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A pea plant that is pure for purple flowers mates with a pea plant thas has white flowers. One of their offspring self-fertilizes and produces 100 offspring. How many would predict turn out to have white flowers?

Of those offspring, 70 are white and 30 are purple. How is this different than your prediction? Isthis possible? Why or why not?

Respuesta :

marianaegarciaperedo

From the cross between two monohybrid plants, 75 are expected to be purple, and 25 plants are expected to be white. The difference between the observed and expected values might be by random chance.

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1st Cross:  purple flowers    x     white flowers

We will assum that this trait is codified by a single diallelic gene, expressing complete dominance, for which

  • P is the dominant allele and codes for purple
  • p is the recessive allele and codes for white

Parentals)   PP    x     pp

F1) 100% Pp, heter0zyg0ous plant expressing purple flowers

2nd Cross: F1 self-fertilizes

Parentals)  Pp   x   Pp

Gametes)  P    p    P    p

Punnett square)     P       p

                       P    PP     Pp

                       p    Pp     pp

F2) Genotype  

  • 1/4 = 25% of the progeny is expected to be h0m0zyg0us dominant PP
  • 2/4 = 50% of the progeny is expected to be heter0zyg0us, Pp
  • 1/4 = 25% of the progeny is expected to be h0m0zyg0us recessive pp

Gentoypic ratio 1:2:1

      Phenotype

  • 75% is expected to have purple flowers  (PP + Pp)
  • 25% is expected to have white flowers (pp)

Phenotypic ratio 3:1

N = 100

100% of the progeny ------------------- 100 individuals

75% Purple flowers-----------------------75 individuals with purple flowers

25% White flowers------------------------25 individuals with white flowers

So the expected number is 75 plants carrying purple flowers and 25 plants carrying white flowers.

But the observed numbers are 70 plants carrying white flowers, while 30 are carrying purple flowers.

This sharp difference between the expected and the observed values are possible, especially in nature and with no artificial selection.

Probably, during the fertilization process, gametes carrying the p recessive allele managed to meet and produce a zygote with pp genotype more frequently than gametes carrying the P dominant allele.

Hence, during combination, by chance, zygotes with the genotype pp were more frequent than zygotes with genotype PP or Pp.

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