CBSE Class 10 Science Heredity And Evolution Notes Set 08

Topics Covered

Accumulation of Variation during Reproduction and Heredity

Chapter Map

Heredity

• Accumulation of Variations
• Heredity
• Traits
  • Acquired
  • Inherited
    • Mendel’s experiments and formulation of laws of genetics
    • How traits get expressed
    • Sex-determination → Chromosomes → Male / Female

Topic 1. Accumulation of Variation During Reproduction and Heredity

Genetics: Branch of science that deals with heredity and variation.

Heredity: It means the transmission of features/ characters/ traits from one generation to the next generation.

Variation: The differences among the individuals of a species/ population are called variations.

There are two types of variations:

Accumulation of Variation during Reproduction

Variations appear during reproduction whether organisms multiply asexually or sexually.

Importance of Variation

• Depending upon the nature of variations different individuals would have different kinds of advantage. For example, bacteria that can withstand heat will survive better in a heat wave.
• It increases the chances of its survival in a changing environment. Free ear lobes and attached ear lobes are two variants found in human populations.
• If one bacterium divides and again the resultant two bacteria divide, the four individual bacteria generated will be highly similar, due to asexual reproduction.
• There will be only minor differences between them, generated due to small inaccuracy in copying of DNA.
• If sexual reproduction is involved, greater diversity will be generated.
• All these variations in the species do not have equal chances of survival.
• Different individuals would have different kind of advantages.
• Evolution involves selection of variants by environmental factors.

Gene: It is a section of DNA that provides information for one protein in a cell.

• If proteins works better and efficiently, the traits get expressed in better way. If protein helps in growth of plant, it will be taller. If protein does not work properly, plant will be shorter. It means gene will control the trait. It is a unit of inheritance.

Chromosome: Each gene set is present as separate independent pieces called chromosomes. Each cell will have two copies of each chromosome, one from each parent. The total number of chromosomes in the offspring will be 46, 23 from mother and 23 from father.

Mendel and His Work on Inheritance

Gregor Johann Mendel (1822-1884): Started his experiments on plant breeding and hybridization

• Mendel was known as the Father of Genetics

Plant selected by Mendel: Pisum Sativum (garden pea). Mendel used a number of contrasting characters for garden pea.

Table of Contrasting Characters (Seven Parts)

• Seven pairs of contrasting characters in garden pea and total 14 varieties of plant were studied by Mendel.

Genetic Terminology

Gene: Mendel used the term factor for a gene. A gene is the unit of DNA responsible for the inheritance of character.

Allele: A pair of genes that control the two alternatives of the same character e.g., TT/tt.

Heterozygous: The organism in which both the genes of a character are unlike e.g., Tt.

Homozygous: The organism in which both the genes of a character are similar e.g., TT, tt.

Dominant: The gene which expresses itself in F₁ generation is known as dominant gene.

Recessive: The gene which is unable to express itself in presence of the dominant gene.

Genotype: It is the genetic constitution (genetic makeup) of an organism which determines the characters.

Phenotype: It is the physical appearance of an individual. (Tall or short)

Mendel’s Experiments

Mendel conducted a series of experiments in which he crossed the pollinated plants to study one character (at a time)

TT, tt —— Pure or homozygous condition
Tt —— Heterozygous condition or Hybrid

Phenotypic ratio: Ratio among numbers of individuals with different external features, e.g., \( 3:1 \)

Genotype: → Genetic makeup [TT, Tt or tt]

Genotypic ratio: Ratio among numbers of individuals with different genetic make up, e.g., \( 1:2:1 \)

Monohybrid Cross

Cross between two pea plants with one pair (monohybrid cross) contrasting characters, e.g., Tall or Short Plants.

Parent Generation: Tall plant × Dwarf plant
TT × tt

Gametes: T, T (Ovules) and t, t (pollen)

F₁ Generation (First Filial Generation):

Genotypic and phenotypic ratio: All hybrid tall plants Tt

Self Pollination: (F₁) × (F₁)
Tt × Tt

Gametes: T, t and T, t

F₂ Generation (Second Filial Generation):

Phenotypic ratio: \( 3 : 1 \)
Genotypic ratio: \( 1 : 2 : 1 \)
Both dominant gene TT : One dominant, one recessive gene Tt : Both recessive gene tt.

