I stand corrected. I had remembered things wrong regarding achondroplasia, and reviewed the texts, and you're both right. however, the cause of death in homozygotes with achondroplasia is listed as thoracic constriction, not skull problems. however, the leading cause of death among young achondroplasts (at least, of those associated with the disease) is cervico-medullary compression, due to malformation of the bones of the skull base. of course, I find the development of the skull to be fascinating, and achondroplasia to be a good illustrator of this development, because the bones of the skull are formed from multiple different embryonic precursors, some of which dependent on cartilage and others not (membranous vs endochondral bone).
but back to the matter at hand, achondroplasia is a completely dominant trait, with 100% penetrance. therefore, if two achondroplasts have kids, you would expect that 2/3 of the surviving children would have achondroplasia, and the remaining 1/3 would be of normal stature (these numbers are based on 1/4 of the offspring being homozygotes and not surviving). but this is often not entirely the case, in that the non-achondroplastic children also have an increased rate of short stature. so again, something more than purely mendellian patterns must be at work.
and I managed to inadvertently make an even better example of the danger of using phenotype to imply genotype. to a casual observer, both of the parents on the learning channel show have a similar phenotype, yet apparently (and I was actually unaware of this because I haven't watched the show other than the commercials), they have different forms of dwarfism, and thus different genotypes.
so anyway, thanks for correcting my errors.
