Chromosomal AbberationDR. ASHISH PATEL

Assistant professorDept. AGB, Veterinary College, AAU,

Anand

In somatic cell of diploid organism two copies of same genome are present (2n), while their gametes contain a single genome (n).

The deviation from the diploid state of chromosome and structure of chromosome is known as chromosomal aberration.

The deviations from the standard chromosomal numbers and morphology (structure) of any species can be detected by Karyotyping (chromosomal Banding).

Broadly the chromosomal abnormalities categorized in to two types:

Numerical abnormality: Change in number of chromosome

Structural abnormality: Change in structure of chromosome

The numerical chromosomal aberration is also known as heteroploidy and individuals who possess other than normal chromosome pair are known as heteroploids.

The heteroploidy is divided in to two categories: Aneuploidy and Euploidy.

Numerical Abnormality:

Aneuploidy A change in number of only single or few

chromosome/s within a set of chromosomes is known as aneuploidy.

In aneuploidy, changed not in whole genome. Reason for Anueploidy is non disjunction of

chromosome during meiosis. The single chromosome is less from the somatic

number of chromosome number (2n - 1) is known as monosomy.

The deficiency of pair of chromosome (2n - 2) is known as nullysomy.

If two missing chromosome are non-homologous (2n-1-1) then it is known as double monosomy.

If individual possess one extra chromosome (2n + 1) are termed as trisomics.

The individual possess an extra pair of chromosome (2n + 2) are referred to as tetrasomics.

If two additional chromosome are non-homologous (2n +1 +1), such individual are known as double trisomics.

Various types of changes in chromosome number

Term Type of change SymbolHeteroploidy change from the 2n stateAneuploidyAneuploidy One or few chromosomes

extra or missing from 2n2n ± one or few

Nullisomy One pair of chromosome missing

2n – 2

Monosomy One chromosome missing 2n – 1Double monosomy Two non-homologous

chromosome missing2n -2

Trisomy one extra chromosome 2n + 1Double trisomy Two extra non homologous

chromosome2n + 1 + 1

Tetrasomy two extra chromosome 2n + 2

Term Type of change SymbolHeteroploidy change from the 2n stateEuploidyEuploidy Change in the chromosomes of entire set (other than

diploid 2n )Monoploidy only one genome present nHaploidy Gametic chromosome

complement n

Autopolyploidy More than two copies of same genome present

3n (AAA), 4n (AAAA), 5n (AAAAA)…….

Autotriploidy Three copies of same genome 3n (AAA)

Autotetraploidy, Autopentaploidy, Autohexaploidy etc (4n, 5n, 6n ….)Allopolyploidy Two or more different genome AABB, AABBCC

Allotetraploidy Two different genome present AABB

Trisomy of Autosomes: Trisomy at 21st chromosomal pair (means in 21st pair

one extra chomosome): Down syndrome (mongolism idiocy): (47, 21+)

Trisomy at 13th chromosomal pair: Patau’s syndrome (47, 13+)

Trisomy at 18th chromosomal pair: Edward’s syndrome (47, 18+) (individual who have such abnormality die within one year)

Trisomy of sex chromosomes

Klinefelter’s syndrome (47, XXY) Triple X syndrome: (47, XXX) Jacob’s syndrome: (47, XYY)Monosomy of sex chromosome: Turner syndrome (45, X)

Common sex chromosome aneuploids in livestock animals and their phenotypic effects are

Monosomy (X0): pig, cat, dog: Ovarian hypoplasia: Infertlity in female

Trisomy (XXX): cattle: Ovarian hypoplasia: Infertlity in female

Trisomy (XXY): Cattle, Buffalo, Sheep, Goat: Testicular hypoplasia: Infertlity in male

Euploidy (Greek ploid means unit, Eu means true/even):

Change in entire set of chromosome is called as Euploidy.

It is also known as heteroploidy. heteroploidy means any alteration in no. of whole set of chromosome from normal 2n number.

Euploidy (Heteroploidy) includes: haploidy (n), triploidy (3n), tetraploidy (4n), pentaploidy (5n) these all are collectively known as polyploidy.

An individual having only one set (n) of chromosome is known as monoploidy or haploidy.

An individual having three or more than three sets of chromosomes is called as polyploidy.

Most commonly triploidy and tetraploidy are observed out of polyploidy (in plants but rarely in animals).

Autopolyploidy: More than two copies of same genome present. chromosome complements of same species/origin

Allopolyploidy: Two or more different genome present. The chromosome complements belong to different origin. e.g. cell hybridization

Reasons for polyploidy Fertilization of egg with more than one speramatozoa

(polyspermy)

Abnormal gamete formation/ meiosis (polar bodies not removed )

Two zygotes fused and produced single individual (tetraploidy)

Structural Abnormality: Any deviation from normal structure of

chromosome is called Structural abnormality or Structural mutation of chromosome.

