In Down syndrome there is impaired fertility of both sexes b

In Down syndrome there is impaired fertility of both sexes, but females are more likely to be fertile than males. Assume that children are born to a female with non- familial Down syndrome and a normal male. What proportion of the offspring would be expected to have Down syndrome assuming no lethality?

all

2/3

0

1/2

1/3

Please explain how you got the answer

Solution

Down syndrome is neither dominant nor recessive. It is caused when there is an error in meiosis ( the formation of gametes ). Non familial down syndrome means a female having all cell in her body an extra copy of 21 chromosome. When she maes with an normal male their progeny will get 50% affected cell from their mother and 50% normal cell from father, that means they show mozaic down syndrome which indicates that they will not show lethality.

Here aa, AA indicates whether the traits is affected or not, it does not indicates whether the trait is dominant or recessive.

Non familial Down syndrome (Female)        X       Normal male

                       aa (affected)                               X             AA (normal)........................................Parent

                                                                Aa     (shows no lethaliy)...................................................Progeny

So, here we see that all the progenies expected to have down syndrome assuming no lethality.

Ans is a). all