In one of the plant species, dissected leaves dominate over whole leaves, and the blue color of flowers dominates over pink. As a result of analyzing crosses, the offspring of four phenotypic classes was obtained:
1) 133 plants with dissected leaves and blue flowers;
2) 362 plants with dissected leaves and pink flowers;
3) 127 plants with whole leaves and pink flowers;
4) 378 plants with whole leaves and blue flowers.
Then the plants of the first phenotypic class were crossed with plants of the second phenotypic class. How many percent of the resulting hybrids will have cut leaves and pink flowers? Whole leaves and blue flowers?
We introduce the notation of genes:
A – dissected leaves;
a – whole leaves;
B – blue flowers;
b – pink flowers.
– As a result of analyzing crosses, four phenotypic classes were obtained in an unequal ratio. The following conclusions can be drawn from this:
1) Genes that determine the type of leaves and color of the flowers are inherited interlinked.
2) The analyzed individual produced 4 types of gametes, i.e. was digeterozygous.
3) The descendants of the first and third phenotypic classes are crossover (they are approximately equally divided and much smaller than individuals of the second and fourth phenotypic classes). The descendants of the second and fourth phenotypic classes are noncross (they are approximately equally divided and much larger than individuals of the first and third phenotypic classes).
4) The crossover frequency between genes A and B is: rfAB = sum of crossover individuals / total number of individuals × 100% = (133 + 127): (133 + 362 + 127 + 378) × 100% = 260: 1000 × 100% = 26%
– We will establish the genotypes of individuals of the first and second phenotypic classes. Both inherited linked ab genes from the recessive homozygous parent ab // ab.
Individuals of the first phenotypic class have dissected leaves (A) and blue flowers (B), which means that they inherited linked AB genes from another parent (analyzed). Thus, individuals of the first phenotypic class have the AB // ab genotype.
Individuals of the second phenotypic class have dissected leaves (A) and pink flowers (b), therefore, they inherited linked Ab genes from another parent (analyzed). Thus, individuals of the second phenotypic class have the Ab // ab genotype.
– In order to find the proportion of individuals in each cell of the Pennet lattice, it is necessary to multiply the fractions of the corresponding gametes by each other.
Therefore, the percentage of plants with dissected leaves and pink flowers is: (0.37 × 0.5 + 0.13 × 0.5 + 0.13 × 0.5) × 100% = (0.185 + 0.065 + 0.065) × 100% = 0.315 × 100% = 31.5%.
The percentage of plants with whole leaves and blue flowers is: 0.13 × 0.5 × 100% = 0.065 × 100% = 6.5%.
Answer: 31.5% of hybrids will have dissected leaves and pink flowers; 6.5% of hybrid plants will have whole leaves and blue flowers.
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