4 doctors weighed in:
Why is there a 50% chance of a woman with Down syndrome producing a normal baby?
4 doctors weighed in

Dr. Saptarshi Bandyopadhyay
Internal Medicine - Hospital-based practice
1 doctor agrees
In brief: Laws of genetics
When a woman with Down's Syndrome forms gametes (unfertilized eggs), all the chromosomes except #21 will split evenly.
Since Down's Syndrome means three copies of chromosome 21 (by dehfinition), the chromosome cannot split "evenly" - half will have 1 copy (normal); the other half will have 2 copies (Down's syndrome carrier). These will pair with 1 copy (usually) of Chromosome 21 from the father.

In brief: Laws of genetics
When a woman with Down's Syndrome forms gametes (unfertilized eggs), all the chromosomes except #21 will split evenly.
Since Down's Syndrome means three copies of chromosome 21 (by dehfinition), the chromosome cannot split "evenly" - half will have 1 copy (normal); the other half will have 2 copies (Down's syndrome carrier). These will pair with 1 copy (usually) of Chromosome 21 from the father.
Dr. Saptarshi Bandyopadhyay
Dr. Saptarshi Bandyopadhyay
Thank
Dr. James Ferguson
Pediatrics
In brief: Its called nature
In deference to the males who are sterile, the females go through normal puberty and can be fertile afterward.
During the complex events forming her eggs, two options emerge (50/50).Her eggs can have 23 chromosomes +an extra 21, or just the normal 23.At conception either a DS fetus or a normal one is formed. Since some DS babies will abort, there is potential for > 50% normal at birth.

In brief: Its called nature
In deference to the males who are sterile, the females go through normal puberty and can be fertile afterward.
During the complex events forming her eggs, two options emerge (50/50).Her eggs can have 23 chromosomes +an extra 21, or just the normal 23.At conception either a DS fetus or a normal one is formed. Since some DS babies will abort, there is potential for > 50% normal at birth.
Dr. James Ferguson
Dr. James Ferguson
Thank
Dr. David Bergman
Obstetrics & Gynecology - Maternal Fetal Medicine
In brief: Downs syndrome
Based on genetics, a women with downs syndrome will have eggs that are either 23 x or 24 x with an extra # 21 chromosome.
Therefore there is a 50 % chance that she will pass on a normal 23 chromosome compliment to her fetus.

In brief: Downs syndrome
Based on genetics, a women with downs syndrome will have eggs that are either 23 x or 24 x with an extra # 21 chromosome.
Therefore there is a 50 % chance that she will pass on a normal 23 chromosome compliment to her fetus.
Dr. David Bergman
Dr. David Bergman
Thank
Get help from a real doctor now
Dr. Bennett Machanic
Board Certified, Neurology
48 years in practice
55M people helped
Continue
107,000 doctors available
Read more answers from doctors