(a) Thalassemia - is an autosomal recessive blood disease which can occur due to deletion of the genes controlling the formation of globin chains (commonly Alpha and Beta) of haemoglobin.

(b) Mother cannot be blamed for the disease as, it is an autosomal recessive blood disorder. The genes for the synthesis of globin chains are present on autosomes.

(i) Formation of Alpha chain is controlled by 2 genes present on chromosome 16 and

(ii) Formation of Beta-chain is controlled by one gene present on chromosome 11manifestation of the disease occurs when the progeny receives defective genes from both the parents (as thalassemia is expressed is homozygous recessive condition only).

(c) Values that can be propagated in families are

(i) One should be aware of such autosomal diseases and should get themselves checked for the same before marriage.

In humans, the blood groups are controlled by a gene called gene I. It has three alleles, namely I^A, I^B and i.  The plasma membrane of the red blood cells has sugar polymers that protrude from the surface and the kind of sugar is controlled by the gene. The alleles I^A and I^B produce a slightly different sugar while i do not produce any sugar. Since humans are diploid, each person can have different combinations of the three alleles. The alleles I^A and I^B are dominant over i therefore a combination of I^A and iproduce blood group A, I^B and I produce blood group B. The combination of I^A, I^B produces blood group AB which has both A and B types of sugar, because of co-dominance. The combination of I^A and I^A produce blood group A, I^B and I^B produce B blood group.

The type of blood group produced by the different combinations is shown in the table below:

The starch synthesis in pea plants is controlled by a single gene. It has two alleles B and b. BB homozygotes produce starch effectively and therefore produce large starch grains. Whereas the bb homozygotes are less efficient in starch production and hence produce smaller grains. After maturation the BB seeds were found to be round and bb was wrinkled. Heterozygotes produce round seeds, and so B was considered to be the dominant allele. But when they were crossed the resultant progeny were intermediate-sized Bb seeds.

P generation                      BB × bb

                                            ↓

F₁ Generation                     Bb × Bb

(a) Monohybrid cross is the cross in which only one trait is taken into account.

 For example, if pea plant with yellow seed coat is crossed with pea plant having green seed coat then in the F1 generation all the plants produce yellow seeds.

(b) Two laws can be derived from such a cross

Law of Dominance- According to this law, characters are controlled by discrete units called factors, which occur in pairs with one member of the pair dominating over the other in a dissimilar pair. It explains the expression of only one of the parental character in F₁generation and expression or appearance of both in F₂ generation.

In the given cross, the allele for yellow seeds is dominant over the allele that produces green seeds. In F₁ generation all offsprings showed yellow colour of seed (dominant character) and expression of both yellow and green in F₂ generation.

Law of Segregation-This law states that the two alleles of a pair segregate or separate during gamete formation such that a gamete receives only one of the two factors. In homozygous parents, all gametes produced are similar; while in heterozygous parents, two kinds of gametes are produced in equal proportions. The alleles for yellow and green seeds in the F1 generation segregate and the gamete either contains yellow allele or green allele giving rise to different combination in the F2 generation. Thus, showing that the gametes receive only one of the two factors.

(a) Thalassaemia and haemophilia are categorised as Mendelian disorders because they occur by mutation in a single gene. Their mode of inheritance follows the principles of Mendelian genetics. Mendelian disorders can be

• autosomal dominant (muscular dystrophy)
• autosomal recessive (thalassaemia)
• sex linked (haemophilia)

Symptoms of Thalassaemia

• Thalassaemia minor results only in mild anaemia, characterised by low haemoglobin level.
• Thalassaemia major is also known as Cooley's anaemia. In this disease, affected infants are normal but as they reach 6 to 9 months of age, they develop severe anaemia, skeletal deformities, jaundice, fatigue, etc.

Symptoms of Haemophilia

• Person suffering from this disease does not develop a proper blood clotting mechanism.
• A haemophilic patient suffers from non-stop bleeding even on a simple cut, which may lead to death.

Pattern of Inheritance of Thalassaemia
Pair of alleles Hb^A and Hb^T controls the expression of this disease.
Conditions for thalassemia:

• Hb^A and Hb^A: Normal
• Hb^A and Hb^T: Carrier