2. Application of Mendel’s Rules assumes:
1. One allele completely dominates the other
2. All genes have 2 allelic forms
3. All traits are monogenic (affected by only one
locus)
4. All chromosomes occur in homologous pairs
5. All genes assort independently
3. Mendel's Law of Independent
Assortment
Allele pairs separate independently during
the formation of gametes. This means that
traits are transmitted to offspring
independently of one another.
5. Independent assortment
An Interpretation from the Dihybrid cross
• During gamete formation, segregating pairs of unit
factors assort independently.
• In other words, segregation of 2 alleles at one genetic
locus has no effect on the segregation of 2 alleles at
another locus.
• For example, the assortment of yellow and green
alleles has no effect on the assortment of round and
wrinkled alleles, and vice versa.
6. Discovery of
Linkage
• William Bateson and R.C. Punnett were
working with several traits in sweet
peas, notably a gene for purple (P) vs.
red (p) flowers, and a gene for long
pollen grains (L) vs. round pollen grains
(l).
7. Bateson and Punnett also studied peas:
Flower Color: P = purple p = red
Pollen seed shape: L = long l = round
True Breeding lines: PPLL x ppll P
PpLl F1
Phenotype Number Exp Ratio Exp Number
Purple long 284 9 215
Purple round 21 3 71
Red long 21 3 71
Red round 55 1 24
Crosses produced a deviation from the predicted
Mendelian independent assortment ratios.
What is going on????
8. • A test cross is a breeding or a mating between an
individual of dominant phenotype, who could be
either homozygous dominant (SS) or heterozygous
(Ss), with an individual that MUST be homozygous
recessive (ss).
9. Test cross F1 to double recessive:
Parents PpLl X ppll
Gametes PL pl
Pl
pL
pl
Expect 1:1:1:1 ratio of phenotypes
Bateson and Punnett observed 7:1:1:7
Some gamete types more common that others…but why???
10. • Because the parental phenotypes
reappeared more frequently than
expected, the researchers hypothesized
that there was a coupling, or connection,
between the parental alleles for flower
color and pollen grain shape
• This coupling resulted in the observed
deviation from independent assortment.
11. • But why are certain alleles linked?
Bateson and Punnett weren't sure.
• In fact, it was not until the later
work of geneticist Thomas Hunt
Morgan that this coupling, or linkage,
could be fully explained.
12. MORGAN’S EXPERIMENTS
• 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).
13. Morgan’s experimental crosses of white-eye and miniature-wing variants
of Drosophila melanogaster
In 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
14. What is Linkage?
• Linkage is defined genetically as the failure of
two genes to assort independently.
• Linkage occurs when two genes are close to each
other on the same chromosome.
• Genes far apart on the same chromosome assort
independently: they are not linked.
• Linkage is based on the frequency of crossing
over between the two genes.
• Crossing over occurs in prophase of meiosis 1,
where homologous chromosomes break at identical
locations and rejoin with each other.
15. Morgan and Crossing Over
• Morgan proposed that the chiasmata visible on
chromosomes were regions of crossing over.
• Occurs between non-sister chromatids.
16. Process of Recombination
• From an evolutionary point of view, the purpose of sex
is to re-shuffle the combinations of alleles so the
offspring receive a different set of alleles than their
parents had.
• Natural selection then causes offspring with good
combinations to survive and reproduce, while offspring
with bad combinations don’t pass them on.
• Genes are on chromosomes. Meiosis is a mechanism
for re-shuffling the chromosomes: each gamete gets
a mixture of paternal and maternal chromosomes.
• However, chromosomes are long and contain many
genes. To get individual genes re-shuffled, there
needs to be a mechanism of recombining genes that
are on the same chromosome. This mechanism is
called “crossing over.
17. • Crossing over occurs in prophase of meiosis 1,
when the homologous chromosomes “synapse”,
which means to pair closely with each other.
DNA strands from the two chromosomes are
matched with each other.
• During synapsis, an enzyme, “recombinase”,
attaches to each chromosome at several
randomly chosen points. The recombinase
breaks both DNA molecules at the same point,
and re-attaches them to opposite partners.
• The result of crossing over can be seen in the
microscope as prophase continues, as X-
shaped structures linking the homologues.
• The genetic consequence of crossing over is
that each chromosome that goes into a gamete
is a combination of maternal and paternal
chromosomes.
23. Linkage Mapping
• Each gene is found at a fixed position on a particular chromosome.
Making a map of their locations allows us to identify and study them
better. In modern times, we can use the locations to clone the genes so
we can better understand what they do and why they cause genetic
diseases when mutated.
• The basis of linkage mapping is that since crossing over occurs at
random locations, the closer two genes are to each other, the less likely
it is that a crossover will occur between them. Thus, the percentage of
gametes that had a crossover between two genes is a measure of how
far apart those two genes are.
• As pointed out by T. H. Morgan and Alfred Sturtevant, who produced
the first Drosophila gene map in 1913. Morgan was the founder of
Drosophila genetics, and in his honor a recombination map unit is called
a centiMorgan (cM).
• A map unit, or centiMorgan, is equal to crossing over between 2 genes in
1% of the gametes.
24. Detecting Linkage through Testcrosses
• Linked genes are used for mapping. They are
found by looking for deviation from the
frequencies expected from independent
assortment.
• A testcross (one parent is homozygous
recessive) works well for analyzing linkage
– If the alleles are not linked, and the second
parent is heterozygous, all four possible
combinations of traits will be present in
equal numbers in the progeny.
– A significant deviation in this ratio (more
parental and fewer recombinant types)
indicates linkage.
25. Sturtevant and Mapping
• Sturtevant, Morgan’s undergraduate student, discovered
frequency of crossing over between each pair of the 3
genes:
– yellow, white 0.5%
– white, miniature 34.5%
– yellow, miniature 35.4%
28. Map Units
• One map unit (centimorgan, cM) = 1% recombination
between two genes
– yellow and white are 0.5 cM apart
– yellow and miniature are 35.4 cM apart
– white and miniature are (35.4-0.5) = 34.9 cM apart
• In Drosophila, crossing over occurs only in females,
never in males.
29. III. Three-Point Mapping
• You can add % recombination between two genes to
find the order of genes pretty well.
• But the only way to be sure of the order of three
genes is by Three-Point Mapping, which considers 3
genes at once.
• You look for rare double-crossover events, and that is
the clue to the gene order.
31. Interference
• There is a second issue with double crossovers:
interference.
• Interference is the inability of 2 crossovers to occur very
close to each other. Think of the chromosome as a thick
rope: it is impossible to bend it too tightly.
• It is possible to measure the amount of interference, by
comparing the actual number of double crossovers to the
number that you would expect based on the number of
single crossovers that occurred.
