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Class 10, Science, Chapter-8, Lecture-1, Heredity (Notes)

 

GENETICS:

The branch of biology in which the ‘continuity of features between generations’ and the ‘occurrence of differences among the individuals’ is studied is termed as genetics.

HEREDITY:

The transmission of traits from parents to the offspring is termed as heredity.

VARIATION:

The occurrence of differences in characteristics among the individuals of the same species is termed as variation.

CAUSE for VARIATION:

In asexual reproduction:

  • inaccuracies in DNA copying.

In sexual reproduction:

  • segregation of chromosomes during meiosis, and
  • independent inheritance of characters.

The creation of variation in a species promotes survival.

Justification:

Variation creates individuals with different characteristics.

In the event of a sudden change in the environment, an individual with suitable characteristics will survive.

For example, an individual that can withstand heat will survive better in a heat wave.

Thus, the Creation of variation in a species promotes survival.

MENDEL’S EXPERIMENT to show the ‘dominant’ and ‘recessive’  nature  of  traits:

Step one: Pea plants with one visible contrasting character – tall plants and short plants – are cross-pollinated.

Observation: In the F1 progeny, all plants are tall.

Step two: The F1 plants (all tall) are self-pollinated.

Observation: In the F2 progeny, some plants are tall, and some plants are short, in the ratio of 3:1.

Conclusion: Traits of shortness are present in F1 plants but only the trait of tallness gets expressed. So, the Tallness trait is dominant and the Shortness trait is recessive.

MENDEL’S EXPERIMENT to show the ‘independent inheritance’ of traits:

Step one: Pea plants with two visible contrasting characters, tallness/shortness, and rounded/wrinkled seeds are cross-pollinated.

Observation: In the F1 progeny, all plants are tall with round seeds.

Step two: The F1 plants (all tall with round seeds) are self-pollinated.

Observation: In the F2 progeny, some plants are tall with round seeds, some plants are tall with wrinkled seeds, some plants are short with round seeds, and some plants are short with wrinkled seeds, in the ratio of 9:3:3:1.

Conclusion: A tall plant can have wrinkled seeds or round seeds. 

So, the Tallness trait and roundness of seeds are independently inherited.

MENDEL’S PRINCIPLE OF INHERITANCE:

  1. Each character is controlled by two copies of factors.
  2. When two plants having contrasting characters are crossed, only the DOMINANT character appears among the next generation.
  3. The DOMINANT, as well as RECESSIVE characters, both appears in the second generation.

GENE:

A segment of DNA that occupies a specific position on a chromosome, and which is a hereditary unit of a specific biological function is termed as a gene.

Genes control traits.

Justification:

The traits of an individual are determined by the synthesis of specific proteins such as hormones.           

Genes are a segment of DNA that are responsible for the synthesis of specific proteins in the body.    

Thus, genes control traits.

MECHANISM OF HEREDITY:

The genes control traits and each gene set is present as a chromosome.

Every diploid cell has two copies of each chromosome, one each from the male and the female parents.

Germ cells are formed by meiotic cell division of diploid cells in which the chromosomes segregate and each daughter cell (haploid germ cells) possesses one set of chromosomes.

When two germ cells (one from each parent) combine to fertilize, the DNA of the resulting cell regains its stability by having two copies of chromosomes (one from each parent)

The dominant traits from each parent get expressed in the progeny.

SEX CHROMOSOMES:

The pair of chromosomes which determine the sex of an organism and which differ in males and females are called sex chromosomes.

SEX DETERMINATION IN HUMAN BEING:

In males (XY)- half of the sperm carry X-chromosomes and half of the sperm, carry Y-chromosomes.

In females (XX)- all the eggs carry X-chromosomes.

During fertilization:

If a sperm carrying a y-chromosome fuses with an egg, the result is a male zygote (XY).

If a sperm carrying an x-chromosome fuses with an egg, the result is a female zygote (XX).