Monohybrid Inheritance (OCR A-Level Biology A): Revision Notes

Monohybrid Inheritance

Introduction to genetic inheritance patterns

Genetic diagrams provide a visual method for tracking how genes and their alleles pass from parents to offspring across generations. These diagrams help predict the genotypes and phenotypes of offspring based on parental characteristics.

A monohybrid cross (also called a monogenic cross) examines the inheritance of a single gene with two alleles. The term "monohybrid" reflects that only one gene locus is being tracked through the cross. Understanding monohybrid inheritance forms the foundation for analyzing more complex inheritance patterns involving multiple genes.

Rules for constructing genetic diagrams

When drawing genetic diagrams, specific conventions ensure clarity and consistency. These standardized rules allow geneticists to communicate inheritance patterns effectively.

Always begin by stating the parental phenotypes, followed by their genotypes if known. This establishes what characteristics are being examined and the genetic basis for those traits.

For example, in Drosophila melanogaster, wing length can be represented by W for the dominant normal (wild-type) long wing and w for the recessive vestigial (shortened) wing. The human ABO blood group system exemplifies multiple allele notation: I represents the gene, with alleles written as $\mathrm{I^{A}}$, $\mathrm{I^{B}}$, and $\mathrm{I^{o}}$. The A and B blood group alleles are both codominant and written as $\mathrm{I^{A}}$ and $\mathrm{I^{B}}$ respectively.

Complete dominance in monohybrid crosses

In classical Mendelian genetics, one allele completely masks the expression of another at the same locus. This pattern, called complete dominance, produces a characteristic 3:1 phenotypic ratio in the F₂ generation when two heterozygotes are crossed.

The wild type refers to the standard phenotype found naturally in a population. In genetic studies, the wild-type allele represents the "normal" version at a locus, contrasted with variant or mutant alleles. For example, in Drosophila, long wings represent the wild-type phenotype.

During meiosis, alleles at a gene locus separate because they occupy positions on different chromosomes of a homologous pair. This separation ensures each gamete receives only one allele from each gene. When a homozygous dominant individual (WW) is crossed with a homozygous recessive individual (ww), all F₁ offspring are heterozygous (Ww) and display the dominant phenotype.

The diagram above illustrates a complete monohybrid cross tracking wing length in fruit flies. The F₁ generation all show the wild-type long wing phenotype despite being heterozygous. When F₁ flies mate, their offspring segregate in the expected 3:1 ratio.

Incomplete dominance

Incomplete dominance occurs when neither allele at a locus is dominant over the other. Instead of one allele masking another, the two alleles blend their effects, producing an intermediate phenotype in heterozygotes.

In snapdragon plants, flower colour demonstrates incomplete dominance. When a plant homozygous for red petals ($\mathrm{C^{R}C^{R}}$) is crossed with one homozygous for white petals ($\mathrm{C^{W}C^{W}}$), all F₁ offspring are heterozygous ($\mathrm{C^{R}C^{W}}$) and display pink flowers. The superscript notation indicates that neither $\mathrm{C^{R}}$ nor $\mathrm{C^{W}}$ is dominant.

Crossing two pink-flowered plants produces an F₂ generation with a 1:2:1 ratio of red:pink:white flowers. This differs from complete dominance because the phenotypic ratio matches the genotypic ratio — heterozygotes are visually distinguishable from both homozygotes.

The figure above shows two crosses involving incomplete dominance. Panel (a) demonstrates a pink × pink cross yielding 1:2:1 offspring, while panel (b) shows a pink × white cross producing 1:1 pink and white offspring.

Codominance

Codominance describes a situation where two alleles at the same gene locus both influence the phenotype of a heterozygous organism. Neither allele is recessive — both are expressed fully and simultaneously. This differs from incomplete dominance, where alleles blend; in codominance, both characteristics appear distinctly.

Human ABO blood groups provide a well-studied example of codominance combined with multiple alleles. Coat colour in cattle represents another codominant system. Like incomplete dominance, codominant crosses produce a 1:2:1 phenotypic ratio in the F₂ generation, but the heterozygote displays features of both alleles rather than an intermediate form.

