Maternal-Newborn and Paediatric Nursing · Chapter 2
One in Four, Every Time
Families reason that having one affected child “uses up” the risk. It does not. Build the Punnett square and watch the arithmetic come out the same for every pregnancy, read four pedigrees for the pattern that fits, and then work the one situation that genuinely changes the number you give the parents — a parental translocation.
Work the genetics
Watch what happens to the number when you add previously affected children. Nothing happens. That is the teaching point, and it is the single most common misunderstanding families bring to the conversation.
Filled symbols are affected. Squares are male, circles are female. Work the pattern from the chromosomes, not from memory.
Patterns identified0 / 0
unaffectedaffected□ male ○ female
The karyotype does not change this infant’s care. It changes what you can honestly tell the parents about the next pregnancy — which is exactly why it is done.
Autosomal dominantOne affected parent, one unaffected