Chapter 5: Chromosome Mapping in Eukaryotes

Question 1

What are the units of transmission in meiosis?

Answer 1

Chromosomes, NOT genes

Question 2

What are linked genes?

Answer 2

Linked genes are genes that are likely to be inherited together because they are physically close to one another on the same chromosome.

Chromosomes have the potential to recombine, If genes are close together, the chances of being recombined are higher than if they are far away from each other.

Question 3

The frequency of crossing over on a single chromosome is proportional to…

Answer 3

Distance between them

Question 4

What does crossing over result in?

Answer 4

Recombination

Question 5

What do chromosome maps indicate?

Answer 5

The relative location of genes on a chromosome

Question 6

Meiotic Consequences: independent assortment

Answer 6

No linkage exhibited

Question 7

Meiotic Consequences: Linkage without crossing over

Answer 7

Complete linkage

Question 8

Meiotic Consequences: Linkage with crossing over

Answer 8

Generates recombinant gametes

Question 9

What is complete linkage?

Answer 9

0% recombination

Parental combinations of alleles always segregate together into the same gamete

Extremely rare

For this to occur, the alleles at the two loci are physically attached to the same chromatid and will nearly always segregate together into the same gamete.

No crossing over between two genes

Produces parental nonrecombinant gametes

Question 10

What is crossing over?

Answer 10

-Occurs between two nonsister chromatids
-Both parental and recombinant (crossover) gametes are produced

Question 11

What is the linkage ratio?

Answer 11

Complete linkage between two genes due to close proximity

-Results in unique F2 phenotypic ration results

Question 12

What is a linkage group?

Answer 12

-Genes on the same chromosome are part of a linkage group

-Number of linkage groups should correspond to haploid number of chromosomes

Question 13

Chiasmata

Answer 13

-Synapsed chromosomes in meiosis wrap around each other
-CHIASMATA: the point at which two non homolog pairs touch when crossing over

-X shaped intersections with points of overlap
-Points of genetic exchange

Question 14

The percentage of offspring resulting from recombinant gametes depends on distance between two genes on ________ chromosome

(same or different?)

Answer 14

Same

Question 15

Two genes that are located close to each other along a chromosome are less likely to have a ____________

Answer 15

Chiasma

Question 16

Sturtevant (this is the name of the scientist dude)

Answer 16

-He compiled data from crosses
-Recombination frequencies between linked genes are ADDITIVE

Question 17

Frequencies of exchange is an estimate of relative ….

Answer 17

distance between two genes

Question 18

Map unit (mu)

Answer 18

1 percent recombination between two genes on a chromosome

Also called the centi-Morgan (cM)

RELATIVE distances, not EXACT

Question 19

Single Crossover (SCO)

Answer 19

-Occurs between two nonsister chromatids
-Recombination is observed in 50 percent of gametes
-In genes that are 50 cM apart, crossing over can be expected between 100 percent of tetrad

Question 20

When a single crossover event takes place in the tetrad stage, what gametes are produced?

Answer 20

-Two noncrossover (parental) gametes are produced

-two crossover (recombinant) gametes are produced

Question 21

What are single crossovers used to determine?

Answer 21

the distance between two linked genes

Question 22

What are double corssovers, and what do they help determine?

Answer 22

-It is the double exchanges of genetic material

-Used to determine the distance between THREE linked genes

-In this case, genes must be heterozygous for two alleles

Question 23

What are the three criteria for three-point mapping?

Answer 23

-Parent must be heterozygous for all three genes under consideration

-Phenotypic class must reflect the genotype of gametes of parents

-Sufficient number of offspring must be produced for a representative sample

Question 24

What is a Three-point cross?

Answer 24

-Determining the distance and order of three genes on one chromosome (doing it at three genes at a time)

Question 25

Noncrossover F2 phenotypes occurs…

Answer 25

in the greatest proportion of offspring

Question 26

Double crossover (DCO) phenotypes occurs…

Answer 26

in the smallest proportion

Question 27

Reciprocal Classes of phenotypes

Answer 27

-F2 phenotypes compliment each other
-Derived from heterozygote
-Have wild type and mutant for all three genes

??

Question 28

Determining gene sequence: two methods?

Answer 28

Method 1: based in the three possible arrangements of genes

Method 2: uses three possible arrangements as basis, BUT it also considers the double-crossover event

Question 29

There are a large number of mutants in organisms like…

These organisms allow for ….

Answer 29

-Drosophila
-Maize
-Mice

Allows for the construction of extensive chromosome mapping

Question 30

What are DNA markers ?

What are the earliest examples of DNA markers?

Answer 30

-Short segments of DNA with known sequence and location

-Useful landmarks for mapping

-Earliest examples of DNA markers:
-RFLPs and microsatellites

Another example: Single-nucleotide polymorphisms

Question 31

What are RFLPs (what does it stand for, etc.)?

Answer 31

-Restriction fragment length polymorphisms

-polymorphic sites
-generated when specific DNA sequences are recognized and cut by restriction enzyme

Question 32

What are microsatellites?

Answer 32

-Short repetitive sequences
-Found throughout the genome

Question 33

What are single-nucleotide polymorphisms? provide an example of what they screen?

Answer 33

-Found throughout the genome
-Used by geneticists to identify and locate related genes
-Used to screen for diseases
Example: Cystic fibrosis

Question 34

What is cystic fibrosis? Give a description and relate it to DNA markers

Answer 34

-Gene located by using DNA markers

-Life-shortening autosomal recessive exocirne disorder

-Gene causing disorder found on chromosome 7

Question 35

During which phase in meiosis does crossing over take place in?

Answer 35

Prophase I of meiosis