Genetic Basis Blood Grouping in human shows the phenomenon ABO blood grouping in human shows the phenomenon of codominance (alleles are able to express themselves independently when present together) and multiple allelism (more than two alleles govern the same character.

Monohybrid cross. It is a cross in which only one character is considered at a time. In a cross between tall and dwarf plant, the size of stem is considered. Mendel made a cross between pure tall
(TT) and a pure dwarf (tt) pea plant as follows:
Mendel selected a tall pea plant represented by genes (TT)and a dwarf pea plant represented by genes (tt). Pea plant is self pollinating. He removed the anthers of tall plant and stigma of dwarf plant, deposited pollen grains of dwarf plant on the stigma of tall plant and obtained seeds.
When the seeds were placed in the soil and allowed to grow he obtained only tall plant (Tt), although he was expecting the plants of an intermediate size. These plants were labelled as plants of First filial generation (\(F_{ 1 }\) 
generation). In \(F_{ 1 }\)  generation all plants were tall because the character tall is dominant over the dwarf.
He allowed the plants ofY~enerl:!tion to self pollinate. Gametes formed by meiosis contained only

\((i)W-I^{ \wedge }I^{ \wedge }\quad or\quad I^{ \wedge }i;\quad X-ii\\ Y-B-\quad group;Z-AB-group\)
(ii) 
(b) Dominance·
 It is the phenomenon in which one allele of a gene expresses itself and suppresses the expression of the other (recessive) allele of the same gene, when they are present together in a hybrid.
The hybrid resembles one of the parents.
Codominance:
It is the phenomenon in which two alleles of a gene are equally dominant and express themselves in the presence of the other.
The hybrid shows characters of both the parents.
Incomplete dominance:
It is the phenomenon in which neither of the two alleles of a gene is completely dominant over the other.
The hybrid is intermediate between the two parents.
(iii)  Y The allele \(I^{ B }\)   dominant over i and hence the phenotype is B-group. Z : The alleles \(I^{ A }\) and  \(I^{ B }\) are codominant and both express themselves and the phenotype is AB- group.

Symptoms of Haemophilia:
1.Since a protein necessary for blood clotting is not formed, the blood does not clot and there is non-stop bleeding in case of an injury in the afflicted individual. Symptoms of sickle-cell anaemia.
2. The RBCs become sickle-shaped.
3. The oxygen transport to the tissues is impaired.
Differences

(i) Sickle-cell anaemia
 It is caused by a change of a single base pair in the gene, leading to substitution of glutamic acid by valine. Inheritance pattern
It is transmitted from parents to the offspring, when both the partners are carriers
(heterozygous) of the disease.
Symptoms
 The RBCsbecome sickle-shaped.
(ii) Phenylketonuria
It is due to a defective allele on the autosome. Inheritance pattern
It is passed on from the parent who are heterozygous for the gene to the offspring.
The affected individual lacks an enzyme that converts the amino acid phenylalanine into tyrosine; consequently,phenylalanine gets accumulated and converted into phenyl pyruvate and other derivatives.
Symptoms
Accumulation of these compounds in the brain results in mental retardation; they are also excreted in the urine.

(a) Dihybrid cross:
1.Since white flowers have appeared in the progeny, both the parents must be heterozygous for flower colour, i.e. Vv.
2.Since all plants in the progeny have axial flowers, the parent with axialm flowers, must be homozygous for the trait, i.e. AA.
3. Genotypes of the parents are as follows: Terminal, violet flowers : aaVv
Axial, violet flowers: AAVv
Parents : Terminal  Violet flowers    X             Axial Violet flowers
                    aaVv                                                  AAVv

4. The phenotypic ratio:
3 Axial, violet flower 1 axial, white flowers is justified
(b) Law of independent assortment It states that when two pairs of traits are combined in a hybrid, segregation of one pair of traits is independent of the other pair of trait.

Thalassemia is categorised as a Mendelian disorder, because it. is an autosomal, recessively-inherited disorder, whose transmission follows Mendel's laws/ principles of inheritance.
(i) The disease is caused by mutation or deletion of the alleles and there is reduced synthesis of alpha or beta globin chain of hemoglobin.
(ii) The symptom is anemia, which is due to less synthesis of hemoglobin.

(a)A female (mother) is homogametic and produces ova, all containing one X-chromosome and 22 autosomes.
A male (father) is heterogametic and produces sperms of two types-50% of them carrying one X-chromosome and the other 50% carrying V-chromosome and 22 autosomes each.
The sex of the child is determined by, the type of sperm fertilising the ovum.
If the ovum is fertilized by a sperm carrying X-chromosome, female child results
If the ovum is fertilised by a sperm carrying V-chromosome, a male child is formed.


(b)