Question about nondisjunction - help please

[Drteeth]

Question about nondisjunction - help please!

My question is related specifically to Trisomy 21. Let's consider spermatogenisis. I know that nondisjunction can occur in either anaphase 1 or anaphase 2, but I was wondering what the outcome of the number of chromosomes is in each of the 4 resulting sperm cells? From what I can see, if the nondisjunction event occurs during anaphase 1, there will be 2 resulting cells with n+1 chromosomes (24) and 2 cells with n-1 (22) chromosomes (where n = the normal haploid number). However, when the nondisjunction events occurs during anaphase 2 I see one n+1 cell resulting, one n-1 cell, and two normal cells with n chromosomes.

Please tell me if this analysis is correct. If not, please give me the correct answer.

Thank you for your help!

 

[jim mcnamara]

Yes, you have it. Other than trisomy 21 and monosomies like cri du chat syndrome, a lot interest in nondisjunction in humans relates to abnormal numbers of sex chromosomes - Turner's, Triple X, etc.

The age of human oocytes is one of many causes of non-disjunction, and is most often mentioned in Freshman Biology. It is generally believed this occurs because oocytes are "parked" in Prophase I. The "parking" starts when the the female is still a fetus. For example: a 40 year old has oocytes that have been in prophase I for more than 40 years.

 

[Revenged]

Yes, you have it. Other than trisomy 21 and monosomies like cri du chat syndrome, a lot interest in nondisjunction in humans relates to abnormal numbers of sex chromosomes - Turner's, Triple X, etc.

Cri du chat is genetic disease where there is a partial deletion of chromosome 5 - it isn't a monozomy (which is where you only have one copy of a chromosome)... Of all the genetic conditions only monozomy X, Turner's syndrome would develop... With all the others it's impossible...

As to the original question - yes you get all sorts of major chromosomal abnomalities but 99% of the time they are fatal or the offspring won't develop into adulthood... This is one of the reasons why it is harder to get pregnant when you are older (chromosomal abnormalities increase with age)...

 

[jim mcnamara]

Correct. cri du chat is a 'missing segment' problem - also thought to be related to meoitic failure.