Question

Bill and Betty have two children with Down syndrome.

Bill’s brother has Down syndrome. His sister has two children with Down syndrome.

On the basis of these observations, indicate which of the following statements are most likely correct and which are most likely incorrect.

Explain your reasoning (Hint: Is the Down syndrome seen here likely familiar Down syndrome?)

a. Bill has 47 chromosomes.

b. Betty has 47 chromosomes.

c. Bill and Betty’s children each have 47 chromosomes.

d. Bill’s sister has 45 chromosomes.

e. Bill has 46 chromosomes.

f. Betty has 45 chromosomes.

g. Bill’s brother has 45 chromosomes.

Answer 1

• Given inheritance is of familial down syndrome, which is caused by robertsonian translocation and it involves autosome chromosome number 21.
• Robertsonian translocation :- It's type of common chromosomal abnormality ,in which certain type of chromosome become attached to another.
• In down syndrome there is abnormal cell division takes place during cell division, which results in an partial or extra full copy of chromosome 21.
• In this given situation of familial down syndrome, Bill and his sister both are phenotypically normal carriers of down syndrome and that's why both of them have 45 chromosome and due to normal carriers both of them thought to be unaffected.
• Children's of Bill and Betty and Bill's sister, who have down syndrome , have 46 chromosome but in these 46 chromosome one of autosome chromosome affected by Robertsonian translocation. Due to this they have an extra copy of long arm of chromosome 21.
• As the above mentioned information there is no any reason to suspect that wife of Bill has any chromosomal abnormality which is mentioned above.

So most correct statement in given options is (d).

Other options -
(a) This statement is wrong because bill is phenotypically carrier of down syndrome and unaffected due to this reason bill has 45 chormosome.
(b) As per given above information there is no reason to suspect that betty has any chromosomal abnormality so we can't decide genotypical constitution of betty
(c) and (f) As we can't not decide the genotypical constitution of betty as per given information. We also can't decide genotypical constitution of bills and betty' s childrens.
(e) This statement is wrong because bill is phenotypically carrier of down syndrome and given situation is off normal cell's genitical constitution that is 2n =46.
(g) As per given above information that Bill's brother has down syndrome so genitical constitution of Bill's brother is 46 chromosome along whit having an extra copy of long arm( q)of 21 no autosomal chromosome due to robertsonian translocation.