Numerical Changes in Chromosomes

Topic: Numerical Changes

Points Covered in This Lecture:

• Aneuploidy
• Euploidy
• Autopolyploidy and Allopolyploidy
• Uniparental Disomy
• Genetic Mosaicism

Chromosome number is one of the most fundamental characteristics of a species. Most species have a characteristic chromosome number that is maintained through mitosis and meiosis. However, errors in chromosome replication, segregation, fertilization or cell division can produce changes in chromosome number. Such changes are collectively called numerical chromosomal abnormalities.

Numerical changes differ from structural chromosomal alterations. Structural alterations change the organization of chromosome material, whereas numerical changes primarily alter the number of chromosomes or complete chromosome sets.

Index

1. 1. Introduction to Numerical Chromosomal Changes
2. 2. Definition of Numerical Changes
3. 3. Classification of Numerical Changes
4. 4. Basic Concept of Ploidy
5. 5. Aneuploidy
6. 6. Nondisjunction
7. 7. Monosomy
8. 8. Trisomy
9. 9. Nullisomy and Tetrasomy
10. 10. Causes and Consequences of Aneuploidy
11. 11. Euploidy
12. 12. Haploidy and Polyploidy
13. 13. Polyploidy
14. 14. Autopolyploidy
15. 15. Allopolyploidy
16. 16. Autopolyploidy vs Allopolyploidy
17. 17. Formation of Polyploids
18. 18. Biological Importance of Polyploidy
19. 19. Uniparental Disomy
20. 20. Types of Uniparental Disomy
21. 21. Mechanisms Producing Uniparental Disomy
22. 22. Uniparental Disomy and Genomic Imprinting
23. 23. Genetic Mosaicism
24. 24. Mechanisms of Mosaicism
25. 25. Mosaicism vs Chimerism
26. 26. Biological and Clinical Significance
27. 27. Detection of Numerical Changes
28. 28. Comparison Table
29. 29. High-Yield Exam Points
30. 30. 10 MCQs with Answers
31. 31. Quick Revision

1. Introduction to Numerical Chromosomal Changes

Chromosomes contain the genetic material required for the growth, development and reproduction of an organism. In a normal diploid organism, chromosome number is maintained because chromosome duplication and segregation are tightly controlled.

During mitosis and meiosis, chromosomes must be distributed accurately to daughter cells or gametes. If chromosomes fail to segregate properly, cells can receive too many or too few chromosomes. Similarly, failure of normal cell division or fusion of abnormal gametes can produce organisms with altered chromosome numbers.

Numerical chromosome abnormalities can involve either:

• A change involving individual chromosomes.
• A change involving an entire chromosome set.

These two situations form the basic distinction between aneuploidy and euploidy.

Aneuploidy → Change in individual chromosome number

Euploidy → Change involving complete chromosome sets

2. Definition of Numerical Changes

A numerical chromosomal alteration is a change in chromosome number relative to the normal chromosome complement of an organism.

For example, if a normally diploid organism has two copies of each chromosome, the gain or loss of one particular chromosome represents an aneuploid condition. In contrast, an organism having three complete chromosome sets instead of two is a triploid and represents a euploid/polyploid condition.

Exam Definition:
Numerical chromosome abnormalities involve alterations in chromosome number rather than changes in the physical structure or sequence organization of a chromosome.

3. Classification of Numerical Changes

Numerical changes can be broadly classified into two major categories:

Category	Meaning	Examples
Aneuploidy	Gain or loss of individual chromosomes	Monosomy, trisomy
Euploidy	Change in complete chromosome sets	Triploidy, tetraploidy

Other important chromosome-number phenomena include uniparental disomy and mosaicism. These do not simply represent conventional whole-organism changes in chromosome number, but they are important concepts in chromosome genetics and numerical abnormalities.

4. Basic Concept of Ploidy

The term ploidy refers to the number of complete sets of chromosomes present in a cell.

• n = one complete chromosome set.
• 2n = two complete chromosome sets, usually called diploid.
• 3n = three complete chromosome sets, called triploid.
• 4n = four complete chromosome sets, called tetraploid.
• 6n = six complete chromosome sets, called hexaploid.

Important distinction:
Ploidy refers to complete sets of chromosomes. Therefore, a triploid cell has three copies of every chromosome type, whereas a trisomic cell has an extra copy of only one particular chromosome.

