In maize three recessive genes z xt and cm are linked on chr

In maize, three recessive genes (z, xt, and cm) are linked on chromosome 3. A homozygous plant for the recessives is crossed with a wild-type plant. The F1 is crossed to get an F2 generation with the following results:

Genotype

Number Observed

z xt cm

444

z⁺ xt cm

59

z⁺ xt cm⁺

74

z⁺ xt⁺ cm

5

z xt⁺ cm

60

z xt cm⁺

6

z xt⁺ cm⁺

65

z⁺ xt⁺ cm⁺

440

What is the coefficient of coincidence for the double crossovers in this scenario?

What does the coefficient of coincidence tell you about interference in this chromosomal region?

Show your work

Genotype	Number Observed
z xt cm	444
z+ xt cm	59
z+ xt cm+	74
z+ xt+ cm	5
z xt+ cm	60
z xt cm+	6
z xt+ cm+	65
z+ xt+ cm+	440

Solution

Answer:

Coefficient of Coincidence = 0.0095/0.0147 = 0.65

Interference = 1-CC = 1-0.65 = 0.35

Based on the interference, the observed double crossover progeny are less than expected double crossover progeny.

Explanation:

Heterozygous genotype z⁺ xt⁺ cm⁺/ z xt cm

1.    If single cross over (SCO) occurs between z+ xt+ & zt xt

Normal order=z+---------xt+ & z-----xt

After cross over= z+-----xt & z------xt+

z+-----xt recombinants are 59+ 74= 133

z------xt+ recombinants are 60+65= 125

Total recombinants = 258

RF = (258/1153)*100 =23.38%

2.    If single cross over (SCO) occurs between xt+ & cm+ and xt & cm

Normal order= xt+---------cm+ & xt----cm

After cross over= xt+-----cm & xt------cm+

xt+-----cm recombinants are 5+60=65

xt------cm+ recombinants are 6+74=80

Total recombinants = 145

RF = (145/1153)*100 = 12.58%

3.    If single cross over (SCO) occurs between z+ & cm+ and zt & cm

Normal order= z+---------cm+ & z------cm

After cross over= z+-----cm & z------cm+

z+-----cm recombinants are 59+5=64

z------cm+ recombinants are 65+6= 71

Total recombinants = 135

RF = (135/1153)*100 =11.71%

% RF = Map unit distance

The order of genes is -----

z------12.58 m.u.--------cm----11.71 m.u.----xt

RF between z & cm = 12.58% = 0.1258

RF between cm & xt = 11.71% = 0.1171

Expected double cross overs = 0.1258 * 0.1171 = 0.0147

Observed double cross overs = 6/1153 + 5/1153

= 0.0052 + 0.0043 = 0.0095

Observed double cross progeny are more than the expected double cross over progeny.

Coefficent of coincidence (CC) = Obeserved dco / Expected dco

CC = 0.0095/0.0147 = 0.65

Interference = 1-CC = 1-0.65 = 0.35

Based on the interference, the observed double crossover progeny are less than expected double crossover progeny.