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Lecture 8

HMB265H1 Lecture Notes - Lecture 8: Gametogenesis, Robin Holliday, Microtubule


Department
Human Biology
Course Code
HMB265H1
Professor
Maria Papaconstantinou
Lecture
8

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Lecture 8 Linkage and Recombination
Tuesday, October 3rd, 2017 3pm-5pm
Hartwell et al. Ch. 3 pp. 94-95, 98, Ch. 4 pp. 120-127
LECTURE 8 OUTLINE:
Thomas Hunt Morgan’s linkage experiments
Genetic linkage based on Morgan’s experiments
Recombination based on Morgan’s experiments
Cytological evidence for crossing over
Physical evidence for recombination
Holliday’s molecular model of recombination
Some Genes on the Same Chromosome Don’t Assort Independently Instead, They’re Linked
In Drosophila, females carry two X chromosomes, and thus two alleles for each X-linked gene, while males have only a single X
chromosome (from the female parent), and thus only a single allele for each of these genes.
Eye colour and body colour as syntenic, because they’re located on the same chromosome
o For the white gene: dominant WT allele w+ specifies red eyes, while the recessive mutant allele w confers white eyes
o For the yellow body colour gene: dominant WT allele y+ specifies brown bodies, while the recessive mutant allele y
confers yellow bodies
The slash (/) is used to separate genes found on chromosomes of a pair (either the X and Y chromosomes, or a pair of X
chromosomes or homologous autosomes)
Detecting Linkage by Analyzing the Gametes Produced by a Dihybrid
In a cross between a female with mutant white eyes and a WT brown body (w y+/w y+) and a male with WT red eyes and a mutant
yellow body (w+ y/Y), the F1 offspring are evenly divided between brown-bodied females with normal red eyes (w y+/w+ y) and
brown-bodied males with mutant white eyes (w y+/Y)
o The male progeny look like their mother because their phenotype directly reflects the genotype of the single X
chromosome they received from her
o F1 females are hybrids, with two alleles for each X-linked gene, one derived from each parent, the dominance relations of
each pair of alleles determine the female phenotype
If these two Drosophila genes for eye and body colour assort independently, the dihybrid F1 females should make four kinds of
gametes, with four different combinations of genes on the X chromosome w y+, w+ y, w+ y+, w y
o These four types of gametes should occur with equal frequency (1:1:1:1 ratio)
o Thus, approximately half the gametes will be of the two parental types, carrying either the w y+ allele combination seen
in the original female of the P generation or the w+ y allele
o The remaining half of the gametes will be of two recombinant types, in which reshuffling has produced either w+ y+ or
w y allele combinations, not seen in the P generation parents of the F1 females
Looking at the F2 males, the relative numbers of the four X-linked gene combinations passed on by the dihybrid F1 females’
gametes reflect a significant departure from the 1:1:1:1 ratio expected of independent assortment
o The largest numbers of gametes carry the parental combinations w y+ and w+ y
o Of the total male flies counted, almost 99% had parental genotypes, while the new combinations w+ y+ and w y made up
little more than 1% of the total
The two genes failed to assort independently because these combinations of alleles might show up most frequently because
they’re parental alleles
o The F1 female inherited w and y+ together from her P generation mother, and w+ and y together from her P generation
father
o The F1 female is then more likely to pass on these parental combinations of alleles, rather than the recombinant
combinations, to her own progeny
Linkage recombination and analysis can be challenging
Focus on first observation of genetic linkage and recombination in Morgan’s
experiments
Potential outcomes of Holliday’s model for the molecular basis of recombination
If genes on different chromosomes assort independently because nonhomologous
chromosomes align independently on the spindle during meiosis I, how do genes on the same
chromosomes assort?
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Bateson and Punnett’s Unusual Observation
Purple flowers, long pollen (PP LL) x red flowers, round pollen (pp ll)
Morgan Also Investigated Two Traits at a Time
British mathematicians interested in genetics and Mendel’s work, studying inheritance
of two genes in sweet peas (flower colour and pollen shape)
Crossed two pure-breeding parents to get a dihybrid
Performed F1 dihybrid cross, and didn’t observe Mendel’s 9:3:3:1 ratio according to
independent assortment
The observed ratios were very different from what they expected
They also noticed some combinations of alleles that showed up more frequently than
expected in the F2 progeny, almost as if the alleles were physically linked
Couldn’t explain this phenomenon at the time
At the same time, Morgan (discovered sex-linkage to support chromosome
theory of inheritance, and his student Bridges proved this to be correct) was
working with fruit flies, Drosophila
He studied inheritance of two autosomal genes one that controlled eye
colour and other than controlled wing shape
Purple gene two mutant alleles give you purple eyes (autosomal recessive)
Vestigial gene two mutant alleles give you vestigial wings (autosomal
recessive)
He also noted a departure from Mendel’s laws, just like Bateson and Punnett
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Morgan Also Had Unusual Results
Morgan’s Observations
Morgan’s Dilemma
Morgan crossed pure-breeding parents, full WT x double mutant and obtained F1
dihybrid
He performed dihybrid testcross (male was tester parent), important because the
tester parent only contributes recessive alleles for genes of interest
So, the phenotype of F2 progeny is direct reflection of the gametes and alleles
contributed by the F1 dihybrid parent
Can focus on meiosis in the dihybrid parent and ignore meiosis in the tester parent,
because they only contribute recessive alleles for those genes
Standard dihybrid crosses require meiosis in female and male parents, but here only
in the female parent
In the F2 progeny, he expected a 1:1:1:1 ratio according to independent assortment,
but he observed a very different ratio
In fact, there were certain combinations that were in the great majority compared to
other allelic combinations
Look at recombinant progeny in F2 generation, formed by fusion of
recombinant gamete with tester gamete
Input gamete is the allelic combination from original parents
According to independent assortment, if genes are found on different chromosomes (non-
homologous chromosomes), they should assort independently
In a dihybrid testcross, you’d get a 1:1:1:1 ratio
Based on his observation, Morgan made his first prediction these two genes that he was
studying (purple and vestigial) were actually located on the same chromosome genetically
linked, attached to each other by the chromosomal material between them
So the parental combinations showed up in the majority of the F2 progeny
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