A plant that produces disc-shaped fruit is crossed with another plant that produces long fruit. All the F1 plants gave disc-shaped fruits. When the F1 were intercrossed, F2 progeny were produced in the following ratio : 9/16 plants with disc-shaped fruits; 6/16 plants with spherical fruits and 1/16 plants having long fruits. Which one of the following options gives correct genotype of spherical fruits obtained in F2?
Understanding Fruit Shape Inheritance
The question describes a genetic cross involving fruit shape in a plant. We start with a parent plant producing disc-shaped fruit and another parent producing long fruit. Their offspring (F1 generation) all have disc-shaped fruits. This indicates that the allele(s) for disc shape are dominant over the allele(s) for long shape.
Analysing the F2 Progeny Ratio
When the F1 plants are crossed with each other (intercrossed), the F2 generation shows a phenotypic ratio of 9/16 disc-shaped : 6/16 spherical : 1/16 long fruits. This ratio (9:6:1) is a modification of the classic dihybrid cross ratio (9:3:3:1). A 9:3:3:1 ratio typically arises when two different genes controlling separate traits are involved and assort independently. The modified 9:6:1 ratio suggests that two genes are involved, but there is gene interaction, specifically additive gene action or complementary gene action leading to a new phenotype (spherical) when certain combinations of dominant and recessive alleles are present.
Genotype and Phenotype Correlation in F2
In a typical dihybrid cross involving two genes, say gene A and gene B, with dominant alleles A and B and recessive alleles a and b, the F2 genotypic ratio is:
- 9/16 A_B_
- 3/16 A_bb
- 3/16 aaB_
- 1/16 aabb
Comparing this with the observed F2 phenotypic ratio:
- 9/16 corresponds to disc-shaped fruits. This suggests that having at least one dominant allele for both genes (A_B_) results in the disc shape.
- 1/16 corresponds to long fruits. This suggests that having recessive alleles for both genes (aabb) results in the long shape.
- The remaining 6/16 corresponds to spherical fruits. This means the two 3/16 classes from the typical ratio (A_bb and aaB_) are combined to produce the spherical phenotype.
Therefore, the genotype combinations that result in spherical fruits are A_bb and aaB_.
Determining Genotypes for Spherical Fruits
Based on the analysis of the F2 ratio:
The phenotype 'disc-shaped' is associated with the genotype A_B_, where '_' means the second allele can be dominant or recessive (AA or Aa, BB or Bb).
The phenotype 'long' is associated with the genotype aabb, meaning both alleles for both genes are recessive.
The phenotype 'spherical' is associated with the genotypes A_bb and aaB_. This means:
- A plant with at least one dominant allele for gene A (A_) and two recessive alleles for gene B (bb) will have spherical fruits (A_bb). This includes genotypes AAbb and Aabb.
- A plant with two recessive alleles for gene A (aa) and at least one dominant allele for gene B (B_) will also have spherical fruits (aaB_). This includes genotypes aaBB and aaBb.
Summary of Genotype-Phenotype Relationship
We can summarize the proposed relationship between genotypes and phenotypes based on the 9:6:1 ratio:
| Genotype | Phenotype | Expected F2 Frequency |
|---|---|---|
| A_B_ | Disc-shaped | 9/16 |
| A_bb | Spherical | 3/16 |
| aaB_ | Spherical | 3/16 |
| aabb | Long | 1/16 |
Adding the frequencies for spherical fruits: \(3/16 (A\_bb) + 3/16 (aaB\_) = 6/16\). This matches the observed ratio for spherical fruits.
Thus, the genotypes responsible for the spherical fruit phenotype in the F2 generation are A_bb and aaB_.
Important Questions from Mendelian principles
In summer squash, white colour fruit (W) is dominant over yellow colour (w) and disc-shaped phenotype (D) is dominant over sphere-shaped phenotype (d). Determine the genotype of the parents if the cross between white, sphere crossed with white, sphere gives 3/4 white, sphere and 1/4 yellow, sphere.
Body weight of rabbits is determined by pairs of alleles at two loci, 'a' and 'b', that are additive and equal in their effects. Rabbits with genotype a-a-b-b- have average 1kg body weight, whereas individuals with genotype a+ a+ b+ b+ have animals that average 3.4 kg in weight. A male rabbit with a- a- b- b- is crossed with a female of genotype a+ a+ b+ b+. What will be predicted average weight of F1 progeny of this cross?
In a mammal, coat colour is governed by gene B, The coat colour is either black or brown, depending on whether the genotype is BB or Bb. It is not known which of these genotypes lead to the black and brown colours. The genotype bb results in albino coat colour. Further, the genotype cc suppresses the expression of coat colour resulting in albino coat colour. An albino male was crossed with a brown female and the resulting progeny had individuals with either black or brown coats. From this observation it can be inferred that the genotype of the male and female that were crossed are:
Assuming that the A, B, C and D genes are not linked, the probability of a progeny being AaBBccDd from a cross between AABbccDd and aaBBccDD parents will be
- What is the probability that a couple heterozygous for albino allele will give birth to an albino son?
