Ch 4Principles of Inheritance and Variation — 2nd PUC Biology, concept-first.

Why does an elephant always give birth to a baby elephant, never anything else? Why does a mango seed only ever grow into a mango tree? And given that offspring resemble their parents, why aren't they…

Gregor Mendel spent seven years (1856–1863) performing hybridisation experiments on garden peas. His work marked the first time statistical analysis and mathematical logic were applied to biology.

Mendel’s first set of experiments looked at the inheritance of a single trait — for example, flower colour, seed shape, or plant height.

The core idea of the Law of Dominance is that when two different versions of a trait come together, one version can hide the other.

The Law of Segregation is Mendel’s first law. It answers a puzzle that troubled biologists for a long time: if a tall plant and a short plant are crossed, why does the first generation (F₁) show only…

Incomplete dominance is a pattern of inheritance where the heterozygous condition produces a phenotype that is a blend — intermediate between the two homozygous parents.

So far, we have seen crosses where the F₁ generation either looked like one of the two parents (dominance) or was a blend between them (incomplete dominance).

Inheritance of two genes is simply the study of how two different traits are passed from parents to offspring at the same time.

The dihybrid cross — a cross between plants differing in two traits — gave Mendel the clue for his second law.

Mendel published his work in 1865, but it was largely ignored until 1900. There were several reasons for this delay.

Morgan’s work on Drosophila (the fruit fly) revealed a major exception to Mendel’s law of independent assortment.

Mendel’s work focused on traits that come in two clear-cut, opposite forms — like purple versus white flowers. But most features in nature are not that simple.

A single gene typically influences one trait, but that is not always the rule. When one gene affects multiple, seemingly unrelated phenotypic traits, the phenomenon is called pleiotropy, and the gene…

The story of how sex is determined begins not with humans, but with insects. For a long time, the mechanism was a puzzle.

The sex of a human child is determined by the chromosomes carried by the sperm that fertilises the egg. This is a clear example of the XY sex-determination system.

The sex of a honey bee is not determined by sex chromosomes like X and Y. Instead, it depends entirely on the number of sets of chromosomes the individual receives — a system called haplodiploidy.

A mutation is any change in the DNA sequence of an organism. This alteration can affect both the genotype (the genetic makeup) and the phenotype (the observable traits).

A genetic disorder is a disease or abnormality caused by an abnormality in an individual's DNA. These disorders can arise from mutations in a single gene (Mendelian disorders) or from changes in the n…

The idea that certain disorders run in families is very old, based on the observation that specific features are passed down through generations.

Mendelian disorders are genetic conditions caused by a mutation in a single gene. They are passed from one generation to the next in patterns that follow Mendel’s laws of inheritance — hence the name.…

Mendelian disorders arise from mutations in single genes. Chromosomal disorders are fundamentally different — they are caused by the absence, excess, or abnormal arrangement of one or more whole chrom…

- Mendel’s laws: Law of Dominance (one allele masks the other in a heterozygote), Law of Segregation (alleles separate during gamete formation), and Law of Independent Assortment (genes on different c…

Higher-order thinking / exemplar-style practice problems.

Sample papers and previous-year board questions for this subject.