Achondroplasia is an homozygous-lethal, autosomal-dominant condition. Therefore, 67% of offspring can be expected to have the achondroplasia phenotype, and 33% will be of average height.
In a homozygous-lethal, autosomal-dominant condition, one can assume that each affected parent is heterozygous for the condition, since the homozygous condition is incompatible with life. Of the zygotes these two heterozygotes create, 25% will be homozygous recessive (average height); 50% will be heterozygous (achondroplasia phenotype), and 25% will be homozygous dominant (usually lethal in utero). Therefore, of the children that two parents with achondroplasia have, it can be expected that 2/3 (66%) will have the achondroplasia phenotype and 1/3 (33%) will be of average height.
Achondroplasia is due to a mutation in the fibroblast growth factor receptor 3 (FGFR3). The mutated gene can be inherited, or it can occur sporadically. Unrelated individuals usually carry the same mutation because a particular sequence in this gene prone to specific mutation. In other words, this gene houses a mutation "hotspot." This particular hotspot has an observed 0.6-1.4*10^-5 frequency of the Gly380Arg mutation, which is much higher than what would be expected by chance alone.
Illustration A shows a patient with achondroplasia as well as a characteristic radiograph.
Illustration B shows a Punnett square depicting a typical homozygous-lethal autosomal-dominant condition, like achondroplasia.
Answer 1: If achondroplasia were not a homozygous-lethal condition and both parents were heterozygotes, one could expect that 25% of offspring would be of average height.
Answer 3: If the patient's husband was of average height, one could expect 50% of their offspring to be of average height.
Answer 4: If achondroplasia were not a homozygous-lethal condition and both parents were heterozygotes, one could expect 75% of offspring to have the achondroplasia phenotype.
Answer 5: If achondroplasia were not a homozygous-lethal condition, it would be impossible to tell if the patient and her husband were heterozygous or homozygous for the condition. In that case it would be impossible to answer the question without more information.
Nussbaum R, McInnes R, Willard H (eds): Genetic variation in individuals: Mutation and polymorphism; in Thompson & Thompson Genetics in Medicine, ed 6.