A Little added challenge Use the following for questions 7-9: In dogs, there is a hereditary deafness caused by a recessive gene, “d” A kennel owner has a male dog (Gilbert) that she wants to use for breeding purposes if possible. The dog can hear. 7) What are the two possible genotypes of Gilbert? 8) If the dog’s genotype is Dd, the owner does not wish to use him for breeding so that the deafness gene will not be passed on. This can be tested by breeding the dog to a deaf female (dd). Fill in these two Punnett squares to illustrate the crosses for your possible male genotypes (from question 9) with the deaf female dog. Possible Cross 1 % of possible Genotypes: DD: Dd: dd: % of possible Phenotypes: Deaf pups: Hearing pups: Possible Cross 2 % of possible Genotypes: FF: FE ff: % of possible Phenotypes: Deaf pups: Hearing pups: 9) Explain how could you tell the genotype of Gilbert’s male dog?
The Correct Answer and Explanation is:
Correct Answer:
7) What are the two possible genotypes of Gilbert?
Gilbert can hear, so he is either DD (homozygous dominant) or Dd (heterozygous).
8) Punnett Squares and Outcomes:
Possible Cross 1: Gilbert = DD, Female = dd
| d | d | |
|---|---|---|
| D | Dd | Dd |
| D | Dd | Dd |
- Genotypes:
- DD: 0%
- Dd: 100%
- dd: 0%
- Phenotypes:
- Deaf pups: 0%
- Hearing pups: 100%
Possible Cross 2: Gilbert = Dd, Female = dd
| d | d | |
|---|---|---|
| D | Dd | Dd |
| d | dd | dd |
- Genotypes:
- DD: 0%
- Dd: 50%
- dd: 50%
- Phenotypes:
- Deaf pups: 50%
- Hearing pups: 50%
9) Explanation: How could you tell the genotype of Gilbert’s male dog?
To determine Gilbert’s genotype, you can perform a test cross with a known genotype — in this case, a deaf female dog with genotype dd (homozygous recessive). This strategy relies on observing the traits of the offspring to deduce the unknown genotype of the hearing male.
If Gilbert is DD, then all offspring from a DD × dd cross will be Dd, and since the “D” allele is dominant for hearing, 100% of the puppies will be able to hear. No deaf pups will be born.
However, if Gilbert is Dd, the cross with dd will produce a 1:1 ratio — 50% Dd (hearing) and 50% dd (deaf). The appearance of any deaf puppies in the litter would confirm that Gilbert carries the recessive allele and is heterozygous (Dd).
This is an example of a monohybrid cross used in genetic testing to determine unknown genotypes by phenotypic ratios in offspring. It is effective because recessive traits only express when both alleles are recessive, making it clear when a recessive allele is present.
So, if any deaf puppies are born, Gilbert must be Dd. If all puppies can hear, especially after several litters, it is highly likely Gilbert is DD and does not carry the deafness gene.