Answer:
The alleles of the genes for height will control the height in each plant. As we can infer from the description in the question, the alleles for the tall trait will be dominant over the alleles of the recessive trait. This is the reason that all the F1 offsprings were observed to be tall. However, the results of the F2 generation interpret that most of the F1 generation plants were heterozygous dominant. When heterozygous dominant plants are crossed, there will be a sound chance for the recessive trait to occur in the F2 generation.
Double recessive epistasis is the interaction between two genes in which either of them in h0m0zyg0us recessive state inhibit the expression of the other gene. It is double recessive epistasis and there are two genes involved.
Observing the phenotypic proportions of the offspring can reveal the inheritance pattern and the number of genes involved in a trait expression.
If more than one gene is involved, we can also determine which interaction is operating in the trait determination.
In the exposed example, the ratio 9:7 belongs to double recessive epistasis.
Epistasis refers to the interaction between two diallelic genes in which the epistatic gene can alter, influence, or suppress the expression of the hypostatic gene.
When the epistatic gene is recessive, the interaction is known as recessive epistasis.
In double recessive epistasis, both genes are epistatic when they are in a h0m0zyg0us recessive state. This is, in the h0m0zyg0us recessive state, either of the genes will inhibit the expression of the other gene. And vice-versa.
Available data:
a tall variety was crossed with a dwarf variety
F1) 100% tall.
F2) 9/16 were tall and 7/16 were dwarf.
Since there are 16 possible genotypes, we can assume there are two genes involved in this trait expression.
Let us name these genes L and T.
- L dominant allele
- l recessive allele
- T dominant allele
- t recessive allele
Since the F2 phenotypic ratio is 9:7, we can assume that these two genes are interacting to determine the expression of the trait. And we can also determine that the inheritance pattern is double recessive epistasis.
Coss: Tall x Dwarf
Parentals) TTLL x ttll
F1) 100% TtLl
Cross) TtLl x TtLl
Gametes) TL, Tl, tL, tl
TL, Tl, tL, tl
Punnett square) TL Tl tL tl
TL TTLL TTLl TtLL TtLl
Tl TTLl TTll TtLl Ttll
tL TtLL TtLl ttLL ttLl
tl TtLl Ttll ttLl ttll
F2) 9/16 tall ⇒TTLL, TtLL, TTLl, TtLL
7/16 swarf ⇒ ttLL, ttLl, TTll, Ttll
the presence of h0m0zygous recessive genotype tt or ll inhibits the expression of the other gene, making plants to express the dwarf phenotype
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