5. Aneuploidy

Aneuploidy is a numerical chromosome abnormality in which the number of individual chromosomes differs from the normal chromosome complement, but the change is not an exact multiple of the complete haploid chromosome set.

In a diploid organism, the normal chromosome constitution is approximately 2n. Aneuploid conditions may be represented as:

• 2n − 1 → monosomy
• 2n + 1 → trisomy
• 2n − 2 → nullisomy
• 2n + 2 → tetrasomy

Aneuploidy = Gain or loss of individual chromosome(s)

Aneuploidy commonly results from errors in chromosome segregation, especially nondisjunction.

6. Nondisjunction

Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during cell division.

It can occur during either meiosis or mitosis.

Meiotic Nondisjunction

During meiosis I, homologous chromosomes normally separate. If they fail to separate, both homologues can move to the same daughter cell.

During meiosis II, sister chromatids normally separate. Failure of this process can also produce abnormal gametes.

Nondisjunction → Abnormal chromosome segregation → Aneuploid cells/gametes

Possible Gametes

A nondisjunction event involving one chromosome can produce gametes containing an extra chromosome or lacking that chromosome.

• n + 1 gamete
• n − 1 gamete

Fertilization involving such gametes can produce aneuploid zygotes.

7. Monosomy

Monosomy is the loss of one chromosome from an otherwise diploid chromosome complement.

Monosomy = 2n − 1

Instead of having two copies of a particular chromosome, the individual has only one copy.

Example

In humans, monosomy X is associated with Turner syndrome. The typical chromosome constitution is 45,X.

• One X chromosome is present.
• The total chromosome number is typically 45.
• The condition occurs in individuals with a female developmental phenotype.

Complete monosomy for many autosomes is generally incompatible with normal human development, although the outcome depends on the chromosome involved.

8. Trisomy

Trisomy is the presence of one extra copy of a particular chromosome.

Trisomy = 2n + 1

A trisomic individual therefore has three copies of one chromosome while the remaining chromosome types generally occur in their usual numbers.

Important Human Examples

Condition	Chromosome	Typical Karyotype
Down syndrome	Chromosome 21	47,+21
Edwards syndrome	Chromosome 18	47,+18
Patau syndrome	Chromosome 13	47,+13

These conditions demonstrate how an extra chromosome can produce significant changes in development because of altered gene dosage.

9. Nullisomy and Tetrasomy

Nullisomy

Nullisomy occurs when both copies of one chromosome are absent.

Nullisomy = 2n − 2

Nullisomy is generally more severe than monosomy because both homologous copies of a chromosome are missing.

Tetrasomy

Tetrasomy occurs when four copies of one chromosome are present in an otherwise diploid complement.

Tetrasomy = 2n + 2

10. Causes and Consequences of Aneuploidy

Major Causes

• Nondisjunction during meiosis.
• Nondisjunction during mitosis.
• Anaphase lag.
• Abnormal chromosome segregation.
• Errors during early embryonic cell division.

Anaphase Lag

Anaphase lag occurs when a chromosome or chromatid fails to migrate properly to a daughter cell during chromosome segregation and is consequently excluded from the daughter nucleus.

This mechanism can contribute to chromosome loss and mosaic aneuploidy.

Consequences

• Altered gene dosage.
• Developmental abnormalities.
• Reduced viability.
• Infertility or reproductive problems.
• Spontaneous pregnancy loss in severe cases.
• Abnormal development.
• Mosaicism when the abnormality arises after fertilization.

11. Euploidy

Euploidy refers to a condition in which the chromosome number consists of complete sets of chromosomes.

The basic haploid number is represented by n. Euploid conditions include:

• Haploid = n
• Diploid = 2n
• Triploid = 3n
• Tetraploid = 4n
• Hexaploid = 6n

When an organism contains more than two complete chromosome sets, the condition is generally referred to as polyploidy.

Key Difference:
Aneuploidy changes individual chromosome number, while euploidy changes the number of complete chromosome sets.

12. Haploidy and Polyploidy

Haploidy

A haploid cell contains one complete chromosome set, represented by n. In many sexually reproducing organisms, gametes are haploid.

Diploidy

Diploid cells contain two homologous chromosome sets, one usually inherited from each parent.

Polyploidy

Polyploid organisms contain more than two complete chromosome sets.

