Chromosome

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But sometime there is an unequal distribution of chromosomes in the daughter cells.

The failure of separation of homologous chromosome is called non-disjunction.

This can occur either during mitosis or meiosis or embryogenesis.

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Mitotic non-disjunction: The failure of separation of homologous chromosomes during mitosis is called mitotic non-disjunction.

It occurs after fertilization.

May happen during first or second cleavage.

Here, one blastomere will receive 45 chromosomes, while other will receive 47.

Meiotic non-disjunction: The failure of separation of homologous chromosomes during meiosis is called mitotic non-disjunction

Occurs during gametogensis

Here, one type contain 22 chromosome, while other will be 24.

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Variation in chromosome number

Organism with one complete set of chromosomes is said to be euploid (applies to haploid and diploid organisms).

Aneuploidy - variation in the number of individual chromosomes (but not the total number of sets of chromosomes).

The discovery of aneuploidy dates back to 1916 when Bridges discovered XO male and XXY female Drosophila, which had 7 and 9 chromosomes respectively, instead of normal 8.

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Nullisomy - loss of one homologous chromosome pair. (e.g., Oat )

Monosomy – loss of a single chromosome (Maize).

Trisomy - one extra chromosome. (Datura)

Tetrasomy - one extra chromosome pair.

More about Aneuploidy

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Uses of Aneuploidy

They have been used to determine the phenotypic effect of loss or gain of different chromosome

Used to produce chromosome substitution lines. Such lines yield information on the effects of different chromosomes of a variety in the same genetic background.

They are also used to produce alien addition and alien substitution lines. These are useful in gene transfer from one species to another.

Aneuploidy permits the location of a gene as well as of a linkage group onto a specific chromosome.

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Trisomy in Humans

Down Syndrome

The best known and most common chromosome related syndrome.

Formerly known as “Mongolism”

1866, when a physician named John Langdon Down published an essay in England in which he described a set of children with common features who were distinct from other children with mental retardation he referred to as “Mongoloids.”

One child in every 800-1000 births has Down syndrome