Extensions of Mendelian Inheritance

Beginner Prerequisites: Mendelian Inheritance Probability IBO USABO genetics transmission-genetics

Overview

Mendel’s seven pea traits were fortunate in showing simple, complete dominance with one gene controlling one trait — but this is a special case, not the general rule. This page covers the inheritance patterns that extend Mendelian analysis without breaking segregation or independent assortment underneath: dominance relationships that aren’t complete, genes with more than two alleles, one gene affecting multiple traits, alleles whose homozygous state is lethal, and the gap between having a genotype and actually expressing its associated phenotype. Every pattern here still obeys segregation at the chromosomal level (see Mendel’s Laws & Probability in Genetics) — what changes is only how genotype maps onto phenotype.

Key Concepts

Incomplete dominance

In incomplete dominance, the heterozygote’s phenotype is an intermediate blend between the two homozygous phenotypes, rather than matching one of them exactly — e.g. a cross between red-flowered (R¹R¹) and white-flowered (R²R²) snapdragons produces pink-flowered (R¹R²) heterozygotes. Critically, this is not a return to pre-Mendelian blending inheritance: the alleles themselves remain discrete and segregate normally (an R¹R² × R¹R² cross still produces a 1:2:1 genotype ratio), only the phenotype ratio changes to match the genotype ratio directly (1 red : 2 pink : 1 white), because there is no dominant allele to mask the other.

Red (RR) × white (ww) snapdragon cross producing pink F1 heterozygotes; the Punnett square below shows the F2 1:2:1 ratio (RR red : Rw pink : Rw pink : ww white). Source: ask.learncbse.in

Codominance

In codominance, both alleles are fully and simultaneously expressed in the heterozygote, rather than blending — the classic example is the MN blood group in humans, where M/N heterozygotes express both M and N antigens on red blood cells simultaneously, distinguishable from either homozygote by direct antigen testing. The distinction from incomplete dominance is precise and frequently tested: incomplete dominance produces a new, intermediate phenotype; codominance produces both parental phenotypes simultaneously and distinguishably.

(No image showing the MN blood group specifically has been sourced yet — the file originally sourced for this slot turned out to depict the ABO system instead, see the note below the ABO table. Flagged HOLD; a genuine MN antigen diagram is still needed here.)

Multiple alleles: the ABO blood group system

A gene can have more than two alleles circulating in a population, even though any single diploid individual still carries only two. The human ABO blood group is the standard teaching example, with three alleles at one locus: I^A and I^B (codominant with each other) and i (recessive to both). This produces four phenotypes from six genotypes:

Genotype(s)	Phenotype
I^A I^A, I^A i	Type A
I^B I^B, I^B i	Type B
I^A I^B	Type AB (codominance)
ii	Type O

This system combines two extensions at once — multiple alleles and codominance between two of them (I^A/I^B) alongside simple dominance of each over the third (i) — and is a frequent source of pedigree/parentage-exclusion exam questions (e.g. two type-A parents, each heterozygous I^A i, can have a type-O child).

ABO blood group reference table: for each blood type (A, B, AB, O), the genotype, the red blood cell surface antigens present, and the plasma antibodies present. Source: jaypeedigital.com

Pleiotropy

Pleiotropy is when a single gene affects multiple, seemingly unrelated phenotypic traits — the opposite direction of complexity from multiple alleles (one gene, many effects, rather than one trait, many alleles). Human sickle-cell disease (a single amino-acid substitution in β-globin, Glu6Val — see Protein Structure, Folding & Function for the molecular mechanism) is a classic pleiotropic example: the same mutation causes anemia, joint pain, organ damage, and increased malaria resistance in heterozygotes — a wide phenotypic footprint traceable to one underlying molecular lesion.

Lethal alleles

Some alleles are lethal in the homozygous state, which distorts the expected phenotypic ratios in a cross. The classic example is coat color in mice: the yellow (A^Y) allele is dominant for coat color but recessive lethal — A^Y A^Y embryos die before birth. A cross between two yellow (heterozygous A^Y A) mice therefore produces a phenotype ratio of 2 yellow : 1 agouti among live births, not the naively expected 3:1, because the A^Y A^Y class is missing entirely. Recognizing a skewed ratio (2:1 instead of 3:1, or a total litter size smaller than expected) as a signature of embryonic lethality, rather than assuming a counting or dominance error, is the key exam skill here.

