So if you noticed, all three of these
children that we discussed had something in common: we had three children who had
a genetic disease in which both parents were seemingly healthy, but the child had
the disease. However, the mechanisms behind these three diseases were
actually different. So in the cases of Lorenzo and Massimo, there was actually a
defective gene that both parents had, and unluckily, each parent passed down the
broken version of the gene to the child, resulting in the disease.
However, in the
case of Sam, both parents actually had perfect copies of this lamin A gene, but
Sam somehow had a mutated copy. So, the distinction is two parents having broken
copies that are recessive and passing down this broken recessive copy each,
resulting in a child that has the disease, versus both parents having
actually two perfect copies, but still having a child that somehow gets a
mutation, and we actually cannot differentiate between these two
mechanisms without sequencing the "ge
nome trio:" the mother, father, and child.
So, some definitions: first we want to talk about "compound heterozygotes." So,
imagine both parents are carriers for some disease: in other words, they have
one functional copy of a gene and one recessive broken copy of the gene. So,
with their dominant functional copy, they display the regular healthy phenotype,
regular healthy characteristics, whereas this recessive broken copy just kind of
hides away in the background. A compound heterozygote is a c
hild who inherits two
recessive broken copies, but these two broken copies are broken in different
ways. So, in this toy example, we have a father who has one healthy copy, the
bottom version, and has one mutated copy, this red A, that's actually broken, and we
also have the mother who is in a similar situation: has one healthy copy, the top
version, but has a broken copy in the bottom version, but her copy that's
broken is broken differently: it has an A in the last position. And the child
inhe
rited each parent's broken copy, but within the child, now you have two
different types of the broken copy, so the two copies are broken in different
ways. So this child would be a compound heterozygote. So, unlike the previous
example, in which both parents had a broken copy of the given gene,
imagine a scenario where both parents actually have two functional copies of
the given gene. However, one of the parents' "gametes," or sex cells, has a new,
or "de novo," mutation, so then when the child
is born, this gamete that gave rise
to the child passes down this mutation to the child. So in this example, we have
a mother who has two functional copies of this gene and passes down a
functional copy to the child, but we have a father who also had two functional
copies of the gene, but in the process of passing it down to the child, the gamete
had a mutation at the second position that ended up breaking the gene. So now,
even though the father had two functional copies, the child has one
bro
ken copy. So if this breaking, this mutation, is dominant, the child will have
the disease.
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