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Mendel's Experiment
Master Mendelian Genetics in Minutes: Learn how traits are passed down from parents to offspring across generations. Master the laws of inheritance, understand monohybrid and dihybrid crosses, and learn how to determine phenotypic and genotypic ratios without confusing formula blocks or LaTeX code.
1.0Learning Outcomes
After completing this lesson, you will be able to:
- Define heredity, variations, traits, alleles, phenotype, and genotype.
- Explain why Gregor Mendel chose the garden pea plant for his experiments.
- Deduce the phenotypic and genotypic ratios of a Monohybrid Cross using a Punnett square.
- Deduce the ratios of a Dihybrid Cross and state the Law of Independent Assortment.
- Formulate Mendel's three fundamental Laws of Inheritance based on experimental outcomes.
2.0Gregor Johann Mendel
Mendel was an Austrian monk and scientist who first worked on heredity experiments and gave theory in 1866 but people of that time couldn't understand him and rejected his theory. Later in 1900, three different scientists from 3 different places also found the same result.
They were Hugo de Vries, Tschermak and Correns. They rediscovered the theory of heredity proposed by Mendel. So, Mendel is called “Father of genetics”.
Mendel said that each character is controlled by a pair of factors which is now known as gene.
3.0Mendelian Experiments
In 1856-57, he started his historical experiments of heredity on pea (Pisum sativum) plant.
Advantages of Pisum Sativum
Pisum sativum is a unique plant selected by Mendel for his experiments which had following importance:
Before starting his experiments, he obtained the pure breed. A pure breed is the one in which a plant is crossed with same allelic type and results in all of its offspring of same kind. Tall plant only results in tall plant for any number of generations and so does other characters too.
Pairs of Allelic Characters Found In Garden Pea Plant
4.0Monohybrid Cross
It is a cross in which only one character is studied at a time.
In his first monohybrid cross, Mendel crossed a pure breed tall plant (TT) and a pure breed dwarf plant (tt).
And he found that in F1 generation only tall plant were born out of these seeds, no intermediate character or mixing or blending was observed by him. Mendel termed tallness as dominant and dwarfness as a recessive character.
He repeated the same experiment taking other characters too and always found same results.
Mendel’s Postulates
To explain the experiment, he said that each character is controlled by a pair of factors.
During gamete formation, the paired factors got segregated and moved to different gametes.
When these gametes fused together, they used to restore paired state again.
In heterozygous conditions, out of the two alleles only one allele is able to express its effect and it is called dominant allele and other which does not show its effect in heterozygous condition is called recessive allele.
When F1 plants are grown and allowed to self-pollinate he got many seeds and when they were grown, he found that the phenotypic ratio of tall plants and dwarf plants was 3:1 respectively.
The dwarfness which was not visible in F1 generation reappeared in F2 generation.
5.0Dihybrid Cross
It is a cross in which 2 characters are simultaneously studied.
In Mendel's first dihybrid cross, he crossed a pure breed plant having yellow and round (RRYY) seeds with another pure breed plant having green and wrinkled (rryy) seeds.
F1 generation had plants with yellow and round seeds.
It means yellow and round seed characters are dominant over green and wrinkled seeds.
He allowed these plants produced in F1 generation to self-pollinate and he expected the same 3 : 1 in F2 generation.
But to his surprise the ratio was all new 9 : 3 : 3 : 1 in which beside 2 parental varieties, 2 new varieties were also produced. So, the ratio was 9 yellow and round ; 3 yellow and wrinkled; 3 green and round ; 1 green and wrinkled.
This could be explained only when beside the parental combination of factors (RY) and (ry), two new combinations of factors are also produced as follows (rY) and (Ry).
Phenotypic Ratio
The occurrence of four types of plants in the F2 generation of dihybrid cross shows that during gamete formation in F2 generation, the two factors of a character are independently assorted and get randomly rearranged in the offspring. So, four types of gametes are formed as RY, rY, Ry, ry from each parent. When they are randomly fused, they resulted following combinations. So, this is how the 9 : 3 : 3 : 1 ratio could be understood.
