Categories Anthropology

I.9.4 Chromosomal aberrations in man

I. CHROMOSOMAL MUTATIONS- Change in number or structure of chromosome.

Types of chromosomal mutation :

  1. Heteroploidy/Genomatic mutation – change in chromosome number
  2. Chromosomal aberration – change in structure of chromosome.
  1. Heteroploidy/Genomatic mutation : Change in number of sets or chromosomes in sets. Two types-
    • a. Euploidy – Change in number of sets.
    • b. Aneuploidy -Change in number of chromosome in set.
  • a. Euploidy : Change in number of sets/loss or addition of sets of chromosomes.
      • i. monoploidy- Loss of one set (2n – n = n)
      • ii. polyploidy- Addition of set

        • Addition of one set – Triploidy
        • Addition of two set – Tetraploidy
        • Ad dition of three set – Pentaploidy
        • Addition of four set – Hexaploidy
      • Polyploidy is of two types :
        1. Autopolyploidy :It is repetition of same set of chromosomes.
        2. Allopolyploidy : It is repetition of  more than one set of chromosomes.
  • b. Aneuploidy : Loss or addition of chromosomes in sets of chromosomes.
    • i. Hypoaneuploidy -Loss of chromosomes in sets of chromosomes.
      1. monosomy:- (loss of one chromosome in one set) 2n – 1
      2. double monosomy (loss of one chromosome from each set, but these are non homologus.) 2n – 1 – 1
      3. Nullisomy (loss of two homologus chromosome)2n – 2
    • ii. Hyperaneuploidy- Addition of chromosomes in sets of chromosomes.
      1. Trisomy: addition of one chromosome in one set. 2n + 1
      2. Double Trisomy: addition of one chromosome in each set. 2n + 1 + 1
      3. Tetrasomy: addition of two chromosome in one set. 2n + 2 =

Cause of aneuploidy is chromosomal non disjunction means chromosomes fail to separate during meiosis, Chances of aneuploidy are more in higher age female due to less activity of oocyte, so chances of syndrome increase in children who are born from higher age female.

2. Chromosomal Aberrations :

a. Deletion : Loss of a part or segment of chromosome which leads to loss of some gene is called as deletion. It is of 2 types:-

i. Terminal deletion- Loss of chromosomal segment from one or both ends. e.g., The cry-du-chat syndrome is an example of terminal deletion in 5th chromosome.

ii. Intercalary deletion- Loss of chromosomal part between the ends.

b. Inversion : Breakage of chromosomal segment but reunion on same chromosome in reverse orders. It leads to change in distance between genes on chromosome or sequence of genes on chromosome so crossing over is affected. It is of 2 types :

i.Paracentric – If inversion occur only in one arm and inverted segment does not include centromere.

ii. Pericentric – In this type of inversion inverted segment include centromere.

c.Duplication : Occurrence of a chromosomal segment twice on a chromosome. If in this segment any recessive gene is present, then it given it’s expression due to homozygous condition. If in this segment any recessive but lethal gene is present, it lead to death of organism.

d. Translocation : A part of the chromosome is broken and may be joined with non homologous chromosome. This is also known as Illegitimate crossing over (illegeal crossing over). Three types of translocation-

      • i.Simple Translocation : When a chromosomal segment breaks and attached to the terminal end of a non-homologous chromosome.
      • ii. Interstitial or shift translocation : If a segment of chromosome breaks and gets inserted in interstitial position of a non homologous chromosome.
      • iii. Reciprocal Translocation : Exchange of segments between two non-homologous chromosome. e.g., Chronic myloid leukemia [C M L] is a type of blood cancer. This disease is a result of reciprocal transiocation between 22 and 9 chromosome.

Examples of trisomies include:2

  • Edward syndrome (trisomy 18)
  • Patau syndrome (trisomy 13)
  • Klinefelter’s syndrome (XXY and other variations)
  • Triple X syndrome (XXX)

Example of monosomy includes:

Examples of chromosomal deletions include

  • Williams syndrome (missing chromosome 7)

    Translocations – There are many examples of translocations including translocation Down syndrome,, Robertsonian translocations.

    Chromosomal deletion syndromes

    Cri du chat syndrome (cat’s cry syndrome, 5p minus syndrome)

    Part of chromosome 5 is missing. The size of the missing part varies, and people who have larger deletions are often more severely affected. Children with this syndrome often have a characteristic high-pitched, mewing cry that sounds like a kitten crying. This cry may be heard immediately after birth, lasts several weeks, and then disappears. However, not all affected newborns have this distinct cry. An infant with this syndrome may have a low birth weight and a small head with many abnormal features, including a round face, small jaw, wide nose, widely separated eyes, crossed eyes (strabismus), and abnormally shaped ears set low in the head. Often the infant seems limp. Webbed fingers and toes (syndactyly) and heart defects are common. There are significant limitations in mental and physical development. Many children with cri du chat syndrome survive to adulthood but have substantial disabilities.


    Prader-Willi syndrome


    About 70% of people who have this syndrome are missing part of chromosome 15. About 30% of people who have this syndrome have problems with the function of chromosome 15. Many symptoms of Prader-Willi syndrome vary according to the child’s age. Newborns with the defect feel limp, feed poorly, and gain weight slowly. Eventually these symptoms resolve. Then, between the ages of 1 and 6, appetite increases, often becoming insatiable. Children rapidly gain weight. The hands and feet remain small, and children remain short. Abnormalities of the face include almond-shaped eyes and a mouth with thin upper lips and down-turned corners. Children have bone disorders (such as scoliosis and kyphosis). Obsessive-compulsive behaviors are common. The function of the reproductive organs is abnormally decreased, which limits growth and sexual development. Boys have undescended testes (cryptorchidism) and an underdeveloped penis and scrotum. Intellectual disability is common. Weight gain continues into adulthood and is excessive, which can lead to other health problems, such as obesity. Obesity can be severe enough to justify gastric bypass surgery.

    Wolf-Hirschhorn syndrome (4p minus syndrome)


    Part of chromosome 4 is missing. Children with this syndrome often have profound intellectual disability. Children may also have epilepsy, a broad or beaked nose, scalp defects, drooping upper eyelids (ptosis) and gaps or fissures (colobomas) of the iris, cleft palate, and delayed bone development. Boys may have undescended testes (cryptorchidism) and a misplaced opening of the urethra (hypospadias). Some children have an immune deficiency, which means their body is less able to fight off infections. Many affected children die during infancy. Children who survive into their 20s typically have severe disabilities.

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