Achondroplasia

Inheritance	autosomal dominant
Occurrence	1 in 26,000 (it affects all races and both genders)
Description	a growth defect causing abnormal body proportions; the arms and legs are very short while the torso is nearly normal in size
Testing	there is a prenatal test
Cause and Location of gene	a mutation in the gene for a fibroblast growth factor receptor; chromosome 4

Punnett Squares

Despite its autosomal dominant inheritance, seven-eighths of new cases are the result of new mutation. This figure reflects both a high rate of mutation for the trait and a reduction in reproductive success of individuals who have achondroplasia.

This is a great example to be used when teaching students about autosomal dominant disorders. Heterozygous individuals have the described condition and symptoms, but homozygous dominant fetuses have such severe skeletal abnormalities that spontaneous abortion occurs. This is why prenatal testing is strongly encouraged if both parents have achondroplasia.

Below are five problems which require an understanding of Punnett squares. These questions provide a way to measure studentsir comprehension and understanding.

(1) Joan and John both have achondroplasia. They are trying to decide whether to have children. What is the probability that any fetus they produce will not have achondroplasia? 25%

(2) Joan and John both have achondroplasia. They are trying to decide whether to have children. What is the probability that any fetus they produce will have achondroplasia? 50%

(3) Joan and John both have achondroplasia. They are trying to decide whether to have children. What is the probability that any fetus they produce will be homozygous dominant for achondroplasia and suffer skeletal abnormalities causing a spontaneous abortion? 25%

(4) Zach's mother has achondroplasia but Zach does not. What is the probability of Zach passing achondroplasia to his children? 0%

(5) Margaret, who is normally statured is married to a man who has achondroplasia. What is the probability that their first child will have achondroplasia? 50%

How do we grow?

Achondroplasia can also be mentioned when discussing how our bones grow. There are many reasons some people do not grow normally; achondroplasia is only one of them. Have students discuss what could go wrong in the body that would prevent normal growth. Allow students to study what causes bones to grow and how they grow. Then have a discussion on other mechanisms that could prevent proper growth.