Genetics Explained — Heredity, Chromosomes, DNA & Sex-linked Traits (JAMB/WAEC Biology)

Genetics is the study of how traits are passed from parents to offspring, covering heredity, the role of chromosomes and DNA, and special inheritance patterns like sex-linked traits. 

It’s one of the more challenging topics in JAMB and WAEC Biology, largely because it combines definitions with problem-solving using genetic crosses. This lesson breaks down heredity, chromosomes, DNA, and sex-linked traits step by step.

Quick takeaways

• Heredity is the passing of traits from parents to offspring through genes.
• Chromosomes are thread-like structures in the nucleus that carry genes; humans have 23 pairs (46 total).
• DNA (deoxyribonucleic acid) is the molecule that makes up genes and carries the genetic instructions for an organism.
• Sex-linked traits are controlled by genes located on the sex chromosomes (X and Y), which is why they’re often inherited differently in males and females.
• Exam questions frequently combine definitions with a genetic cross problem — practice both, not just the theory.

What is heredity, and what basic terms should you know?

Heredity is the process by which traits are passed from parents to their offspring through genes. A few key terms come up repeatedly in this topic:

• Gene – a unit of heredity that controls a specific trait
• Allele – an alternative form of a gene (e.g., the allele for tall vs the allele for short)
• Genotype – the genetic makeup of an organism (e.g., Tt)
• Phenotype – the observable physical characteristic resulting from the genotype (e.g., tall)
• Dominant/Recessive – a dominant allele expresses itself even with one copy; a recessive allele only expresses itself with two copies

What are chromosomes, and what role do they play?

Chromosomes are thread-like structures found in the nucleus of a cell, made of tightly coiled DNA, and they carry the genes responsible for inherited traits. Humans have 23 pairs of chromosomes (46 in total) – 22 pairs of autosomes and one pair of sex chromosomes (XX in females, XY in males). 

Because genes are located on chromosomes, understanding chromosome behavior during reproduction is essential to understanding how traits are inherited.

What is DNA, and how is it related to genes and chromosomes?

DNA (deoxyribonucleic acid) is the molecule that carries an organism’s genetic instructions, structured as a double helix made of nucleotide building blocks. 

Genes are specific segments of DNA, and many genes together, tightly coiled, make up a single chromosome. In simple terms: DNA is the material, genes are functional segments of that material, and chromosomes are the structures that organize and carry many genes together.

What are sex-linked traits, and why do they behave differently?

Sex-linked traits are traits controlled by genes located on the sex chromosomes, most commonly the X chromosome. Because males have only one X chromosome (XY) while females have two (XX), a recessive allele on the X chromosome is far more likely to be expressed in males, since there’s no second X chromosome to potentially carry a dominant, masking allele. 

This is why conditions like color blindness and hemophilia are classic JAMB/WAEC examples of sex-linked traits, occurring much more frequently in males than females.

Common mistakes students make with genetics

• Confusing genotype (genetic makeup, e.g. Tt) with phenotype (observable trait, e.g. tall)
• Assuming dominant traits are automatically more common – dominance refers to expression, not frequency in a population
• Forgetting that sex-linked conditions are carried on the X chromosome, and applying autosomal inheritance rules instead
• Skipping practice with actual genetic cross diagrams, then struggling when a calculation-based question appears

Frequently asked questions

Q: What is the difference between genotype and phenotype? 

Genotype refers to an organism’s genetic makeup (its combination of alleles), while phenotype refers to the observable physical trait that results from that genotype.

Q: How many chromosomes do humans have? 

Humans have 23 pairs of chromosomes, totaling 46, made up of 22 pairs of autosomes and one pair of sex chromosomes.

Q: What is the relationship between DNA, genes, and chromosomes? 

DNA is the molecule that carries genetic information, genes are specific functional segments of DNA, and chromosomes are the tightly coiled structures that organize and carry many genes together.

Q: Why are sex-linked conditions like color blindness more common in males? 

Because these conditions are typically carried on the X chromosome, and males have only one X chromosome, a single recessive allele is enough to cause the condition, whereas females would need two copies since they have two X chromosomes.

This is one topic from our full JAMB & WAEC Biology course — see everything covered →