Question

In a given plant, the flower colour is governed by a single gene locus. The flower can be either white, red, blue or purple colour. However, a single flower will never have different coloured petals. Multiple crosses were carried out between plants with different flower colours (white, red, blue or purple) and the observations on the progeny phenotype are tabulated.

Cross	Progeny Phenotypes
White X White	All flowers are white
Red X Red	All flowers are red or some are red and some are white (number of red > number of white)
Blue X Blue	All flowers are blue or some are blue and some are white (number of blue > number of white)
Purple X Purple	A mix of red, blue and purple flowers (number of purple flowers > numbers of red and blue flowers and equal number of red and blue flowers)
White X Red	All flowers are red or some are red and some are white (equal number of red and white flowers)
White X Blue	All flowers are blue or some are blue and some are white (equal number of blue and white flowers)
White X Purple	Blue and red flowers (equal number of red and blue flowers)
Red X Blue	All flowers are purple or some are red, some are blue and some are white
Red X Purple	Mostly red and purple flowers and some are blue
Blue X Purple	Mostly blue and purple flowers and some are red

Based on this information, the correct option is:

• A. There are two alleles governing flower colour.
• B. Purple allele is dominant over Red allele, and Red allele is dominant over Blue allele.
• C. Red allele and Blue allele show incomplete dominance.
• D. White allele is dominant over purple allele.

Answer 1

Answer: (C)

Red allele and Blue allele show incomplete dominance.

Answer 2

The problem describes the inheritance of flower color in a plant, which is governed by a single gene locus with multiple alleles. We need to deduce the relationship between the different color alleles based on the provided crosses.

Let's analyze the crosses step-by-step:

1. White X White → All flowers are white.
   This indicates that white is a pure-breeding phenotype. It suggests that the allele for white is recessive. Let's denote the allele for white as 'w'. So, the genotype for white flowers is 'ww'.

2. Red X Red → All flowers are red OR some are red and some are white (number of red > number of white).

   • If all flowers are red, the parent red plants were homozygous (e.g., RR).
   • If red and white flowers appear (in a ratio like 3 Red : 1 White, implied by "number of red > number of white"), it indicates that the red parent plants were heterozygous (e.g., Rw x Rw). This confirms that Red is dominant over White.

3. Blue X Blue → All flowers are blue OR some are blue and some are white (number of blue > number of white).
   Similar to Red, this indicates that Blue is dominant over White.

From points 1, 2, and 3, we establish that white (w) is recessive to both red and blue.

4. White X Red → All flowers are red OR some are red and some are white (equal number of red and white flowers).

   • If all red, the red parent was homozygous (RR x ww → all Rw, which are Red).
   • If equal numbers of red and white (1:1 ratio), the red parent was heterozygous (Rw x ww → 1 Rw : 1 ww, i.e., 1 Red : 1 White). This further confirms Red is dominant over White.

5. White X Blue → All flowers are blue OR some are blue and some are white (equal number of blue and white flowers).
   Similar to Red, this confirms Blue is dominant over White.

6. Purple X Purple → A mix of red, blue and purple flowers (number of purple flowers > numbers of red and blue flowers and equal number of red and blue flowers).
   This is a crucial cross. The progeny ratio of purple > red and blue, with red = blue, strongly suggests a 1:2:1 phenotypic ratio (1 Red : 2 Purple : 1 Blue). This ratio is characteristic of a cross between two heterozygotes where the heterozygous genotype produces an intermediate phenotype, and the two homozygous genotypes produce distinct phenotypes. This implies that Purple is the result of the interaction between the Red allele (R) and the Blue allele (B). If R and B are alleles of the same gene and show incomplete dominance (or codominance), then the genotype RB would produce Purple.
   So, if Purple is RB, then RB x RB cross would yield:
   1 RR (Red) : 2 RB (Purple) : 1 BB (Blue). This perfectly matches the observation.

7. White X Purple → Blue and red flowers (equal number of red and blue flowers).
   If White is 'ww' and Purple is 'RB', then ww x RB would yield:
   1 Rw (Red) : 1 Bw (Blue). This perfectly matches the observation of equal numbers of red and blue flowers, with no white or purple progeny. This further confirms our hypothesis that Purple is RB and white is recessive to both R and B.

8. Red X Blue → All flowers are purple OR some are red, some are blue and some are white.

   • If pure Red (RR) X pure Blue (BB) → All RB (Purple). This is consistent.
   • If heterozygous Red (Rw) X heterozygous Blue (Bw) →
     Progeny: 1 RB (Purple) : 1 Rw (Red) : 1 Bw (Blue) : 1 ww (White).
     This accounts for "some are red, some are blue and some are white" (and also purple). This is consistent.

9. Red X Purple → Mostly red and purple flowers and some are blue.

   • If pure Red (RR) X Purple (RB) → 1 RR (Red) : 1 RB (Purple).
   • If heterozygous Red (Rw) X Purple (RB) → 1 RR (Red) : 1 RB (Purple) : 1 Rw (Red) : 1 Bw (Blue). This simplifies to 2 Red : 1 Purple : 1 Blue. This matches "mostly red and purple flowers and some are blue". Consistent.

10. Blue X Purple → Mostly blue and purple flowers and some are red.

    • If pure Blue (BB) X Purple (RB) → 1 RB (Purple) : 1 BB (Blue).
    • If heterozygous Blue (Bw) X Purple (RB) → 1 RB (Purple) : 1 BB (Blue) : 1 Rw (Red) : 1 Bw (Blue). This simplifies to 1 Purple : 2 Blue : 1 Red. This matches "mostly blue and purple flowers and some are red". Consistent.

Conclusion from all crosses:

• There is a single gene locus with three alleles: R (Red), B (Blue), and w (white).
• The 'w' allele for white is recessive to both R and B alleles.
• The R and B alleles show incomplete dominance (or codominance) when present together, resulting in the Purple phenotype (RB genotype).

Now let's evaluate the given options:

(A) There are two alleles governing flower colour.
Incorrect. Our analysis shows there are three alleles (R, B, w).

(B) Purple allele is dominant over Red allele, and Red allele is dominant over Blue allele.
Incorrect. Purple is a phenotype resulting from the combination of R and B alleles (RB genotype), not a distinct dominant allele. There is no simple dominance hierarchy between R and B; they show incomplete dominance.

(C) Red allele and Blue allele show incomplete dominance.
Correct. When R and B alleles are together (genotype RB), they produce the Purple phenotype, which is an intermediate color between red and blue. This is the definition of incomplete dominance.

(D) White allele is dominant over purple allele.
Incorrect. White is recessive to both Red and Blue alleles. White X Purple (ww x RB) produces Red (Rw) and Blue (Bw) flowers, with no white flowers, indicating white is recessive.