Lecture 8: Linkage and Recombination

Question 1

what unusual observations did Bateson and Punnett make?

Answer 1

when crossing purple flowers, long pollen (PPLL) with red flowers, round pollen (ppll)
they found more parental-type combinations and fewer recombinants than expected.

however, according to Mendel’s law of independent assortment, they expected a 9:3:3:1 ratio in the F2 generation.

Question 2

what unusual observations did Morgan make?

Answer 2

crossed red eyed, full size wing (pr+vg+/pr+vg+) with purple eye, vestigial wing (pr vg/pr vg)

again, parental combinations appeared more often than recombinant ones and recombinant types were less frequent than expected by chance.

Question 3

what did Frans Janssens observe in 1909? what did Morgan suggest relating to this?

Answer 3

chiasmata, regions in which nonsister chromatids of homologous chromosomes cross over each other. Morgan suggested these were sites of chromosome breakage and exchange resulting in genetic recombination

Question 4

cytogenetics

Answer 4

The study of chromosomes inside cells, especially during cell division.

Question 5

crossovers

Answer 5

• physical exchanges among nonsister chromatids; visualised cytologically as a chiasma (plural = chiasmata)
• typically, several crossing-over events occur within each bivalent or tetrad in each meiosis

Question 6

function of chiasmata

Answer 6

physically hold homologous chromosomes together and assure proper segregation at anaphase I

Question 7

Morgan’s first prediction and second prediction relating to crossing over

Answer 7

First:
- Crossing over occurs during meiosis between homologous chromosomes, allowing genes to be exchanged.
- This explains how linked genes can sometimes produce recombinant offspring

Second:
- The frequency of crossing over between two genes is proportional to the distance between them on the chromosome.
- Genes closer together have less chance of crossover (more likely inherited together).

Question 8

Mendel’s fruit fly test cross progeny ratios

Answer 8

> 50% of progeny have parental type (1/4 one, 1/4 other)
<50% have recombinant phenotype

Question 9

genetic recombination depends on

Answer 9

the reciprocal exchange of parts between maternal and paternal chromosomes

Question 10

what did McClintock and Creighton observe?

Answer 10

• They studied corn (maize) chromosomes with visible physical markers (like a knob on one chromosome end and a translocation on the other).
• They tracked the inheritance of these physical chromosome markers alongside genetic traits.
• this provided visual confirmation that chromosomes cross over
• correlation between genetic crossover and chromosomal crossover
• this was verified in drosophila by Stern

Question 11

Meiosis contributes to genetic diversity in 2 ways

Answer 11

• independent assortment of non homologous chromosomes creates different combinations of alleles among chromosomes
• crossing over between non sister homologous chromatids creates different combinations of alleles within each locus

Question 12

when does recombination take place?

Answer 12

at the four-chromatid (four-strand) stage of Meiosis I (prophase I)

Question 13

5 stages of crossing over

Answer 13

1. leptotene: thread-like chromosomes begin to condense, becoming visible as discrete structures, although the sister chromatids cannot yet be distinguished
2. zygotene: chromosomes are clearly visible and begin pairing with homologous chromosomes along the synaptonemal complex to form a bivalent, or tetrad
3. pachytene: the homologs synapse fully, recombination nodules appear along the synaptonemal complex
4. diplotene: the bivalent pulls apart slightly, but homologous chromosomes remain connected due to recombination at crossover sites (chiasmata)
5. diakinesis: the bivalent condenses further

Question 14

synaptonemal complex

Answer 14

zipper-like elaborate protein structure that aligns chromosomes base pair by base pair

Question 15

terminalization

Answer 15

process where the chiasmata move from the middle of the homologous chromosomes toward the ends (telomeres) as meiosis progresses, especially during diplotene and diakinesis stages

Question 16

what happens after terminalization?

Answer 16

anaphase I

Question 17

Holliday’s model for mechanism of recombination

Answer 17

1. Homologues physically break, exchange parts, and rejoin
2. Breakage and repair create reciprocal products of recombination
3. Recombination events can occur anywhere along the length of a DNA molecule, but at some locations with a higher frequency than others
4. The exchange is precise - no gain or loss of nucleotide pairs occurs– thus preventing mutation from occurring
5. Gene conversion - where small segments of information from one homologous chromosome transfers to the next - may result in unequal yield of the two alleles

Question 18

draw a table showing the meiotic products based on crossover between genes

Answer 18

slide 27

Question 19

genetic recombination

Answer 19

mixing of genes during gametogenesis produces gametes with combinations of genes that are different from the combinations received from parents

Question 20

unlinked vs linked genes

Answer 20

• genes on non homologous chromosomes (unlinked genes) assort independently
• genes on the same chromosome co-segregate

Question 21

crossovers result in recombination between

Answer 21

linked genes

Question 22

crossover frequency is a function of

Answer 22

the distance between two loci

Question 23

can crossover ever occur when genes are completely linked?

Answer 23

no; the genes are too close

Question 24

recombination frequency can never exceed

Answer 24

50% (ie more recombinant types than parental types)

Question 25

single crossover vs double crossover

Answer 25

single: parental and recombinant double: all parental

Question 26

how do you test for linked vs unlinked genes?

Answer 26

test cross to distinguish between linked and unlinked genes, you'd typically cross a heterozygous individual with a homozygous recessive individual for the traits being studied. 2 genes unliked = 1:1 between parental genotypes and recombinant genotypes 2 genes linked = more parental genotypes than recombinant genotypes