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 640 seedlings, what would be the expected phenotypes, and in what numbers would they be expected?

Answer 1

From the given information, the genotypes:

PP' has a phenotype of medium sized leaves, PP has a phenotype of large leaves, P'P' has a phenotype of small leaves, RR has a phenotype of red flowers, RR' has a phenotype of pink flowers and R'R' has a phenotype of white flowers. A cross is made between two plants with medium sized leaves and pink flowers which means that a cross is made between the genotypes PP'RR' x PP'RR'. The progeny obtained from this cross is indicated in the table below:

	PR	PR'	P'R	P'R'
PR	PPRR large leaves, red flower	PPRR' large leaves, pink flower	PP'RR Medium leaves, red flower	PP'RR' Medium leaves, pink flower
PR'	PPRR' Large leaves, pink flower	PPR'R' large leaves, white flower	PP'RR' medium leaves, pink flower	PP'R'R' medium leaves, white flower
P'R	PP'RR medium leaves, red flowers	PP'RR' Medium leaves, pink flower	P'P'RR small leaves, red flower	P'P'RR' small leaves, pink flower
P'R'	PP'RR' medium leaves, pink flower	PP'R'R' medium leaves, white flower	P'P'RR' small leaves, pink flower	P'P'R'R' small leaves, white flower

The phenotypes observed are in the ratio: large leaves, red flower: large leaves, pink flower: large leaves, white flower: medium leaves, red flower: medium leaves, pink flower: medium leaves, white flower: small leaves, red flower: small leaves, pink flower: small leaves, white flower = 1:2:1:2:4:2:1:2:1.

Hence the 640 seedlings would be expected to be in the same above ratio and so they would be in the expected numbers of:

Large leaves, red flower = ratio of phenotype * total number of progeny/16 = 1*640/16 = 40

Large leaves, pink flower = 2*640/16 = 80

Large leaves, white flower = 1*640/16 = 40

medium leaves, red flower = 2*640/16 = 80

medium leaves, pink flower = 4*640/16 = 160

medium leaves, white flower = 2*640/16 = 80

small leaves, red flower = 1*640/16 = 40

small leaves, pink flower = 2*640/16 = 80

small leaves, white flower = 1*640/16 = 40

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