Respuesta :
Answer:
1/64
Explanation:
This cross is a trihybrid cross because it involves three genes. The cross is said to be a self-fertilization of plant heterozygous for the three traits i.e. two plants which have a combination of dominant and recessive alleles for each trait is crossed.
Let's say the three genes involved are genes P, Q and R. Where P, Q and R alleles are dominant over p, q and r alleles respectively. Thus, a heterozygous plant will possess PpQqRr genotype.
In concordance with Mendel's law of independent assortment, the alleles get sorted into gametes independently of one another. Meiosis occurs and each heterozygous plant (PpQqRr) produces 8 possible combinations of gametes viz; PQR, PQr, PqR, Pqr, pQR, pQr, pqR and pqr.
These gametes are crossed using a punnet square (see attached image) to produce a total of 64 possible offsprings i.e. 8 (male gametes) × 8 (female gametes).
A phenotypic ratio of 27:9:9:9:3:3:3:1 will result from a trihybrid cross involving two species heterozygous for the three genes/traits. (Check attached image for details of their phenotype)
According to the question, the fraction of offsprings expected to be homozygous dominant for the three traits is 1 out of 64 possible offsprings = 1/64
Note that, homozygous dominance is a state where the alleles for each gene are the same type i.e PPQQRR. Since P, Q and R are the dominant alleles.
Although 27 of the 64 offsprings are dominant for each trait but only one is homozygous dominant (PPQQRR) for the three traits, the other 26 are dominant but had one or more recessive genes i.e. are heterozygous for some/all genes