L7

Question 1

what do you call a group of four cells that have retained all the products of a single meiosis

Answer 1

tetrad

Question 2

how do tetrads aid genetic analyses

Answer 2

• it allows the direct observation of genes during meiosis
• it gives a clearer insight into chromosome behaviour during meiosis
• enables the mapping of genes in relation to the centromere

Question 3

what are meiotic products of fungi called

Answer 3

spores

Question 4

where are spores stored

Answer 4

in a tube like array called ascus

Question 5

what is first division segregation

Answer 5

where there is no crossing over between the gene and the centromere

Question 6

what is second division segregation

Answer 6

where crossing over does occur between the gene and the centromere

Question 7

How do we measure the map distance of a gene from the centromere; in Neurospora

Answer 7

by using the frequency of crossing over; by measuring the frequency of crossing over between the centromere and a heterozygous gene locus by observing the frequency of first division segregation and second division segregation

Question 8

when does chiasma formation occur

Answer 8

during the four strand stage; involves the breakage and reunion of non sister chromatids

Question 9

Who are Beadle and Tatum and what organism did they use to demonstrate their hypothesis

Answer 9

Beadle and Tatum proposed the one gene one enzyme hypothesis, they used an organism called Neurospora crassa

Question 10

What is the Neurospora crassa and where does it grow

Answer 10

• can reproduce sexually or aesexually
• found in many parts of the world growing on dead vegetation and is a haploid fungi (n=7)
• when the diploid nuclei under goes meiosis and mitosis it produces eight ascospores

Question 11

true or false, each meiocyte produces a linear array of eight ascospores called octad

Answer 11

true

Question 12

true or false, centromeres are not genes but they are regions of DNA

Answer 12

true

Question 13

how come we cannot map centromeres in eukaryotes

Answer 13

this is because in most eukaryotes they show no heterozygosity, however in fungi that produce linear tetrads the centromere can be mapped

Question 14

What does centromere mapping involve

Answer 14

involves estimating the distance from a locus to a centromere

Question 15

What are the two basic patterns which can be observed from the meiosis of a meiocyte; resulting octad

Answer 15

4:4 or 2:2:2:2

Question 16

what is the ratio of the progeny during a first division segregation (M1)

Answer 16

4:4 ; equal number of A and a; arises when there is no crossing over between the gene and the centromere

Question 17

what is the ration of the progeny during a second division segregation (M2)

Answer 17

2:2:2:2 pattern; this arises when there is a cross over between the gene and the centromere

Question 18

how do we calculate the map distance of the gene from the centromere

Answer 18

you will need to divide the percentage of the second division asci by two as only half of the products of any meiosis with a single crossover will be recombinant

Question 19

How did Mclintock and Creighton demonstrate that crossing over involves the breakage and reunion of chromatids

Answer 19

The used ‘marked’ chromosomes in maize, the chromosome had an extra large segment at one end and a ‘knob’ at the other end; they knew they were studying the two genes of corn that were on chromosome 9; they conducted a test cross and found that all recombinants inherited one or other of the bottom two alleles

Question 20

What did Tease and Jones do to prove that the chiasmata is the crossing over site

Answer 20

In 1978 they used harlequin chromosomes; showed that there is a physical exchange of light and dark stained chromatid segments; they also showed that tetrads contained four different allele combinations thus crossing over must occur at 4 chromatid stage

Question 21

what does a double cross over involve

Answer 21

three chromatids

Question 22

what does a single cross over involve

Answer 22

two chromatids

Question 23

true or false, crossovers do occur between sister chromatids, but they are rare and do not produce new allele combinations

Answer 23

true