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What is heredity?

The continuity of features from one generation to another is known as heredity. It is also defined as the transmission of traits from parents to the offsprings.


Name the plant on which Mendel performed his experiments.

Mendel performed his experiments on the plant, Pisum sativum - the garden pea plant.


Define variation.

Children do not resemble their parents completely. They possess characters obtained from both the parents. These changes in the phenotypic and genotypic characters are known as variations. Thus a given population of a species has indefinite variants.


Define a gene.

A gene is a small portion of the DNA, with codes for a particular polypeptide or a protein. In other words, it is the functional unit of the DNA. It is also responsible for the transmission of hereditary characters from the parents to the offspring.


Write the expanded form of DNA.

The expansion for DNA is Deoxyribonucleic Acid.


What are the components of the chromosome?

A chromosome consists of two Chromatids joined together at the centromere. The chromaid consists of the DNA material wound over small protein molecules called histones.


What is a retrovirus?

A retrovirus is a virus that has RNA as its genetic material instead of DNA. HIV virus is a retrovirus.


What is a sex chromosome?

Chromosomes that are responsible for the determination of sex in an individual are known as sex chromosomes. There are two types sex chromosomes - the X chromosome and the Y chromosome. Males possess one X and one Y chromosome and females possess two X chromosomes.


How is sex determined in human beings?

The male sex gametes have X and Y chromosomes, whereas the female sex gametes have two X chromosomes. When a sperm containing a Y chromosome fuses with the ovum containing X chromosome, the zygote develops into a male. When a sperm containing the X chromosome fuses with an ovum containing X chromosome, the zygote develops into a female. Thus the sex of an individual is determined by the sex chromosomes X and Y, which is present in the male chromosomes.


Define homologous organs.

Homologous organs are those organs, which have similar origin and basic plan of development, but may or may not differ in their functions. The forelimbs of a human, a bird and a horse are homologous organs.


Explain Darwin’s theory of evolution.

Darwin’s theory of evolution is also known as the Theory of Natural Selection or Darwinism. Darwin explained that despite having enormous potential of fertility, the population size of any kind of organism remains within a limit. It is due to the struggle between members of the same species and different species for food, space and mate. This struggle eliminates the unfit individuals. In other words, the fit organisms possess some variations, which are favorable and can leave the progeny to continue the variations. This is called Natural Selection.


Define Genetics. What is the contribution of Mendel in the field of genetics?

The branch of biology that deals with the study of heredity and variations is known as Genetics. Gregor Johann Mendel was the first person to carry out experiments regarding the heredity of certain characters from one generation to another in a scientific manner. He worked mainly on the garden pea plant. His observations regarding the occurrence of contrasting characters in various generations of garden pea led him to interpret that these are controlled by units which he called, factors. These factors are today known as genes. He is also known as the Father of Genetics.


Where are the genes located? What is the chemical nature of genes?

Genes are segments of the DNA, which is wound compactly into chromosomes. A gene is composed of a specific sequence of nucleotides. A nucleotide is made up of a nitrogenous base, a sugar molecule and a phosphate group.


During which stage can the chromosomes be seen clearly? Write the features of the eukaryotic and prokaryotic chromosomes.

Chromosomes are distinctly visible during the Metaphase stage of mitosis of a cell.
The features of eukaryotic and prokaryotic chromosomes are as follows

Eukaryotic chromosomes
1. They are present in large numbers.
2. In higher organisms it occurs in paired condition.
3. They are visible distinctly only during the metaphase stage of mitosis.
4. Each chromosome consists of two chromatics attached together by a centromere.

Prokaryotic chromosomes
1. They are simple in composition when compared to the eukaryotic chromosomes.
2. They are generally circular in shape.
3. There is only one chromosome in a cell.


Who provided the evidence of DNA as the genetic material? Write the names of the components of the DNA molecule.

Frederick Griffith, Avery, Mc Leod and Mc Carty established that DNA was the carrier of the genetic information.

The DNA molecule is a polynucleotide. A nucleotide is made up of a nitrogenous base, a sugar molecule and a phosphate group.


What do you understand by the double helical structure of DNA? Who proposed this structure?

James Watson and Francis Crick proposed the double helical structure of the DNA. According to this structure,

DNA molecule consists of two polynucleotide strands forming a double helix. Each helical turn has a length of 3.4nm in which ten nucleotides are present.

Each polynucleotide strand has a backbone of sugar and phosphate. The nitrogenous base is attached to the sugar.

The nitrogenous bases of the two strands of a double helix form a pair with the help of hydrogen bonds. Adenine pairs with thymine by two hydrogen bonds, whereas guanine pairs with cytosine by three hydrogen bonds.

The hydrogen bonds hold the two strands of the helix together.


Describe the different types of chromosomes.

The different types of chromosomes are,
It is a chromosome with the centromere near the middle and the two chromatics are of equal lengths.

Here the centromere is situated slightly closer to one end than the other. Thus one chromatic is slightly longer than the other.

Here the chromosome is situated near one end of the chromosome. Thus one chromatic is very long while the other is very small.

Here the centromere is situated at the tip of the chromosome.


How do embryological studies provide evidence for evolution?

The embryology of different vertebrates provide very strong evidence favoring organic evolution. The early embryos of different vertebrates show striking similarities. This indicates common origin and ancestry of different vertebrates. Thus embryological studies provide direct evidence for evolution.


Define evolution. Describe the contribution of Lamarck.

Evolution is referred to as the changes acquired by a species or a certain population of a species gradually over a long period of time. These changes should be heritable.

Contribution of Lanark
According to the Theory of inheritance of acquired characters or Lamarckism, put forward by Lanark, the use and disuse of an organ leads to acquiring change in the features of that organ. These changes are also inherited by the offspring's. The favorable variations caused due to use and disuse after a considerably long period of time, results in evolution of a new species.


How do homologous organs provide evidence in support of evolution?

The presence of homologous organs indicates that all vertebrates have a common ancestry. Similarly, all organs and systems of the vertebrates show fundamental similarities, which point towards common ancestry.

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