6.0Mendel's Laws of Inheritance
Human beings were always interested to know more about the law of inheritance. On the basis of Mendel's work, 3 basic laws of inheritance were proposed.
(i) Law of Dominance
(ii) Law of Segregation
(iii) Law of Independent Assortment
Law of Dominance
In crossing between organisms pure for contrasting characters of a pair, only one character of the pair appears in the F1 generation. This character is termed as dominant while the one which does not express itself in F1 generation is termed as recessive.
Law of Segregation
Different alleles or genes of a character remain together in an individual and segregate randomly at the time of gamete formation. This is also known as the Law of purity of gametes.
Law of Independent Assortment
This law states that when individuals differing in two or more than two pairs of contrasting characters are crossed, the inheritance of any one pair is not affected by the presence of the other.
E.g., The inheritance of seed shape character is not related to the seed colour character. Rather, the two characters inherit independently of each other.
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9.0Supporting Study Materials
This study material, including CBSE Notes and NCERT Solutions for the Chapter "Heredity" on Mendel's Experiment topics, is designed according to the latest CBSE Class 10 Science syllabus and NCERT guidelines. It provides clear explanations of key concepts, definitions, examples, and important questions to help students understand monohybrid and dihybrid crosses, the laws of inheritance, and traits expression, and prepare effectively for examinations.
10.030-Second Revision
- Heredity = Passing traits from parents to offspring.
- Genotype = Genetic code (TT, Tt, tt); Phenotype = Physical look (Tall, Dwarf).
- Monohybrid F2 Phenotypic Ratio = 3:1 (3 Tall, 1 Dwarf).
- Monohybrid F2 Genotypic Ratio = 1:2:1 (1 TT, 2 Tt, 1 tt).
- Dihybrid F2 Phenotypic Ratio = 9:3:3:1 (Shows independent assortment).
- Dominant Trait = Expresses itself in both homozygous (TT) and heterozygous (Tt) forms.
- Recessive Trait = Expresses itself only in homozygous condition (tt).
11.0PREVIOUS YEAR QUESTIONS (PYQs)
Q1. A green-stemmed rose plant denoted by GG is crossed with a brown-stemmed rose plant denoted by gg.
a) What will be the color of the stem in their F1 progeny?
b) What percentage of plants will have brown stems if F1 plants are self-pollinated?
c) State the ratio of genotypes GG and Gg in the F2 generation. (CBSE Board)
Answer
- a) Color of F1 Progeny: All plants in the F1 generation will have Green stems (Genotype: Gg) because green (G) is the dominant trait over brown (g).
- b) Percentage of Brown Stems in F2: When F1 plants are self-pollinated (Gg x Gg), the F2 phenotypic ratio is 3 Green : 1 Brown. Therefore, 25% of the plants will have brown stems.
- c) Ratio of Genotypes GG and Gg: In the F2 generation, the genotypic breakdown is 1GG : 2Gg : 1gg. Thus, the ratio of GG to Gg is 1:2.
Q2. Why did Mendel choose the garden pea plant for his experiments? List any three reasons. (CBSE Board)
Answer
Mendel selected the garden pea plant (Pisum sativum) due to the following three key reasons:
- Clear Contrasting Characters: The plants possessed easily distinguishable, opposite traits (such as distinct tall vs. dwarf heights, or round vs. wrinkled seed shapes) with no intermediate types.
- Short Lifecycle: The pea plant is an annual plant with a short growth span, enabling Mendel to study and analyze multiple generations within a few years.
- Ease of Controlled Pollination: The flowers are naturally self-pollinating due to their enclosed structure, but they can easily be cross-pollinated artificially by removing the stamens manually when desired.
12.0Recommended Next Topics
- Sex Determination in Human Beings
- Molecular Basis of Inheritance (DNA and Genes basics)
- Evolution and Classification concepts
- Sexual Reproduction in Flowering Plants