A test-cross is used to determine the genotype of an individual with a functiona

Question

A test-cross is used to determine the genotype of an individual with a functional phenotype. An organism with a functional phenotype (but unknown genotype) is mated to a homozygous individual with a non-functional phenotype and the progeny are examined. If a red-flower producing plant is test-crossed to a homozygous white-flower producing plant, what would the phenotypic ratios be if: A) The red flowers were RR B) The red flowers were Rr?

Please answer questions in bold. Thank you.

The red flowers were RR. If a red-flower producing plant is test-crossed to a homozygous white-flower producing plant, RR would be the dominant trait and rr would be the recessive trait.

R

R

r

Rr

Rr

r

Rr

Rr

offspring will be 100% Red having the genotype RR

Phenotypic ratio: 4:0 or 0:4 Which one would be correct? Is the answer correct?

The red flowers were Rr. R would be the dominant trait and r would be the recessive trait.   Genotypes would be Rr X rr

R

r

r

Rr

rr

r

Rr

rr

Rr = 2/4 or ½ would be red

rr = 2/4 or ½ would be white

Phenotypic ratio: 1:1

A homozygous white-flowered plant is crossed with a homozygous dark purple-flowered plant. All of the seeds grow into pale purple-flowered plants. A) If there is no dosage effect, is it more appropriate to write the genotype for the white-flowered plant “WW” or “pp”? B) What is your reasoning?

Uppercase letters are used to represent dominant genes and lowercase letters are used to represent recessive genes. All of the seeds grew into pale purple-flowered plants, the dominant gene would be purple and the recessive gene would be white. You would write the genotype for the white-flowered plant as “pp” instead of “WW” because the white would be the recessive gene. How would I explain that they are both homozygous? Is this explanation correct?

Purple flower = PP

White flower = pp

P

P

p

Pp

Pp

p

Pp

Pp

R

R

r

Rr

Rr

r

Rr

Rr

Explanation / Answer

1. RR X rr
     Rr (All are red 4:0)

   Rr X rr
    Rr rr Ratio 1:1 (Red : White)
   

2. pp Dark Purple X WW White

            Wp (Pale Purple)
Two genes are in incomplete dominance. Both expressed together to form new color.

No, we can't convert WW to pp. Because,
PP Purple X pp White

          Pp (All are in Dark purple not pale purple)

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