class 12 genetic gene interaction Human Karyotype
The arrangement of human chromosomes into groups according to the pairing, a number of centromeres, position of the centromere, absence/presence of satellite.
· The Human Karyotype was first made by Tijo & Levan of Sweden in 1956.
· Pictorial Representation of Karyotype is called an Idiogram which was made by Nettie Stevens.
A popular method of classification of chromosomes is called the Denver Method of classification of chromosomes.
X – Chromosome belongs to Group C .
Y – Chromosome belongs to Group G but has no satellite.
Steps Above the Rest —
1. Chromosome number 2 has been formed by a mutation which has led to Centric fusion of 2 chromosomes of Apes. (2n = 48)
2. Additions and 9 Pericentric types of inversion have been known to occur in Human Chromosomes. Thus we have evolved from apes through chromosomal aberrations.
The other types of chromosomes known to occur are :
1. B-chromosomes :
á These are called supernumerary chromosomes.
á They are extra chromosomes apart from Autosomes and Heterosomes.
á They are smaller than normal.
á They are slow to replicate.
á They are genetically unnecessary.
2. M-chromosomes :
á They are minute chromosomes.
á They are usually 0.5 mm.
á They are present in Bryophytes and Insects.
3. L-chromosomes /E-chromosomes:
á They are large in size.
á They are present only in germ cells.
á They are eliminated during the formation of somatic cells.
á They are constant in a given species.
CLASSIFICATION OF CHROMOSOMES
I. On The Basis of Type of Genes
1. Chromosomes having genes determining only somatic (body characteristics) Autosomes.
2. Chromosomes having genes determining the sex (gender) of the organism – Allosomes / Idiochromosomes
II. On The Basis of the Number of Centromeres
- Monocentric = Chromosomes having 1 centromere
Most of the chromosomes are monocentric.
2. Dicentric = Chromosomes having 2 centromere
For example, Maize (Zea mays) plant
- Polycentric = Chromosomes having more than 2 centromeres.
For example, parascaris equorum.
4. Acentric = Chromosomes without a centromere.
It is formed by breaking off one arm of the chromosome, usually by mutation.
III. On the Basis of Position of Centromere.
1. Metacentric. The centromere is present exactly in the centre of the chromosome.
2.Submetacentric. The centromere is present just away from the centre of the chromosome.
3. Acrocentric. The centromere is present close to one end of the chromosome.
- Telocentric. The centromere is present at the tip of the chromosome.
IV. On the Basis of Size of the Arms
1. Isobrachial. When the 2 arms of the chromosome are equal in size
as seen in metacentric chromosome
2. Heterobrachial. When the 2 arms of the chromosome are unequal in size
seen in submetacentric and acrocentric chromosomes.
V. On the Basis of the Extent of Coiling
1. Euchromatin. Loosely coiled chromatin material which can be uncoiled easily for transcription of RNA.
2. Heterochromatin. Tightly coiled (supercoiled) chromatin material containing a large amount of DNA; however it cannot be uncoiled and therefore cannot be transcribed to RNA.
Heterochromatin stains are darker than normal. This phenomenon is called Heteropycnosis (the term is given by Heitz).
Heterochromatin may be:-
(a) Constitutive Heterochromatin:- The chromatin which always remains transcriptionally inactive.
(b) Facultative Heterochromatin:- The chromatin material undergoes heterochromatization only at a certain stage in life. Example: Barr Body is facultative heterochromatized X-chromosome.