How is it possible for monosomies to occur?

Question

I was taught in biology class that a cell dies if it doesn't have at least one copy of a chromosome "type". If this is the case, that means that:1. Zygotes without a copy of a specific chromosome (for example chromosome 21) die.But this would *also* mean:2. Gametes without a copy of a specific chromosome would also die.If this is the case, how is it possible for monosomies to occur? Wouldn't there have to be at least one haploid cell with (n-1) chromosomes when fertilization occurs?

Answer 1

Humans contain two copies of each chromosome (n=23) for a total of 46 each. A monosomy is a type of nondisjunction (when chromosomes do not evenly separate) that occurs during an anaphase event during meiosis resulting in 2n-1= 45 chromosomes. Most monosomes are lethal in humans except for Turner's Syndrome. In this condition (affecting females) the woman only has one sex chromosome, an X. All other chromosomes are present in the correct numbers. There is usually one dominant X chromosome which can mask the expression of genes on the other X chromosome. Without the second X, there is only one chance to receive a functional X chromosome.

The gametes themselves aren't what dies. The embryo produced on fusion of two gametes is usually nonviable and spontaneously aborts.

Hope this helps!

Answer 2

Hi Skylar,

To answer your question, zygotes will typically die if they do not have the correct number of chromosomes but not always. As you may remember from class, the chromosomes are numbered from largest to smallest where chromosome 1 is the biggest and chromosome 21 being the smallest. The main reason cells die with a monosomy or trisomy is that you will either have 50% more or 50% less representation of all the genes on those chromosomes so if you imagine all of the genes on chromosome 1 being over-expressed/under-expressed because there is an entire extra copy of the chromosome either present or missing, it isn't too difficult to see how this can lead to fatal issues in the organism.

However, chromosome 21 being the smallest chromosome only has a handful of genes on it so, in a way, it gives the organism more leeway in that an extra copy of chromosome 21 (trisomy 21 or more commonly known as Down's Syndrome) is not catastrophic for the individual though it will cause health problems. Likewise a missing copy of chromosome 21 (monosomy 21) may not cause the organism to die immediately.

In summary to answer your question, as with all things in biology: it depends. Not all chromosomes are created equal in size so the presence of an extra or absence of one the chromosomes will have varying effects depending on how large it is ranging from immediate zygote death to health problems in a full term individual. There are a number of different genetic disorders that result from chromosome abnormalities that also include the sex chromosomes such as XXY (Klinefelter Syndrome) or Monosomy X (Turner Syndrome). I hope this answers your question and if not please feel free to let me know!