MITOCHONDRIAL INHERITANCE AS A CASE OF MATERNAL INHERITANCE
Apart from the nucleus, mitochondria and chloroplasts also possess DNA. Till now you have studied about genes on the chromosomes present in the nucleus. Since mitochondria come into the zygote from the egg, inheritance of mitochondrial DNA or DNA is said to be a case of maternal inheritance.
In fact, certain disease and therefore the genes responsible for them are due to defects in mitochondrial DNA and can be traced to the mother’s family. Human karyotype is the arrangement of chromosomes in seven groups according to the types of chromosomes and their size. It is prepared by arranging chromosomes seen at mitotic metaphase. (i) Total no. of chromosomes or 2n 46 (23 pairs)
(ii) Number of autosomes 44 (22 pairs)
(iii) Sex Chromosomes X and Y = 2
(iv) Depending on size, location of centromere, and bands obtained by special staining methods, human chromosomes are grouped into 7 groups A to G.
Presence of Y is necessary for maleness. Absence of Y chromosomes makes the individual a female
CHROMOSOMAL ABNORMALITIES AND GENETIC IN HUMANS
Any change from the normal number or structure of chromosomes causes abnormalities. Following are some of them.
- Mongoloidism or Down’s syndrome The individual has 47 chromosome because of one extra chromosome in the 21st pair (Trisomy of Chromosome 21) The outcome is that they are
- Mentally retarded
- have a thick tongue
- and a drooping (false expression of pleasure) face.
The possibility of giving birth to a mongol child is far greater in pregnant mothers above the age of forty.
Individual is a male with 47 chromosomes with one extra X chromosome. (44 autosomes + XXY).
Typical features of a Klinefelter are
- Tall, mentally retarded male:
- Sterile and shows breast development or gynaecomastia (gynae female; massere : Mammary glands).
3. Turner’s syndrome
Individual is a female with 45 chromosomes with only one X, chromosome (22 pairs of autosomes + XO). The characteristic features are
- Mentally retarded
- web like skin on neck.
4. Colour blindness and Hemophilia (Bleeder’s diseases) Both these are sex linked disorders. Inheritance of colour blindness. In male, the single X- chromosome is received from the mother. Hence a defective, gene (for colour blindness or haemophilia) on X chromosome of the mother, is passed on to the son and expressed as a defect.
The daughter receives the X – chromosome from the mother and the other X from the father. The defective gene received from the mother may be masked by normal allele on the other X. The daughter is then called a carrier.
Colour blind males are unable to distinguish between red-green colours. In haemophilia afflicted male, blood does not clot easily and the patient may bleed to death. Its mode of inheritance is exactly like that of colour blindness.
5. Thalassemia
It is a disorder in which haemoglobin is not synthesised properly. So, frequent blood transfusions are required for survival. The defective gene is recessive and therefore heterozygous parents may not show the disorder. The child who gets the defective genes from both the parents (homozygous recessive) suffers from Thalassemia.
6. Sickle Cell Anemia
This is another hereditary abnormality due to mutation of a single gene in which red blood corpuscles lose their shape and become sickle shaped because of defective Haemoglobin. Individuals possessing two defective genes (homozygous recessive), cannot survive. In the heterozygous individuals, one gene is normal and so half the number of total red blood corpuscles are normal containing normal haemoglobin while the others are defective. A boon in disguise against malaria for children with one defective Haemoglobin gene can survive as they are less affected by malaria because the malaria parasite cannot thrive inside the defective RBC’s.
7. Rh factor
Rh factor is an antigen ( a protein) present on the surface of red blood corpuscles. About 15% of all women do not have the gene for Rh antigen. They are Rh-negative. Men can also be Rh negative, but the problem which this traits creates is in Rh- negative women. A pregnant Rh- negative women whose husband is Rh+ may bear a child who may have inherited the Rh + gene from the father. If the foetal blood enters mother’s body stream, her immune system produces antibodies against Rh antigen of the embryo. Rh antigens cause no problems in first pregnancy. But in a subsequent early pregnancy, the mother’s antibodies against Rh antigen may enter the foetal blood stream and destroy its red blood corpuscles causing severe anemia which may even be fatal. (erythroblastosis foetalis) Now a days Rh- negative mother of a Rh+ positive foetus is treated immediately after delivery, to destroy Rh antigens in her blood stream.
Amniocentesis in a technique by which hereditary disorders due to defects in genes can be detected. If incurable genetic defects are detected. If incurable genetic defects are detected, pregnancy can be terminated. It is illegal to use amniocentesis for detecting the sex of the unborn.
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