Structural changes usually take place either during interphase or early prophase.

Structural changes occur due to breakage and reunion of the chromosome.

The chromosomal breaks are of two types -restituted and non‑restituted. Two non‑restituted breaks in one chromosome can lead to deficiency, duplication and inversion.

A non‑restituted break in each of two non‑homologous chromosomes may lead to reciprocal translocations.

Structural changes lead to alteration in phenotype, fertility, viability and karyotype of an individual.

Types of Structural ChangesTwo types:(1) Alteration in gene number in the chromosome-

deletion &duplications (2) Alteration in the sequence of genes in the

chromosome - translocations and inversionsDeletion Deletion refers to loss of a portion of segment from

a chromosome. It is also known as deficiency. Depending upon the location, deletions are of two

types:Terminal Deletion Loss of either terminal segment of a chromosome is

known as terminal deletion. Interstitial Deletion Loss of a segment of chromosome from the

intermediate portion or between telomere and centromere is known as interstitial or intercalary deletion.

Cri-du-chat syndrome in human

In Cri-du-chat syndrome, deletion of terminal segment of the short arm (q) of chromosome number 5.

In this syndrome, infants crying like cat crying, which is due to malformation of larynx

Affected children show mental retardation and die at early age or some may attain adulthood

Di-george syndrome in human: deletion of small segment of the short arm (q) of chromosome number 22.

Angelman syndrome: deletion of small segment of the short arm (q) of chromosome number 15.

Pseudo-dominance: Due to loss of dominance loci in deletion its alternative recessive allele act as dominance and produce phenotype, which is called as pseudo-dominance.

Duplication

Occurrence of a segment twice in the same chromosome. Duplication is also known as repeat. or

Presence of segment of chromosome in excess of the normal is called as duplication.

It results in addition of one or more genes to a chromosome.

Duplications are of four types: Tandem: Adjacent duplications Reverse tandem: Adjacent duplications Displaced: Non‑adjacent duplications Reverse displaced: Non‑adjacent duplications

Tandem Sequence of genes in the duplicated segment is

similar to the sequence of genes in the original segment of a chromosome.

Reverse Tandem The sequence of genes in the duplicated segment

is reverse to the sequence of genes in the original segment of a chromosome.

Tandem and reverse tandem duplications are known as adacent duplications, because they are adacent to the original segment.

Displaced When the duplication is found away from the

original segment but on the same arm of the chromosome, it is known as displaced duplication.

Reverse Displaced Such duplication is also away from the original

segment but on the other arm of a chromosome.

Inversion Inversion refers to structural change in a

chromosome in which a segment is oriented in a reverse order. Thus the inverted segment is rotated to a full 180°.

It leads to change in arrangement of gene loci. Depending upon whether centromere is involved

or not inversions are of two types - paracentric and pericentric.

Paracentric Inversion Centromere is not involved. Both breaks occur in one arm of the chromosome.

Pericentric Inversion Centromere is involved. Break occurs in each of the two arms of a chromosome. Centromere is included in the detached segment

resulting in a pericentric inversion.

Translocation One way or reciprocal transfer of segments

between non‑homologous chromosomes is known as translocation.

Differences between translocation and crossing over:

Translocations involve non‑homologous chromosomes, whereas crossing over involves non‑sister chromatids of homologous chromosomes.

Translocation homozygotes change the linkage map, whereas crossing over does not change the linkage map.

Translocations involve breakage and reunion, whereas crossing over involves chiasma formation.

Three types of translocations: (1) Simple Translocation (2) Shifts(3) Reciprocal Translocations

(1) Simple Translocation When a segment from one chromosome is

transferred and attached to the end of a non‑homologous chromosome, it is known as simple translocation.

In such translocation only a simple break occurs in one chromosome

(2) Shifts Transfer of an intercalary segment from one

chromosome to the intercalary position in a non‑homologous chromosome is referred to as shift.

Thus two breaks occur in a loser chromosome and one break in the gainer chromosome for transfer and integration of such segment.

(3) Reciprocal Translocations Mutual exchange of segments between

non‑homologous chromosomes is known as reciprocal translocation.

In such translocations, one break occurs in each chromosome before exchange of segments.

Reciprocal translocations are of two types- homozygotes and heterozygotes.

A reciprocal translocation between two acrocentric chromosomes may produce a dot or microchromosome and metacentric chromosome this phenomenon is known as Centric fusion or Robertsonian translocation.

If unequal translocation between a dot and a metacentric chromosome, would produce two acrocentric chromosomes. This process is known as Dissociation.

1/29 translocation reported first time in Swedish red and white cattle by Gustavassam and it is most common types of translocation livestock species.

1/29 translocation result into reduction in fertility, early embryonic death, interfere in embryonic development in female, poor libido, inferior quality semen in bull