Crossing over and all test including inbit it is very helpful for stuff

1. LINKAGE, CROSSING-OVER, AND GENE
MAPPING IN EUKARYOTES

2. Linkage
• The Manner or style of being united.
• Linkage is defined genetically: the failure of
two genes to assort independently.
• Linkage occurs when two genes are close to
each other on the same chromosome.

3. • It reduces the chance of recombination of
genes and thus helps to hold parental
characteristics together.
• It does not permit the breeders to bring the
desirable characters in one variety.
• It helps organism to maintain its parental
characters.

4. Discovery of Genetic Linkage
• Genes on non-homologous chromosomes assort
independently
• Genes on the same chromosome may instead be
inherited together (linked), and belong to a linkage
group

8. Linkage in the Sweet Pea
• Expected results of F1 X F1 cross if genes for
flower color & pollen size were to assort
independently
• F1 generation: diploid, Genotype Pp Ll
Phenotype Purple flowers, long pollen
• Expected genotypes of possible F1 gametes
haploid ¼ PL, ¼ Pl, ¼ pL, ¼ pl

9. • Expected phenotypic ratios of F2 generation
Purple long Purple round Red long Red round
9/16 3/16 3/16 1/16
% 56.25 18.75 18.75 6.25
• Observed results:
69.5 5.6 5.6 19.3
• Observed results indicated partial linkage of genes.
Dominant purple and long characters occurred toge
ther more often than predicted
• F1 haploid gametes
44% PL 6% Pl 6% pL 44% pl

10. • Classical genetics analyzes the frequency of
allele recombination in progeny of genetic
crosses
New associations of parental alleles are
recombinants, produced by genetic
recombination
Testcrosses determine which genes are linked
and a linkage map (genetic map) is
constructed for each chromosome
Genetic maps are useful in recombinant DNA
research and experiments dealing with genes
and their flanking sequences

11. • Current high resolution maps include both:
Gene markers from testcrosses
DNA markers composed of genomic
regions that differ detectably between
individuals

12. TYPES OF LINKAGE :
• There are two types of linkage depending upon the
presence or absence of new combinations or non-
parental combinations.
1. Complete Linkage
2. Incomplete Linkage

13. Complete Linkage:
• If two or more characters are inherited together and
consistently appear in two or more generations in their
original or parental combinations
• Do not produce non-parental combinations.
• During synapsis exchange of segments takes place. In
such condition the possibility of separation of two genes
situated close together is greatly reduced.When genes are
closely associated and tend to transmitt together,it is
called complete linkage.

15. Incomplete Linkage:
• It is exhibited by those genes which produce some
percentage of non-parental combinations. Such
genes are locate distantly on chromosomes.
• When linked genes are situated at long distance in
chromosomes and have chances of separation by
crossing over is called incompletely linked genes and
phenomenon of the their inheritance is called
incomplete linkage.

18. Arrangement of genes:
• There are two ways by means of which genes
arranged.
1. Cis arrangement of genes:
2. Trans arrangement of genes:

19. Cis arrangement of genes:
If the dominant alleles of two linked genes are
present on same chromosome and their recessive
alleles are present on homologous chromosomes the
arrangement of genes is called cis arrangement.

20. Trans arrangement of genes:
If one dominant gene and other recessive gene present
on one chromosome and their allele type on the
chromosome this arrangement of genes is called trans
arrangement of genes.

21. Morgan’s Linkage Experiments with
Drosophila
• Both the white eye gene (w) and a gene for miniature
wing (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 are white eyed with miniature
wings and all females are wild-type for both eye color
and wing size
• Morgan concluded that during meiosis, alleles of some
genes do not assort independently but assort together
because they lie near one another on the same
chromosome

22. • F1 interbreeding is the equivalent of a
testcross for these X-linked genes, since the
male is hemizygous recessive
• In the F2, the most frequent phenotypes for
both sexes are the phenotypes of the parents
in the original cross (white eyes with
miniature wings, and red eyes with normal
wings)
• Non-parental phenotypes occurred in about
37% of the F2 flies below the predicted 50%
for independent assortment
• This indicates non-parental flies result from
recombination of linked genes

24. • Morgan proposed that:
During meiosis alleles of some genes assort
together because they are near each other on
the same chromosome
Recombination occurs when genes are
exchanged between the X chromosomes of the
F1 females

25. TERMINOLOGY
• A chiasma (plural chiasmata) is the site on
the homologous chromosomes where
crossover occurs
• Crossing-over is the reciprocal exchange of
homologous chromatid segments, involving
the breaking and rejoining of DNA
• Crossing-over is also the event leading to
genetic recombination between linked genes
in both prokaryotes and eukaryotes

26. Mechanism of Crossing-Over

27. GENE RECOMBINATION AND THE ROLE
OF CHROMOSOMAL EXCHANGE

28. Corn Experiments
• Creighton and McClintock (1931) performed crosses
with corn plants and found cytological evidence that
crossing-over occurs during meiosis and is associated
with the physical exchange of parts between
homologous chromosomes

29. • The study used a corn strain heterozygous for two
genes on chromosome 9
• One gene determines seed color (C for colorled
seeds, c for colorless)
• The other gene is involved in starch synthesis:
Wild-type allele (Wx) produces amylose, and
amylose + amylopectin = normal starch
Waxy mutant (wx) lacks amylose
• In this corn strain, the appearance of each
chromosome 9 homolog correlated with its
genotype

