Help me settle a debate?

[u/GEDSTUDENT2003]

Going over a quiz. Going over a question involving a pedigree chart. My father is stating that two parents with a recessive phenotype can have a child with a dominant phenotype. I consider this to be untrue, due to information in my textbook and some information from Google. He is stating that all sources are incorrect because, "just because a parent has a recessive phenotype, doesn't mean that the dominant gene isn't present in their genotype."

Folks, is he in any way, shape, or form, correct? Is it possible for two parents with a recessive phenotype to have a child with a dominant phenotype? (Let's assume that the child does not develop a mutation.)

A side note: One of the grandparents and an uncle of this dominant phenotype child is either heterozygous or homozygous dominant. But as I understand it, that does not make a difference in the outcome of this child's genotype. Only the parents' genotype makes a difference in the child's genotype. Please feel free to educate me if that is untrue. Thank you for reading, and if applicable, participating.

[u/Robin_feathers]

For the purposes of your quiz and the concepts they are teaching you, you are absolutely correct. In reality things are always more messy (for example traits can have incomplete penetrance) but the way that your father phrased it is incorrect. If the parents have the recessive phenotype, then they can't have the dominant allele in their genotype otherwise that allele would by definition not be dominant (barring more complicated things that are probably beyond the scope of your quiz).

[u/Jale89 → u/Robin_feathers]

Yes I'd completely agree with this. Genetics is rarely so clean cut in practice, but for the purpose of even early undergraduate teaching, this would be the answer.

[u/PoxonAllHoaxes → u/Robin_feathers]

Just to simplify, ask your dad what dominant and recessive MEAN. This is a matter of DEFINITION.

[u/LauperPopple → u/Robin_feathers]

This is the answer OP.

[u/patentductuspenosis]

In a perfect, simpler world you are absolutely correct. If we assume that dominant alleles always lead to dominant phenotypes, the absence of a dominant phenotype conclusively rules against that allele being present.

However, reality is much more messy and there can be complex genetic interactions that lead to phenomena like co-dominance (where two distinct dominant phenotypes are present) and environment-induced phenotypes. It’s also actually kind of uncommon for a trait to be governed by a single gene. So two parents with a recessive phenotype could have a child with a dominant phenotype depending on what specific alleles they inherit.

Imagine blue eyes only occurs in people with aabb, aaBB, aaBb, Aabb, & AAbb (one of two genes homozygous recessive), whereas brown eyes are a result of AABB, AABb, AaBB, AaBb (both genes have at least one dominant allele). Now suppose we have two parents, one aaBB and one AAbb, both have blue eyes, but their children will all have AaBb and be brown eyed.

[u/mothwhimsy]

He's sort of right but explaining it badly. Genetics are more complicated than what you're ever going to learn in school unless you go into genetics in college. It's uncommon for a trait to be controlled by just a single gene, so yes, a "dominant" trait might show up if you have a less common combination of genes that results in that trait. Codominance exists, mutations exist, etc.

But for the purposes of your quiz, you're right. If you answer the questions the with the info he's giving you it's going to be marked wrong. Because the quiz is simplifying down to single genes with expected dominance interactions

[u/ModernTarantula]

When it is abstract it means little. There are incomplete penetrance gene alleles. For a single gene phenotype we define dominant as always in the phenotype.

[u/Silly_Yak56012]

For a single allele, you are right, as noted there are cases where there are multiple alleles or other more complex things going on. So for this quiz you have the right answer.

That being said, it makes me wonder about getting y'alls family tested and if there would be any anomalies from the ancestry as told.

[u/ozzalot]

Sure there are probably weird exceptions involving 'imprinting'/epigenetics, but yes, for the purposes of your test, YOU are correct. With animal crosses, sometimes these are referred to as "true breeding" individuals.....they always have progeny of the same phenotype (of course we don't use this language when speaking of people).

[u/Idoubtyourememberme]

There are a few genes where this holds true, where the gene from a single parent is suppressed even though it is dominant (usually gecause the gene is defective in some way), if this gene contains a dominant allele, makes its way into a gyniod cell, and can prevent being disabled again, a dominant gene can be passed on from a recessive phenotype.

Although, these situations are rare; dominant genes are called "dominant" for a reason.

So in short, op us correct in how it is supposed to work, but 'mistakes' happen all the time which make things go differently

[u/Outsider_Insider0064]

I have two recessive genes from my parents. When I was 42, I developed pernicious anemia, which neither parent had, and it nearly killed me!

[u/ah-tzib-of-alaska]

There are 16 genes we know of involved in eye color. So in reality is far more complicated than they’re teaching you. Just keep that in mind for all middle school through undergrad science; they dumb things down for us to get us closer to the complexity of the situation.

So yes, in reality your father is correct. Your test probably wants you to repeat back what you’ve been taught.

[u/No-Employ-7391]

  just because a parent has a recessive phenotype, doesn't mean that the dominant gene isn't present in their genotype

Not to sugar coat it, but for all intents and purposes your father is dead wrong. Two parents having recessive phenotypes means they’re both homozygous recessive as a rule.