Observations

• All F₁ progeny were tall.
• F₂ progeny \( \frac{1}{4} \) were short and, \( \frac{3}{4} \) were tall.
• Phenotypic ratio: F₂ — \( 3 : 1 \)
• Genotypic ratio: F₂ — \( 1 : 2 : 1 \)

Conclusions

TT and Tt both are tall plants while tt is a short plant.

• A single copy of T is enough to make the plant tall, while both copies have to be t for the plant to be short.
• Character/ Trait like ‘T’ is called dominant trait (because it expresses itself) and ‘t’ is recessive trait (because it remains suppressed).

Dihybrid Cross

A cross made between two plants having two pairs of contrasting characters is called dihybrid cross.

Parent Generation: Round Green seeds × Wrinkled Yellow seeds
Genotypes: RRYY × rryy
Gametes: RY and ry
F₁ Generation: RrYy [round, yellow]

F₁ × F₁: RrYy × RrYy

F₂ Generation:

Phenotypic Ratio:
Round yellow : 9
Round green : 3
Wrinkled yellow : 3
Wrinkled green : 1

Genotypic Ratio:
RRYY : 1
RRYy : 2
RrYY : 2
RRyy : 1
RrYy : 4
Rryy : 2
rrYY : 1
rrYy : 2
rryy : 1
Ratio: \( 1 : 2 : 2 : 1 : 4 : 2 : 1 : 2 : 1 \)

Observations

• When RRYY was crossed with rryy in F₁ generation all were RrYy, round and yellow seeds.
• Self pollination of F₁ plants gave parental phenotype together with two mixtures (recombinants). Round wrinkled and green yellow seeded plants which appeared in the ratio of \( 9:3:3:1 \) in F₂ generation.
• Conclusions: Round and yellow seeds are Dominant characters
• Occurrence of new phenotypic combinations show that genes for round and yellow seeds are inherited independently of each other.

Mendel’s laws of inheritance

Law of dominance

• Some alleles are dominant while others are recessive. An organism with at least one dominant allele will display the effect of the dominant allele.

Law of segregation

• During gamete formation, the alleles for each gene segregate from each other so that each gamete carries only one allele for each gene.

Law of independent assortment

• Genes for different traits can segregate independently during the formation of gametes.

Mechanism of Heredity

Most of the characters or traits of an organism are controlled by the genes. Genes are actually segments of DNA guiding the formation of proteins by the cellular organelles. These proteins may be enzymes, hormones, antibodies, and structural components of different types of tissues. In other words, DNA/ genes are responsible for structure and functions of a living body. Genotype of an individual controls its phenotype.

• Gene T → Responsible for synthesis of efficient enzyme (Proteins) → More production of growth hormone → Results in Tall Plants
• Gene t → Responsible for synthesis of less efficient enzyme → Less production of growth hormone → Results in short plants

Since we inherit half of our chromosomes [genes] from each of our two parents, hence we resemble both of them or their side of family members. For the same reasons, siblings resemble each other too. But if there are errors during DNA copying, genes may get altered and variation may appear in the form of a defect, disease or simply changed phenotype. Sexual reproduction is also a source of variation.

Sex Determination

Phenomenon of decision or determination of sex of an offspring.

Factors Responsible for Sex Determination

Environmental: In some animals the temperature at which the fertilised eggs are kept decides the gender, e.g., in snails, some lizards and turtle.

In some animals like humans gender of individual is determined by a pair of chromosomes called sex chromosomes.

Sex Chromosomes: In human beings there are 23 pairs of chromosomes. Out of these 22 pairs of chromosomes are called autosomes and the last pair of chromosomes that help in deciding gender of that individual are called sex chromosomes. XX – female; XY – male.

Sex determination in Human beings

• Male produces two genetic types of sperms, half with X and other half with Y chromosome. The female produces only one genetic type of ova, all carrying X chromosome.
• This shows that:
  • All children will inherit an X chromosome from their mother regardless whether they are boys or girls.
  • Thus sex of children will be determined by what they inherit from their father, and not from their mother.