1. What are the chances?
In humans, acondroplasia “dwarfism” (D) is dominant over normal (d). A
homozygous dominant (DD) person dies before the age of one. A heterozygous
(Dd) person is dwarfed. A homozygous recessive individual is normal. A
heterozygous dwarf man marries a dwarf heterozygous woman.
1. Work out the genotypes of the parents (parent’s genes).
Mother 
Father 
2. Set up a Punnett square with possible gametes (sex cells).
Explanation 1: During meiosis (the formation of sex cells) one member (allele) of each
gene pair separate. In this example, the male (Db) produces some sperm containing
“D” (the allele for dwarfism) and some sperm containing “d” (the allele for normal).
Fill in the circles
only and go to
step 3.
3. Fill in the Punnett squares above to work out the possible combination for
the offspring.
Explanation 2: During fertilization sperm meets the egg. The Punnett square show us
the possible combination of genes during fertilization.
4. Count the number of possible genotypes.
 Number of DD = ______
 Number of Dd = ______
 Number of dd = ______
5. What is the predicted phenotypic ratio for the offspring?
 Number of dwarf child/offspring = ______
 Number of normal child = ______
What is the probability of having a child that dies at one from this
disorder? _____________