Work out the crosses between:
Normal female and Haemophilic male
Carrier female and Normal male
(III) Carrier female and Haemophilic male
Write the conclusions you draw from these crosses. Comment on the type of inheritance of the disease.
(Use: $X$ - Normal, $X^h$ - Haemophilic)
Normal female and Haemophilic male
Carrier female and Normal male
(III) Carrier female and Haemophilic male
Write the conclusions you draw from these crosses. Comment on the type of inheritance of the disease.
(Use: $X$ - Normal, $X^h$ - Haemophilic)
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Solution and Explanation
(I) Cross between Normal female and Haemophilic male
Parents:
\[ \text{Female }(XX)\times\text{Male }(X^hY) \]
Gametes:
\[ \text{Female: }X,\;X \] \[ \text{Male: }X^h,\;Y \]
Punnett Square:
\[ \begin{array}{c|cc} & X^h & Y\\ \hline X & XX^h & XY\\ X & XX^h & XY \end{array} \]
Conclusion:
- All daughters are carriers \((XX^h)\).
- All sons are normal \((XY)\).
- No haemophilic child is produced.
(II) Cross between Carrier female and Normal male
Parents:
\[ \text{Female }(XX^h)\times\text{Male }(XY) \]
Gametes:
\[ \text{Female: }X,\;X^h \] \[ \text{Male: }X,\;Y \]
Punnett Square:
\[ \begin{array}{c|cc} & X & Y\\ \hline X & XX & XY\\ X^h & XX^h & X^hY \end{array} \]
Conclusion:
- \(25\%\) Normal daughters \((XX)\).
- \(25\%\) Carrier daughters \((XX^h)\).
- \(25\%\) Normal sons \((XY)\).
- \(25\%\) Haemophilic sons \((X^hY)\).
(III) Cross between Carrier female and Haemophilic male
Parents:
\[ \text{Female }(XX^h)\times\text{Male }(X^hY) \]
Gametes:
\[ \text{Female: }X,\;X^h \] \[ \text{Male: }X^h,\;Y \]
Punnett Square:
\[ \begin{array}{c|cc} & X^h & Y\\ \hline X & XX^h & XY\\ X^h & X^hX^h & X^hY \end{array} \]
Conclusion:
- \(25\%\) Carrier daughters \((XX^h)\).
- \(25\%\) Haemophilic daughters \((X^hX^h)\).
- \(25\%\) Normal sons \((XY)\).
- \(25\%\) Haemophilic sons \((X^hY)\).
Overall Conclusions:
- Haemophilia is caused by a recessive gene located on the X chromosome.
- Males are more commonly affected because they possess only one X chromosome.
- Females are usually carriers and become haemophilic only when they inherit the defective gene from both parents.
Type of Inheritance:
\[ \boxed{\text{Haemophilia shows X-linked recessive (sex-linked recessive) inheritance.}} \]
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