Question

You have crossed two Mexican hairless dogs, and the offspring are 1/3 hairy and 2/3 hairless. Given this phenotypic ratio, draw the Punnett square for this cross. Indicate the phenotype for each of the possible offspring. What are the genotypes of the P1 and F1 dogs in this crosList the predicted genotype as well as the phenotype for each of the offspring. Which genotypes are hairless and which are hairy? List the predicted genotype as well as the phenotype for each of the offspring. Which genotypes are hairless and which are hairy? Can you design a genetic cross that would yield a true breeding hairless line-where all offspring is hairless?

Answer 1

- Punnett square:

H - h

H - HH (lethal) Hh (hairless)

h - Hh (hairless) hh (hairy)

- Cross: Hh × Hh (all genotypes are Hh)

- Genotypes of the P1 dogs: heterozygous (Hh)

- Genotypes of the F1 dogs: 1/3 hh; 2/3 Hh

- It is not possible to design a cross where all offspring are hairless because the H allele for hairless is lethal in homo-zygous condition (the maximum possible is 2/3 hairless individuals)

Step-by-step explanation:

As you can see in the Punnett square above, the 1:2 ratio (i.e., 1/3 hairy and 2/3 hairless) is indicative that the hair phenotype is a single (monogenic) trait associated with a lethal allele (H allele). In this case, it should be impossible to design a true-breeding cross because the homo-zygous HH genotype is lethal. ​Alleles that cause the death of an organism are known as lethal alleles (in this case, the H allele). A recessive lethal allele is only lethal in the homo-zygous individuals, thereby heterozygous individuals survive and they may exhibit a normal/altered (non-lethal) phenotype.