7. In Mendel's breeding experiment on garden pea, the F2 generation yielded the offspring in a ratio of 25% which produced pure yellow pods, 50% which produced hybrid seed pods and 25% pure green pods.
(a) Which of the two colours of pods is
dominant?
(b) What are the phenotypes of the parents
of the F1 generation?
Answers
Answer:
Mendelian Domination
Mendel identified traits that dominate other traits. For example, smooth peas demonstrate a dominant trait, while wrinkled peas display a recessive trait. In Mendel’s work, if an individual plant has at least one smooth-pea factor, it will have smooth peas. It must have two wrinkled-pea factors to have wrinkled peas.
This can be expressed with an “S” for smooth peas and an “s” for the wrinkled variety. The genotype SS or Ss creates smooth-pea plants, while ss is needed for wrinkled peas.
Purebred Peas: F1 and F2 Generation
Mendel numbered his generations of pea plants. The original parents from generation F0 created F1 offspring. Self-fertilization of F1 individuals produced the F2 generation. Mendel was careful to first breed several generations of pea plants to ensure that the F0 generation was purebred -- that is, had two of the same factors.
Today, scientists would say the F0 parents were homozygous for the pea-shape gene. The F0 crossings were SS X ss -- pure smooth crossed with pure wrinkled.
A Generation of Hybrids
All of the F1 peas were smooth. Mendel understood that each F1 individual had one S factor and one s factor -- in modern parlance, each F1 individual was heterozygous for pea shape. The genotype ratio of generation F1 was 100 percent Ss hybrid, which yielded 100 percent smooth peas since that factor is considered dominant.
By self-fertilizing those F1 individuals, Mendel was creating the Ss X Ss cross.
The resulting F2 genotype ratios were 25 percent SS, 50 percent Ss and 25 percent ss, which can also be written as 1:2:1. Because of dominance, the phenotype, or visible trait, ratios were 75 percent smooth and 25 percent wrinkled, which can also be written as 3:1.
Mendel got similar results with other pea plant traits, such as flower color, pea color, and size of the pea plants.
Domination Variations
Alleles can have relationships beyond the classic Mendelian dominant-recessive one. In codominance, both alleles are equally expressed. For example, crossing a codominant red-flowered plant with a white-flowered one produces offspring having red and white spotted flowers. In a red vs. white cross of a plant with incomplete dominance, the resulting offspring will be pink.
In multiple allele variations, an individual’s two alleles for a trait come from a population of more than two possible traits. For example, the three human blood alleles are A, B and O. A and B are codominant, while O is recessive.
Answer:
(a) The green colour of the pods is dominant.
(b) The plants of the F₁ generation will produce hybrid pods which will be green in colour.
Explanation:
Traits considered by Mendel for his experiments on the Pea plant:
Mendel selected 14 true-breeding varieties of pea plants. He considered 2 plants as a pair which were similar in all aspects except one pair of contrasting traits for a particular character.
Mendel selected 7 characters altogether for carrying out his hybridisation experiments. These are:
- Seed Shape, where the round shape was dominant and wrinkled shape was recessive.
- Seed colour, where the yellow colour was dominant and green colour was recessive.
- Flower colour, where the violet colour was dominant and white colour was recessive.
- Pod shape, where the inflated shape was dominant and constricted shape was recessive.
- Pod colour, where the green colour was dominant and yellow colour was recessive.
- Flower position, where the axial position was dominant and terminal position was recessive.
- Stem height, where the tall stem was dominant and the short stem was recessive.
Now, let us consider the pod colour as the character of our concern.
Let us represent,
The green-coloured dominant trait as R, and,
The yellow-coloured recessive trait as r.
Now, when two pure breed varieties are crossed, i.e., RR × rr, then, the F₁ generation plants will have the genotype Rr, following the law of segregation, and the F₁ generation plants would bear green-coloured pods following the law of dominance.
When two such plants of the F₁ generation are crossed, i.e., Rr × Rr, then, the F₂ generation will have 25% pure green pods, 50% hybrid seed pods which will be green-coloured, and, 25% pure yellow pods.
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