I know scant little about genetics, dominant and recessive genes, and all that. If a man and woman, each with Down’s Syndrome, were to have a baby, would that baby necessarily have Down’s Syndrome as well?
Down’s Syndrome isn’t controlled by a single gene, so you can’t narrow it down to dominant and recessive precisely in the same manner as you can with eye color, for example.
That being said, I’m not sure if two parents with DS are guaranteed to have children with it.
Some relevant data in a chart here:
http://www.ds-health.com/issues.htm
A man with Down syndrome will almost certainly produce no children at all, so no.
Oh wow, really? I didn’t know that. But a woman with DS could still have children?
Theoretically, you could have several different outcomes. DS is caused by trisomy 21 - that is, three copies of chromosome 21 instead of two. During meiosis, those three chromosomes will split up unevenly - two to one gamete, and one to another. (Other options are available, of course, but these will be the two most common) So, the egg could have either one or two chromosomes, and the sperm could have either one or two. If a egg with two (E2) meets a sperm with two, (S2), you’d have a zygote with four chromosomes, which is lethal. If E1 meets S2, or E2 meets S1, then the zygote will have three chromosomes and have Down syndrome, if it lives. If, however, E1 meets S1, then you should get a normal child with two copies of chromosome 21.
Other factors, like the aforementioned reduced fertility, will complicate matters somewhat, but it is at least theoretically possible to get a chromosomally normal child out of the match.
Down Syndrome is produced by have three copies of chromosome 21 (all or in part). As has been mentioned, males are almost invariably sterile, although there have been a small number of cases of Down males fathering children. Females have reduced fertility but may produce either Down or non-Down children.
Whether the offspring will be Down or not will depend on how the chromosomes segregate in the eggs or sperm. Normally eggs and sperm are haploid, and have one copy of each chromosome. A Down individual could theoretically produce eggs or sperm with either one or two copies of chromosome 21. If two gametes with one copy unite, the offspring will be normal. If one that has one, and one that has two, unite the offspring will be Down.
So the chances are very small that a Down couple will produce children at all. However, it is possible that if a child were produced it would be normal, if it got only one copy of chromosome 21 from both parents.
Are there any other chromosomes that are sometimes fucked up in new children? Like, is there a condition for having three copies of chromosome 20? Or 19?
According to the wikipedia article, survivable trisomies include 8, 9, 12, 13, 18, 21, 22, and X.
Yes, other forms of trisomies exist (see Wikipedia article). Many lead to spontaneous miscarriages. The most benign of the full trisomies is the trisomy of the X chromosome (XXX), because the extra X-chromosome gets shut off.
Apparently one symptom of Trisomy 18 is intestines extending outside of the body… 
Most trisomies are lethal and do not result in live births. The vast majority of Downs syndrome (trisomy 21) conceptions result in early miscarriages, often too early to be noticed. The only other autosomal (non-sex chromosome) trisomies which result in live births are trisomy 13 and 18, known as Patau and Edwards syndromes respectively. These are both much nastier than Downs and are almost always fatal within the first year of life. Other chromosomes may be partially duplicated or missing part of one copy, with varying results.
Notice that 21 is one of the smallest chromosomes, which probably explains why it’s the least lethal trisomy.
There are also a number of sex chromosome numerical abnormalities, include X, XXX, XXY, XYY, among others.
Omphalocele is probably one of the least serious symptoms of Trisomy 18. When omphalocle is present in chromosomally normal infants as an isolated defect, it can generally be treated surgically with good long term results.
From what I have read, all people with Down’s syndrome have reduced fertility, particularly the men. There is at least one confirmed case of a man with Down’s syndrome fathering a normal child, so it’s certainly possible.
The woman have difficulty conceiving, but when they do it’s 50/50 if their child has Downs. I’m presuming that’s conceiving with a normal partner. With a Downs partner the chance of conception is much reduced (as most of the men are effectively sterile) and then it’s probably 25% non-viable, 50% Downs, 25% normal.
Of course, women with Downs frequently have problems other than mental development delays or impairments - the various other congenital abnormalities often found with Downs can greatly complicate a pregnancy.
For a really rare scenario that would probably produce the highest odds of having two Down’s Syndrome Parents and a Normal Offspring- I believe it would be a 25% chance (post fertilization obviously).
The scenario:
if one of the parents had Down’s Syndrome via aRobertsonian Translocation, while the other had trisomy 21 “the regular way”, it would lead to a better chance of the child not having Down’s syndrome.
