CBSE Class 10 Science Heredity And Evolution Notes Set 07

HEREDITY AND EVOLUTION

• The process of transmission of characters from parents to offspring is known as inheritance. This is the basis of heredity.

• Genetics is the science that deals with heredity and variation.
• Variation: Small changes / modifications in a particular character that are visible between parents and Offsprings
• Gregor Johann Mendel is known as the "father of genetics".

• Heterozygous: Two different alleles are present together. E.g.- Tt
• Genotype: It is the genetic makeup of an individual. E.g.- TT, tt, Tt
• Phenotype: It is an observable feature. E.g.- tall, dwarf
• Monohybrid cross: Cross to observe a single character. E.g.- height of the plant
• Dihybrid cross: Cross to observe two characters at the same time. E.g. colour and shape of seed

Types of Variation

1- Somatic- occur in vegetative cell and not inherited
E.g. Boring of pinna by Indian women, hair style etc.

II- Germinal variations:
Occur in special gamete forming cells only
Inherit in next generation
E.g.: Human skin colour, shape of nose, etc.

Importance of variations

• Variation enables organisms to adjust and adapt better according to the changing conditions of the environment (Survival advantage).
• Different kinds of variations in organisms lead to the development of new species.

• Mendel worked on Pea plant (Pisum sativum).
• Advantages of using pea plant are- availability of pure line plant, clearly visible observable characters, contrast characters of same features, easily pollinated (self and cross) etc.
• He worked on 7 contrasting features of pea plant. E.g. Height of plant, flower colour, seed colour, seed shape, pod colour, pod shape and position of flower.
• He conducted monohybrid and Dihybrid cross.

MENDEL’S LAW OF INHERITANCE

• The Law of Dominance
• The Law of Segregation
• The Law of Independent Assortment.

Law of Dominance: When parents having pure contrasting characters are crossed then only one character expresses itself in the F1 generation. This character is the dominant character and the character/factor which cannot express itself is called the recessive character.

Law of segregation: The phenomenon of separation of the two alternating factors of one character, during gamete formation so that one gamete receives only one factor of a character is called as 'Law of Segregation'.

Law of Independent Assortment- 'When two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair of characters'.

• Dihybrid cross. He cross breed pea plants bearing round green seed (RRyy) with plants bearing wrinkled and yellow seeds (rrYY).
• In the F1 generation he obtained all round and yellow seeds it means round and yellow traits of seeds are dominant features while wrinkled and green are recessive.
• He self-crossed the plants of F1 and found that in F2 generation four different types of seeds round yellow, round green, wrinkled yellow and wrinkled green in the ratio of \( 9 : 3 : 3 : 1 \) are present.

HOW DO TRAITS GET EXPRESSED?

• DNA is regulating the authority to making of proteins in the cell.
• Gene provides information for one particular protein.
• E.g. the height of a plant depends upon the growth hormone which is in turn controlled by the gene.
• Both parents contribute equally to the DNA of next-generation during sexual reproduction.

SEX DETERMINATION IN HUMAN

The process of determining the sex of an individual, based on the composition of the genetic makeup is called sex determination.

• Human has 23 pair of chromosomes.
• Autosome: 22 pairs (44)
• Sex chromosomes: 01 pair (02). They may be either-
  • i- Homogametic – XX for female (44 + XX)
  • ii- Heterogametic XY for male (44 + XY)

In some organism-environment also plays a crucial role in the determination of sex-

• In some Reptiles: The temperature at which a fertilized egg is incubated governs the gender.
• Snails: A particular animal can change gender within one's lifetime.