Biology · Meiosis & Patterns of Inheritance
Sex-linked, cytoplasmic and non-Mendelian inheritance
A variant on an X-specific region, a mitochondrial variant and an allele with an intermediate heterozygote phenotype travel through different accounts. Each still needs a clear rule. Start by asking where the hereditary material is and how the phenotype is scored.
Begin with the cell-division lesson and Heredity, Variation & Evolution: recognise homologous chromosomes, sister chromatids, a locus and an allele. DNA, Replication & Repair explains how copying makes two sister DNA molecules. Gene Expression helps with the later distinction between transmitting a sequence and expressing a trait. Here, fractions and conditional probabilities are explained alongside the cases. All chromosome-count examples use a specified diploid model; the final lessons introduce extensions rather than requiring advanced genetics beforehand.
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Apply specified chromosome and expression rules to extended inheritance cases without treating every departure from a 3:1 phenotype ratio as a failure of segregation.
A locus's chromosome location changes the transmission paths
Use a familiar XX/XY model and place an invented N/v locus on X outside the region shared with Y. An XY individual has one copy at that locus, a hemizygous state; Y supplies no corresponding allele in this example. This differs from the two-copy autosomal crosses.
Our egg-producing parent is XᴺXᵛ and sperm-producing parent XᴺY. Assume equal gamete contributions and a simplified recessive phenotype rule: XᵛY shows the trait, while XᴺY, XᴺXᴺ and XᴺXᵛ do not. The four unions each have probability 1/4. Among the XY unions, half are XᵛY; among all unions, one quarter are.
The sperm-producing parent contributes X to the XX unions and Y to the XY unions. This explains the absence of direct father-to-son transmission for a locus in this X-specific model. Loci in pseudoautosomal regions shared by X and Y need a different account.
The drawing is a stated chromosome pathway, not a complete model of sex development or trait expression. Other species use other sex-chromosome systems. Even within a familiar XX/XY setting, X inactivation, gene-specific dosage and variable expression can make a heterozygote's phenotype more complicated than the assigned recessive rule.
State the X-specific locus and the denominator
Read the diagram in words
The four unions are XᴺXᴺ, XᴺXᵛ, XᴺY and XᵛY, each with probability 1/4. Only XᵛY shows the assigned trait. Of the two XY paths, one is XᵛY, giving the conditional probability 1/2.
| Egg contribution | Sperm contribution: Xᴺ | Sperm contribution: Y |
|---|---|---|
| Egg contribution: Xᴺ | XᴺXᴺ; Trait absent; 1/4 | XᴺY; Trait absent; 1/4 |
| Egg contribution: Xᵛ | XᴺXᵛ; Trait absent in this simplified model; 1/4 | XᵛY; Trait present; 1/4 |
- Invented locus on X; absent from Y
- Outside the pseudoautosomal region
- 1/4 of all unions are XᵛY
- 1/2 of XY unions are XᵛY
- Specified simplified recessive phenotype rule
Some inherited DNA follows an organelle pathway
Mitochondria carry DNA distinct from nuclear chromosomes. In the usual human mitochondrial inheritance pattern, the egg supplies the mitochondria transmitted to the developing offspring. A mitochondrial sequence variant can therefore follow an egg-line transmission pattern rather than the equal nuclear-allele grid.
A cell can carry a mixture of mitochondrial DNA variants, called heteroplasmy. Which copies reach descendants and how their proportions change can affect expression. Receiving a variant does not establish identical symptoms or severity in every carrier. Organelle inheritance across other organisms also varies, so the familiar human pattern is not a universal rule for all mitochondria and chloroplasts.
Genomic imprinting concerns a different account: at some nuclear loci, expression depends on whether a copy came through the egg or sperm pathway. The DNA copies can still be transmitted through chromosomes while their activity differs with parent of origin. Imprinting does not mean all genes from one parent are silent, nor that the base sequence must have changed to alter expression.
Change phenotype grouping without changing allele identities
With incomplete dominance, a heterozygote has an intermediate value for the specified trait relative to the two homozygotes. If a toy pigment measurement is 0 units for C¹C¹, 5 for C¹C² and 10 for C²C², the intermediate phenotype does not fuse C¹ and C² into a new allele. The heterozygote can still transmit either version.
With codominance, both allele-associated features can be distinguished in the heterozygote, rather than one hiding the other in the scored phenotype. A population may also contain more than two alleles at a locus even though an ordinary diploid individual carries only two copies there. These are separate ideas from linkage.
