Question

On a trihybrid cross of a tall, purple-flowered pea plant with round seeds (TtPpRr) with a short, white-flowered pea plant with round seeds (ttppRr), where you will use the Fork-Line Method to determine the probability of three different characteristics.

Answer 1

Trihybrid cross is the cross between three different genetic characteristics of an organism to determine the genotype and phenotype of the offspring generation.

To perform this cross we can 1st breakdown the individual genotypes to determine their probabilities then perform the forkline method to find the probability of each combination of the genotype involving all the characteristics.

1st trait cross Tall vs Short

Alleles T t

Cross Tt x tt

	T	t
t	Tt	tt
t	Tt	tt

Probabilities: Tt = 1/2

tt = 1/2

2nd trait Purple coloured flower vs White coloured flowers

Alleles P p

Cross Pp x pp

	P	p
p	Pp	pp
p	Pp	pp

Probabilities: Pp = 1/2

pp = 1/2

3rd trait Round seeds

Alleles R r(allele for wrinkled seed)

Cross Rr x Rr

	R	r
R	RR	Rr
r	Rr	rr

Probabilities: RR = 1/4

Rr = 1/2

rr = 1/4

Using forkline method now we determine the probabilities of combined genotypes

Parental generation:    TtPpRr      vs         ttppRr

F1 generation probabilities:

                                                                                ¼ RR         ½ x ½ x ¼ = 1/16 TtPpRR

                                              ½   Pp        ½ Rr       ½ x ½ x ½ = 1/8 TtPpRr

                                                                                 ¼ rr       ½ x ½ x ¼ = 1/16 TtPprr

½           Tt   

                                                                                    ¼ RR       ½ x ½ x ¼ = 1/16 TtppRR

                                              ½   pp       ½ Rr        ½ x ½ x ½ = 1/8 TtppRr

                                                                                     ¼ rr        ½ x ½ x ¼ = 1/16 Ttpprr

                                                                                ¼ RR         ½ x ½ x ¼ = 1/16 ttPpRR

                                              ½   Pp    ½ Rr          ½ x ½ x ½ = 1/8 ttPpRr

                                                                                 ¼ rr         ½ x ½ x ¼ = 1/16 ttPprr

½           tt  

                                                                                    ¼ RR    ½ x ½ x ¼ = 1/16 ttppRR

                                              ½   pp      ½ Rr       ½ x ½ x ½ = 1/8 ttppRr

                                                                                     ¼ rr         ½ x ½ x ¼ = 1/16 ttpprr

Hence you determine all the probabilities of different genotypes resulting from the above trihybrid cross by forkline method.