Dioxin is known to be toxic in humans (can cause cancers, damage the immune system, cause developmental problems) when present in the environment at concentrations as low as 15 parts per trillion. In one study on the genetic effects of dioxin using health department records from several US States bordering the Great Lakes, investigators examined the consequences of dioxin exposure in one of the two parents, on the proportion of their offspring that are male or female. The results are as follows:

• The proportion of female babies born to parents whose father was exposed to dioxin but not the mother was 0.565. Total number of babies born in this group was 186.
• The proportion of female babies born to parents whose mother was exposed to dioxin but not the father was 0.455. Total number of babies born in this group was 220.
• The total number of babies born was 406, of which 205 were female.

Use a z Testto determine if there is a difference in the proportion of female babies born when either only the father or only the mother was exposed to dioxin.