Ploidy	Chromosome Sets
Haploid	n
Diploid	2n
Triploid	3n
Tetraploid	4n
Hexaploid	6n

13. Polyploidy

Polyploidy is the presence of more than two complete chromosome sets in a cell or organism.

Polyploidy is extremely important in plant genetics and evolution. Many cultivated plants have polyploid genomes.

Examples of Ploidy

• Triploid = 3n
• Tetraploid = 4n
• Hexaploid = 6n
• Octoploid = 8n

Polyploidy can arise through errors in meiosis, chromosome doubling, fertilization involving unreduced gametes, or hybridization followed by chromosome doubling.

14. Autopolyploidy

Autopolyploidy occurs when multiple complete chromosome sets originate from the same species.

For example, if a diploid species has chromosome complement 2n and its chromosome number doubles without hybridization with another species, the result may be an autotetraploid with 4n chromosomes.

Autopolyploidy = Multiple chromosome sets from the same species

Formation

Autopolyploidy can arise through chromosome doubling in somatic cells or through formation and fusion of unreduced gametes.

Important Features

• Chromosome sets are derived from the same species.
• Homologous chromosomes may have multiple pairing possibilities during meiosis.
• Meiosis can therefore be more complex.
• Fertility may be affected depending on the species and chromosome configuration.
• Autopolyploidy is important in plant breeding and genome evolution.

15. Allopolyploidy

Allopolyploidy occurs when chromosome sets from two or more different species become combined in a hybrid, often followed by chromosome doubling.

Allopolyploidy = Chromosome sets derived from different species

A classic conceptual example involves two species:

Species A: AA
Species B: BB

Hybrid: AB

Chromosome doubling → AABB

The doubled hybrid may have homologous partners for each chromosome and can therefore become more fertile than the original sterile hybrid.

Importance

• Combines genetic characteristics of different species.
• Can restore fertility in some interspecific hybrids after chromosome doubling.
• Has played an important role in crop evolution.
• Can generate new polyploid species.

16. Autopolyploidy vs Allopolyploidy

Feature	Autopolyploidy	Allopolyploidy
Origin	Same species	Different species
Chromosome sets	Multiple copies of same genomic set	Different genomic sets
Hybridization	Not required	Usually important
Example concept	AAAA	AABB
Meiosis	Can involve multivalent pairing	More regular pairing may occur after chromosome doubling

Memory Trick:
AUTO = AUTO = Same source
ALLO = OTHER = Different source

17. Formation of Polyploids

Polyploidy can arise through several mechanisms.

1. Failure of Meiosis

Errors during meiosis can generate unreduced gametes. These gametes retain the chromosome number of the parent rather than undergoing the normal reduction.

2. Fusion of Unreduced Gametes

Fusion of two unreduced gametes can produce a polyploid zygote.

2n gamete + 2n gamete → 4n zygote

3. Chromosome Doubling

Chromosome duplication without normal cell division can increase chromosome number within a cell.

4. Hybridization Followed by Chromosome Doubling

This mechanism is particularly important for allopolyploid formation.

18. Biological Importance of Polyploidy

Polyploidy has enormous importance in plant evolution and agriculture. Polyploid plants may show changes in cell size, organ size, growth, fertility and environmental tolerance.

• Can contribute to the origin of new species.
• Can combine useful characteristics from different species.
• Can alter plant morphology.
• Can increase genetic diversity.
• Can be useful in crop improvement.
• Can contribute to adaptation to different environments.

Important Examples

Wheat is a well-known example of a polyploid crop. Bread wheat is hexaploid, with genomic constitution commonly represented as AABBDD.

Cotton includes important allopolyploid species. Many cultivated crop species illustrate the evolutionary significance of genome duplication and hybridization.

Exam Point:
Polyploidy is much more common and evolutionarily important in plants than in humans.

19. Uniparental Disomy

Uniparental disomy (UPD) occurs when both copies of a particular chromosome, or chromosome region, are inherited from the same parent rather than one copy from each parent.

Normal disomy → One chromosome from mother + one from father

UPD → Both homologues from the same parent

The total chromosome number may remain normal in a person with UPD. The important abnormality is the parental origin of the chromosome copies.

Important:
Uniparental disomy is not necessarily associated with an abnormal chromosome number. It is primarily a problem of parental origin and chromosome inheritance.

20. Types of Uniparental Disomy

There are two major forms of uniparental disomy.

Maternal Uniparental Disomy

Both copies of a chromosome are inherited from the mother.

