when to use XX/XY cross versus standard mendelian cross?

[bleach121]

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Can someone tell me when to use a standard mendelian cross (e.g.: Aa x Aa) versus when to use the X^AX^a x X^A Y notation. For example, if Phenylketonuria is a autosomal recessive disease, should we only use the standard Aa x Aa cross as opposed to using an XX and XY cross?

The question I had came from TBR FL 3:
"A woman who expresses the PKU phenotype marries a man whose father did not carry the PKU gene, but whose mother did express the disease. What is the probability that their child will NOT show any symptoms of PKU?"
A. 0.25
B. 0.50
C. 0.75
D. 1.00


I initially answered it using the XX and XY notation in which I assigned these values:
Woman: X^a X^a since she expresses PKU
Man: X^a Y (because his father did not carry the gene (X^A Y) but his mother did (X^a X^a) so the resultant cross of those can only give X^a Y for the Man.
Thus the answer according to this notation would be 0%, which is incorrect

When you use the standard Aa x Aa notation, the woman is aa and the man is Aa and the cross gives a probability of 50% for Aa, which is the correct answer.

Can I always assume to use the standard A- x A- notation when dealing with autosomal diseases and then use the XX x XY notation when dealing with SEX LINKED diseases? Please correct me if I am wrong.

Thanks.

 

[JMMTB]

Humans have 23 sets of chromosomes. 22 are autosomal and 1 set are sex chromosomes. Autosomal chromosomes do not play into the gender of an individual, just sex chromosomes (XX v. XY). If an allele is located on an autosomal chromosomes, it is inherited like standard mendelian genetics because gender has no role in its inheritance. If an allele is sex linked that means it is on either the X or Y chromosome, but not both. An example is the SRY gene found only on the Y chromosome.

A rare exception are alleles that are located in the pseudoautosomal region of the sex chromosomes. This is a region where the allele is on both X and Y chromosomes and inheritance follows a standard autosomal pattern. It is also this region that allows X and Y chromosomes to pair as homologues during meiosis I.