Biology Crossing Over

The microscopically visible region where crossing over has occurred between homologous non-sister chromatids

The recombination or new combination of genes is possible only due to exchange of genetic material between

Is a recombination of genes due to exchange of genetic material between homologous pairs

It’s is the mutual exchange of segments of genetic material between non-sister chromatids of two homologous chromosomes so as to produce

The non-sister chromatids in which exchange of segments have occurred are called recombinations or cross-overs while the other chromatids in which crossing over has not taken place are known as

A new grouping of genes or a new combination of characters which is different from the parental types is called

What is produced due to crossing over that occurs during meiosis prior to gamete formation

The frequency of recombination is directly related to the frequency of

An individual contains two homologous chromosomes of each

The two chromosomes are obtained from the two different

These two chromosomes are called

If there is no crossing over at the time of gamete formation or meiosis only ____ types of gametes are produced one carrying the ______ (AB) and the other _______ (ab) genes.

Thus ____ types of chromatids will be produced after one crossing over.

Of the 4 chromatids produced two of the are _____ (AB, ab) and two ________ (Ab, aB).

After meiosis the four types of chromatids segregate and pass on to four different gametes, ___ parental and ___ recombinant.

What are the factors that influence crossing over

The physical distance between two genes _________________________ between two genes.

The frequency of crossing over _____ with the increase in the physical distance between the two genes.

Increase in age _______ the degree of crossing over in most of the cases.

Exposure to x-rays ______ the incidence of crossing over.

Variations in temperature _______ the frequency of crossing over.

A number of _____ present in the food have been found to change the degree of crossing over in animals.

Is a means of introducing new combinations of genes and hence traits.

Crossing over increases ______ which is useful for natural selection under changed environment.

Since the frequency of crossing over depends upon the distance between the two genes, the phenomenon is used for preparing

Chromosome maps have proved that genes lie in a ______ fashion in the chromosome.

For obtaining cross-overs between closely linked genes, a very ____ population is required.

Types of crossing overs

Crossing over occurs at one point between two non-sister chromatids of a homologous chromosome pair. There are two parental types and two recombinants.

Crossing over occurs at two points in a homologous pair of chromosomes.

Three or more points of crossing over occur in the same homologous chromosome. Double cross-overs and parental types may or may not occur.

Chromosomes get replicated in S phase of interphase

Replicated homologous chromosomes come to lie side by side with similar genes of the two chromosomes exactly opposite. It occurs in prophase 1.

It occurs in prophase __ stage (chromosomes are in a four strand stage at this point) with the help of enzymes. There is a breakage of chromatid segments, exchange of non-sister chromatids segments and later their fusion in new places.

Four stand stage

Synaptinemal complex begins to dissolve except in the region of crossing over. Therefore, chromosomes separate and chromatids become distinct at most of the places. As it appears to have four chromatids now, it is called a ____ _____. The synaptinemal attachment points between the homologous chromosomes called _________.