Maternal UPD = two maternal copies

Paternal Uniparental Disomy

Both copies are inherited from the father.

Paternal UPD = two paternal copies

Isodisomy

Isodisomy refers to inheritance of two copies that are genetically identical or derived from the same parental chromosome through a meiotic mechanism, particularly relevant to meiosis II or mitotic events depending on context.

Heterodisomy

Heterodisomy refers to inheritance of both homologous chromosomes from the same parent, generally reflecting a meiosis I nondisjunction event.

Type	Basic Meaning
Maternal UPD	Both chromosome copies from mother
Paternal UPD	Both chromosome copies from father
Isodisomy	Two copies derived from one parental homolog
Heterodisomy	Both homologues inherited from one parent

21. Mechanisms Producing Uniparental Disomy

UPD can arise from chromosome segregation errors followed by mechanisms that restore the chromosome number.

Trisomy Rescue

Suppose a zygote initially contains three copies of a chromosome. If one chromosome is subsequently lost during early development, the remaining two copies may both have originated from the same parent.

Trisomic zygote → Loss of one chromosome → Disomic cell
If the lost chromosome is from one parent → Possible UPD

Monosomy Rescue

A monosomic cell containing one chromosome may duplicate that chromosome. The resulting two copies can originate from the same parental chromosome.

Gamete Complementation

Rarely, two abnormal gametes can combine in a way that produces a disomic chromosome pair from one parent.

High-Yield Mechanism:
Trisomy rescue is an important mechanism that can result in uniparental disomy.

22. Uniparental Disomy and Genomic Imprinting

One of the most important reasons UPD can have biological consequences is genomic imprinting.

Genomic imprinting is an epigenetic phenomenon in which the expression of certain genes depends on whether the allele was inherited from the mother or father.

Therefore, inheriting two copies from one parent can produce abnormal gene expression even when the total number of chromosome copies is normal.

UPD → Abnormal parental origin
↓
Imprinted gene expression can be altered
↓
Possible phenotype

UPD can also have consequences when a recessive pathogenic variant is inherited in two copies through isodisomy. Thus, UPD can reveal recessive genetic conditions in some circumstances.

23. Genetic Mosaicism

Genetic mosaicism is a condition in which an individual contains two or more genetically different cell populations that originated from the same fertilized egg.

One zygote → Post-zygotic genetic change → Different cell populations

Mosaicism can involve chromosome number, chromosome structure, DNA sequence or other genetic differences.

Chromosomal Mosaicism

When different cell populations have different chromosome numbers, the condition is called chromosomal mosaicism.

For example, an individual may contain both normal diploid cells and aneuploid cells.

Cell population 1 → 46 chromosomes
Cell population 2 → 47 chromosomes

The proportion and distribution of the different cell populations can influence the phenotype.

24. Mechanisms of Mosaicism

Post-Zygotic Nondisjunction

If nondisjunction occurs after fertilization during an early mitotic division, the resulting daughter cells can carry different chromosome complements.

Normal zygote → Mitotic nondisjunction → Normal + aneuploid cell lines

Anaphase Lag

Loss of a chromosome during anaphase can create a cell line with fewer chromosomes.

Somatic Mutation

A mutation arising after fertilization can produce a genetically distinct cell population.

Timing Matters

The earlier a post-zygotic mutation occurs during development, the greater the potential number of descendant cells carrying that mutation.

Important:
An alteration occurring very early in embryonic development can produce a larger proportion of affected cells than an alteration occurring much later.

25. Mosaicism vs Chimerism

Mosaicism and chimerism both involve genetically different cell populations, but their origins are different.

Feature	Mosaicism	Chimerism
Origin	One zygote	Two or more genetically distinct zygotic lineages
Mechanism	Post-zygotic mutation or chromosome error	Fusion or contribution of cells from different zygotes
Genetic populations	Derived from the same original zygote	Derived from different zygotic origins
Example concept	Normal + mutant cell lines	Cells derived from two embryos

Memory Trick:
Mosaic = One embryo, multiple cell populations.
Chimera = More than one embryonic genetic origin.

26. Biological and Clinical Significance

Numerical chromosomal abnormalities can have major effects on development, fertility and health. The severity depends on the chromosome involved, the size of the dosage imbalance, the presence of mosaicism and other genetic and environmental factors.

Aneuploidy

• Can cause developmental disorders.
• Can result in pregnancy loss.
• Can influence fertility.
• Can alter gene dosage.