Epistasis concerns the effect of one locus on the phenotype associated with another. In a hypothetical pigment pathway, let pp block pigment formation entirely; where P_ permits pigment, Q_ gives a dark category and qq a light one. For an independently assorting PpQq × PpQq cross, the phenotype categories become dark 9/16, light 3/16 and no pigment 4/16. The transmission still follows the original four genotype classes; two have been grouped by the pathway rule.
A pathway rule groups two independently produced classes
Read the diagram in words
Independent transmission produces four genotype classes with probabilities 9/16, 3/16, 3/16 and 1/16. The assigned pp block groups the last two as no pigment, giving dark:light:no-pigment probabilities 9/16:3/16:4/16.
| Combination class | Probability | Specified phenotype |
|---|---|---|
| P_Q_ | 9/16 | Dark pigment |
| P_qq | 3/16 | Light pigment |
| ppQ_ | 3/16 | No pigment |
| ppqq | 1/16 | No pigment |
| Specified phenotype | Probability |
|---|---|
| Dark | 9/16 |
| Light | 3/16 |
| No pigment | 4/16 |
- P_ permits pigment production
- pp blocks the pigment step
- Where pigment is permitted: Q_ dark, qq light
- Independent allele transmission retained
- Phenotype grouping changes to 9:3:4
- Underscore means either allele at the second copy
Worked example
Keep transmission separate from the scored result
For the X-linked grid, multiply 1/2 for an Xᵛ egg contribution by 1/2 for a Y sperm contribution to obtain 1/4 for XᵛY. Conditioning on Y-contributing unions leaves XᴺY and XᵛY equally likely, so the probability becomes 1/2 within that subset. The two denominators answer different questions.
In the pigment pathway example, ppQ_ has probability 3/16 and ppqq 1/16. Both lack pigment because pp blocks the earlier step. Add them to obtain 4/16. This grouping changes the phenotype ratio from 9:3:3:1 to 9:3:4 without changing the assumed independent assortment of P/p and Q/q. A real ratio alone would not prove that particular pathway mechanism.
Check your understanding
Why is the XᵛY probability one quarter among all unions but one half among XY unions?
Show reasoning
The first denominator includes four equally likely union paths; one is XᵛY. Conditioning on XY removes the two XX paths, leaving two equally likely possibilities, one of which is XᵛY.
Must a mitochondrial variant produce the same phenotype in all people who inherit it?
Show reasoning
No. Variant mixture and proportions, relevant tissues, genetic background and other conditions can affect expression. Transmission and severity are different records.
In the 9:3:4 pigment model, has the pp genotype prevented Q/q alleles from segregating?
Show reasoning
No. The independent allele combinations still occur. The assigned pp pathway block makes ppQ_ and ppqq share the same no-pigment phenotype, changing how the outcomes are grouped.
What to carry forward
Locate the inherited material and state its expression rule before extending a nuclear cross.
References
Resources supporting this lesson’s explanations and examples.
National Library of Medicine · MedlinePlus Genetics2 sources
- Help Me Understand Genetics — Inheritance and expression (Live guide)National Library of Medicine · MedlinePlus GeneticsScientific education guideHelp Me Understand Genetics — Inheritance and expression (Live guide)Reduced penetrance and variable expressivityGenomic imprinting and uniparental disomyHeritability
- Mitochondrial DNA (Live reference)National Library of Medicine · MedlinePlus GeneticsGenetic reference overview
- Biology 2e — Inheritance and probability (2018)Mary Ann Clark, Matthew Douglas and Jung Choi · OpenStaxOpen textbook sectionsBiology 2e — Inheritance and probability (2018)Characteristics, traits and dominanceSegregation and independent assortment
- Introduction to Genetics — Chromosomes, crosses and mapping (2023 adaptation of Open Genetics Lectures)Natasha Ramroop Singh · Thompson Rivers UniversityOpen university textbookIntroduction to Genetics — Chromosomes, crosses and mapping (2023 adaptation of Open Genetics Lectures)DNA amount and chromosome numberAssumptions of a dihybrid ratioConditional pedigree probabilitiesAllele phaseLinkage and recombinant fractionsGenetic maps and their limitsTwo-point and three-point crossesTypes of dominanceBiochemical basis of dominanceGene interactions and phenotype groupingPseudoautosomal regions