gabeyeap
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When it asks you to give evidence of why an allele for a genetic disease is on or not on the X chromosome from a family tree, what evidence should I be looking out for? (I.e. Males only inherit Y chromosomes from their father)
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macpatgh-Sheldon
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Hi,

Sex-linked recessive conditions e.g. haemophilia (where there is a greater propensity to bleed from relatively minor injuries/lacerations due to a deficiency of one of the clotting factors (VIII), AND which is present in our royal family through history) OR red/green colour blindness (protanopia - due to complete absence of red-detecting cones [look up trichromatic theory of colour vision]) are diseases that are transmitted from a man (grandfather) through his daughter(s) [who would be (a) carrier(s), unless homozygous for the abnormal allele] to his grandsons. The abnormal allele, being recessive, does not manifest a full-blown symptomtology in heterozygous females but does so in males since the Y chromosome is referred to as a "sleeping partner" [with reference to a business partner, not a sexual partner], meaning it has no influence on the phenotype apart from determining male sex.

So if you notice in the family tree that an abnormality is being transmitted missing alternate generations, and only manifesting itself clinically in males, chances are it is a sex-linked recessive condition.

Sex-linked dominant conditions are extremely rare, and there is some dispute as to whether they exist at all - an example that was quoted some years ago is Vitamin D resistant rickets.

Thanks,
M (ex-medic)
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