Four live mice showing real yellow (A^Y/+) and agouti (a/a) coat-color phenotypes, with genotypes labeled for each animal. Source: researchgate.net (“Coat colors: C57BL/6J A^y/a and C57BL/6J a/a mice have yellow and black pelage”)

Penetrance and expressivity

Even a fully accounted-for genotype does not always guarantee a predictable phenotype:

Penetrance is the proportion of individuals with a given genotype who show the phenotype at all — a condition is incompletely (reduced) penetrant if some genotypically affected individuals show no phenotype whatsoever (e.g. certain dominant conditions “skip” an apparently unaffected carrier generation in a pedigree).
Expressivity is the degree or severity of the phenotype among individuals who do express it — variable expressivity means affected individuals can range from mildly to severely affected despite carrying the identical genotype.

Pedigree showing an apparently skipped generation: an affected woman’s children (a son and daughter, both apparently unaffected) go on to have an affected grandchild, consistent with reduced penetrance in the middle generation rather than non-paternity or a pedigree error. Source: ResearchGate

Both are frequently modulated by genetic background (modifier genes) and environment, and both matter directly for reading real pedigrees: reduced penetrance can make a dominant pedigree pattern look recessive by producing an apparently-skipped generation (see Pedigree Analysis & Human Genetic Disorders).

Norm of reaction

The norm of reaction is the full range of phenotypes a single genotype can produce across different environments — formalizing the point that genotype sets a range of possible outcomes, not a fixed single phenotype. Himalayan rabbits’ coat-color pattern (dark extremities, pale body) is a standard example: the underlying temperature-sensitive pigment enzyme produces different local phenotypes depending on local skin temperature, all from one genotype.

Comparative Structures

Pattern	Heterozygote phenotype	Genotype:phenotype ratio relationship	Example
Complete dominance	Matches one homozygote	Phenotype ratio simpler than genotype ratio (3:1 from 1:2:1)	Pea seed shape
Incomplete dominance	Intermediate/blended	Phenotype ratio matches genotype ratio (1:2:1)	Snapdragon flower color
Codominance	Both parental phenotypes shown simultaneously	Phenotype ratio matches genotype ratio (1:2:1)	MN blood groups

Common Exam Questions

“Is this incomplete dominance or codominance?” — always check whether the heterozygote shows a new blended phenotype (incomplete dominance) or both original phenotypes simultaneously and distinguishably (codominance); a pink flower is blended, an AB blood type is both A and B markers present at once.
A skewed ratio that’s consistently missing one class (2:1 instead of 3:1, or a reduced total litter/brood size) should immediately suggest a recessive lethal allele, not a counting error or an exception to dominance.
“Why did an obviously affected parent have an apparently unaffected child who later has affected children of their own?” — tests recognition of reduced penetrance, not a mistaken pedigree or non-paternity.
ABO parentage-exclusion logic (a type-AB parent cannot have a type-O child; a type-O parent cannot have a type-AB child) is a frequent applied-genetics question — always reason from the i allele’s strict recessiveness to both I^A and I^B.

Visual Reference

Interactive

An incomplete-dominance-vs-codominance-vs-complete-dominance comparator: pick a dominance mode and a cross, and see the resulting phenotype ratio and a visual representation of the heterozygote’s actual appearance.

(Static images are placed inline in Key Concepts above, next to the concept each one illustrates, rather than collected here. Two gaps remain open: a genuine MN blood group codominance image, and a genuine A^Y lethal-allele cross/Punnett-square diagram — see the notes inline above.)

Practice Problems

1. A type-A woman and a type-B man have a child who is type O. Using the ABO genotype table, determine the genotypes of both parents and explain how a type-O child is possible.

Show answer

Both parents must be heterozygous: the mother is I^A i (type A) and the father is I^B i (type B). Since i is recessive to both I^A and I^B, each parent can pass either their dominant allele or their hidden i allele. A child who inherits i from both parents (i i) is type O, even though neither parent is phenotypically type O — the i allele was masked in each heterozygous parent.

2. In a cross between two curly-tailed cats (a trait caused by a dominant allele that is homozygous lethal), 90 kittens are born: 60 curly-tailed, 30 straight-tailed. Explain why this ratio is not 3:1, and predict how many kittens would be expected to have died as embryos if the litter size had not been reduced by lethality.

3. A patient has a genotype associated with a dominant, incompletely penetrant disorder (penetrance = 70%), but shows no symptoms. Explain how this is possible, and predict what proportion of this patient’s children (assuming the other parent is unaffected and non-carrier) who inherit the disease allele would be expected to show symptoms.