All examples of codominance represent discontinuous variation, where phenotypes fall into distinct categories rather than showing a continuous range.

Dihybrid inheritance and independent assortment

Dihybrid inheritance tracks two different characteristics simultaneously, each controlled by a separate gene. When these genes are located on different, non-homologous chromosomes, they are described as unlinked. Unlinked genes segregate independently during meiosis, allowing all possible combinations of alleles to form in gametes.

Consider a cross examining both body colour and wing type in Drosophila. Body colour has two alleles: E (dominant grey, wild-type) and e (recessive ebony/black). Wing shape also has two alleles: N (dominant normal, wild-type) and n (recessive curly). When pure-breeding flies with grey bodies and normal wings (EENN) are crossed with flies having ebony bodies and curly wings (eenn), all F₁ offspring are heterozygous for both genes (EeNn) and show both wild-type phenotypes.

Crossing two F₁ individuals produces an F₂ generation with the characteristic 9:3:3:1 ratio. Examining each trait separately reveals a 3:1 ratio for body colour and 3:1 for wing type, confirming the genes assort independently.

A Punnett square for this cross shows nine different genotypes but only four phenotypes due to dominance. The explanation for these results lies in independent assortment — the principle that genes on different chromosomes separate randomly during meiosis I, producing all possible allele combinations in gametes with equal probability.

Gene linkage and crossing over

Linkage describes genes located on the same autosomal chromosome (an autosome is any chromosome other than a sex chromosome). Linked genes do not follow independent assortment because they are physically connected on the same DNA molecule. This connection disturbs the expected phenotypic ratios and reduces genetic variation in offspring.

A test cross determines the genotype of an individual by crossing it with a homozygous recessive individual. For two unlinked genes, a test cross between a double heterozygote and a double homozygous recessive produces a 1:1:1:1 ratio of phenotypes. However, linked genes produce a 1:1 ratio if they remain together throughout meiosis.

The frequency of recombination reflects the distance between genes — the farther apart two genes are, the more likely crossing over will occur between them.

Cross	Genotypes and ratios	Explanation
1	ABab (52) : abab (48)	Complete linkage with no crossing over between genes
2	ABab (46) : Abab (5) : aBab (6) : abab (43)	Partial linkage; genes positioned close together on chromosome
3	ABab (34) : Abab (12) : aBab (16) : abab (38)	Partial linkage; genes more distant than in cross 2
4	AaBb (24) : Aabb (26) : aaBb (29) : aabb (21)	Unlinked genes showing independent assortment (approximately 1:1:1:1)

The table above compares test cross results for linked versus unlinked genes. Notice how the recombinant classes (Abab and aBab) increase in frequency as the distance between genes increases, until genes are far enough apart (or on different chromosomes) to assort independently.

Sex linkage

Sex linkage refers to genes present on the X chromosome. This inheritance pattern has nothing to do with determining biological sex itself. The X chromosome carries hundreds of genes coding for many important characteristics beyond sex determination. In contrast, the Y chromosome contains relatively few genes, primarily those involved in male sexual development.

Females possess two X chromosomes (XX), carrying two alleles for each sex-linked gene. For a recessive X-linked trait to appear in females, both X chromosomes must carry the recessive allele. When females have one recessive and one dominant allele, they are carriers — they display the dominant phenotype but can pass the recessive allele to offspring.

The diagram illustrates sex-linked inheritance in Drosophila melanogaster. Eye colour in fruit flies demonstrates sex linkage, with red eyes ($\mathrm{X^{R}}$) dominant over white eyes ($\mathrm{X^{r}}$).

Males		Females
Genotype	Phenotype	Genotype	Phenotype
$\mathrm{X^{R}Y}$	Red eyes	$\mathrm{X^{R}X^{R}}$	Red eyes
$\mathrm{X^{r}Y}$	White eyes	$\mathrm{X^{R}X^{r}}$	Red eyes (carrier)
		$\mathrm{X^{r}X^{r}}$	White eyes

Notice that males need only one copy of the recessive allele to display white eyes, while females require two copies. Carrier females ($\mathrm{X^{R}X^{r}}$) appear red-eyed but can produce both red- and white-eyed sons.

