Meiosis is a very complex type of cell division and requires precision at each step. However, mistakes or errors during meiosis is not so rare in humans. The main cause of this error is the failure of separation of homologous chromosomes during Meiosis I or failure of separation of sister chromatids during Meiosis II and both these cases are called Non-disjunction of chromosomes.
Non-disjunction of chromosomes lead to the formation of gametes that contain an abnormal number of chromosome- either an extra copy of a chromosome or a missing chromosome. When these abnormal gametes fuse with a normal gamete, this fusion leads to the formation of a zygote having an abnormal number of chromosome and cause serious consequences.
In most of the cases, these abnormal zygote develops into an abnormal embryo by division but fails to develop into a complete offspring during some stage of development. However, in few cases, the zygote develops into an offspring having odd number of chromosomes from the usual number in its species. This condition is called Aneuploidy.
TYPES OF ANEUPLOIDY
Monosomy (2n-1)= Cells with one chromosome of a homologous pair missing.
Double Monosomy (2n-1-1)= Cells with one chromosome missing from 2 homologous pairs.
Nullisomy (2n-2)= Cells with both chromosomes of a homologous pair missing.
Trisomy (2n+1)= Cells with one extra copy of a chromosome.
CONSEQUENCES OF ANEUPLOIDY
The consequences of Aneuploidy depends on which chromosome or chromosomes are affected.
The absence of one autosomal chromosome (Monosomy) always become lethal, regardless of the affected chromosomes number, either during embryonic development or during fetal development. The reason is absence of a chromosome results in absence of 2nd set of alleles of the genes related to that chromosome number. This results in expression of all the genes of that chromosome whether dominant or recessive. This may lead to expression of harmful mutations that otherwise were not able express in normal conditions.
The gain of a chromosome (Trisomy) does not have any survival advantage anyway. Out of the 22 autosomal chromosomes in humans, only zygotes with trisomy of 21st chromosome can survive beyond first few weeks or months of life. Trisomy of chromosomes 13 and 18 are born alive but do not survive more than a few weeks due to severe abnormalities.
However, presence of an abnormal number of sex chromosomes do not have such lethal effects on human development.
CHANCES OF ANEUPLOIDY
Chances of non-disjunction leading to development of abnormal gametes is more in females than in males. As the age of female increases, chances of giving birth to an abnormal offspring increases too. For Down's syndrome it increases from 0.05% in 19 year old mothers to >3% for 45 years or older mothers.
Non-disjunction is more commonly observed during Meiosis I rather than Meiosis II. The reason, according to a hypothesis is, as the older oocytes of females remain arrested in Meiosis I for too long the meiotic spindles are less able to hold together a bivalent which is weakly constructed. There is also a possibility that cohesin which prevents chiasmata from sliding off the end of the chromosome cannot be maintained for extended periods resulting in premature separation of homologous chromosomes.
More than a 25% of the fetus that spontaneously abort carry chromosomal trisomy.
In humans more than 20% of the oocytes are aneuploid.
Examples of trisomy (47 chromosomes)
1. DOWN'S SYNDROME
Discovered by Langdon Down in 1866 and occurs due to trisomy of 21st chromosome. Chromosome number 21 is the smallest chromosome in humans having approximately 400 genes. Additional copy of the chromosome leads to abnormal expression of the genes of 21st chromosome. But it can also affect expression of genes of other chromosomes as well as many genes are concerned with coding proteins/ enzymes involved in regulation of genes present on other chromosomes as well.
Symptoms of Down's Syndrome are:
Mental retardation
Short stature
Round head (Mongol face)
Protruding tongue
Open mouth
Multiple finger loops
Webbed neck
Congenital heart disease
Increased susceptibility to infectious diseases
High risk of leukemia
Early onset of Alzheimer's disease
Males/females are fertile
2. KLINEFELTER'S SYNDROME
Klinefelter's syndrome is trisomy of Sex chromosomes in which a male is born with an extra copy of X chromosome resulting in (44AA+XXY) organization of chromosomes.
Symptoms of Klinefelter's syndrome are-
Male with female like characters
Gynecomastia- Development of breast in males
Under-development of genitalia
Sterile males
Female pattern of pubic hair growth
3. Jacob'S SYNDROME
Trisomy of sex chromosome resulting in chromosomal organization of (44AA+XYY). Males are taller than average. Other features like aggressive, antisocial and criminal behavior are just hypothesis.
EXAMPLE OF MONOSOMY (45 chromosomes)
TURNER's SYNDROME
Monosomy of sex chromosome resulting in chromosomal organization of (44AA+XO) i.e. females having only one copy of X chromosome.
Symptoms of Turner's syndrome are-
Abnormal body structure
Arrested development of genitalia
Rudimentary (poorly developed) ovaries