Mendelian Genetics – Concise Lecture Note
1. Introduction to Mendelian Genetics
Genetics is the branch of biology that studies how traits are passed from
parents to offspring.
Gregor Johann Mendel (1822–1884), known as the Father of Genetics,
was the first to explain how traits are inherited using experiments with
pea plants (Pisum sativum).
Before Mendel, people believed traits blended like paint — for example,
tall and short parents would always produce medium-height children.
Mendel proved that traits are passed as separate units (now called
genes) that do not mix, but are inherited independently.
2. Mendel’s Experiments and Method
Mendel used pea plants because they had clear, opposite traits (like tall
vs. short, round vs. wrinkled seeds) and could self-pollinate or cross-
pollinate easily.
What Mendel did:
1. Selected pure-breeding plants (those that always produced
offspring with the same trait).
2. Crossed plants showing opposite traits (e.g., tall × short).
3. Collected seeds and planted them to study how the traits
appeared in the next generations.
4. Counted and analyzed results mathematically — something no
one had done before!
, 3. Mendel’s Laws of Inheritance
Mendel explained inheritance through three major laws:
A. Law of Segregation
Each organism carries two alleles (forms) of a gene, one from each
parent.
During the formation of gametes (sperm or egg), these two alleles
separate so that each gamete gets only one.
Example:
A tall plant (Tt) produces gametes carrying either T or t, not both.
B. Law of Independent Assortment
Genes for different traits are passed on independently of one another.
This means that the inheritance of one trait (like height) does not affect
another (like seed color).
Example:
The genes for seed shape and seed color assort independently to form
combinations like round-yellow, round-green, wrinkled-yellow, and
wrinkled-green.
C. Law of Dominance
When two different alleles occur together, one may hide the effect of the
other.
The stronger one is called dominant, and the weaker one is recessive.
Example:
Tall (T) is dominant over dwarf (t), so Tt plants are tall.
1. Introduction to Mendelian Genetics
Genetics is the branch of biology that studies how traits are passed from
parents to offspring.
Gregor Johann Mendel (1822–1884), known as the Father of Genetics,
was the first to explain how traits are inherited using experiments with
pea plants (Pisum sativum).
Before Mendel, people believed traits blended like paint — for example,
tall and short parents would always produce medium-height children.
Mendel proved that traits are passed as separate units (now called
genes) that do not mix, but are inherited independently.
2. Mendel’s Experiments and Method
Mendel used pea plants because they had clear, opposite traits (like tall
vs. short, round vs. wrinkled seeds) and could self-pollinate or cross-
pollinate easily.
What Mendel did:
1. Selected pure-breeding plants (those that always produced
offspring with the same trait).
2. Crossed plants showing opposite traits (e.g., tall × short).
3. Collected seeds and planted them to study how the traits
appeared in the next generations.
4. Counted and analyzed results mathematically — something no
one had done before!
, 3. Mendel’s Laws of Inheritance
Mendel explained inheritance through three major laws:
A. Law of Segregation
Each organism carries two alleles (forms) of a gene, one from each
parent.
During the formation of gametes (sperm or egg), these two alleles
separate so that each gamete gets only one.
Example:
A tall plant (Tt) produces gametes carrying either T or t, not both.
B. Law of Independent Assortment
Genes for different traits are passed on independently of one another.
This means that the inheritance of one trait (like height) does not affect
another (like seed color).
Example:
The genes for seed shape and seed color assort independently to form
combinations like round-yellow, round-green, wrinkled-yellow, and
wrinkled-green.
C. Law of Dominance
When two different alleles occur together, one may hide the effect of the
other.
The stronger one is called dominant, and the weaker one is recessive.
Example:
Tall (T) is dominant over dwarf (t), so Tt plants are tall.
