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Nick's wife Maria has a history of red-green colour blindness in her family - HSC - SSCE Biology - Question 24 - 2005 - Paper 1
Question 24
Nick's wife Maria has a history of red-green colour blindness in her family. Jack, their two-year-old son, may be red-green colour blind. Maria's brothers, Vincent a... show full transcript
Worked Solution & Example Answer:Nick's wife Maria has a history of red-green colour blindness in her family - HSC - SSCE Biology - Question 24 - 2005 - Paper 1
Step 1
Construct a family pedigree to show the inheritance of this sex-linked genetic disorder.
Answer
To construct a family pedigree that illustrates the inheritance of red-green color blindness:
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Key Symbols: Use standard symbols for males (squares) and females (circles). Indicate affected individuals with shaded symbols, and carriers with a half-shaded symbol.
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Individual Representation:
- Draw a square for Nick and a circle for Maria. Shade Maria's symbol partially to indicate her carrier status.
- For Maria's brothers, represent Vincent and Paul as shaded squares.
- Draw an unshaded square for James (not color blind) and an unshaded circle for Anne (their mother) indicating she is a carrier.
- John, Maria's father, is represented as an unshaded square.
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Connect the Relationships:
- Connect Nick and Maria with a line representing their marriage.
- Link them with a circle for their child, Jack, which may be shaded depending on his predicted status.
This pedigree will effectively demonstrate the transmission of the trait, highlighting the sex-linked nature of the disorder.
Step 2
Predict whether Jack will be colour blind. Justify your answer.
Answer
Jack's color blindness can be predicted based on his mother's genetic background. Since Maria is a carrier of the red-green color blindness gene (X^c) and Nick's genotype is assumed to be normal (X^N), the possible genotypes for their son Jack are:
- If Jack inherits the normal X from his father and the color blindness X from his mother, he will be color blind (X^cY).
- If he inherits the normal X from both parents, he will not be color blind (X^NY).
Given that Maria has one X chromosome for color blindness (X^c) and one normal (X^N), there is a 50% chance that Jack will inherit the X^c from Maria. Therefore, the prediction is that Jack has a 50% chance of being color blind, supported by the inheritance pattern observed in the pedigree.