The basics of Heredity, and Mendel's laws of Inheritance

1. Introduction to Heredity

2. DNA
Deoxyribonucleic Acid, the bio molecule present in the cell’s nucleus, containing information
for all the characters that we have.
And this DNA or the blueprint, for building an offspring is equally contributed by both the
parents. During zygote formation, each parent contributes one set of 23 chromosomes.
This transmission of characters from parents to offsprings is called Heredity.

3. How is the transmissiontakes place ? Inwhat form do the characters carried
to the next generation?
What is the link betweenthe genes andthe expressionshown?
And since for each character the baby has the pair of gene, soWhich gene
will express itself- paternal gene or maternal gene?
To clarify all this Gregor Johann Mendel did a few experiments and gave the
Laws of inheritance
G.J. Mendel is known as the father of Genetics
Experimental planttaken- Pisum sativum/Pea plant
WhyMendel chose pea plant as his experimental plant?
1. Pea plant has 7 characters with onlytwo contrasting traits each, which were very easyto
observe and compare.
2. Pea plants reproduce quickly,so one couldsee 2 to 3 generationsresultin one year.
3. It was easyto do artificial Cross pollination/ emasculationin pea plants.
A list of Contrasting Traits studied by Mendel

4. MonohybridCross
Male plant Female plant
Tall Dwarf
Parents 1 TT X tt
Gametes. T T t t
Fertilisation
F1 Tt Tt Tt Tt
All tall plants in first generation
Parents 2 Tt X Tt
Gametes. T t T t
F2. TT Tt Tt tt
Phenotypic Ratio- 3 Tall : 1 Dwarf
Genotypic Ratio – 1TT : 2 Tt : 1 tt
1. Law of Segregation- It states that during gamete formation, the
alleles of each gene separate fromeach other and only one gene of the
pair coding for a character, passes tothe gamete. E.g. In Tt pair coding
for height character, either Tor t will pass to the next generation.
Law of dominance
Definition- It states that out of the two contrasting traits, the gene
which expresses itself, eveninthe presenceof the gene for contrasting
trait is dominant, and the other one whichis able to express itself only
in the homozygous conditionis the recessive trait. E.g. Phenotype of TT
& Tt is tallness, while the phenotype of only tt is dwarfness.

5. Dihybrid Cross
Male plant Female plant
Parents 1 RRYY X rryy
Roundand yellow Wrinkledand green
Gametes. RY RY ry ry
Fertilisation
F1 RrYy RrYy RrYy RrYy
All plants infirst generationare withroundshape and yellow colour.
Parents 2 RrYy X RrYy
Gametes. RY Ry rY ry RY Ry rY ry
Fertilisation
F2. RY Ry rY ry
RY RRYY RRYy RrYY RrYy
Ry RRYy RRyy RrYy Rryy
rY RrYY RrYy rrYY rrYy
ry RrYy Rryy rrYy rryy

6. Phenotypic Ratio: 9: Round and Yellow( RRYY, RrYy)
3: Round and green ( RRyy, Rryy)
3: wrinkled and yellow ( rrYy, rrYY)
1: wrinkled and green ( rryy)
2. Law of independence assortment- It states that in a
dihybrid cross, during gamete formation or fertilisation,
the genes coding for one character assort or distribute
themselves independent of the genes coding for the
second character.