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

Dominance:
Every gene contains the information to express a particular trait.
In a diploid organism, there are two alternate forms of a gene, or a pair of alleles.
The two alleles may be similar (homo-zygous) or may be different (heterozygous).
One of them may be different due to some changes that it has undergone, which modify the information of that particular allele.
The phenotype will be dependent on the original unmodified allele.

\((i)W-I^{ \wedge }I^{ \wedge }\quad or\quad I^{ \wedge }i;\quad X-ii\\ Y-B-\quad group;Z-AB-group\)
(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) The gene for blood group character exists in three allelic forms, lA, IB, and i.
(ii) It is the phenomenon of multiple allelism, as there are more than two  allelic forms of a gene.
 Any individual carries two of the three alleles.
(iii) The allele \(I^{ A }\)  codes for glycoprotein A and the allele IBcodes for glycoprotein B, that are found on the surface of RBCs; the allele i does not produce any glycoprotein.
(iv) The allele \(I^{ A }\)  is dominant over i and IB' is also dominant over i.
(v) When the alleles \(I^{ A }\) and  \(I^{ B }\) are together. they are equally dominant and both the glycoproteins A and B are produced, making the blood group AB; this phenomenon, where both the alleles are equally dominant, is known as codominance.
(vi) The blood group is determined by the glycoprotein(s) on the RBCs.
(vii) There are six genotypes and four phenotypes as given in table.                  

(i) The gene for blood group character exists in three allelic forms, lA, IB, and i.
(ii) It is the phenomenon of multiple allelism, as there are more than two  allelic forms of a gene.
(iii) Any individual carries two of the three alleles.
(iv) The allele \(I^{ A }\)  codes for glycoprotein A and the allele IBcodes for glycoprotein B, that are found on the surface of RBCs; the allele i does not produce any glycoprotein.
(v) The allele \(I^{ A }\)  is dominant over i and IB' is also dominant over i.
(vi) When the alleles \(I^{ A }\) and  \(I^{ B }\) are together. they are equally dominant and both the glycoproteins A and B are produced, making the blood group AB; this phenomenon, where both the alleles are equally dominant, is known as codominance.
(vii) The blood group is determined by the glycoprotein(s) on the RBCs.
(viii) There are six genotypes and four phenotypes as given in table.

(a)

(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.

(a) It is due to trisomy of 21st chromosome.
The symptoms include
Short stature and small round head with a flat back.
Rartially open mouth with furrowed tongue.
Broad, flat face with slanting eyes.
Broad palm with characteristic palm crease.
Retarded physical, mental and psychomotor development.
It is caused by the non-segregation of the 21st chromosomes during meiosis in the gamete formation; when such an ovum is fertilised by a normal sperm, the individual comes to possess three copies (trisomy) of 21st chromosome.
(b)