanking
regions. CO recombination is thought to occur by the Double
Holliday Junction (DHJ) model, illustrated on the right,
above. NCO recombinants are thought to occur primarily by
the Synthesis Dependent Strand Annealing (SDSA) model,
illustrated on the left, above. Most recombination events
appear to be the SDSA type.