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Principles of Inheritance and Variation

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Class 10 Class 12
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Explain the law of dominance using a monohybrid cross.

Law of Dominance states that
i. characters are present as dicrete units called Factors.
ii. Factors occur in pairs.
iii. In the condition where dissimilar factors are present one expresses itself and is called Dominant whereas the other does not express itself and is called recessive.

The law explains the expression of only one factor in a monohybrid cross.
For example
If we cross a  Pure tall( TT) and a Pure dwarf (tt) with each other then in the First filial generation (F1) we get only Tall plants (Tt) even when two discrete factors T and t are present. Thus we can say that T factor is dominant over t factor as it expresses itself even in the dissimilar condition. The t factor is recessive as it expresses only in the homozygous condition (tt) in the Second filial generation where Tt and Tt are selfed.  

. A Punnett square used to understand  law of Dominance using monohybrid cross.


Differentiate between the following :

(a) Dominance and Recessive

(b) Homozygous and heterozygous

(c) Monohybrid and dihybrid.

Dominance Recessive
1. When the allele expresses itself in both heterozygous as well as homozygous condition. 1. When the allele expresses itself only in homozygous condition.
2. The allele is unmodified and functional. 2. The allele is modified and  non-functional.

Homozygous Heterozygous
1.Condition where both the alleles of the gene are same. 1.Condition where the alleles are different.
2.The gametes formed from them are same. 2. The gametes formed from them are different.
eg. TT or tt eg. Tt

Monohybrid Dihybrid   
1. When an individual is heterozygous for genes controlling one character it is called monohybrid 1. When an individual is heterologous for genes controlling two characters it is called dihybrid.
2. In F2 generation  it produces a phenotypic ratio of 3:1. 2. In F2 generation it produces a phenotypic ratio of 9:3:3:1

Mention the advantages of selecting pea plant for experiment by Mendel.

Advantages of selecting pea plant as experimental material

1. Many varieties were available with observable and contrasting characters.

2. Peas normally self pollinate so pure lines could be obtained for experimental purpose

3. It was easily available.

4 The life span of pea is short and it produced large number of offsprings

5. They were easy to cultivate and did not require much care.



Using a Punnett Square, work out the distribution of phenotypic features in the first filial generation after a cross between a homozygous female and a heterozygous male for a single locus.

If we cross a  Bb (heterozygous male) and
BB (homozygous female) the distribution of phenotypic features in the first filial generation will be -:

Punnett Square of a cross between Bb and BB



A diploid organism is heterozygous for 4 loci, how many types of gametes can be produced ?

16 types of gametes.