Question

The trait for medium-sized leaves in iris is determined by the genetic condition PP'. Plants with large leaves are PP, whereas plants with small leaves are P'P'. The trait for red flowers is controlled by the genes RR, pink by RR', and white by R'R'. A cross is made between two plants each with medium-sized leaves and pink flowers. If they produce 320 seedlings, what would be the expected number of phenotype (medium and pink)? Assume no linkage.

1) 10
2) 20
3) 40
4) 60
5) 80

Answer 1

A cross between irises with medium-sized leaves (PP') and pink flowers (RR') is expected to yield 1/4 offspring with the combined medium-leaf and pink-flower phenotype. This predicts 80 out of 320 seedlings to have medium leaves and pink flowers.

Step-by-step explanation:

The student asked about the expected phenotypic ratio in iris seedlings with medium-sized leaves and pink flowers when two such plants are crossed, based on the given genotypes. For medium-sized leaves, the genotype is PP', and for pink flowers, the genotype is RR'. Since we are dealing with a dihybrid cross of independent genes and assuming no linkage, we use the Mendelian principles of inheritance to determine the phenotypic ratio.

To find the expected number of offspring with medium-sized leaves and pink flowers, we first identify the possible gametes from each parent for leaf size (P or P') and flower color (R or R'). We use a Punnett square to determine the outcomes. Each parent can produce PR, PR', P'R, and P'R' gametes. The possible genotype combinations for medium leaf size, PP' or P'P, and pink flowers, RR' or R'R, lead us to expect a 1:1:1:1 ratio.

Out of the four phenotypic combinations in the offspring, one of them (PP'RR') will show the desired traits of medium-sized leaves and pink flowers. Therefore, 1/4 of the total offspring (320 seedlings) are expected to have the desired phenotypic combination, which is 320 / 4 = 80 seedlings.