If we were to try to go w/ Mendelian sort of genetics for that one (the assumption that a child has a 50-50 chance of inheriting a chromosome from either the mother or the father), the child would have a 25% chance of being normal.
He would need to inherit the normal 21 hopefully from the Robertsonian Down’s Syndrome Patient , and if he could get the normal Chromosome 14 or 13 from the Trisomy 21 parent (again a 50-50 split of does the child get the mother or the father’s genes), the kid would be born thus with a normal 13/14 chromosome, and a normal 21 chromosome.
So the odds in THAT specific scenario of the child not having down’s syndrome would be .5x.5= basically a 25% chance of a normal offspring- probably the best case scenario for not passing the trait down to the child.
An Example:
45XY t(14,21) : Father, 2 normal chr.21 but a Robertsonian Translocation Down’s Syndrome
47XX +21: Mother: Trisomy 21 Down’s Syndrome but normal chromosome 14
Father gives: either normal 21 chromosome to child. (50% chance)
Mother gives: either normal 14 chromosome to child. (50% chance)
Child: 46XX or 46XY, and has normal pairings of all chromosomes (25% chance)
Also, if anyone can point out why my scenario doesn’t work (ie: anyone more familiar with the Robertsonian Translocation) feel free to let me know what I ignored/left out.
Because otherwise, I’m really proud of thinking up that answer and nailing down a specific % and all- it may be my finest answer that I’ve been able to give in GQ.
I wish my Professors could have seen that answer! 
I think you got the right answer, but made one small mistake:
Let’s go through your example again:
Father 45XY t(14;21): 50% chance of passing on a normal chromosome 14. 100% chance of passing on a normal 21.
Mother 47XX +21: 100% chance of passing on a normal chromosome 14 (because hers are normal). 50%? chance of passing on one chromosome 21; 50%? chance of passing on two chromosome 21s.
The question marks are because I don’t know if the single 21 and double 21 gametes are equally viable. Let’s assume for the sake of argument that 50% is correct. You posted that the mother only had a 50% chance of passing on a normal 14 - that’s not correct. Both of her 14s are normal. She has a roughly 50% chance of passing on a normal complement of chromosome 21.
So it’s still .5^2 = .25, but not quite for the reasons you posted.
If I’m reading the Wikipedia article correctly, I think the odds of a mother with Down syndrome having a non-Down kid change if her condition is a result of Mosaicism (which is the case for 1-2% of those with Down syndrome). If her unaffected body cells include her gametes, then her children would be no more likely to have Down than any other mother, as it would require a new translocation/non-disjunction occurring.
Is that correct?
Yes, but probaly not "full Downs’ Syndrome. Either mosaic or the kind that’s the result of a translocation. Or what about partial trisomy? Does partial trisomy of 21 cause Down’s?
Yes, that’s what my geneticist said for me.
Yes. The most common autosome cause of miscarriage is Trisomy 16.
Many fetuses with T-21 do not survive b/c of profound birth defects. But then again that’s true for almost ALL chromosome disorders. The effect of a chromosome disorder can vary tremendously. Just wanted to say that while T-18 and 13 can be really bad, (ie cause early death or cause profound intellectucal disabilty, some kids with it, (especially some partial and mosaic ones) are still living and some even have normal IQs!
Some kids and indivduals with the other chromosome disorders , are the stereotypical profoundly intellectucally disabled kid, with a billion medical problems and are deaf and blind. And then they are SLOWLY discovering that not all autosome abnormalities translate into really extreme medical and developmental disorders. For example, I have something called 18q- syndrome. I have the FULL version, as it was discovered with the technology of 30 years ago. I wasn’t told I had it until I was sixteen. I do have a TON of medical problems, but nothing on a par with what is described in the textbooks. (my syndrome is described as "worst case scenerio persistant vegetative state, and best case scenerio severe mental retardation) Instead of all that I am hard of hearing (no earcanals or drums) low muscle tone, learning disabilty, thyroid, history of seizures and foot problems and other stuff I’m prolly forgetting. I am in term of functioning, like someone with a mild learning disabilty or mild CP. I am far from alone. I know of adults with it who are basicly “mildly LD” functioning, as well as a girl who was dx with it inutereo. They tested her after she was born, and she didn’t have it. They kept testing her and testing her, and then one measly little blood sample indicated the 18q-.
There is a syndrome called 18p- , where the only “obvious” symptom is severe apraxia. They’re thinking that maybe a lot of the “LD kids” who speak late may in fact have 18p-
Yepper.