What is the phenotypic ratio in the F1 generation when PP is crossed with PP?

Problem 9: Incomplete dominance

In Mendel's experiments, if the gene for tall (T) plants was incompletely dominant over the gene for short (t) plants, what would be the result of crossing two Tt plants?

Tutorial

A cross of two F1 hybrids, heterozygous for a single trait that displays incomplete dominance is predicted to give a 1:2:1 ratio among both the genotypes and phenotypes of the offspring. 1/4 will have the dominant phenotype (tall, TT), 1/2 will have the intermediate phenotype (Tt) resembling the parents of this cross, and 1/4 will have the recessive phenotype (short, tt.) When incomplete dominance occurs, it is not necessary to do a test cross to identify the heterozygotes.

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Thursday, October 1, 1998
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Hint:A phenotype is a physical character of an organism such as flower color, seed shape, leaf color, etc. A phenotypic ratio is the relative number of certain physical characteristics of the offspring.
A monohybrid cross is a cross between two organisms that are having variations and differ in one trait only. For example a cross between a pea plant having green seeds and another pea plant having yellow seed. Here, the trait differs in yellow and green seeds.

Complete answer:
Each of the given options are discussed below:
- All white- In a cross between a pea plant with purple flower (PP) and a pea plant with white flower (pp), the offspring will be 4 Pp (heterozygotes) pea plants where the allele P will be dominant over allele p. The allele P is responsible for purple color and allele p is responsible for white color. Since all are of Pp genotype and purple phenotype therefore the flowers cannot be all white.
- All purple- The offsprings are of Pp (heterozygote) genotype. According to Mendel’s law of dominance, in a heterozygote one trait will hide or mask the characteristics of another trait of the same characteristic. Only the dominant allele can express itself. The recessive allele can only express itself in monozygotic conditions. Here, the purple color trait is dominant therefore hides the white color trait for the same characteristic that is flower color. Hence, all the flowers are purple.
- Half purple, half white- The offspring all have a Pp genotype. According to Mendel’s law of dominance, only purple color flowers will be expressed in the offspring, not half purple and half white.
- 1 PP, 2 Pp, 1 pp- This genotypic ratio for the offspring is not possible as the cross produces only Pp genotypes.
- All pp- Since the p allele is recessive therefore pp genotype gives white flowers. This recessive genotype is not possible in the given cross.

Therefore the correct answer is (B)- All purple.

Note:
The genotype is the genetic characteristic of an organism. For example here, PP, Pp, and pp are the genotypes that can produce purple, purple, and white phenotypes respectively. The dominant trait can express itself both in heterozygotes and homozygotes whereas the recessive can only express itself in homozygotes. The genotypic trait is non-observable whereas the phenotypic traits can be observed on the body of a particular organism.

QUESTION 1: In Mendel's original experiments with purple and white pea flowers, why were only purple flowers seen in the F1 generation? Why was there a 3:1 ratio of purple:white in the next generation (the F2)?

From AS: Q1 = In the F1 generation the white pea flower was "masked", which resulted in the appearance of all purple flowers. There was a 3:1 ratio of purple:white flowers in generation F2 because the "masked" (recessive) genes from the F1 generation were allowed to pair, however they were only allowed to pair one time leaving the other three times to appear with purple flowers.

From SP: Q1 = Purple flowers were only seen in the F1 generation because the purple gene was dominant and all of the offspring included that gene so they were purple. The second generation had a 3:1 ratio of purple:white because when the offspring (F1 generation) reproduced they had the recessive white gene which was able to be expressed because during mitosis two white genes paired up and thus one white flower was produced.

From AG: Q1 = The two flowers that he crossed in his original experiment were a homozygous dominant and a homozygous recessive. Because of this each of the offspring in the F1 generation were all heterozygous making them purple. The offspring from the F1 generation was crossed with another heterozygous plant, and the result was one homozygous dominant (purple), two heterozygous (purple), and one homozygous recessive (white).

Note From Dr. Marrs: Great answers. The 'key' feature is that homozygous dominant (PP) x homozygous recessive (pp) always means that ALL F1 offspring will be heterozygous (Pp)- that is the ONLY possibility. If two heterozygous individuals then reproduce to make an F2 generation, there will be a 25% chance of being PP (purple), a 50% chance of being Pp (also purple) and a 25% chance of being pp - the only possiblilty for white. (PS: Figs come from the Encyclopedia Brittanica)

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QUESTION 2: If you and your spouse are carriers for sickle cell disease (Ss), that means that you have a 25% chance (1 in 4 chance) of producing a child with sickle cell disease (ss - if both of you donated the recessive allele in the egg and sperm). If you have 4 children, does this mean that one of then WILL have sickle cell disease? Why or why not?

From KW: Q2 = No, not necessarly. None of the children or all of the children could have the disease. During every pregnancy the parents give the gene and it just depends what egg and sperm are together. It can be different every time.

From Emer: Q2= Not neseccarily because each time you try to have a kid the ratio and precentige is that same. Its not that once you have a kid the ratio changes. it is the same for every pregnancy. Therefore the chances will always be one in four chances.

From AS: Q2 = No this does not mean that if you have a 25% chance of having a child with sickle cell that once you have four children one of them must have the disease. The reason for this is that everytime the sperm and egg meet it is a roll of the dice. Every pairing has a 25% chance of creating a diseased child.

From Stud Muffin: Q2 = Out of the four children, one may have sickle cell disease, just according to the odds. With each baby the chance is one in four. There is nothing to say that any will come out with sickle cell disease.

From AC: Q2 = No, This just means that for each child you have there is a 25% chance that that child will have sickle cell disease. You could have a hundred children and none of them have sickle cell, which would be unlikely but could happen, or all the children could have sickle cell. It is basically a probability question.

From Dr. Marrs: Great answers. It's sometimes hard to think of conception as a 'roll of the dice', but really thats what sexual reproduction has done for life on earth!!!!

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QUESTION 3: What do you think makes a gene a 'dominant' gene? What makes a gene a recessive gene?

From Jo: Q3 = Dominant gene are traits thatwill be observerd in the organisms appearance or physiology.
The recessive cannot be observed unless the individual has two copies of the recessive allele.

From Walker: Q3 = Dominant means stronger - I guess that particular gene is a stronger protein. Recessive means less dominant and I'm assuming the protein is less strong also.(?)

From Cari: Q3 = I have read the information covering this and I do not fully understand what makes a gene dominant or recessive. Maybe I am just thinking about it too hard, but I am just not sure. Could you please explain this in class. Thanks.

From JB: Q3 = I think a dominant gene has a more efficient allele and DNA sequence than a recessive gene. A gene is recessive when two recessive alleles are present.

From LB: Q3 = I'm not sure but I am sure that it has something to do with the way that the alleles are arranged or mutated.

What is the genotypic ratio in the F1 generation when PP is crossed with PP?

In what ratio would you find PP and PP?

What is the phenotype for PP?

What would the ratio of purple flowers to white flowers be if PP and PP were crossed?