BIOLOGY:- PRINIPLES OF INHERITANCE AND VARIATION:
1. Heredity: Transmission of characters from one generation to another (similarity of offsprings to
parents).
2. Variation: The differences between parents and offsprings and among themselves.
3. Character: It’s a feature of the individual.
e.g. stem height
4. Trait: An inherited character and its detectable variant.
e.g. tall/ dwarf
5. Gene (Mendelian factor): It is a functional unit of DNA. It’s a basic or elementary unit of inheritance.
6. Allele: Alternative forms of a gene.
e.g. T/ t
7. Gene locus: It is the position or region on chromosome representing a gene.
8. Phenotype: It is the observable external appearance.
E.g. tall, dwarf
9. Genotype: it is the genetic constitution of an organism.
E.g. TT, Tt, tt
10. Homozygous: The organism in which both the alleles of a gene are identical.
e.g. TT, tt
11. Heterozygous: Organism in which both the alleles of the character are different.
e.g. Tt
12. Dominant allele: An allele that influences the appearance of the phenotype even in the presence of
an alternative allele.
e.g. T
13. Recessive allele: An allele that influences the appearance o the phenotype only in the presence of
another identical allele.
e.g. tt
14. Monohybrid cross: It is a cross that involves the inheritance of one character (one allelic pair).
e.g. inheritance of stem height.
15. Dihybrid cross: It is a cross that involves the inheritance of two characters (two allelic pair).
e.g. inheritance of seed color and shape.
16. Test cross: In a test cross the F1 generation is crossed to a recessive parent.
17. Genome: Total set of genes.
18. Gene pool: The sum total of all alleles or genes in a population forms the gene pool.
19. Homozygous vs Heterozygous.
HOMOZYGOUS
HETEROZYGOUS
1. Homozygous individual possess similar alleles
for a particular trait.
1. Heterozygous individual possess dissimilar
alleles for a particular trait.
2. Possess either dominant or recessive allele
never both.
2. Always with both dominant and recessive
allele.
3. It produces only one type of gametes.
3. It produces different types of gametes.
4. They breed true for that character.
4. They don’t breed true.
5. They don’t show extra vigour.
5. They show extra vigor(hybrid vigor or
heterosis)
6. e.g., TT or tt
e.g., Tt
20. Why did Mendel choose Garden pea (Pisum sativum)?
Ans: a. Pea plant is small & easy to grow.
b. Its an annual plant.
c. Plant being bisexual is self fertilizing in nature but can be easily cross pollinated experimentally.
d. It produces a large no. of offsprings and completes its life cycle in one season.
e. A large no. of true breeding pea varieties are available.
f. Garden peas possess many contrasting heritable characters.
21. Reasons for Mendel’s success:
a. He selected garden pea as his experimental material.
b. He kept accurate records of his experiments.
c. He used statistical methods for finalizing his results.
d. Mendel studied the inheritance of one or two traits at a time.
e. Mendel was fortunate that the characters he selected for his breeding experiments did not show
linkage, incomplete dominance etc.
22. Why Mendel’s observations remained unnoticed?
Ans: a. He published his works in an obscure journal.
b. Communication was not easy in those days and his work could not be wildly publicized.
c. His concept of genes as stable and discrete units that controlled the expression of traits and of
the pair of alleles which don’t blend with each other was not accepted by his contemporaries as an
explanation for the apparently continuous variation seen in nature.
d. Mendel’s approach of using mathematics to explain biological phenomenon was totally new and
unacceptable to many of the biologists of his time.
e. Though his work suggested that factors were discreet units he could not provide any physical
proof fro the existence of the factors.