Introduction

In the vast majority of cases, when a zygote contains an incorrect number of chromosomes as a result of a gamete produced through meiosis in which a nondisjunction event occurred, that zygote will be non-viable and will be spontaneously aborted.  However, in rare instances, a zygote can survive and develop into a fetus, such as is the case with Down Syndrome.

Let us explore how this occurs now.

Learning

Down Syndrome is known to geneticists as ‘Trisomy 21’, indicating that individuals with Down Syndrome have three copies of chromosome #21 instead of the normal two copies.

Down Syndrome occurs in about 1 in 800 births. It is interesting to note that the rate of Down Syndrome occurrences increases with the age of the mother. This would suggest that nondisjunction of chromosome #21 in the eggs occurs more frequently in women over the age of 40.

Other common (relatively speaking) nondisjunction events occur with the sex chromosomes. Let’s consider what happens when non-disjunction of the X-chromosomes occurs in the female. In this case, if the egg is fertilized, we will find the following sex-chromosome combinations.

Scenario 1 – Trisomy XXX

In this situation, the resulting zygote has three copies of the X Chromosome. The phenotype of this individual is that they are female and have no distinguishing characteristics from ‘normal’ females. About 1 in every 1,000 women has three copies of the X chromosome. Interestingly, these individuals are fertile and can give birth to normal children.

Scenario 2 – Turner Syndrome (X0)

In Turner Syndrome, the resulting zygote has only 1 sex chromosome – an X chromosome obtained from the father’s sperm. The phenotype of this individual is female, however they suffer from hormone deficiencies at puberty and lack the development of secondary sexual characteristics. This condition affects about 1 in every 3,000 women. In most cases, individuals with Turner Syndrome are infertile.

Scenario 3 – Klinefelter Syndrome (XXY)

In Klinefelter Syndrome, the resulting zygote has (2) copies of the X-chromosome AND (1) copy of the Y-chromosome.  Since we already learned that the Y-chromosome is responsible for determining gender in humans, you can probably guess that the phenotype of this individual is male; the presence of the extra X-chromosome results in the presentation of mixed secondary sexual characteristics such as partial breast development and a reduction in the size of the testicles.  This condition affects about 1 in every 1,000 men and these individuals are sometimes infertile.

Scenario 4 – Nonviable (OY)

In the final quadrant in our Punnett square, we can see the genotype YO – meaning there is no X-chromosome in this individual. In this case, the zygote is non-viable and would not develop into a fetus.

Summary

In this section, we have learned:

• In rare instances, a zygote containing an irregular number of chromosomes can survive and develop.
• In Down Syndrome, an individual inherits (3) copies of Chromosome #21, a condition also referred to as ‘Trisomy 21‘
• Nondisjunction of the sex chromosomes can result in the inheritance of too few or too many sex chromosomes. Not all permutations are viable and/or fertile.

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Sources:

Trisomia 18.jpg. Image of Karotype, Dec 17, 2010. Wikimedia Commons,  GNU Free Documentation License, Version 1.2 45,X.jpg.

Image of Turner syndrome karyotype. Oct 17, 2006.  GNU Free Documentation License, Version 1.2

Klinefelter syndrome karyotype, Source: MedlinePlus, National Library of Medicine. Sept 8, 2020. https://medlineplus.gov/genetics/condition/klinefelter-syndrome/