Meiosis, Linkage, Crossing over

1. MEIOSIS.
LINKAGE.
and.
CROSSING OVER

2. LINKED GENES
• Genes located close together on the same
chromosome
• Linked genes travel together during meiosis,
eventually arriving at the same destination
(the same gamete), and are not expected to
assort independently

3. LINKAGE
• Describes the tendency of genes to be
inherited together as a result of their location
on the same chromosome; measured by
percent recombination between loci.
• Linked genes do not assort independently

4. Concepts
• Recombination is the sorting of alleles into
new combinations.
• Inter-chromosomal recombination, produced
by independent assortment, is the sorting of
alleles on different chromosomes into new
combinations.
• Intra-chromosomal recombination, produced
by crossing over, is the sorting of alleles on
the same chromosome into new
combinations.

5. Notation for Crosses with Linkage
• In analyzing crosses with linked genes, we
must know not only the genotypes of the
individuals crossed, but also the arrangement
of the genes on the chromosomes

6. Complete Linkage Compared
with Independent Assortment

7. Crossing Over with Linked Genes
• Linkage is rarely complete—usually, there is
some crossing over between linked genes
(incomplete linkage), producing new
combinations of traits
• Crossing over, which takes place in prophase I
of meiosis, is the exchange of genetic material
between non-sister chromatids

9. Coupling and Repulsion
• In crosses for linked genes, the arrangement
of alleles on the homologous chromosomes is
critically important in determining the
outcome of the cross

10. Coupling
• Wild-type alleles are found on one
chromosome and mutant alleles are found on
the other
• Allelic arrangement in which mutants are on
the same chromosome and wild-type alleles
on the homologue
• The cis configuration

11. Coupling phase (cis)

12. Repulsion
• Each chromosome contains one wild-type and
one mutant allele
• Allelic arrangement in which each homologue
chromosome has mutant and wild-type allele
• The trans configuration

13. Repulsion phase (trans)

14. Calculation of Recombination
Frequency
• The percentage of recombinant progeny
produced in a cross is called the
recombination frequency, which is
• calculated as follows:

15. Diskusi
Suatu organisme pentahibrid yang mempunyai genotip
Aa Bb Cc Dd Ee. Diketahui bahwa gen B dan C
berangkai tidak sempurna secara coupling phase pada
satu kromosom, sedangkan gen D dan E berangkai
sempurna secara repulsion phase pada kromosom
lainnya.
Adapun gen A tidak berangkai dengan gen-gen lainnya ,
pada kromosom yang lain
a) Apabila individu tersebutmembentuk gamet, ada
berapamacamgamet yang dibentuk dan sebutkan macam-
macamgamet tersebut
b) Gambarkan secara jelas kemungkinan-kemungkinan
sinapsis yang terjadi saatmetafase I (gambar tersebut
berupa skema kromosomdengan gen-gen yang dibawa)

16. A two-point test cross to
detect linkage in fruit flies

18. RESULTS AND CONCLUSION
• In the 2300 offspring (bottom row) of an
actual cross, about 1909 of the offspring
belong to each of the two parental classes
(83% total), and 391 belong to each of the two
recombinant classes (17% total)
• Thus, loci for wing length and body color are
linked on a homologous chromosome pair

19. Calculating the frequency of crossing over
reveals the linear order of linked
genes on a chromosome

20. Concepts
• During a single meiotic division, crossing-over
may occur at several different points along the
length of each homologous chromosome pair
• In general, crossing-over is more likely to occur
between two loci if they lie far apart on the
chromosome and less likely to occur if they lie
close together
• A genetic map of the chromosome can be
generated by converting the percentage of
recombination to map units
• 1% recombination between two loci equals a
distance of 1 map unit

21. Result
• By convention, 1% recombination between
two loci equals a distance of 1 map unit, so
the loci in our example are 17 map units apart

22. Gene Mapping
• Gene order (that is, which locus lies between
the other two) is determined by the
percentage of recombination between each of
the possible pairs

23. Gene Mapping
• In this hypothetical
example, the
percentage of
recombination between
locus A and locus B is
5% (corresponding to 5
map units) and that
between B and C is 3%
(3 map units).
• There are two
alternatives for the
linear order of these
alleles.

24. Predicting the Outcomes of
Crosses with Linked Genes
• In cucumbers,
– smooth fruit (t) is recessive to warty fruit (T) and
– glossy fruit (d) is recessive to dull fruit (D)
• Geneticists have determined that these two
genes exhibit a recombination frequency of
16%.

25. Predicting the Outcomes of
Crosses with Linked Genes
• Suppose we cross a plant homozygous for
warty and dull fruit with a plant homozygous
for smooth and glossy fruit and then carry out
a testcross by using the F1
• What types and proportions of progeny will
result from this testcross?

26. Solution
• Four types of gametes will be produced by the
heterozygous parent
– two types of non-recombinant gametes ( T D and t
d )
– two types of recombinant gametes ( T d and t D )
• The recombination frequency tells us that 16%
of the gametes produced by the heterozygous
parent will be recombinants

27. Solution
• Because there are two types of recombinant
gametes, each should arise with a frequency
of 16% : 2 = 8%
• All the other gametes will be
nonrecombinants; so they should arise with a
frequency of 100% - 16% = 84%
• Because there are two types of
nonrecombinant gametes, each should arise
with a frequency of 84% : 2 = 42%

28. Solution
• The progeny of the cross result from the union
of two gametes, producing four types of
progeny
• The expected proportion of each type can be
determined by using the multiplication rule,
multiplying together the probability of each
uniting gamete