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Mendel's Laws of Inheritance

Mendel's laws describe the regular patterns by which contrasting traits are inherited through discrete hereditary units called alleles, as shown by pea plant crosses.

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Student-friendly explanation

Mendel studied clear contrasting traits in pea plants, such as tall versus dwarf stem and round versus wrinkled seeds. From controlled crosses, he proposed that hereditary factors occur in pairs, one from each parent, and separate during gamete formation. The law of dominance explains why one allele may express in a heterozygote. The law of segregation explains equal separation of allele pairs into gametes. The law of independent assortment applies when two gene pairs assort independently, usually because they are on different chromosomes or far apart. In exams, the laws are tested through ratios, genotypes, and reasoning from crosses.

How to write this in exams

  1. 1

    Start with the exact idea

    Mendel's laws describe the regular patterns by which contrasting traits are inherited through discrete hereditary units called alleles, as shown by pea plant crosses.

  2. 2

    Then show how to use it

    Identify the parental genotypes. Write the gametes produced by each parent. Combine gametes in a Punnett square. Count genotypes and phenotypes separately. Link the result to the correct Mendelian law.

  3. 3

    Add one concrete example

    In a cross between true-breeding tall pea plants and true-breeding dwarf pea plants, all F1 plants are tall. Selfing the F1 plants gives an F2 phenotypic ratio close to 3 tall : 1 dwarf, showing dominance and segregation.

  4. 4

    Avoid this incomplete answer

    Writing that the law of dominance causes the 3:1 ratio by itself is incomplete; the 3:1 ratio depends on segregation during gamete formation.

Definition

Mendel's laws describe the regular patterns by which contrasting traits are inherited through discrete hereditary units called alleles, as shown by pea plant crosses.

Example

In a cross between true-breeding tall pea plants and true-breeding dwarf pea plants, all F1 plants are tall. Selfing the F1 plants gives an F2 phenotypic ratio close to 3 tall : 1 dwarf, showing dominance and segregation.

Rule to remember

Key rules: dominance explains expression in heterozygotes, segregation gives pure gametes for one gene pair, and independent assortment gives new allele combinations for two independently assorting gene pairs.

Memory hook

Dominance shows what is seen in F1; segregation explains why the hidden allele returns in F2.

Examples and method

Worked example

For TT x tt, all F1 are Tt and tall. Selfing Tt x Tt gives TT, Tt, Tt, tt, so the genotypic ratio is 1:2:1 and the phenotypic ratio is 3 tall : 1 dwarf.

Method to apply

Identify the parental genotypes. Write the gametes produced by each parent. Combine gametes in a Punnett square. Count genotypes and phenotypes separately. Link the result to the correct Mendelian law.

Diagram support

A Punnett square for a monohybrid cross should show parental alleles, gametes, F1 genotype, F2 genotype ratio, and phenotype ratio. Students should notice that ratios arise from allele combinations, not guesswork.

How CBSE asks it

Questions may ask students to state Mendel's laws, explain a 3:1 or 9:3:3:1 ratio, identify dominant and recessive traits from a cross, or justify why a trait reappears in F2.

Avoid common mistakes

Common confusion

Students often write that recessive traits disappear in F1. The recessive allele is present in heterozygous F1 plants but is masked by the dominant allele.

Common wrong answer

Writing that the law of dominance causes the 3:1 ratio by itself is incomplete; the 3:1 ratio depends on segregation during gamete formation.

Exam tip

When explaining a Mendelian law, connect the statement to gamete formation and support it with a ratio or cross, rather than only naming the law.

Quick check

Why does the recessive trait reappear in the F2 generation of a monohybrid cross?

The recessive trait reappears because the two alleles separate during gamete formation in F1 plants. When two recessive alleles come together in an F2 offspring, the recessive phenotype is expressed.

Answer writing and exam use

1-mark answer

Define the law, mention alleles in pairs, describe gamete formation, show a small cross or ratio, and state how the observation supports the law.

2-mark answer

Define the law, mention alleles in pairs, describe gamete formation, show a small cross or ratio, and state how the observation supports the law.

3-mark answer

Define the law, mention alleles in pairs, describe gamete formation, show a small cross or ratio, and state how the observation supports the law.
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