Page 3 - 2-Lesson Notes-Accumulation of Variations, Inerited Traits

• He made three basic laws of inheritance – The Law of Dominance, The Law
of Segregation and The Law of Independent Assortment.
Some Important definitions associated with Heredity
Allele:
An allele is one of usually two or more forms of genes (generally a group of genes).
An allele is an alternative form of a gene that is located at a specific position on a
specific chromosome. For example: TT, Tt, tT, tt
Each ‘t’ or ‘T’ is an allele (Capital letter denotes Dominant allele, whereas Small
letter denotes Recessive allele)

Traits:
Traits are characters that are inherited by the progeny. Every person has two copies
of each gene, one inherited from each parent.

Dominant trait :
The traits that express themselves in an organism in every possible combination and
can be seen are called Dominant traits. (Ex- TT)
• In Mendel’s experiment, we see that tall trait in pea plants tends to express
more than the short trait.
• Therefore, the tall trait of the plant is said to be dominant over the short trait.

Recessive trait:
A trait which is not expressed in presence of a dominant allele is known as
recessive. (Ex- tt)
• So, recessive character/trait is present in an organism but cannot be seen if a
dominant allele exists.

Homozygous:
Consists of two copies of the same allele that codes for a particular trait. (Ex- TT or
tt)

Heterozygous:
Consists of two different copies of alleles that code for a particular trait. (Ex- Tt)

Phenotype:
It refers to the observable physical properties of an organism; these include the
organism's appearance, development, and behaviour.

Genotype:
It refers to the genetic makeup of an organism; in other words, it describes an
organism's complete set of genes.

MONOHYBRID CROSS:
Mendel used a number of contrasting visible characters of garden peas –
round/wrinkled seeds, tall/short plants, white/violet flowers and so on. He took pea
plants with different characteristics – a tall plant and a short plant, produced progeny
from them, and calculated the percentages of tall or short progeny.

3