6. Gene Mapping and Linkage

Question 1

What did Thomas Morgan win the Nobel prize for

Answer 1

Physiology/Medicine in 1933
- Sex-linked traits in fruit flies
- Genetic linkage and recombination

Question 2

The 3 theories that Morgan validated

Answer 2

1. CHROMOSOMAL THEORY OF INHERITANCE: genes are carried on chromosomes and are in a specific order
2. GENES ARE PHYSICAL ENTITIES THAT ARE INTEGRAL ON CHROMOSOMES: genes are composed of nucleotides in which recombination may occur
3. CLOSELY RELATED SPECIEIS HAVE SIMILAR NUMBER AND ARRANGEMENTS OF CHROMOSOMES: supports the theory of evolution, recombination is a mechanism for divergence

Question 3

Chromosomes and independent assortment (gamete perspective)

Answer 3

• HOMOLOGOUS RECOMBINATION: the crossing over phase during meiosis 1 (prophase) - results in recombinant chromosomes aka non-parental chromosomes
• NO CROSSING OVER: parental chromosomes aka nonrecombinant chromosomes

Question 4

Do linked genes assort independently?

Answer 4

No, there is genetic linkage

Question 5

Syntenic Genes

Answer 5

• Genes that are located on the same chromosome

Question 6

What happens when syntenic genes are located close together

Answer 6

• Alleles can’t independently assort and causes genetic linkage (linked genes)

Question 7

What does gene linkage produce

Answer 7

Produces a distinctive pattern of gamete genotypes since they recombine less during meiosis

Question 8

What happens when syntenic genes are located far apart?

Answer 8

• They assort following Mendels law of independent assortment (unlinked gene)

Question 9

How does genetic linkage affect gamete frequencies

Answer 9

It creates different expected gamete frequencies because theres a greater number of parental gametes than recombinants compared to independent assortment - smaller number of recombinant gametes

Question 10

Difference of crossing over in linked genes that are close together and far apart

Answer 10

Linked genes that are closer together are less likely to have crossing over compared to linked genes that are further apart.
Frequency of cross-over is roughly proportionate to the distance between a gene

Question 11

What are the observed frequencies of genotypes and phenotypes of linked genes compared to expected frequencies in independent assortment

Answer 11

• If they’re linked, parental gametes will be produced more than by chance (>50%) AKA showing more parental phenotype than expected. Recombinant gametes will be produced less than by chance (<50%)
• If they’re unlinked, parental gametes with parental alleles will produce the same amount as recombinant gametes. Produces 4 genetically different gamete combinations (1:1:1:1)

Question 12

What is complete linkage?

Answer 12

No recombination occurs at all - produces 2 equally frequent gametes with only parental allele combinations

Question 13

What is incomplete linkage?

Answer 13

Recombination between the homologs produces a mix of parental and recombination gametes - produces 4 genetically different gametes (2 parents and 2 recombinants)
- >50% are parental, <50% is recombinant

Question 14

Recombination frequency equation

Answer 14

r = (number of recombinants offspring)/(total number of offspring)

Question 15

If crossing over rate is high/low what does that mean?

Answer 15

High = genes are further apart
Low = genes are closer together

Question 16

If a gene has completely unlinked genes, what is the frequency?

Answer 16

Parental: 50% (25% and 25%)
Recombinants: 50% (25% and 25%)

Question 17

Anything over/under 50%, linked and unlinked

Answer 17

Over 50 = unlinked
Under 50 = linked

Question 18

William Bateson and Reginald Punnett - Gene linkage

Answer 18

• Helped discover genetic linkage
• When crossing flower colour and pollen grain shape, found that F2 is showed more parental phenotypes compared to recombinants. - not a 9:3:3:1 ratio.

Question 19

Thomas Morgan and gene linkage

Answer 19

• Confirmed genetic linkage
• Constructed the first genetic maps
• Found that parental phenotypes are produced in F1 female gametes predominantly contain X chromosomes with 1 of the original parental sets of alleles
• Recombination occurs on homologous X chromosomes

Question 20

Two point test crosses

Answer 20

• Males have no crossovers because they only have 1 X chromy, so they produce parental gametes
• Females have single crossing over - producing 2 parental gametes and 2 recombinant gametes

Question 21

Morgans expected vs observed ratios

Answer 21

• Expected was 1:1:1:1 but observed was 1:1 parental and 1:1 recombinants
• parental ratio > recombinants
• parental ratio > 50%
• recombinants <50%

Question 22

Harriet Creighton and Barbara McClintock

Answer 22

• First demonstrated cytological evidence of recombination in corn
• Chromosome 9 testing colour and starch type
• Observed a physical exchange of genetic information of chromosomes

Question 23

Three point test cross

Answer 23

• A test-cross designed to identify genetic linkage between 3 genes and provides data for determining recombination frequency between linked genes
• 3 genes results in 8 genetically different gametic genotypes from each parents 2 parentals, 4 single crossovers, 2 double crossovers.

Question 24

Factors that cause recombination to vary

Answer 24

1. SEX - Heterogametic organisms have lower recombination because theres a smaller recomb map.
2. HOTSPOTS - recomb occurs more often than average based on the number of nucleotides present
3. COLDSPOTS - recomb occurs less often than average based on the number of nucleotides present
4. GENETIC RECOMBINATION MAP - relative position of genes based on recombination freq
5. PHYSICAL GENETIC MAP - actual position of the genes based on nucleotide sequence