Red Flowers in a Shady Corner
A gardener keeps a population of a flowering plant in which flower colour is controlled by a single gene with two alleles: R gives red flowers and is dominant over r, which gives white flowers. Cross A (red × red) produced 78 red and 26 white offspring. Cross B (red × white) produced 41 red and 39 white offspring. The gardener also notices that plants growing in a shaded corner of the garden have paler flowers than plants of the same stock grown in full sun, even though the seeds came from the same parent plants. She collects seeds from one of the pale-flowered plants and sows them in a sunny bed.
1. Deduce the genotypes of both parents in cross A and both parents in cross B, showing how the numbers lead you to each conclusion.
Read the worked answer
Cross A: 78 : 26 simplifies to 3 : 1. A 3 : 1 ratio appears only when both parents are heterozygous, so both red parents are Rr × Rr. The white offspring (rr) can only appear if each red parent carries a hidden r allele.
Cross B: 41 : 39 is close to 1 : 1. That is what a heterozygous parent crossed with a homozygous recessive parent gives: Rr × rr, producing Rr (red) and rr (white) in equal numbers. If the red parent had been RR, no white offspring could have appeared at all.
2. The white-flowered offspring from cross A is crossed with the red parent from cross B. Predict the expected ratio of red to white offspring and explain your reasoning.
Read the worked answer
The white offspring from cross A must be rr, since white is recessive and only appears in the homozygous state. The red parent of cross B was shown to be Rr.
Cross: rr × Rr → ½ Rr (red) and ½ rr (white).
Expected ratio 1 red : 1 white, so roughly half of the offspring should be white whatever the total number. Every pollen or egg from the white parent carries r, so the colour of each offspring is decided entirely by which allele the Rr parent passes on.
3. The shaded plants have paler flowers than sunlit plants of the same parentage. Does this mean the gardener has bred a new pale variety? What will the seeds she collected look like when grown in full sun, and how would she find out the genotype of the pale parent?
Read the worked answer
No new variety has been produced. The paler colour is a modification caused by the environment - less light means less pigment - not a change in the alleles. Genotype plus environment together produce the visible phenotype.
The seeds still carry the same alleles, so grown in full sun they will show the colour their genotypes dictate, not pale flowers.
To find the pale plant's genotype she should test-cross it with a white-flowered (rr) plant: if any white offspring appear, the pale plant was Rr; if all offspring are red, it was RR. Only the offspring ratios reveal the hidden allele.