how did you learn pedigrees (M5 IQ4)? (1 Viewer)

[miss.chanel]

hey gng, could you guys pls provide me w some advice on how to understand pedigrees? also, what type of questions are most commonly assessed within this subtopic? thank you sm!

 

[Blacklight5555]

For me, the key was breaking pedigrees into small chunks: first identify the trait type (dominant, recessive, X-linked), then label every individual with possible genotypes. I redrew the pedigree multiple times and wrote out the Punnett squares for tricky generations. Most assessed questions are about: identifying carriers, determining the probability that offspring will inherit a trait, spotting the inheritance pattern, and sometimes filling in missing genotypes. Doing lots of past M5 IQ4 questions really helped me see the patterns and common twists.

 

[Eagle Mum]

@ChroneDragon26, clarification required to your statement:

”Check males:female (if strong preference -> X-linked)” should instead be
’Check males:female (if strong preference -> X-linked recessive).

X-linked dominant conditions are very rare, but an affected female has 50/50 chance of passing the condition to each of her offspring, male or female, whereas an affected male only passes the condition to his daughters, so in fact there’s a preponderance of females over males amongst the affected.

You do have these facts correctly stated in the table, but I just didn’t want any reader, who may not know the subject material as well as you do, to get confused.

 

[Eagle Mum]

The sample pedigree is a typical example of a pedigree of phenotype (ie. individuals are only classified as having or not having the condition) so carriers are not differentiated from non carriers. You are more likely to get this in an exam because in real life, unless most of the family gets tested, not all the carriers are known. A pedigree by genotype makes it much easier to work out the mode of inheritance.

In a pedigree by phenotype, individuals are presumed to be carriers if their offspring are affected (leaving de novo variants aside, as they are outside the scope of the syllabus).