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Several errors in sex chromosome number have been characterized. Individuals with three X chromosomes, called triplo-X, are phenotypically female but express developmental delays and reduced fertility. The XXY genotype, corresponding to one type of Klinefelter syndrome, corresponds to phenotypically male individuals with small testes, enlarged breasts, and reduced body hair. More complex types of Klinefelter syndrome exist in which the individual has as many as five X chromosomes. In all types, every X chromosome except one undergoes inactivation to compensate for the excess genetic dosage. This can be seen as several Barr bodies in each cell nucleus. Turner syndrome, characterized as an X0 genotype (i.e., only a single sex chromosome), corresponds to a phenotypically female individual with short stature, webbed skin in the neck region, hearing and cardiac impairments, and sterility.

Duplications and deletions

In addition to the loss or gain of an entire chromosome, a chromosomal segment may be duplicated or lost. Duplications and deletions often produce offspring that survive but exhibit physical and mental abnormalities. Duplicated chromosomal segments may fuse to existing chromosomes or may be free in the nucleus. Cri-du-chat (from the French for “cry of the cat”) is a syndrome associated with nervous system abnormalities and identifiable physical features that result from a deletion of most of 5p (the small arm of chromosome 5) ( [link] ). Infants with this genotype emit a characteristic high-pitched cry on which the disorder’s name is based.

 Photos show a boy with cri-du-chat syndrome. In parts a, b, c, and d of the image, he is two, four, nine, and 12 years of age, respectively.
This individual with cri-du-chat syndrome is shown at two, four, nine, and 12 years of age. (credit: Paola Cerruti Mainardi)

Chromosomal structural rearrangements

Cytologists have characterized numerous structural rearrangements in chromosomes, but chromosome inversions and translocations are the most common. Both are identified during meiosis by the adaptive pairing of rearranged chromosomes with their former homologs to maintain appropriate gene alignment. If the genes carried on two homologs are not oriented correctly, a recombination event could result in the loss of genes from one chromosome and the gain of genes on the other. This would produce aneuploid gametes.

Chromosome inversions

A chromosome inversion    is the detachment, 180° rotation, and reinsertion of part of a chromosome. Inversions may occur in nature as a result of mechanical shear, or from the action of transposable elements (special DNA sequences capable of facilitating the rearrangement of chromosome segments with the help of enzymes that cut and paste DNA sequences). Unless they disrupt a gene sequence, inversions only change the orientation of genes and are likely to have more mild effects than aneuploid errors. However, altered gene orientation can result in functional changes because regulators of gene expression could be moved out of position with respect to their targets, causing aberrant levels of gene products.

An inversion can be pericentric    and include the centromere, or paracentric    and occur outside of the centromere ( [link] ). A pericentric inversion that is asymmetric about the centromere can change the relative lengths of the chromosome arms, making these inversions easily identifiable.

Questions & Answers

in what type of cells is meiosis taking place?
Rhyeann Reply
applications of biology
Namawejje Reply
what is dormancy?
Aliyu Reply
hello guys what's the difference between prokaryotes and eukaryotes
Nwachukwu Reply
hlo what are the applications of biology?
Namawejje
deviation from mendelian
Ogali Reply
what is lethal allele
Ogali
Explain how chemical , bioligical and physical interaction between themselves and the non living components ?
Beyan Reply
what is Tissues
Faith Reply
Group of similar cells performing some related functions We have some type of tissues Connective Muscle Epithelial Nervous tissues
peter
group of cells that is called tissue.
Nikita
what is abiotic?
Williams Reply
state 2 abiotic factors that affect the rate of transpiration in plants?
Benenge
non living things
Chris
what is a cell
Esther Reply
structural and functional group is called cells
Nikita
the structural and functional unit of life is called cell. the cell was discovered by Robert hook in 1665Ad.
ROHIT
The structural functional of unit is called cell
Anas
The structural and functional unit of cell
Anas
Halim I am. A biological cell is the basic unit of life made up of protoplasm, organelles, a mitochondria where ATP, adenosinetriphosphate produces our energy, and a membrane barrier that envelopes the entire cell separating the internal and the external like our epidermis skin.
halim
is isn't the cell
Amarachi
what is contained in a smooth endoplasmic reticulum?
Bleyd Reply
sperm
Chris
cool
Jo
sorry i meant the structure within the smooth endoplasmic reticulum
Bleyd
ribosomes
Ogali
what is cell made of
Jo Reply
cytoplasm,cell wall and nucleus
Ogali
what is sponge
Joseph Reply
sponge are the multicellular that have bodies full of pore. and do not have nervous, digestive and circulatry system. the studies of sponge is known as spongiology.
ROHIT
what is a photometer
Abigail Reply
photometer is an electrical device which use to measure the strength of electromagnetic rediation.
ROHIT
what is respiration
Alao Reply
Is the release of energy from food substance in a living organism
innocent
What is cytoplasm
Nitta Reply
it is a gell like structure found in cell it contains the mitochondria, nucleus, lysosome etc
Ogali
in other words it's the material that contains the organelles of an organism
Nwachukwu

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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