Polyploidy

• Common and important in plants.
• Can contribute to crop evolution.
• Can produce changes in cell size and plant morphology.
• Can contribute to speciation.

UPD

• Can alter expression of imprinted genes.
• Can reveal recessive variants through isodisomy.
• Can arise following chromosome rescue events.

Mosaicism

• Can produce variable clinical phenotypes.
• Phenotype may depend on tissue distribution.
• Can result from post-zygotic chromosome errors.

27. Detection of Numerical Changes

Several laboratory methods are used to detect chromosome-number abnormalities.

Karyotyping

Karyotyping allows visualization of chromosomes and can identify many numerical abnormalities such as trisomies and monosomies.

FISH

Fluorescence in situ hybridization uses chromosome-specific probes to detect particular chromosome abnormalities in cells.

Chromosomal Microarray

Chromosomal microarray can identify many copy-number changes across the genome. However, it does not directly detect every type of balanced rearrangement.

Genomic and Molecular Methods

Modern molecular techniques can identify chromosome dosage, mosaicism and parental origin depending on the assay used.

UPD Testing

UPD can be investigated using genetic markers and molecular approaches that compare parental and offspring alleles to determine the parental origin of chromosomal regions.

28. Comparison Table

Condition	Basic Change	Notation/Concept	Key Point
Monosomy	Loss of one chromosome	2n − 1	One copy instead of two
Trisomy	Gain of one chromosome	2n + 1	Three copies of one chromosome
Nullisomy	Loss of both homologues	2n − 2	Both copies absent
Tetrasomy	Gain of two copies of one chromosome	2n + 2	Four copies of one chromosome
Triploidy	One additional complete set	3n	Three complete sets
Tetraploidy	Two additional complete sets	4n	Four complete sets
Autopolyploidy	Multiple sets from same species	Example: AAAA	Same genomic origin
Allopolyploidy	Sets from different species	Example: AABB	Hybrid origin
UPD	Both chromosome copies from one parent	Maternal or paternal	Parental origin altered
Mosaicism	Different cell populations in one individual	Post-zygotic origin	One zygote

29. High-Yield Exam Points

• Aneuploidy involves gain or loss of individual chromosomes.
• Euploidy involves complete chromosome sets.
• Monosomy = 2n − 1.
• Trisomy = 2n + 1.
• Nullisomy = 2n − 2.
• Tetrasomy = 2n + 2.
• Nondisjunction is failure of chromosomes or chromatids to separate properly.
• Triploidy = 3n.
• Tetraploidy = 4n.
• Polyploidy means more than two complete chromosome sets.
• Autopolyploidy involves chromosome sets derived from the same species.
• Allopolyploidy involves chromosome sets from different species.
• Allopolyploidy commonly involves hybridization followed by chromosome doubling.
• Polyploidy is especially important in plants.
• Uniparental disomy means both copies of a chromosome are inherited from one parent.
• UPD can be maternal or paternal.
• Heterodisomy generally reflects inheritance of both homologues from one parent.
• Isodisomy involves two copies derived from the same parental homolog.
• Trisomy rescue can result in UPD.
• UPD can cause abnormal phenotypes because of genomic imprinting.
• UPD can also unmask recessive variants through isodisomy.
• Mosaicism involves genetically different cell populations derived from one zygote.
• Post-zygotic nondisjunction can produce chromosomal mosaicism.
• Anaphase lag can contribute to mosaic chromosome loss.
• Earlier developmental mutations can affect a larger fraction of cells.
• Mosaicism and chimerism are not the same.
• Mosaicism usually originates from one zygote, while chimerism involves more than one embryonic genetic origin.

30. 10 MCQs with Answers

Q1. Which of the following represents monosomy?

1. 2n + 1
2. 2n − 1
3. 3n
4. 4n

Correct Answer: B — 2n − 1
Monosomy represents the loss of one chromosome from a diploid chromosome complement.

Q2. Which condition represents trisomy?

1. 2n − 1
2. 2n − 2
3. 2n + 1
4. 3n

Correct Answer: C — 2n + 1
Trisomy is the presence of one additional copy of a particular chromosome.

Q3. Failure of homologous chromosomes or sister chromatids to separate properly is called:

1. Crossing over
2. Nondisjunction
3. Translocation
4. Replication

Correct Answer: B — Nondisjunction
Nondisjunction is an error in chromosome segregation during cell division.

