1. Chromosome Structure and Mutations part-2
Genetics

2. Inversions
A third type of chromosome rearrangement is a chromosome inversion, in which a chromosome segment is inverted—turned 180 degrees.
If a chromosome originally had segments AB•CDEFG, then chromosome AB•CFEDG represents an inversion that includes segments DEF.
For an inversion to take place, the chromosome must break in two places.
Inversions that do not include the centromere, such as AB•CFEDG, are termed paracentric inversions (para meaning “next to”),
whereas inversions that include the centromere, such as ADC•BEFG, are termed pericentric inversions (peri meaning “around”).
Inversion heterozygotes are common in many organisms, including a number of plants, some species of Drosophila.

3. Effects of inversions
Individual organisms with inversions have neither lost nor gained any genetic material; just the DNA sequence has been altered.
These mutations often have pronounced phenotypic effects.
An inversion may break a gene into two parts, with one part moving to a new location and destroying the function of that gene.
Even when the chromosome breaks are between genes, phenotypic effects may arise from the inverted gene order in an inversion.
Many genes are regulated in a position-dependent manner; if their positions are altered by an inversion, they may be expressed at inappropriate times or in inappropriate tissues, an outcome referred to as a position effect.

4. Translocations
A translocation entails the movement of genetic material between non-homologous chromosomes or within the same chromosome.
Translocation should not be confused with crossing over, in which there is an exchange of genetic material between homologous chromosomes.
In a nonreciprocal translocation, genetic material moves from one chromosome to another without any reciprocal exchange.
Consider the following two non-homologous chromosomes: AB•CDEFG and MN•OPQRS.
If chromosome segment EF moves from the first chromosome to the second without any transfer of segments from the second chromosome to the first, a nonreciprocal translocation has taken place, producing chromosomes AB•CDG and MN•OPEFQRS.

5. More commonly, there is a two-way exchange of segments between the chromosomes, resulting in a reciprocal translocation.
A reciprocal translocation between chromosomes AB•CDEFG and MN•OPQRS might give rise to chromosomes:
AB•CDQRG and MN•OPEFS.

6. Effects of translocations
Translocations can affect a phenotype in several ways.
First, they can physically link genes that were formerly located on different chromosomes.
These new linkage relations may affect gene expression (a position effect): genes translocated to new locations may come under the control of different regulatory sequences or other genes that affect their expression.
Second, the chromosomal breaks that bring about translocations may take place within a gene and disrupt its function. Molecular geneticists have used these types of effects to map human genes.

7. A human monosomic zygote has 45 chromosomes.
Types of Aneuploidy
We will consider four types of common aneuploid conditions in diploid individuals: nullisomy, monosomy, trisomy, and tetrasomy.
Nullisomy is the loss of both members of a homologous pair of chromosomes. It is represented as 2n – 2, where n refers to the haploid number of chromosomes.
Thus, among humans, who normally possess 2n = 46 chromosomes, a nullisomic zygote has 44 chromosomes.
2. Monosomy is the loss of a single chromosome, represented as 2n – 1.
A human monosomic zygote has 45 chromosomes.

8. 3. Trisomy is the gain of a single chromosome, represented as 2n + 1.
A human trisomic zygote has 47 chromosomes.
Down syndrome result from trisomy of chromosome 21.
4. Tetrasomy is the gain of two homologous chromosomes, represented as 2n + 2.
A human tetrasomic zygote has 48 chromosomes.
Tetrasomy is not the gain of any two extra chromosomes, but rather the
gain of two homologous chromosomes; so there will be four homologous copies of a particular chromosome.

9. Aneuploidy in Humans
Sex-chromosome aneuploids The most common aneuploidy seen in living humans has to do with the sex chromosomes.
Aneuploidy of the human sex chromosomes is better tolerated than aneuploidy of autosomal chromosomes.
Both Turner syndrome and Klinefelter syndrome result from
aneuploidy of the sex chromosomes.
Autosomal aneuploids resulting in live births are less common than sex-chromosome aneuploids in humans.
Most autosomal aneuploids are spontaneously aborted, with the exception of aneuploids of some of the small autosomes such as chromosome 21. Because these chromosomes are small and carry fewer genes, the presence of extra copies is less detrimental than it is for larger chromosomes.
Down syndrome The most common autosomal aneuploidy in humans is trisomy 21, also called Down syndrome

10. Polyploidy Is the presence of more than two sets of chromosomes
Most eukaryotic organisms are diploid (2n) for most of their life cycles, possessing two sets of chromosomes.
Occasionally, whole sets of chromosomes fail to separate in meiosis or mitosis, leading to polyploidy, the presence of more than two genomic sets of chromosomes.
Polyploids include triploids (3n), tetraploids (4n), pentaploids (5n), and even higher numbers of chromosome sets.

12. References
Benjamin A. Pierce, Genetics: A Conceptual Approach, 4th Edition. 4th Edition. W. H. Freeman.

13. The End