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How to test or prove the character is sex linked ？
I think it would be more helpful if you can think it through by yourself. See if you can try out the following question:
2011 AL Paper 2 Q.1(d)
In a family of four, both the father and the mother are normal but both their son and daughter suffer from a genetic disease. Without using genetic diagrams,
(i) deduce whether the genetic defect is a dominant trait or a recessive trait. (4 marks)
(ii) deduce whether the gene for the genetic defect is located on the X-chromosome. (4 marks)
(i )Son has genetic disease so he must bear at least one allele for genetic disease which is inherited from either mother or father.
So at least one of parents must bear one allele for genetic disease. Both parents are normal so they must bear at least one allele for normal condition. Thus at least one of parents is heterozygous, dominant character is expressed under heterozygous condition. Therefore , normal condition is dominant trait while genetic defect is recessive trait.
(ii) father has only one X chromosome and it will be inherited to his daughter. Mother will inherit her one of X chromosome to her daughter too.
The daughter has genetic defect so she must have homozygous recessive trait with bearing two recessive allele for genetic defect on both her X chromosomes. So recessive allele is located on X chromosome.
Full marks for (i)
Your deduction in (ii) is incorrect. Because following your logic, if the father pass the only one X-chromosome which contains the allele for the genetic defect to his daughter, this implies the father himself will also have the genetic disease despite the allele for the genetic disease being recessive , because he has the X-chromosome carrying the recessive allele for the genetic defect, which cannot be masked. But the father does not have the disease.
The approach of answering (ii) is similar to 2016 Q5(b).
You first assume the statement (the gene for the genetic defect is located on the X-chromosome) to be true,
then suggest the theorectical outcome based on the fact that the statement is true.
Then point out the fact that the theortical outcome (father has the disease) is contradictory to the actual outcome (father does not have the disease).
Finally, draw your conclusion (the allele for the genetic defect is not located on the X-chromosome)
Normally, it is harder to prove a disease is X-linked than it is not. So you have to bear this approach in mind to solve sex-linkage problem.
Model answer:
If it is X-linked, the daughter should have two recessive alleles for the genetic disease. (1)
One of the X-chromosome bearing the recessive allele must be inherited from her father. (1)
As the father only possess one X-chromosome, he should have manifested the disease if it is X-linked. (1)
However, since the father is normal, the allele for the genetic defect is not located on X-chromosome. (1)
OR
If it is X-linked, the X-chromosome of the father must carry an allele for being normal (1)
which must be inherited to his daughter. (1)
Since the allele for being normal is dominant, the daughter must be normal. (1)
As the daughter has the disease, the allele for the genetic defect is not located on X-chromosome. (1)
What is the significance of phototropism in root ? 2012 dse mc q30 said it is not for water absorption ?
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温唔切
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有冇人tip今年E1出咩topic?
温唔切
按pattern:
出thermoregulation and mentrual cycle
Menstural cycle 你可以參考2012 Al 實驗題
Dse e1 hormonal cycle仲未試過用實驗題考
用實驗題考，又係冚家鏟...
有冇人tip今年E1出咩topic?
温唔切
按pattern:
出thermoregulation and mentrual cycle
Menstural cycle 你可以參考2012 Al 實驗題
Dse e1 hormonal cycle仲未試過用實驗題考
用實驗題考，又係冚家鏟...
都係離唔開Negative feedback
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Full marks for (i)
Your deduction in (ii) is incorrect. Because following your logic, if the father pass the only one X-chromosome which contains the allele for the genetic defect to his daughter, this implies the father himself will also have the genetic disease despite the allele for the genetic disease being recessive , because he has the X-chromosome carrying the recessive allele for the genetic defect, which cannot be masked. But the father does not have the disease.
The approach of answering (ii) is similar to 2016 Q5(b).
You first assume the statement (the gene for the genetic defect is located on the X-chromosome) to be true,
then suggest the theorectical outcome based on the fact that the statement is true.
Then point out the fact that the theortical outcome (father has the disease) is contradictory to the actual outcome (father does not have the disease).
Finally, draw your conclusion (the allele for the genetic defect is not located on the X-chromosome)
Normally, it is harder to prove a disease is X-linked than it is not. So you have to bear this approach in mind to solve sex-linkage problem.
Model answer:
If it is X-linked, the daughter should have two recessive alleles for the genetic disease. (1)
One of the X-chromosome bearing the recessive allele must be inherited from her father. (1)
As the father only possess one X-chromosome, he should have manifested the disease if it is X-linked. (1)
However, since the father is normal, the allele for the genetic defect is not located on X-chromosome.
OR
If it is X-linked, the X-chromosome of the father must carry an allele for being normal (1)
which must be inherited to his daughter. (1)
Since the allele for being normal is dominant, the daughter must be normal. (1)
As the daughter has the disease, the allele for the genetic defect is not located on X-chromosome. (1)