The document discusses genetic linkage, a phenomenon where DNA sequences close together on a chromosome tend to be inherited together, first discovered by Bateson and Punnett in 1906. It explains the concepts of recombination, factors affecting recombination frequency, types of linkage, and methods of chromosome mapping, highlighting experiments by Creighton, McClintock, and Morgan. The document concludes with an analysis of linkage groups and genetic mapping techniques, emphasizing the relationship between recombination and the inheritance of traits in organisms.

1. ACTIVITY
HEAD, SHOULDER,
KNEES AND PEN

2. LINKAGE
AND
RECOMBINATION

3. Genetic Linkage
Is the tendency of DNA sequences that are close together a
chromosome to be inherited together during the meiosis
phase of sexual reproduction.

4. Why genetic linkage?

5. When and who discovered?
• Shortly after Mendel’s work was rediscovered, exceptions to this rule were
found. In 1906, the British geneticists William Bateson and Reginald
Punnett discovered the phenomenon of linkage in sweet pea (lathyrus
odoratus).
• Genetic linkage is the most prominent exception to Gregor Mendel’s Law
of Independent Assortment.

6. How it was discovered?

7. WHAT IS RECOMBINATION?
• Recombination is the process through which genes present on
the same chromosomes can be separated and new combination
of genes could be formed.
• Recombination is caused by a physical exchange between
paired homologous chromosomes early in prophase of the first
meiotic division after chromosomes have duplicated (crossing
over).
• At any one point along a chromosome, the process of exchange
(crossing over) involves only two of four chromatids in meiotic
tetrad.

8. Who provided the evidence that crossing
over crosses recombination and how?
• In 1931 Harriet Creighton and Barbara McClintock obtained
evidence that genetic recombination where associated with a
material exchange between chromosomes.
• They studied homologous chromosomes in maize that where
morphologically distinguishable.
• Two forms of chromosome 9 was available for analysis, one
was normal, and other had cytological aberrations at each
end --- a heterochromatic knob at one end and a piece of a
different chromosome at other.

9. Recombination Frequency
• The frequency of the recombinant progeny produced by the heterozygous F1
plants is called recombination frequency. It can be use to measure the
intensity of linkage between genes.
• Genes that are tightly linked seldom recombine, that is there recombination
frequency in low.
• To calculate frequency of recombinants = (total no. of recombinants
observed/total no. plants observed) in a test cross*100)
• Recombination frequency never exceeds above 50%.

10. Factors affecting recombination frequency
• Distance between the genes
• Sex: heterogametic sex shows relatively lower recombinations (males) than
homogametic sex (females). Disophila males present the extreme example as
they show no recombination between linked genes.
• Age: progressive decline with age
• Temperature: lowest in drosophila 22 C
⁰
• Nutrition
• Chemicals: antinomycin D promotes recombination
• Radiation: X-rays and gamma rays increases recombination
• Chromosomal abbreviations: paracentric iversions, translocation, deletions
• Distance from centromere: centromere tends to suppress
recombination while recombination rates are much higher near
telomeric regions.

11. Types of Linkage
• Complete linkage: when only parental character combinations are
recovered in test cross progeny, it is called linkage ex: male drosophila
• Incomplete linkage: when recombinant types are also recovered with
parental types in test cross progeny it is called incomplete linkage
• A case of incomplete linkage id tight linkage in when genes are so
closely located that they may show very low frequency of
recombination.
• Linkage is also classified as coupling phase (dominant alleles of linked
genes are present together) and repulsion phase (dominant allele of
gene is linked with recessive allele of other gene).
• Sex linkage: linkage in genes present in sex chromosomes.

12. Linkage Group
• All genes that are linked together form a linkage group
• The no. of different linkage groups in a species is, as a rule, equal
to its gametic chromosome no.
• Drosophila has 5 linkage group and human beings have 24 linkage
groups.

13. Chromosome Map
• A straight line drawing (supposing chromosome are linear), depicting the linked
genes and the distances (recombination frequencies) between them is known as
chromosome map.
• The procedure for chromosomal mapping was invented by Alfred H. Sturtvent in
1911.
• Geneticists construct chromosome maps by counting the number of crossovers
that occur during meiosis. Since crossing over event can’t be seen directly. So
they must estimate the no. of crossovers have taken place by counting either
chiasmata (cytological analysis) or recombinant chromosomes (genetic
analysis).
• The distance between two points on genetic map of a chromosome is the
average number of crossovers between them.

14. Recombinant mapping with a two point testcross
• A two point test cross uses 2 loci to determine the distance
between them using the amount of average recombinants
obtained from the test cross.

16. Limitations of a two point cross
• We can’t determine the relative order on the chromosome of the
loci
• As well as we cannot analyse the effect of multiple crossovers as
two crossovers between two loci can cause the chromosome to
look as if no cross overs took place, causing to underestimate map
distances.

17. Recombination mapping with a three point
cross
• Analysis of three loci, each segregating two alleles, is referred to as a three-
point cross.
• We can determine order of loci or genes in the chromosome.
• We can determine relative distance between the genes.
• But we cannot distinguish right and left hand of chromosome.

19. Interference & Coefficient of Coincidence
As we know 3 point cross has an important advantage, that is it allows the
detection of double crossovers, permitting us to determine if exchanges in
adjacent regions are independent of each other or does one cross over inhibit
the occurrence of another nearby
If one crossover inhibit the occurrence of another nearby, the phenomenon is
known as interference.
And the extent of the interference is customarily measured by the coefficient of
coincidence(c), which is the ratio is observed frequency of double cross over to
the expected frequency.
The level of interference(I) is calculated as I=1-C.
Coefficient of coincidence 1 would imply no interference at all. But negative
interference means that the occurrence of a crossing over promotes other.

21. • Recombination involves product of many genes, some of which
play role in chromosome pairing, others catalyse the process
of exchange, and still others help to rejoin broken chromatid
segments.
• Inversions suppress recombination as it suppress crossing
over
• Recombination can bring favorable mutation together.

22. • In Drosophila, both the white eye gene (w)
and a gene for miniature wings (m) are on
the X chromosome.
• Morgan (1911) crossed a female white
miniature (w m/w m) with a wild-type male
(w+m+/Y).
• In the F1, all males were white-eyed with
miniature wings (w m/Y), and all females
were wild-type for eye color and wing size
(w+m+/w m).
EXPERIMENT OF T H MORGAN

23. MORGAN’S EXPERIMENTAL CROSSES OF WHITE
EYE AND MINIATURE WING VARIANTS Of
Dorsophilia Melanogaster

24. In the F2, the most frequent phenotypes for both sexes were the
phenotypes of the parents in the original cross (white eyes with
miniature wings, and red eyes with normal wings.
Non-parental phenotypes (white eyes with normal wings or red
eyes with miniature wings) occurred in about 37% of the F2 flies.
Well below the 50% predicted for independent assortment, this
indicates that non-parental flies result from recombination of linked
genes.