River crossing

A ford (crossing)

A bridge.

A tunnel.

Anytypeof ferry. A cable ferry. A reaction ferry. A water taxi.

Types of Crossing Over(ii) DoubleCrossing Over: In thistypetwo chiasmata are formed along the entire length of the chromosome leading to breakage and rejoin of chromatids at two points. The gametes produced are called double crossovergametes

Crossing over is the process of exchange of genetic material or segments between non-sister chromatids of two homologous chromosomes. Crossing over occurs due to the interchange of sections of homologous chromosomes.

1. Meaning of Crossing Over:

Crossing over refers to the interchange of parts between non-sister chromatids of homologus chromosomes during meiotic prophase (pachytene). In other words, crossing over results from exchange of genetic material between non-sister chromatids involving breakage and reunion at precise point. The term crossing over was first used by Morgan and Cattell in 1912.

2. Feature of Crossing Over:

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The main features of crossing over are given below:

1. Crossing over takes place during meiotic prophase, i.e., during pachytene. Each pair of chromosome has four chromatids at that time.

2. Crossing over occurs between non-sister chromatids. Thus one chromatid from each of the two homologus chromosomes is involved in crossing over.

3. It is universally accepted that crossing over takes place at four strand stage.

Crossing Overon the Chromosomes: Mechanisms,Kinds, Factors and Significance!Crossing overis the process of exchange of genetic material or segments between non-sister chromatids of two homologous chromosomes. ... The chromosomes usually undergo breakage during gametogenesis.

Crossing Overon the Chromosomes: Mechanisms,Kinds, Factors and Significance!Crossing overis the process of exchange of genetic material or segments between non-sister chromatids of two homologous chromosomes. ... The chromosomes usually undergo breakage during gametogenesis.

Crossing Overon the Chromosomes: Mechanisms,Kinds, Factors and Significance!Crossing overis the process of exchange of genetic material or segments between non-sister chromatids of two homologous chromosomes. ... The chromosomes usually undergo breakage during gametogenesis.

(i)Single Crossing Over:In this type of crossing over only one chiasma is formed all along the length of a chromosome pair. Gametes formed by this type of crossing over are called single cross over gametes .

(ii)Double Crossing Over:In this type two chiasmata are formed along the entire length of the chromosome leading to breakage and rejoin of chromatids at two points. The gametes produced are called double cross over gametes .

(iii)Multiple Crossing Over:In this type more than two chiasmata are formed and thus crossing over occurs at more than two points on the same chromosome pair. It is a rare phenomenon.