Revise Inheritance for IB Biology (D3.2)

Genetics has a precise vocabulary, and examiners award — or withhold — marks on exactly these words. A gene is a length of DNA that codes for a characteristic and occupies a fixed position, or locus, on a chromosome. An allele is one of the alternative versions of a gene. Because chromosomes come in pairs, an individual carries two alleles for each gene; that pair is its genotype. A genotype with two identical alleles (PP or pp) is homozygous, and one with two different alleles (Pp) is heterozygous. The phenotype is the observable characteristic the genotype produces. A dominant allele (capital letter) is expressed in the phenotype whenever it is present, even in a heterozygote, while a recessive allele (lower-case letter) is expressed only when homozygous, because a dominant allele masks it. A heterozygote that shows the dominant phenotype but carries a hidden recessive allele for a condition is called a carrier.

Mendel's central insight is the principle of segregation: the two alleles of a gene separate from one another during the formation of gametes, so that each gamete receives only one allele of the pair. Biologically this happens at meiosis, when the members of each homologous pair of chromosomes are pulled into different cells. A homozygous parent (PP or pp) can therefore make only one kind of gamete for that gene, while a heterozygous parent (Pp) makes two kinds — half carrying P and half carrying p — in equal proportion. Fertilisation then brings one allele from each parent together at random, restoring the paired genotype in the offspring. This is why a genetic diagram always shows single alleles in the gametes, never pairs.

A monohybrid cross follows the inheritance of a single characteristic. To predict its outcome we lay out a genetic diagram: state and define the allele symbols, write the two parental genotypes, work out the gametes each parent can make (one allele each, by segregation), then combine them in a Punnett square — one parent's gametes along the top, the other's down the side — and fill every cell to list the possible offspring. Reading the completed square gives two ratios: the genotype ratio (how many of each allele combination) and the phenotype ratio (how many show each observable trait). Keep them separate: a Pp × Pp cross gives a genotype ratio of 1 PP : 2 Pp : 1 pp but a phenotype ratio of 3 dominant : 1 recessive, because PP and Pp look the same.

An organism showing a dominant phenotype hides an ambiguity: its genotype could be homozygous (PP) or heterozygous (Pp), and you cannot tell by looking. A test cross resolves this by crossing the individual with a homozygous recessive partner (pp), which can only contribute recessive alleles. If the tested individual is homozygous dominant, all offspring inherit a dominant allele and show the dominant phenotype; if it is heterozygous, about half the offspring are homozygous recessive and show the recessive phenotype. So the appearance of even one recessive offspring proves the parent was heterozygous — the homozygous recessive partner acts as a genetic 'developer' that reveals the hidden allele.

Not all alleles show simple dominance. In codominance, both alleles of a heterozygote are expressed fully and separately in the phenotype, so both characteristics appear at once. The standard example is the human ABO blood group system, which also illustrates multiple alleles — a gene with more than two alleles in the population. There are three ABO alleles: $I^A$ and $I^B$, which produce the A and B antigens and are codominant with each other, and $i$, which is recessive to both. Any one person still carries only two of the three. This gives four blood groups: group A ($I^A I^A$ or $I^A i$), group B ($I^B I^B$ or $I^B i$), group AB ($I^A I^B$, both antigens expressed), and group O ($ii$, neither antigen). Because $I^A$ and $I^B$ are codominant, an $I^A I^B$ person is genuinely AB — not an intermediate blend.

In humans, sex is determined by the sex chromosomes: females are XX and males are XY. Eggs always carry an X, whereas sperm carry either an X or a Y, so it is the sperm that determines the sex of the offspring, and the expected ratio is 1 male : 1 female. The X chromosome is much larger than the Y and carries genes that have nothing to do with sex; a gene on the X is described as sex-linked (X-linked). Because a male has only one X, a single recessive allele on it is expressed with no second copy to mask it, so X-linked recessive conditions — such as haemophilia and red-green colour blindness — affect many more males than females. A female must inherit the recessive allele on both X chromosomes to be affected; a female with just one copy is an unaffected carrier. X-linked alleles are written as superscripts on the X, with a bare Y for the Y chromosome.

A pedigree chart is a family tree that tracks a single trait across generations, and analysing its pattern lets you deduce the mode of inheritance. By convention, circles are females and squares are males, a horizontal line joins a mating pair, vertical lines lead down to their offspring, and a shaded symbol marks an affected individual. Work through the chart asking a few diagnostic questions: does the trait appear in every generation, or does it skip? Do two unaffected parents ever produce an affected child? Are affected individuals overwhelmingly male? The answers point to whether the condition is dominant or recessive, and autosomal or X-linked.

A genetic-diagram question is marked analytically: the marks are split between method marks (M) for setting the cross out correctly and answer marks (A) for the ratios you read off it. Crucially, error-carried-forward (ECF) applies — if you make an early slip but your Punnett square is then completed correctly from the gametes you wrote, you still earn the method and any ratios that follow consistently. Study how each mark below is tied to a specific part of the diagram, not to loose wording.

The genetic-diagram habit built here — define alleles, segregate them into gametes, recombine in a Punnett square, count the ratios — is the template for every inheritance problem you will meet. At HL it extends to dihybrid crosses, where two genes are followed at once and the classic 9:3:3:1 phenotype ratio appears, and to more complex patterns of inheritance. Master the single-gene method now, keeping genotype and phenotype ratios distinct and laying every step out on the page, and those harder crosses become just more of the same careful bookkeeping.