Disruption of meiosis during gametogenesis may result in the formation of chromosomally abnormal gametes. eg nondisjunction which can lead to a numerical chromosomal abnormality such as in down’s syndrome.

There is an increase in the likelyhood of chromosomal abnormalities in the embryo after the mother reaches 35 years of age. The ideal maternal age for reproduction is considered to be from 18 to 35 years.

Older mothers have a high risk of Down’s syndrome (Or other trisomies).

There is also an increase in the incidence of fresh gene mutations with increase in age.

A higher risk of Achondroplasia is associated with the increasing age of the father.

Can Grossly abnormal (Morphologically abnormal) sperms cause birth defects?

It is beleived that most abnormal sperms are unable to pass through the mucus in the cervical canal, due to lack of normal motility.

Up to 10% of sperms may be grossly abnormal (eg with two tails or two heads etc).

Abnormal sperms are not believed to affect fertility, unless their number exceeds 20%.

Some of the causes of grossly abnormal sperms include X-Rays, severe allergic reactions and antispermatogenic agents.

Are there grossly abnormal oocytes (Human eggs)?

Some oocytes may have two or more nuclei. The incidence of grossly abnormal oocytes is much less than in sperms. It is believed that such abnormal oocytes never mature and get expelled during ovulation.

What is Nondisjunction?

During meiotic cell division, nondisjunction is the failure of homologous chromosomes to seperate and go to opposite poles of the germ cells.

This results in one gamete having 24 chromosomes while the other gamete from the same meiotic cell division has 22 chromosomes (The normal is 23 chromosomes per gamete).

If a gamete with 24 chromosomes unite with a normal gamete which has 23 chromosomes ,the zygote will have 47 chromosomes (They have three representatives of a particular chromosome – Therefore this condition is called trisomy).

References

Chapter 2, The Beginning of Human Development .The Developing Human. Clinically Oriented Embryology 6th edition by Keith L. Moore, Phd, FIAC, FRSM and TVN Persaud MD, PhD, DSc, FRCPPath (Lond).

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