The question is incomplete. The complete question is:
Achondroplasia is a common cause of dwarfism in humans. All individuals with achondroplasia are thought to be heterozygous at the locus that controls this trait. When two individuals with achondroplasia have children, the offspring occur in a ratio of 2 achondroplasia:1 wild type. What is the most likely explanation for these observations
A. The allele that causes achondroplasia is a late-onset lethal allele.
B. The allele that causes achondroplasia is a recessive lethal allele.
C. The allele that causes achondroplasia is a dominant lethal allele.
D. Achondroplasia is incompletely dominant to the normal condition.
E. Achondroplasia is codominant to the normal condition.
Answer:
B. The allele that causes achondroplasia is a recessive lethal allele.
Explanation:
According to the given information, achondroplasia is a genetic disorder and the individuals with the disorder are heterozygous. The individuals with homozygous recessive genotype do not survive as the allele for the disorder is lethal in the homozygous condition. Let's assume that the allele "a" causes achondroplasia while the allele "A" imparts normal height to the individuals. Two heterozygous parents each with genotype "Aa" would produce a progeny in the following ratio=
Aa x Aa= 1 AA (normal height): 2 Aa (achondroplasia): 1 aa (do not survive).
