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 Tay sachs disease  



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  #1

[left]3. A married couple is screened to assess the risk for[/left] [left]Tay-Sachs disease in their children. The activities of[/left] [left]hexosaminidase A in the sera of the mother and father[/left] [left]are 45% and 55%, respectively, of the reference value.[/left] [left]The couple has one child. What is the probability of the[/left] [left]child possessing one or more alleles of the Tay-Sachs[/left] [left]mutation?[/left] [left](A) 0[/left] [left](B) 0.25[/left] [left](C) 0.5[/left] [left](D) 0.75[/left] [left](E) 1.0[/left]




  #2

Tay Sachs is Autosomal Recessive inheritance. So the chances of the baby being a carrier (haing one allele) or diseased (having both alleles) is 75%.

The ans is D.



  #3

will u please explain Mr IVIL?


  #4

Hi Sridevi,
The genetic defect in Tay-Sachs disease results in the lack hexosaminidase A. Because it is a recessive disorder, only people who receive two defective genes will actually have the disease. People who have only one defective gene will be carriers. Their enyme activity will be reduced but not completely absent as seen in the question stem.
When a carrier and a non-carrier have children, none of their children will actually have Tay-Sachs. It is likely that 50% of their children will be carriers themselves. When two carriers have children, their children have a 25% chance of having normal genes, a 50% chance of being carriers of the defective genne, and a 25% chance of having two defective genes. The two defective genes cause the disease itself.

Tay-Sachs is caused by a defective gene. Genes are located on chromosomes, and serve to direct specific development/processes within the body. The genetic defect in Tay-Sachs disease results in the lack of an enzyme, called hexosaminidase A. Without this enzyme, gangliosides cannot be degraded. They build up within the brain, interfering with nerve functioning. Because it is a recessive disorder, only people who receive two defective genes (one from the mother and one from the father) will actually have the disease. People who have only one defective gene and one normal gene are called carriers. They carry the defective gene and thus the possibility of passing the gene and/or the disease onto their offspring.

When a carrier and a non-carrier have children, none of their children will actually have Tay-Sachs. It is likely that 50% of their children will be carriers themselves. When two carriers have children, their children have a 25% chance of having normal genes, a 50% chance of being carriers of the defective genne, and a 25% chance of having two defective genes. The two defective genes cause the disease itself.

So the chances of the baby being a carrier (having one allele - 50%) or diseased (having both alleles - 25%) is 75%.

And BTW I am no MR.




  #5

thank u ivil


  #6

nice explanation...really helpful


  #7

thank you, I'd go also for 75% (d)


  #8

the 4 possibilities are-------AA,Aa,Aa,aa

so probability of having ds(aa),or being a carrier(Aa,Aa) is

3/4=75% (ie. total ds or carriers/total possibilities)smiling face


  #9

Thank u Ms. Ivil!!!


  #10

D

easy one





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