Question

1. You carry out a dihybrid cross in petunias. You are unaware of the linkage relationship between the genes DBL and CAS. One parent has single flowers (dbl) and has and upright growth habit (cas) while the other parent is wildtype, having double flowers (DBL) and a cascading growth habit (CAS). You cross the two parents to produce a wild-type F1 and then carry out a testcross. (a) Write the genotypes, using the correct notation, of the two individuals used in the testcross. (2 marks) F1: Tester: (b) What percentage of the testcross progeny will be single flowering and have an upright growth habit if the two genes are: i) Unlinked? (1 mark) ii) Completely linked (no crossing over). (1 mark) iii) If the two genes are 30 map units apart? (1 mark) (c) If DBL and CAS are 20 map units apart and there are 360 test cross progeny in total, how many of the testcross progeny will be single flowering and have a cascading growth habit? (1 mark)

Answer 1

Answer:

a).

D= Double; d = Single

Cascading = C; Upright = c

CD/cd (F1) x (tester) cd/cd ---parents

b).

i). If unlinked:

	cd
CD	CD/cd (cascading, double)—25%
cd	cd/cd (upright, single)---25%
Cd	Cd/cd (cascading, single)—25%
cD	cD/cd (upright, double)—25%

ii).

If completely linked:

	cd
CD	CD/cd (cascading, double)—50%
cd	cd/cd (upright, single)---50%

iii).

Distance between genes (mu) = Recombination frequency (%)

CD/cd genotype produces 4 types of gametes as below:

Recombinant gametes (30%):

Cd = 15%

cD = 15%

Non-recombinant gametes (70%):

CD = 35%

cd = 35%

cd/cd genotype produces only one type of gametes, cd.

	Cd
CD (35%)	CD/cd (cascading, double)—35%
cd (35%)	cd/cd (upright, single)---35%
Cd (15%)	Cd/cd (cascading, single)—15%
cD (15%)	cD/cd (upright, double)—15%

c).

CD/cd genotype produces 4 types of gametes as below:

Recombinant gametes (20%):

Cd = 10%

cD = 10%

Non-recombinant gametes (80%):

CD = 40%

cd = 40%

cd/cd genotype produces only one type of gametes, cd.

	Cd
CD (40%)	CD/cd (cascading, double)—40%
cd (40%)	cd/cd (upright, single)---40%
Cd (10%)	Cd/cd (cascading, single)—10%
cD (10%)	cD/cd (upright, double)—10%

Number of single flowering, cascading growth habit = 10/100 * 360 = 36