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Thursday, October 1, 2009

Colour Blindness (Short Note)

Thursday, October 1, 2009 - 0 Comments

It is a sex linked inheritance found in human beings. Persons suffering from colour blindness have difficulty in distinguishing red from green. It is rather common trait and more common in males. It is because Y chromosome is inert for this trait. Only one gene of this trait will render a colour blind man. While on the other hand a woman must have two genes for this trait to become colour blind.

This trait of colour blindness can easily be detected by using special charts made up of a number of coloured dots so arranged that colour blind person sea different pattern than other persons do.

Inversions (Short Note)

It is a type of chromosomal aberration in which a segment of chromosome is turned around 180ยบ and is reinserted into the chromosome. An inversion does not involve a loss of genetic information but simply rearranges the linear gene sequence. An inversion involves two breaks along the length of the chromosome prior to the reinsertion of the inverted segments. The inverted segment may be short or quite long and may or may not include the Centromere. If the Centromere is not the part of the rearranged chromosome segment, the inversion is said to be Para centric, if the Cetromere is a part of the inverted segment, the inversion is known as Peri centric.

The organisms in which one inverted chromosome and one non inverted homologue is present are called Inversion heterozygotes. Normal linear pairing between such chromosomes is not possible until they form inversion loop. In other cases no loop can be formed and the homologues are seem to synapse every where but along the length of the inversion, where they appear separated if crossing over does not occur within the inverted segment of the inversion heterozygote, the homologues will segregrate normally.

Delection or Deficiency (Short Note)

When a portion of a chromosome is lost, the missing piece is referred to as a Deletion or deficiency. It may be terminal if it is produced as one end of the chromosome or it may be intercalary if it occurs from interior of the chromosome. Both results from one or more breaks in the chromosome. The portion of the chromosome retaining the region of centromere will survive and the portion without the Centrosome will be lost after mitosis or meiosis in the resultant cells. During synapsis between structural heterozygotes (one normal complete chromosome and one with a deficeinty), the unpaired region f the normal homologue “buckles out”. Such a configuration is called Deficiency loop or Compensation loop. Loops can be observed in salwary gland chromosomes of Drosophila.

Chromosomal Translocations (Short Note)

Translocations involve the movement of a segment of a chromosome to new place in the genome. The translocation may occur within single chromosome or it may be Reciprocal Translocation. If it occurs between two non homologous chromosomes. The arms of two non homologous chromosomes come close to each other so that the exchange is facilitated. Two breaks are required if the translocation involves internal segment of the chromosome, two in each chromosome. The genetic information is not lost or regained as a result of translocation. Rather there is only rearrangement of genetic material. The presence of translocation does not therefore directly alter the viability following fertilization. Translocation like inversions may also produce position effect because they may realign certain genes in relation to other genes or to heterochromatic area. This exchange does however create new genetic linkage relationship which can be detected experimentally.

Sex Chromosome (Short Note)

Chromosomes which are similar in males and females of same species are called autosomes and the chromosomes which are different in them are called sex chromosomes. Each individual possesses a pair of sex chromosomes which are designated as X and Y. The male possesses XY and female possesses XX chromosomes but in some animals the position is reversed e.g. moths, birds and some fishes. In them it is the female which possesses a pair of unlike chromosomes whereas male has like chromosome. To avoid the confusion, the sex chromosome in such animals is called Z instead of X and the other member is called W. thus the genetic constitution in female is ZW and male ZZ.

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