In contrast to our bizarre new world of “alternate facts,” science is both logical and rational. If an observation seems to counter dogma, then we investigate and get to the truth. That’s what happened for Millie and Hannah, whose stories illustrate two ways that genetic disease can seem to veer from the predictions of Mendel’s first law: that genes segregate, one copy from each parent into sperm and ova, and reunite at fertilization. (I’ll get to embryo engineering at the end.)
Millie’s situation is increasingly common – exome or genome sequencing of a child-parent “trio” reveals a new (“de novo”), dominant mutation in the child, causing a disease that is genetic but not inherited.
Hannah’s situation is much rarer: inheriting a double dose of a mutation from one parent and no copies of the gene from the other. (more…)