30. Evidence of the association of gene recombination
with chromosomal exchange in corn

31. Drosophila Experiments
• Shortly after Creighton and McClintock
published their results, Stern reported identical
results for experiments done with Drosophila
• He reported on two linked gene loci
• From experiments such as these, it became
apparent that genetic recombination results
from physical crossing-over between
chromosomes

33. Crossing-Over at the Tetrad Stage of
Meiosis
• Crossing-over occurs at the four-chromatid
(tetrad) stage in prophase I during meiosis
• In Neurospora crassa (orange bread mold) forms
eight haploid spores. Their arrangement in the
ascus refects the orientation of chromatids in the
metaphase tetrad of meiosis
• To determine when crossing-over occurs, crosses
were made between haploid Neurospora strains
of different mating types (A and a)

34. Life cycle of
the haploid,
mycelial-
form fungus
Neurospora
crassa

35. Experiment
showing that
crossing-over
occurs at the
four-chromatid
stage of meiosi

36. LOCATING GENES ON CHROMOSOMES: MAPPING
TECHNIQUES
1. Detecting Linkage Through Testcrosses
2. Gene Mapping by Using Two-Point Testcrosses
3. Generating a Genetic Map
4. Double Crossovers
5. Three Point Crosses

37. Detecting Linkage Through Testcrosses
• To construct a genetic map or linkage map,
the genes in question must be linked, or
located on the same chromosome
• To test for linkage, a testcross is used
• Testcrosses are between one individual of
unknown genotype and a homozygous
recessive individual

38. Detecting Linkage Through Testcrosses
• If two genes are not linked, a testcross
should show a 1:1:1:1 phenotypic ratio
• Statistically significant deviations from the
expected results indicate that the genes
are linked and have recombined (chi-
square test)
• Two-linked genes→ too many parental
types and too few recombinant types

39. Testcross to show that
two genes are linked

40. CONCEPT OF A GENETIC MAP
• In an individual heterozygous at two loci,
there are two arrangements of alleles:
 Cis (coupling) arrangement has both wild-
type alleles on one homologous
chromosome and both mutants on the
other (w+ m+ and w m)
 Trans (repulsion) arrangement has one
mutant and one wild-type on each
homolog (w+ m and w m+)

41. • A crossover between homologs in the cis arrangement
results in a homologous pair with the trans
arrangement and vice versa
• Frequency of recombinants is the same, regardless of
how the alleles of the two genes involved are arranged
relative to each other on the homologous
chromosomes
• Geneticists use recombination frequencies to make a
genetic map
• A 1% crossover rate is a genetic distance of 1 map unit
(mu) = 1 centimorgan (cM)
• Genetic distances are additive

42. Gene Mapping Using Two-Point Testcrosses
• The percentage of recombinants resulting from
crossing-over is used as a measurement of the genetic
distance between two linked genes
• In all cases, a two-point testcross should yield a pair of
parental types that occur with equal frequency and a
pair of recombinant types that also occur equally
• The value for the percentage of recombinants (#
recombinant/total # of progeny X 100) is usually directly
converted into map units

43. TWO-POINT TESTCROSSES
• Autosomal Recessive Test-cross
a+ b+/a b X a b/a b Homozygous Recessive
• Autosomal Dominant WT alleles are recessive
A B/A+ B+ X A+ B+/A+ B+
• X-Linked Recessive
a+ b+//a b X a b/
• X-Linked Dominant
A B//A+ B+ X A+ B+/

44. Generating a Genetic Map
• A genetic map can be constructed by
estimating the number of times a crossover
event occurred in a particular segment of the
chromosome
• The map distance between two genes is based
on the frequency of recombination between
the two genes

45. • The recombination frequency between genes in
chromosomes can be computed as the percentage of
progeny showing the reciprocal recombinant
phenotypes
• The closer the recombination frequency paralleles
the crossover frequency, the closer the genes are
• Multiple crossovers may result: use product rule to
calculate its probability
ex: p= 0.2 for one crossover
p= 0.2 X 0.2 = 0.04 of double crossover

47. • For any testcross, the percentage of
recombinants cannot exceed 50%
• Independent assortment → equal # of
recombinants and parents = 50%
• Unlinked genes → 50% recombination
The two genes are on different
chromosomes
The two genes are on the same
chromosome but are so far apart → map
other genes in the linkage group to
determine whether the former genes are on
the same or different chromosome

48. Crossing over
• A random exchange of DNA between two non-
sister chromatids of homologous
chromosomes.
• Results in recombination of genetic material.

53. Single Crossing over
• In this type, a single chiasma is formed all
along the length of a chromosome pair.

54. DOUBLE CROSSOVERS
• When the distance between two genes on a
chromosome increases > 10 mu, the incidence
of multiple crossovers causes the
recombination frequency to be an
underestimate of the crossover frequency and
map distance
• The effects of multiple crossovers can be
corrected to provide a more accurate estimate
of map distance

55. Progeny of single and double crossovers

56. THREE POINT CROSSES
• One way to overcome the problem of genetic
mapping when multiple crossover events occur
between linked genes is to perform a three-
point testcross, which involves three genes on a
short region of the chromosome
• In a three-point testcross, a triple heterozygous
is crossed with a homozygous recessive for all
three genes

57. Consequences of a double crossover in a
triple heterozygote for three linked genes

58. Three-point mapping,
showing the testcross used
and the resultant progeny

59. Rearrangement of the three genes to p j r

60. Rewritten form of
the testcross and
testcross progeny
based on the
actual gene order
p j r

61. Genetic map of the p-j-r region of the
chromosome computed from the
recombination data of the previous slide