Thanks for stopping by, today we are going to discuss how to determine inheritance patterns, so essentially how to read a pedigree chart. Like this one. If you are unfamiliar with pedigree charts or Mendelian genetics, you should watch some other videos before this one.
We are going to create a flowchart to solve pedigrees, so you’ll simply take any given pedigree chart through a series of questions until you determine the inheritance pattern.
And a note here, this flowchart we are going to build has a high degree of accuracy, but is not foolproof, the random nature of genetics make it impossible to be 100%.
So let’s get started, the first two questions you’ll ask are, Are only males affected? AND are all sons of an affected father affected?
If the answer is yes, then the pattern is Y linked, meaning a genetic disorder affiliated with the y chromosome. Only males possess a Y chromosome, so that means all males receive their Y chromosome from their father. A Y linked trait will never show up in males without affected father. If you reach this point, you have your solution.
If the answer is no,
Then we ask if there are there any cases were affected children do not have at least one affected parent.
If yes, it’s recessive though we don’t know what type. Being recessive, they must have two affected alleles in order to express the disorder, therefore, an affected child can have unaffected parents because they're both carries. However, it is possible that their parents could still be affected.
If the answer is no, then it is a dominant disorder, and thus an affected child must have an affected parent. You should note that dominant patterns usually see someone affected in every generation in each affected linage, this means that once you see an affected parent, you will usually see it in every generation thereafter. It doesn’t skip generations very often.
Moving down the recessive line we need to ask two more questions.
Are all sons of affected mother infected? And are more males affected in general?
If yes, it is X-linked recessive, meaning it’s attached to the X chromosome, since males only have one X chromosome, they are more likely to show, whereas females have two Xs, so a good one can mask the mutated one. Also, Males always receive their X chromosome from their mother, so if she is affected, she will have two X chromosomes with the mutation, and all sons are guaranteed to end up with it.
If the answer is no, then it is autosomal recessive by default. Essentially, males and females are affected evenly, and affected mothers have unaffected sons.
Moving on down the dominant line, we ask
Are all of the daughters of an affected father affected?
If yes, then it is X linked dominant, males only have one X chromosome, which goes to their daughter, so all their daughters will have that mutation.
If no, then it is autosomal dominant by default. If there is a case where an affected father has an unaffected daughter, or an affected daughter has an unaffected father, you know it can’t be X linked, because the father only has one X and that will always to go to the daughter.
So this is the whole flow chart, eventually I’ll make a website and have a downloadable PDF available. I’ll link that in the comments whenever I get to making it. In the meantime, subscribe to my channel so that you’re around when I do create my website.
Thanks for watching, I’ll catch you next time.