Sex Linkage (Leaving Cert Biology): Revision Notes
📚 Revision Notes
Sex Linkage
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Sex linkage occurs when a gene is located on a sex chromosome.
Sex Chromosomes:
- Females: Have two X chromosomes (XX).
- Males: Have one X chromosome and one Y chromosome (XY).
Examples of Sex-Linked Traits in Humans:
- Red-green colour blindness: Inability to distinguish between red and green.
- Haemophilia: Inability to form blood clots properly.
Features of Sex-Linked Traits:
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Genes on X Chromosome:
- Most genes related to sex-linked traits are located on the X chromosome, as the Y chromosome has very few genes.
- This makes sex-linked traits more likely to appear in males.
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Why Sex-Linked Traits are More Common in Males:
- Males only need one recessive gene on their single X chromosome to express the trait (e.g., colour blindness).
- Females need two recessive genes (one on each X chromosome) to express the trait.
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Carrier Females:
- Females with one normal allele and one abnormal allele (heterozygous) do not express the trait but are carriers.
- Carriers can pass the trait to their offspring.
Example: Genotypes for Colour Vision
- N: Represents the normal allele for vision.
- n: Represents the allele for colour blindness (abnormal allele).
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Normal Vision
- Male (Normal Vision): Genotype: XN Y
- The XN on the X chromosome carries the allele for normal vision.
- The Y chromosome has no allele for colour vision.
- Female (Normal Vision): Genotype: XN XN
- Both X chromosomes carry the normal allele for vision.
- Female (Carrier): Genotype: XN Xn
- One X chromosome carries the normal allele (N) and the other X chromosome carries the allele for colour blindness (n).
- The female does not have colour blindness but can pass the allele to her children.
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Colour Blindness:
- Male (Colour Blind): Genotype: Xn Y
- The Xn on the X chromosome carries the allele for colour blindness.
- The Y chromosome has no allele for colour vision, so males only need one n allele to be colour blind.
- Female (Colour Blind): Genotype: Xn Xn
- Both X chromosomes carry the allele for colour blindness.
- The female is colour blind because no normal allele (N) is present.
Example Cross: Carrier Female (XN Xn) x Normal Vision Male (XN Y)
Let's determine the possible genotypes and phenotypes for the offspring of a carrier female (XN Xn) and a normal vision male (XN Y).
Step 1: Gametes
- Female (XN Xn) produces two types of eggs: XN and Xn.
- Male (XN Y) produces two types of sperm: XN and Y.
Step 2: Punnett Square
| XN (Sperm) | Y (Sperm) | |
|---|---|---|
| XN (Egg) | XN XN (Normal Female) | XN Y (Normal Male) |
| Xn (Egg) | XN Xn (Carrier Female) | Xn Y (Colour Blind Male) |
Step 3: Results
Genotypes:
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XN XN: Female with normal vision.
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XN Xn: Carrier female (normal vision).
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XN Y: Male with normal vision.
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Xn Y: Male with colour blindness. Phenotypes:
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Normal Vision Female: 25% (1 out of 4).
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Carrier Female (Normal Vision): 25% (1 out of 4).
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Normal Vision Male: 25% (1 out of 4).
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Colour Blind Male: 25% (1 out of 4).
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Key Points:
- Sex-linked traits are controlled by genes on the X chromosome.
- Males are more likely to express sex-linked traits because they only have one X chromosome.
- Females can be carriers of sex-linked traits without expressing them.
- Examples of sex-linked traits include red-green colour blindness and haemophilia.