C
CraftExam
medium difficulty7 min

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.

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

Concept Practice Coming Soon

Understand it

  • 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.

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.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

  1. 1For TT x tt, all F1 are Tt and tall.
  2. 2Selfing 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 you can reuse

  1. 1Identify the parental genotypes.
  2. 2Write the gametes produced by each parent.
  3. 3Combine gametes in a Punnett square.
  4. 4Count genotypes and phenotypes separately.
  5. 5Link the result to the correct Mendelian law.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

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

Think first, then show answer
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.

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.

Don't mix this up

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.

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.

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

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.

Answer plan

Define the law, mention alleles in pairs, describe gamete formation, show a small cross or ratio, and state how the observation supports the law.
  1. 1

    Enough for 1 mark

    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

    Add for 2 marks

    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.

  3. 3

    Add for 3 marks

    Mendel studied clear contrasting traits in pea plants, such as tall versus dwarf stem and round versus wrinkled seeds.

Practice

Concept practice is coming soon

Login to access concept-level MCQs and weak-concept practice as they become available.

10 MCQs5 MinutesInstant Results
Login

Help improve this page

Found something confusing, incorrect, or missing?

Open Learning Map