Question

) Interstitial chiasmata are found in
i)
Univalents
ii) Bivalents
iii) Uni- and bivalents
iv) none of these.​

Answer 1

Answer:

diakinesis is generally used as a cytological

measure for the estimation of the total length of

the genome. The estimation is based on the

hypothesis that there is a one to one correspondence between genetic crossing over and chiasmata

(reviewed by HENDERSON 1969a and WHITEHOUSE

1969). If this hypothesis is correct chiasma counts

might, as suggested by HALDANE (1931), also give

information on genetic interference since in the

case of positive interference one chiasma would

reduce the likelihood with which another chiasma

is formed in its vicinity. Statistically this would

be recognized as a non-random distribution of

chiasmata within a chromosome, i.e. a deviation

from a Poisson distribution. Consequently the

mean chiasma frequency for each autosome

would be larger than the variance.

MATHER (1937, 1938, 1940) proposed that the

distribution of chiasmata in a cell is regulated by

two mechanisms, the interference of chiasmata

within each bivalent and the competition for

chiasmata between bivalents. Interference was

believed to imply that chiasmata are formed

serially along the chromosome. On the basis of

an analysis of the relationship between chromosome length and chiasma frequency in species

with a large variation in chromosome size and

one obligate chiasma on each bivalent, MATHER

suggested that the serial formation of chiasmata

starts at a specific distance from a fixed point on

the bivalent. In analogy with his findings on

crossing over in Drosophila melanogaster ( MATHER

1936) he believed that this fixed point most

probably was the centromere. The distance

between the centromere and the initial chiasma,

the so-called differential distance, was suggested

to be either bivalent-specific or proportional to

the arm length. Subsequent chiasmata were

considered to be formed at a general, species

specific interference distance, which would be

equal for all chromosomes. The interference

distance was defined as the chromosome length

required to obtain an increase in the mean

chiasma frequency of 1 .O chiasmata per bivalent.

In order to test the above hypothesis of

MATHER the chiasma distribution within individual chromosome arms has been thoroughly