Maharashtra Board Class 9 Science Chapter 16 Heredity and Variation PDF Download

Heredity And Variation

Do all the boys and girls of your class look alike?

Think about the following characteristics and note similarities and differences. (Teachers should help in this activity.)

Earlier, we have seen that there is great variation within every species in nature. In this chapter, we shall study the factors that give rise to these variations.

Inheritance

The branch of biology which studies the transfer of characteristics of organisms from one generation to the next, and genes in particular, is called 'genetics'.

New progeny is formed through the process of reproduction. Except for a few minor differences, the offspring shows great similarities with parents. Organisms produced by asexual reproduction show minor variations. However, offspring produced through sexual reproduction, show comparatively greater variations.

Teacher's Note

Did you know that children always look like their parents? This is because of inheritance. For example, if your mother has curly hair, you might also have curly hair.

Exam Trick

Remember: Asexual = one parent = less change. Sexual = two parents = more change.

Points to Remember

Inheritance means passing traits from parents to children.
Asexual reproduction shows small changes in babies.
Sexual reproduction shows big changes in babies.
Biology is the science that studies inheritance.
All living things have different traits from each other.

Heredity

Transfer of characteristics from parents to offspring is called heredity. It is due to heredity that puppies are similar to dogs, squabs are similar to pigeons and infants are similar to humans.

Inherited Traits And Expression Of Traits

How do specific traits or characteristics appear in organisms?

Though there are many similarities between parents and their offsprings there are some differences too. These similarities and differences are all the effect of heredity. Let us study the mechanism of heredity. Information necessary for protein synthesis in the cell is stored in DNA. The segment of DNA which contains all the information for synthesis of a particular protein is called a 'gene' for that protein. It is necessary to know the relationship of these proteins with the characteristics of organisms.

To understand the concept of heredity let us consider the characteristic 'plant height'. We know that there are growth hormones in plants. Increase in height of plants depends upon the quantity of growth hormones.

The quantity of growth hormones produced by a plant depends upon the efficiency of the concerned enzyme. Efficient enzymes produce a greater quantity of the hormone due to which the height of the plant increases. However, if the enzymes are less efficient, a smaller quantity of hormone is produced leading to a stunting of the plant.

Teacher's Note

Heredity is why children look like their parents. For example, a tall father often has tall children because height is inherited.

Exam Trick

Remember: Heredity = what you get from your parents. Like inheriting a house from grandparents!

Points to Remember

Children get traits from their parents through heredity.
Puppies look like dogs because of heredity.
Genes control how traits appear in the body.
Enzymes help make proteins that show traits.
Plant height depends on growth hormone amount.

Chromosomes

The structure in the nucleus of cells that carries the hereditary characteristics is called the chromosome. It is made up mainly of nucleic acids and proteins. During cell division chromosomes can be clearly seen under the compound microscope. 'Genes' which contain the information about hereditary characteristics in coded form are located on chromosomes. Each species has a specific number of chromosomes.

Each chromosome is made up of DNA and it appears dumbell-shaped midway during cell division. There is a constricted region on each chromosome. It is called the 'Primary constriction' or 'Centromere'. This divides the chromosome into two parts. Each part is called an 'arm'. The centromere has a specific position in each chromosome. Depending upon this, there are four types of chromosomes.

Teacher's Note

Chromosomes are like instruction books in every cell of your body. Humans have 46 chromosomes, just like how a library has many books.

Exam Trick

Remember: Centromere = center of chromosome. Like the center of a wheel!

Points to Remember

Chromosomes are found in the nucleus of cells.
They carry hereditary information.
Chromosomes look like dumbbells during cell division.
The centromere divides the chromosome into two arms.
Each species has a specific number of chromosomes.

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