Hey. For pedigrees, how do we know if a trait is dominate or recessive or whether it is X-linked dominant or X-linked recessive?
On a pedigree, there are clues you can look for when identifying if a trait is X-linked or autosomal and dominant or recessive. It is mainly a common sense thing but you can ask yourself questions to work through it.
X-Linked or AutosomalDoes this trait occur equally in men and women?
(If carriers are shown, are there carriers of both sexes?)
If the trait occurs equally in both genders, then it is most likely autosomal.
Dominant or RecessiveDoes every affected child have an affected parent? (Then it is most likely dominant.)
Does the trait disappear from a family line and reappear (skip a generation)? (Then it is recessive.)
Are there carriers? (Recessive.)
In Summary:X-Linked Dominant: Occurs more often in women, no carriers, cannot skip generations, every child has affected parents.
X-Linked Recessive: Occurs more in men, female carriers (but no male carriers), can skip generations.
Autosomal Dominant: Equal in both genders, no carriers, cannot skip generations, every child has affected parents.
Autosomal Recessive: Equal in both genders, carriers (but these may not be shown directly), can skip generations.
If it’s dominant, it would be represented by a capital letter, if recessive a lower case. I’m not too sure about the other two.
This is true if they give you the genotype.
eg. Dominant allele (B) and recessive allele (b). Then genotypes could be (BB, Bb or bb)
If it is X-linked, the alleles and genotypes would look like this: X
R or X
r. Then genotypes could be ( X
R X
R, X
RX
r or X
rX
r in females. In males they could be X
RY or X
rY.
Note: Remember men would only get one copy of the X chromosome.