Chapter 7 - Linkage, Recombination, and Eukaryotic Gene Mapping

Question 1

How do genes on the same chromosome travel during meiosis?

Answer 1

Genes that are on same chromosome travel through meiosis together

Question 2

What is crossing-over?

Answer 2

Alleles of chromosomally linked genes can be recombined by crossing-over

Question 3

What did Morgan and Sturtevant observe about linked genes?

Answer 3

• Some genes do not segregate randomly
  MORGAN
• Genes are linked if present on same chromosome
• Divergence from linkage is called recombination
  STURTEVANT
• Frequency of recombination reflects distance between genes

Question 4

What is Mendel’s Law of Independent Assortment?

Answer 4

The inheritance pattern of one trait does not affect the inheritance pattern of another trait (Ratio 9:3:3:1)

Question 5

What did the Bateson and Punnett experiment reveal?

Answer 5

Some pairs of genes do not segregate independently (i.e. they do not produce distribution of 9:3:3:1)

Question 6

What is a possible explanation for genes that do not segregate independently?

Answer 6

• R and L genes are linked on same chromosome
• RL and rl alleles segregate as linked genes
• There is some infrequent exchange between chromosomes that harbor RL and rl
• Exchange = recombination

Question 7

How do you calculate the frequency of recombination?

Answer 7

recombinants/(total # gametes) = Frequency of recombination

Question 8

How can you tell when genes are not linked?

Answer 8

F1 generation has 4 classes of offspring with equal frequencies

Question 9

What will the F1 generation look like if the genes are linked?

Answer 9

• 2 major classes of offspring with equal frequencies

- 2 minor classes of offspring as a result of recombinant gametes

Question 10

How does recombination take place? When in the cell cycle?

Answer 10

• Crossing-over between homologous chromosomes
• Prophase I of meiosis
• Tetrad/bivalent is formed
• A chiasma (physical junction b/n homologous chromatids) is where recombination occurs

Question 11

Can multiple crossovers happen simultaneously over the same tetrad?

Answer 11

Yes

Question 12

What is the relationship between distance b/n genes on a chromosome and recombination?

Answer 12

The closer genes are to each other on the chromosome, the more likely they are to crossover together (decreasing likelihood of recombination)

Question 13

What is chromosome mapping?

Answer 13

Simple calculating of probabilities
Ex.
# recombinants/total offspring x 100 = map units
100/400 x 100 = 25 map units

Question 14

Calculate the genetic distance b/n genes A and B.
AB//ab = 18 progeny
Ab//ab = 72 progeny
aB//ab = 64 progeny
ab//ab = 10 progeny

Answer 14

17 map units

Question 15

What are the 3 possible scenarios for a dihybrid cross (AaBb x AaBb)?

Answer 15

• Genes A and B are independently assorting (4 classes of offspring with 9:3:3:1 ratio)
• Genes are tightly linked so crossing over does not happen (2 classes of offspring with 3:1 ratio)
• Genes are linked and crossing-over occurs (variations in 3:1 ratio)

Question 16

If the ratio is 24:1:1:7, what does that tell us?

Answer 16

• R and L are linked
• R and L recombine at rate of 0.06
• R and L are 6 units apart

Question 17

A map distance greater than ____ cM leads to independent assortment.

Answer 17

About 50 map units

Question 18

Unspotted (A) is dominant to spotted (a) and short hair (L) is dominant to long hair (l). Dihybrid females were crossed to a homozygous male to yield:
- 75 spotted/short-haired
- 66 unspotted/long-haired
- 10 unspotted/short-haired
- 7 spotted/long-haired
Are the genes linked? What is the genotype of the dihybrid parent? What is the genetic distance b/n genes A and L?

Answer 18

Yes, and the genotype of the dihybrid parent is Trans (because the dominant and recessive alleles are found together). The genetic distance is 10.8 map units.

Question 19

What are the 2 possible configurations for a dihybrid parent?

Answer 19

Cis (AL//al)

Trans (aL//Al)

Question 20

Why is the relationship b/n linkage and pedigrees important?

Answer 20

• Huge amount of info about phenotypes of related individuals
• Use this info to generate elaborate genetic maps and to track inheritance of certain genes, genetic disorders and other traits

Question 21

Are pedigrees always successful in locating genes?

Answer 21

• Clear sign of heritable trait in prognatism, but detailed genetics are unknown
• Majority of pedigrees suggest autosomal dominant
• Incomplete penetrance
• Major gene that influences prognatism with signs of Mendelian inheritance and multifactorial component
• Pedigrees can have limited success in complex genetic traits