Few questions I had on Biology... PLEASE HELP :)?

Question

I had a few questions on genotypes related questions.

How could non disjunction result in an individual with Down's syndrome?


Could an individual with Down’s syndrome ever have normal children... why??

What are the chances that a 40 year old pregnant woman could have a child with Down's syndrome? why or why not??

In women with Turner's syndrome the incidence of colorblindness is similar to the percentage expected in normal males. why is this??

if you guys can help me out with any of these questions i had been pondering about, it would be great

Answer 1

You would be wise to put each question in a separate post. You will get more answers that way. You are asking for too much information in one post.

Answer 2

1. Non disjunction is where 2 chromosomes (in Down's Syndrome is number 21) DO NOT separated in meiosis meaning instead of having 1 of each of the 23 chromosomes, you have 2 of 1 and 1 of the other 22....this means at fertilisation, another number 21 chromosome will be added meaning you have 3 (called trisomy) and get downs syndrome.

2. I don't know, I thought they are all sterile (unable to have children)...but even if they could they still have 3 copies of 1 chromosome and would have a much higher chance of having children also with the same condition, even without non-disjunction happening again, they would have most gamtes with 2 x 21 chromosomes!

3. the chances are higher for an older woman to have down syndrome children....I beleive its about 1% chance, its about 1 in 2000 for around 20year old mothers.

4. For Turners syndrome the girl has XO (ie a missing X) so effectively they have the same situation as a male (which is XY...the Y has no part to play in colourblindness and cannot mask the effects of it like an X with the dominant allele (for normal eye "colour seeing")...so if the turner girl has an X chromosome carrying the colourblindness allele she will show it, rather than carrying it like most females..and therefore do not get the condition....males either get it or not, like turners girls.

Answer 3

1. Nondisjunction causes homologous chromosomes to stay together in the same gamete, so of the four gametes produced, two have an extra chromosome and two are missing a chromosome. Down's syndrome results when there are three copies of chromosome number 21. If a gamete has two 21s instead of one like it's supposed to have, and that gamete joins with a normal gamete with one 21, then the zygote and resulting offspring will have three 21s (two from one parent, one from the other parent).

2. According to the article cited below:
"Men with Down's syndrome appear to be uniformly sterile. That is, they are unable to have children. Women with Down's syndrome, however, are usually able to have babies. About half of these babies will also be born with Down's syndrome."

3. The same article says that at age 35, a woman's risk of having a child with Down's syndrome is 1 in 400. At age 45, the risk increases to 1 in 35.

4. A woman with Turner's syndrome has a single X-chromosome instead of two X-chromosomes. Since color-blindness is governed by a recessive allele on the X-chromosome, a woman with Turner's syndrome who gets one copy of the allele on her X-chromosome does not have a second X-chromosome with a second chance of having the allele for normal color vision. In this way, the woman's situation is like that of a male who also has just one X-chromosome. The Y-chromosome (that the male has but the woman does not have) has nothing to do with color-blindness, so is not a factor in this case.

Answer 4

1. down's syndrome is caused by the trisomy of chromosome 21. its thought that 95% of the cases it is caused by non disjunction. non disjunction causes a difference in the chromosome number. it usually takes place in meiosis I (even though it can also occur in meiosis II) and its due to chromosomes (or sister chromatids) that fail to separate - so they align in the equator of the cell during metaphase but instead of each one going to one pole of the cell, two of them go to the same cell/zygote. this results in games having trisomy (three) 21 and games having monosomy (one) 21 in the case of individuals with down syndrome.

2. yes, an individual with down syndrome can have children with the normal number of chromosomes 21. this is because when the sex cells are formed, during meiosis I one of the resulting cells will end up having 2 chromosomes 21 (that if fertilized will lead to down's syndrome) and one will have only 1 chromosome 21 (that if fertilized will give the normal diploid number).

3. the figures i know are that the incidence are 1/2300 for mothers ages 20 and rise to 1/40 for mothers aged 45. the reason for this figures its not clearly understood but it's thought that it is related to the mothers sex cells aging.

4. turner's syndrome results in an XO configuration. colour blindness is an X-linked condition and hence its equally common in man and women with turner's disease. why? i'm not really sure. there are suggestions about the fact that maybe the interaction between the two X-chromosomes in a "normal" female genotype "inhibits" the colour-blindness.

hope this is what you were looking for:)

Answer 5

non-disjunction is when a chromosome doesn't fully divide during reproduction. this results in one cell dividing with both semi-copies and the other cell dividing without that specific chromosome. in the case of down's syndrome, the imperfect cell can still survive, thus altering the genetic makeup of the individual. this extra chromosome sends incorrect instructions to the body because it actually is two chromosomes (think siamese twins)

i don't know exact percentiles, as women age, the chances of having a child with down's syndrome grows. i forget why.. something along the lines of that the reproductive system begins to slow and eventually stop at menopause.

if i remember correct, turner's syndrom involves something with the XY chromosome.

hope i helped a little

Answer 6

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