Multiple alleles

Many genes exist in more than two allelic forms within a population, a phenomenon called multiple allelism. Although numerous alleles may exist across a population, any diploid individual can only carry two alleles (one from each parent).

The human ABO blood group system exemplifies multiple alleles. Three alleles exist at this locus on chromosome 9: $\mathrm{I^{o}}$, $\mathrm{I^{A}}$, and $\mathrm{I^{B}}$. These alleles control production of antigens (specific proteins) on red blood cell surfaces.

Blood group	Possible genotypes
A	$\mathrm{I^{A}I^{A}}$ or $\mathrm{I^{A}I^{o}}$
B	$\mathrm{I^{B}I^{B}}$ or $\mathrm{I^{B}I^{o}}$
AB	$\mathrm{I^{A}I^{B}}$
O	$\mathrm{I^{o}I^{o}}$

The alleles $\mathrm{I^{A}}$ and $\mathrm{I^{B}}$ are both dominant over $\mathrm{I^{o}}$. Therefore, individuals with genotypes $\mathrm{I^{A}I^{o}}$ display blood group A, while $\mathrm{I^{B}I^{o}}$ individuals show blood group B. However, $\mathrm{I^{A}}$ and $\mathrm{I^{B}}$ are codominant to each other — when present together in genotype $\mathrm{I^{A}I^{B}}$, both A and B antigens appear on red blood cells, producing blood group AB.

Epistasis

Epistasis occurs when two or more genes interact to influence a single phenotypic characteristic. Although these genes may segregate independently during meiosis (if on different chromosomes), their interactions modify the expected phenotypic ratios. One gene effectively suppresses or masks the expression of another gene.

Epistatic interactions frequently involve metabolic pathways where each gene codes for a different enzyme. If an individual is homozygous recessive for a gene coding a particular enzyme, that enzyme is non-functional. This stops the metabolic pathway at that step, preventing the characteristic from being expressed even if dominant alleles exist at other loci in the pathway.

The figure above illustrates four different epistatic interactions involving enzyme pathways for pigment production. In each pathway, genes A and B code for enzymes that convert substrates in a sequence. When an individual is homozygous recessive (aa or bb), no functional enzyme is produced, blocking that step in the pathway.

Different types of epistasis produce characteristic F₂ ratios when dihybrid heterozygotes are crossed:

Type of epistasis	F₂ phenotypic ratios				Example explanation
	$\mathbf{A\_B\_}$	$\mathbf{A\_bb}$	$\mathbf{aaB\_}$	$\mathbf{aabb}$
No epistasis (independent assortment)	9	3	3	1	Standard dihybrid ratio
Recessive epistasis (Example 1)	9 purple	3 red		4 white	Both dominant alleles (A and B) needed for purple; A_bb produces red; aa blocks all pigment
Recessive epistasis (Example 2)	9 purple		7 white		Both dominant alleles required for any pigment production
Dominant epistasis (Example 3)	12 purple		3 red	1 white	Dominant allele A produces purple regardless of B; only aaBb produces red
Dominant epistasis (Example 4)	15 purple			1 white	Either dominant allele (A or B) produces purple pigment

Using chi-squared analysis

The chi-squared ($\chi^{2}$) statistical test evaluates whether observed data differ significantly from expected values. In genetics, this test determines if experimental results match theoretical predictions based on inheritance patterns.

The test compares observed offspring numbers in each category against expected numbers predicted by a hypothesis (the null hypothesis). The null hypothesis typically states that genes assort independently or follow a particular inheritance pattern.

The chi-squared value is calculated using:

$\chi^{2} = \sum \frac{(O - E)^{2}}{E}$

where:

• $\Sigma$ means "sum of"
• $O$ represents observed values from the experiment
• $E$ represents expected values from the hypothesis

Remember!