Course of Meiosis

Monosomy means that the zygote is missing one chromosome from a homologous pair. Instead of 2n, it has 2n−1.

It typically occurs when, during meiosis, non-disjunction causes a gamete to not receive a chromosome (a nullisomic gamete, with n=1 therefore 0 instead of 1 chromosome of this pair).

Why are the red-marked examples monosomies?
In the marked cases, the gamete contains no chromosome from this autosome pair. After fertilization by a sperm cell (with 1 chromosome from this pair), a zygote with only 1 instead of 2 chromosomes is formed - this is a monosomy (2n−1).

• Possibility 1: non-disjunction in anaphase I → two gametes without this chromosome (marked).
• Possibility 2: non-disjunction in anaphase II → one gamete without this chromosome (marked).

Note: with trisomy, the opposite occurs - the gamete contains 2 instead of 1 chromosome from this pair (zygote: 2n+1).

Trisomy means that one chromosome is present three times in the zygote. Instead of 2n, it has 2n+1.

It typically occurs when, during meiosis, non-disjunction causes a gamete or sperm cell to receive an additional copy of this chromosome.

Why are the marked examples trisomies?
In the marked cases, there is one additional copy of this chromosome overall:
• Red: the gamete contains n+1 (two chromatids/chromosomes from this pair) and is fertilized by a normal sperm cell (n).
• Yellow: the sperm cell contains n+1 (two chromatids/chromosomes from this pair) and fuses with a normal gamete (n).
In both cases, a zygote with 3 copies of this chromosome is formed - a trisomy (2n+1).

• Possibility 1: non-disjunction in anaphase I (reduction division). The homologous chromosomes do not separate, producing gametes with n+1 or n−1.
• Possibility 2: non-disjunction in anaphase II (equational division). The sister chromatids do not separate, producing one gamete with n+1 and one with n−1 in addition to normal gametes.