Genetics and Inheritance

The method, step by step

1. 1

   Get the vocabulary in order

   DNA → chromosome → gene → allele

   DNA is the molecule, packaged into chromosomes. A gene is a section of DNA coding for a characteristic, and an allele is one version of that gene. Questions often depend on using these words precisely.

2. 2

   Distinguish genotype from phenotype

   genotype Bb → phenotype brown eyes

   The genotype is the alleles carried; the phenotype is the observable characteristic. Two different genotypes — BB and Bb — can give the same phenotype, which is exactly why recessive traits can skip generations.

3. 3

   Apply dominance

   B dominant, b recessive → BB and Bb both show the dominant trait

   One dominant allele is enough to produce the dominant phenotype. The recessive characteristic appears only in bb individuals, where no dominant allele is present to mask it.

4. 4

   Draw a Punnett square

   Bb × Bb → BB, Bb, Bb, bb → 3 : 1 dominant to recessive

   Put one parent's alleles along the top and the other's down the side, then fill each cell with the combination. The four cells give the possible offspring genotypes and their relative likelihood.

5. 5

   Read ratios as probabilities, not guarantees

   3 : 1 means each offspring has a 3 in 4 chance

   A ratio describes the probability for each individual offspring, not a promise about a particular family. Four children of Bb parents can easily all show the dominant trait.

The distinction that most questions turn on

Genotype is what alleles an organism carries; phenotype is what you can observe. A person with genotype Bb has brown eyes and carries the blue allele without showing it, which is why two brown-eyed parents can have a blue-eyed child.

That scenario is one of the most frequently examined in the whole topic, and it is impossible to explain without the distinction. It is worth establishing before any Punnett squares are attempted.

• DNA → chromosome → gene → allele
• Genotype = alleles carried; phenotype = trait shown
• Homozygous = two identical alleles; heterozygous = two different
• Dominant shows whenever present; recessive needs both
• Punnett square ratios are probabilities

Carriers and skipped generations

A heterozygous individual carries a recessive allele without displaying it — a carrier. This is how recessive conditions appear to skip generations: the allele is passed on invisibly until two carriers have a child who inherits it from both.

Genetic diagrams for inherited disorders such as cystic fibrosis rely on this, and questions typically ask for the probability that two carriers produce an affected child. The Punnett square gives one in four.

How sex is inherited

In humans, sex is determined by one pair of chromosomes: XX in females and XY in males. Since mothers can only pass on an X, the father's contribution determines the outcome.

A Punnett square with XX and XY gives a 50:50 ratio, which matches the observed near-equal split in populations. It is a good first Punnett square because the answer is already familiar.

How we teach genetics

We fix the vocabulary chain — DNA, chromosome, gene, allele — before anything else, because imprecise use of these words is what makes otherwise correct answers lose marks.

We also stress that ratios are probabilities. Students frequently write that three of four children will show the dominant trait, which is a misreading, and examiners mark it as one.