Mendel's Laws
Monohybrid and dihybrid crosses giving dominance, segregation and independent assortment. (Biology › Principles of Inheritance and Variation, NEET UG syllabus.)
What is Mendel's Laws?
A unit of heredity that is transferred from a parent to offspring and is held to determine some characteristic of the offspring.
Key formula / rule: Monohybrid Cross F2 Phenotypic Ratio
Key points
- Explain Mendel's experimental approach and reasons for his success.
- Define and differentiate between dominant and recessive alleles, genotype and phenotype.
- Describe the Law of Dominance, Law of Segregation, and Law of Independent Assortment.
- Perform monohybrid and dihybrid crosses using Punnett squares.
Common exam trap
Confusing the Law of Segregation with the Law of Independent Assortment.
Definitions
- Term
Gene
- Meaning
A unit of heredity that is transferred from a parent to offspring and is held to determine some characteristic of the offspring.
- Term
Allele
- Meaning
One of two or more alternative forms of a gene that arise by mutation and are found at the same place on a chromosome.
- Term
Dominant Allele
- Meaning
An allele that expresses its phenotypic effect even when heterozygous with a recessive allele; it completely masks the expression of the recessive allele.
- Term
Recessive Allele
- Meaning
An allele that produces its characteristic phenotype only when its paired allele is identical (homozygous recessive).
- Term
Genotype
- Meaning
The genetic constitution of an individual organism.
- Term
Phenotype
- Meaning
The set of observable characteristics of an individual resulting from the interaction of its genotype with the environment.
- Term
Homozygous
- Meaning
Having two identical alleles of a particular gene or genes.
- Term
Heterozygous
- Meaning
Having two different alleles of a particular gene or genes.
Learning objectives
Explain Mendel's experimental approach and reasons for his success.
Define and differentiate between dominant and recessive alleles, genotype and phenotype.
Describe the Law of Dominance, Law of Segregation, and Law of Independent Assortment.
Perform monohybrid and dihybrid crosses using Punnett squares.
Predict phenotypic and genotypic ratios for Mendelian crosses.
Explain the concept of a test cross and its significance.
Formulae
- Name
Monohybrid Cross F2 Phenotypic Ratio
- Note
Ratio of dominant phenotype to recessive phenotype.
- Expression
3:1
- Name
Monohybrid Cross F2 Genotypic Ratio
- Note
Ratio of homozygous dominant : heterozygous : homozygous recessive.
- Expression
1:2:1
- Name
Dihybrid Cross F2 Phenotypic Ratio
- Note
Ratio of (dominant-dominant) : (dominant-recessive) : (recessive-dominant) : (recessive-recessive).
- Expression
9:3:3:1
Prerequisites
Basic understanding of cell division (meiosis).
Knowledge of basic genetic terminology (gene, allele, chromosome, locus, genotype, phenotype, homozygous, heterozygous).
Understanding of probability.
Common mistakes
Confusing the Law of Segregation with the Law of Independent Assortment.
Assuming all traits follow Mendelian inheritance (e.g., incomplete dominance, codominance, multiple alleles are exceptions).
Incorrectly applying Punnett squares for complex crosses.
Forgetting to consider all possible gamete combinations in dihybrid crosses.
Misinterpreting phenotypic vs. genotypic ratios.
Keywords
Mendel
Genetics
Inheritance
Dominance
Segregation
Independent Assortment
Monohybrid
Dihybrid
Punnett Square
Allele
Gene
Genotype
Phenotype
Test Cross
Practice preview
Which of Mendel's laws states that alleles for each gene segregate from each other during gamete formation such that each gamete carries only one allele for each gene?…
easy
If a pea plant with round seeds (R) is crossed with a pea plant with wrinkled seeds (r), and all offspring have round seeds, which allele is dominant?…
easy
Mendel's Law of Independent Assortment can be demonstrated by which of the following crosses?…
medium