D3.2 Inheritance

Why do you have your mother’s eyes but your father’s height? The science of inheritance answers this by tracking how genes pass from parents to offspring in predictable, countable ratios. The foundations were laid by Gregor Mendel breeding pea plants long before anyone knew DNA existed, and his rules still work because they describe how chromosomes — and the alleles they carry — are sorted during meiosis and recombined at fertilisation. With a few clear definitions and the humble Punnett square, you can predict the outcome of a cross and explain patterns from blood groups to colour blindness.

Genes, alleles and the language of genetics

A gene is a length of DNA that codes for a particular characteristic; its position on a chromosome is its locus. Most genes come in alternative versions called alleles. Because chromosomes occur in homologous pairs in a diploid organism, each individual carries two alleles of every gene — one inherited from each parent. The precise vocabulary matters in this topic:

• Genotype: the alleles an organism carries (for example, Bb).
• Phenotype: the observable characteristic that results (for example, brown eyes).
• Homozygous: the two alleles are the same (BB or bb).
• Heterozygous: the two alleles are different (Bb).
• Dominant: an allele expressed even when only one copy is present (shown by a capital letter).
• Recessive: an allele expressed only when two copies are present (shown by a lower-case letter).

Using the correct term, and consistent letters where the capital and lower-case forms are clearly different, is the foundation of every genetics answer.

Monohybrid crosses and Punnett squares

A monohybrid cross follows the inheritance of a single gene. The standard tool is the Punnett square, which lays out the possible gametes from each parent and combines them to show the expected genotypes of the offspring. For example, crossing two heterozygous parents (Bb × Bb) gives offspring in the ratio 1 BB : 2 Bb : 1 bb, which produces a 3:1 phenotypic ratio of dominant to recessive — the classic Mendelian result.

It is vital to read these ratios as probabilities, not guarantees: a 3:1 ratio means each offspring has a 3 in 4 chance of showing the dominant phenotype, but small numbers of real offspring will vary by chance. A test cross — crossing an individual showing the dominant phenotype with a homozygous recessive individual — is used to find out whether the dominant individual is homozygous or heterozygous, by examining the offspring.

Beyond simple dominance: codominance and multiple alleles

Not all inheritance follows a neat dominant/recessive pattern. In codominance, both alleles in a heterozygote are fully expressed, so neither masks the other. The human ABO blood group system is the syllabus example and also illustrates multiple alleles — there are three alleles (I^A, I^B and i) for a single gene in the population, although any one person still carries only two.

• I^A and I^B are codominant, so genotype I^AI^B gives blood group AB.
• Both I^A and I^B are dominant over the recessive i, so group O occurs only in genotype ii.

Working through such crosses requires care with the genotype notation, but the Punnett-square method is exactly the same as for a simple monohybrid cross.

Sex determination and sex linkage

In humans, sex is determined by the sex chromosomes: females are XX and males are XY. Genes carried on the X chromosome show sex linkage, because males have only one X and therefore only one copy of those genes. A male will express a recessive X-linked allele even with a single copy, since there is no second X to mask it. This is why conditions such as red–green colour blindness and haemophilia are far more common in males than in females.

When setting out a sex-linkage cross, write the alleles as superscripts on the X chromosome (for example X^H and X^h) and remember that the Y carries no copy of the gene. A female can be an unaffected carrier (heterozygous) and pass the allele to her sons. Finally, this topic ties back to variation: the combination of meiosis and random fertilisation means each individual inherits a unique mixture of alleles, which is the basis of the genetic variation on which evolution acts.