Chapter 6_Genetic Linkage And Mapping In Eukaryotes

Question 1

Genetic Map

Answer 1

A diagram that describes the order of genes along a chromosome.

Question 2

Synteny

Answer 2

Two or more genes are located on the same chromosome. Genes that are syntenic are phsically linked to each other, because each eukaryotic chromosome contains a single, continuous, linear molecule of DNA.

Question 3

Genetic Linkage

Answer 3

The phenomenon in which genes that are close together on the same chromosome tend to be transmitted as a unit. Therefore, genetic linkage has an influence on inheritance patterns.

Question 4

Linkage Groups

Answer 4

An alternate name for chromosomes, because a chromosome contains a group of genes that are physically linked together.

Question 5

Genetic Recombination

Answer 5

Grouping of linked alleles have changed, leading to a new combination of alleles.

Question 6

Null Hypothesis

Answer 6

Assumes there is no real difference between the observed and expected values.

Question 7

High vs. Low Chi Square value

Answer 7

• Low = Cannot reject null hypothesis, so original hypothesis is valid
• High = Hypothesis is rejected.

Question 8

Why is genetic mapping useful?

Answer 8

• It allows geneticists to understand the overall complexity and genetic organization of a particular species.
• Genetic maps are useful from an evolutionary point of view.
• Many human genes that play a role in human disease have been genetically mapped; can be used to diagnose and treat inherited human diseases. Also helps predict likelihood of disease in offspring.

Question 9

What are most test crosses between?

Answer 9

Most test crosses are between an individual that is heterozygous for two or more genes and an individual that is heterozygous for two or more genes and an individual that is recessive and homozygous for the same genes.

Question 10

What is the goal of the testcross?

Answer 10

To determine if recombination has occurred during meiosis in the heterozygous parent.

Question 11

Positive Interference

Answer 11

The occurrence of a crossover in one region of a chromosome decreases the probability that a second crossover will occur nearby.

Question 12

Coefficient of Coincidence (C)

Answer 12

To provide interference with a quantitative value, we first calculate the coefficient of coincidence (c)

C = Observed number of double crossovers / Expected number of double crossovers

Question 13

How is interference expressed?

Answer 13

I = 1 - C

Question 14

Positive vs. Negative values of Interference

Answer 14

• Positive value is positive interference. It means that X percent interference is the expected number of crossovers did not occur. An interference of 0.60 would mean that 60% of the expected number of crossovers did not occur.
• A negative interference value suggests that a first crossover enhanced the rate of a second crossover in a nearby region.

Question 15

Unordered vs Ordered Tetrad/Octad

Answer 15

• If the ascus provides enough space for the tetrads or octads of spores to randomly mix together, then the tetrad can be unordered.
• Other species have a tight ascus that prevents spores from randomly moving around, creating an ordered tetrad or octad.

Question 16

In species that make ordered tetrads or octads…

Answer 16

…experimenters can determine the genotypes of the spores within the asci and map the distance between a single gene and the centromere.

Question 17

First-Division Segregation (FDS)

Answer 17

(M1 Pattern) A crossover has not occurred, so the octad contains a linear arrangement of four haploid cells. Creates a 4:4 arrangement. It is called first division because the A and a alleles have segregated from each other after the first meiotic division.

Question 18

Second-Division Segregation (SDS)

Answer 18

If a crossover occurs between the centromere and the gene of interest, the ordered octad will deviate from the 4:4 pattern. It will be either 2:2:2:2 or 2:4:2. It is an M2 pattern. In this case, the A and a alleles do not segregate until the second meiotic division.

Question 19

Give an accurate equation for determining the map distance of an unordered tetrad.

Answer 19

[(T + 6NPD) / Total number of asci] x 0.5 x 100