What is crossing over and how crossing?

Question

What is crossing over and how crossing over premoted genitic variability among individuals?

q2sing diagrams,illistrate some examples of nondisjunction.

3)describe the pattren of pairing of nucletides.

Answer 1

1.Crossing over occurs during Phophase I of Meiosis. homologous pais of chromosomes align together and adjacent chromatids from the homologous pairs can cross over at points called chiasma exhanging a portion of the chromosomes and hence genes.  The pairs are separated during Metaphase I and then sister chromatids are separated during Metaphase II.

The result of crossing over during meiosis is the production of four non identical haploid gametes for sexual reproduction.

2. Non disjunction occurs due to one or more spindle fibres failing during  Meiotic cell division. This results in production of some gametes with an extra copy of a chromosome and others (usually non viable) with one missing. This abnormal gamete can be fertilsed by a normal gamete to produce an individual with an extra copy of one chromosome.

eg Down's Syndrome is caused by an individual receiving

an extra copy of chromosome 21.

Non disjunction can also occur in the s*x chromosomes. two examples of these are Kleinfelter's Syndrome and Turner's Syndrome.

Persons with Kleinfelter's Syndrome are males with an extra copy of the X chromosome. They are infertile and may develop slight female characteristics. Their Genotype is XXY instead of the normal XY for males.

Persons with Turner's Syndrome are females who are missing an X chromosome. Theya are also infertile and have underdeveloped sexual organs. their genotype is XO instead of the normal XX for females.

Hope this helps!