Meiosis is an essential event in the sexual cycle of any organism because:
(i) It maintains the number of chromosomes fixed (2n) from generation to generation. Haploid gametes are formed as a result of meiosis. Diploid (2n) condition is restored during fertilization.
(ii) It increases the genetic diversity and variation due to crossing over between homologous chromosomes and their random segregation.

Characteristics of blastodermic vesicle
1. Rearrangement of blastomeres takes place.
2. A central cavity called blastocoel is formed.
3. Zona pellucida breaks down.
4. Blastula grows.
5.Differentiation of inner cell mass, trophoblast and outer wall of blastocyst occur.
6. Implantation of blastocyst in the wall of uterus.

Asexual Reproduction. It is a type of multiplication in which an offspring is formed a specialised or unspecialised part of a parent with or without the formation of sex cells but always without the fusion of sex cells.

(i) In Vallisneria, the male flowers are released on the surface of water and female flowers reach the surface by the long stalk.
(ii) Pollen grains are protected from wetting by a mucilaginous covering.

According to Central Dogma of molecular biology there is unidirectional flow of genetic information from DNA to m RNA and from here to protein.
\(DNA\underset { Transcription }{ \longrightarrow } mRNA\quad \underset { Translation }{ \longrightarrow } Protein\)
But in HIV (Human immuno-dificiency virus) there is central dogma reverse i.e. the flow of genetic information is in reverse direction. It is because the virus contains the genetic RNA and produces an enzyme reverse transcriptase. This enzyme helps to synthesize DNA from genetic RNA of HIV by a process called as reverse transcription or Teminism. This newly synthesized DNA functions as master copy producing mRNA through transcription & RNAs controlling translation to synthesize protein.
\(RNA\quad \overset { Reverse }{ \underset { Transcriplase }{ \longrightarrow } } DNA\quad \overset { Transcription }{ \longrightarrow } mRNA\quad \overset { Translation }{ \longrightarrow } Protein\)
The phenomenon of reverse transcription was discovered by Temin & Baltimore (1970) in Retrovirus.

(a) Motivate smaller families.
(b) Use media/poster showing a happy couple with two children with slogan 'hum do hamare do'.
(c) Statutory raising of marriageable age of the female to 18 years and male to 21 years.

Mendel performed his experiments on pea plants,i.e pisum sativum.After eight years of tedious experiments with pea plants, he proposed three fundamental principles of inheritance.These principles eventually assisted clinicians in human disease research,e.g after a few years of the discovery of Mendel's work,Archibald Garrod applied Mendel's principles to his study of alkaptonuria.Today, Mendalwhether we talk about pea plants or humans,genetic traits that follow the rules of inheritanceMendel proposed are called Mendelian Principles.

A diagrammatic representation of meiosis-I and meiosis-II (stage(a-e) represent prophase-I)is given below

(a) Ovary is superior
(b) Flower is hypogynous because all the floral arise from thalamus below the level of ovary
(c) Ovule is anatropous because it is completely inverted. The micropyle is near the hilum and faces down towards the placenta.The embryo sac is straight.
(d) The ovary of the flower gets converted into fruit after fertilization.
(e) A = Stigma, B = Style, C = Ovary and D = Ovule 

Tandemness in repeats provides many copies of the sequence for DNA fingerprinting and variability in nitrogen base sequences presents in them. Being individual-specific, this proves to be useful in the process of DNA fingerprinting.

The \(F_{ 1 }\) individuals are pink-flowered which is a case of incomplete dominance, it is a phenomenon in which neither of the two alleles of a gene is completely dominant over the other and the Fl hybrid is an intermediate between the two parents.
- The alleles of the hybrid segregate during gamete formation and the parental characters reappear without
any change; hence the phenotypic and genotypic ratios of the F2are the same, i.e.

Alfred Hershey and Martha Chase proved that DNA is the genetic material.
Experiment of Hershey and Chase
1. They made two different preparations of the phage; in one, the DNA was made radioactive with \(^{ 32 }P\) and in the other, the protein coat was made radioactive with \(^{ 35 }{ { S } }\).
2. These two phage preparations were allowed to infect the bacterial cells separately.
Soon after infection, the cultures were gently agitated in a blender to separate the adhering protein coats of the virus from the bacterial cells.
3. The culture was also centrifuged to separate the viral coat and the bacterial cells.
4. It was found that when the phage containing radioactive DNA was used to infect the bacteria, its radioactivity was found in the bacterial cells (in the sediment) indicating that the DNA has been injected into the bacterial cell.
5. So, DNA is the genetic material and not proteins.