BIOLOGY Write the short note.

(1) Thalassemia is an autosomal-eeessive disorder. The synthesis of alpha ciiains are controlled by two genes, (HBA1 and HBA2) on chromosome 16. Beta chain synthesis is controlled by gene HBB located on chromosome 11. Two alpha chains and two beta chains together form four polypeptide chains that make heterotetrameric haemoglobin molecule. But when there is defective gene on either of chromosome 16 or 11, there is quantitative abnormality of polypeptide globin chain synthesis. This results into thalassemia.

(2) Depending upon which chain is affected, thalassemia is classified as, alpha (α) thalassemia and beta (β) thalassemia.

(3) The clinical symptoms of thalassemia are as follows:

• Pale yellow skin.
• Anaemia due to inability to synthesize haemoglobin.
• Slow growth and development.
• Variation in the shape and size of RBCs.

(4) Patients need regular blood transfusions to cope with the disorder.

1. German biologist, Henking in 1891, was studying spermatogenesis of the squash bug (Anasa tristis).
2. He noted that 50% of sperms receive the unpaired chromosomes while other 50% sperms do not receive it.
3. Henking gave a name to this structure as the X-body. He was unable to explain its role in sex determination.
4. Further investigations by other scientists led to conclusion that the ‘X-body’ of Henking was a chromosome and gave the name as X-Chromosome to X-body.

1. Holandric means entirely of male sex. Y-linked genes are called holandric genes because they are located on non-homologous region of Y chromosome.
2. The Y-linked genes are inherited directly from male to male.
3. These genes are never seen in females due to lack of Y chromosome in them.
4. Hyper Mchosis and ichthyosis are examples of holandric genes.
5. Hypertrichosis means excessive development of hair on pinna of ear. This character is transmitted directly from father to son.
6. Ichthyosis person with rough skin.

1. Environmental sex determination is shown by lower organisms such as Bonellia viridis.
2. In this animal the environmental factors decide the sex of an offspring.
3. There is extreme sexual dimorphism in this worm. Female is about 10 cm long while male is tiny and parasitic in the reproductive parts of mature female.
4. If larva is reared in vicinity of mature female then it becomes a male. By settling on the proboscis of mature female, larva becomes parasitic, enters the female’s mouth and then takes permanent shelter in the female uterus. Such males then produce gametes and fertilize the eggs.
5. If larvae are drifted away from mature female or if they settle on the sea bottom, they develop into females. Thus determination of sex is due to environmental factors.

1. Widow’s peak is a prominent ‘V’ shaped hairline on forehead.
2. It is due to autosomal dominant gene.
3. Widow’s peak occurs in homozygous dominant (WW) and also heterozygous (Ww) individuals.
4. Individuals with homozygous recessive (ww) genotype do not have widow’s peak but have a straight hair line.
5. Both males and females have equal chance of inheritance.

1. Transmission of body characters occurs due to autosomes. They are not concerned with sex determination or sex linkage.
2. All the body characters from parents are passed on to their offspring through autosomes. This is called autosomal inheritance.
3. Some autosomal characters are due to dominant genes while some other are due to recessive genes. E.g. Widow’s peak and Huntington’s disease is also autosomal dominant character, etc.
4. Phenyl ketonuria (PKU), Cystic fibrosis and Sickle-cell anaemia are autosomal recessive traits.

1. Multiple alleles are more than two alternative alleles of a gene in a population situated on the same locus on a chromosome or its homologue.
2. Multiple alleles arise by mutations of the wild type of gene. Series of multiple alleles are formed due to several mutations that take place in the wild type of allele. This series show alternative expression.
3. Different alleles in a series show dominant-recessive relation or may show co-dominance or incomplete dominance among themselves. Among all the wild type is the most dominant one over all other mutant alleles.
4. In Drosophila, a large number of multiple alleles are known. E.g. The size of wings from normal wings to vestigial wings is due to one allele (vg) in homozygous condition. The normal wing is dominant and wild type while vestigial wing is recessive type.
5. Human blood groups A, B, AB and O Eire also due to series of multiple alleles.

1. Linkage is the tendency of genes to be inherited together because they are present in the same chromosome.
2. All the genes on a chromosome are linked to one another. In the linkage group some of the genes are included.
3. The number of linkage groups of a particular species corresponds to its haploid number of chromosomes present in the organism.
4. In human beings, there are 23 linkage groups which correspond to the pairs of chromosomes found in each cell.
5. Linkage groups can be separated only at the time of crossing over during meiosis. The linkage group can form a new combination of genes after crossing over.
6. Linkages Eire of two types, viz, complete linkage and incomplete linkage.
7. Morgan discovered linkage in animals while Bateson and Punnett discovered it in plants.