Q4. Which of the following represents a complete extra chromosome set?

1. Trisomy
2. Monosomy
3. Triploidy
4. Tetrasomy

Correct Answer: C — Triploidy
Triploidy is 3n and therefore contains three complete chromosome sets.

Q5. Autopolyploidy involves:

1. Chromosome sets from unrelated species only
2. Multiple chromosome sets derived from the same species
3. Loss of one chromosome
4. Only mitochondrial chromosomes

Correct Answer: B — Multiple chromosome sets derived from the same species
Autopolyploidy results from multiplication of chromosome sets within the same species.

Q6. Allopolyploidy is most closely associated with:

1. Only chromosome deletion
2. Hybridization between different species followed by chromosome doubling
3. Loss of all chromosomes
4. Point mutation

Correct Answer: B — Hybridization between different species followed by chromosome doubling
Allopolyploidy combines chromosome sets from different species and often involves subsequent chromosome doubling.

Q7. Uniparental disomy means:

1. Both copies of a chromosome come from the same parent
2. One chromosome is deleted
3. Three copies of every chromosome are present
4. Chromosome structure is inverted

Correct Answer: A — Both copies of a chromosome come from the same parent
In UPD, both copies of a particular chromosome or chromosome region are inherited from one parent.

Q8. Which mechanism can produce uniparental disomy?

1. Trisomy rescue
2. Transcription
3. Translation
4. Crossing over only

Correct Answer: A — Trisomy rescue
Loss of one chromosome from an initially trisomic zygote can result in both remaining copies originating from the same parent.

Q9. Genetic mosaicism usually refers to:

1. Only one genetically uniform cell population
2. Two or more genetically different cell populations derived from one zygote
3. Two different species in one ecosystem
4. Only mitochondrial inheritance

Correct Answer: B — Two or more genetically different cell populations derived from one zygote
Mosaicism results when a genetic change occurs after fertilization and creates distinct descendant cell populations.

Q10. Which statement correctly distinguishes mosaicism from chimerism?

1. Mosaicism always involves two zygotes.
2. Chimerism always involves one zygote.
3. Mosaicism generally originates from one zygote, whereas chimerism involves different zygotic origins.
4. There is no difference between them.

Correct Answer: C
Mosaic cell populations generally arise from one zygote through post-zygotic genetic changes, while chimerism involves genetically distinct cells with different embryonic origins.

31. Quick Revision

Aneuploidy

• Individual chromosome gain or loss.
• Monosomy = 2n − 1.
• Trisomy = 2n + 1.
• Nullisomy = 2n − 2.
• Tetrasomy = 2n + 2.
• Nondisjunction is a major cause.

Euploidy

• Change involving complete chromosome sets.
• 3n = triploid.
• 4n = tetraploid.
• Polyploidy is particularly common in plants.

Autopolyploidy

• Multiple chromosome sets from the same species.
• Can arise through chromosome doubling or unreduced gametes.

Allopolyploidy

• Chromosome sets originate from different species.
• Often involves hybridization followed by chromosome doubling.
• Important in crop evolution and speciation.

Uniparental Disomy

• Both copies of a chromosome come from one parent.
• Maternal UPD = both copies from mother.
• Paternal UPD = both copies from father.
• Can arise through trisomy rescue.
• Important because of genomic imprinting.

Genetic Mosaicism

• Two or more genetically different cell populations.
• Derived from one zygote.
• Usually caused by a post-zygotic genetic or chromosome event.
• Post-zygotic nondisjunction can produce chromosomal mosaicism.
• Different tissues can contain different proportions of cell populations.

MASTER MEMORY TRICK

ANEUPLOIDY = Individual chromosome change

EUPLOIDY = Entire chromosome-set change

AUTO = Same species

ALLO = Different species

UPD = Both copies from One Parent

MOSAIC = One Zygote → Different Cell Populations

Final Concept Map

Numerical Chromosomal Changes

↓

Aneuploidy → Individual chromosome gain/loss
Monosomy | Trisomy | Nullisomy | Tetrasomy

Euploidy → Complete chromosome sets
Haploid | Diploid | Triploid | Tetraploid | Polyploid

Polyploidy
Autopolyploidy → Same species
Allopolyploidy → Different species

Uniparental Disomy
Both chromosome copies → Same parent

Genetic Mosaicism
One zygote → Multiple genetically different cell populations