Phenotypic Ratio For The Offspring :What And How To Find

In this post, you will find the how to find the phenotypic ratio for the offspring using monohybrid and dihybrid cross.

Phenotype ratio for the offspring refers to the characters which are observable and determined by functional genes or traits.

Phenotypic ratio for the offspring can determined by using Mendel’s cross-

Monohybrid Cross

Monohybrid cross deals with only one character. Gregor J Mendel was the first who studied the inheritance of a single character. Using monohybrid cross we can determine the phenotypic ratio of an individual.

F1 Generation:

Mendel cross-pollinated a pure tall pea plant(100-120cm) and a pure dwarf pea plant (22-44cm). He called them parental generation. He collected the seeds produced by this cross and grew plants from them. All of these plants were found to be tall. None was dwarf or intermediate in size. They constituted the F1 generation.

phenotypic ratio for the offspring
Phenotypic Ratio For The Offspring :F1 generation

F2 Generation:

The F1 plants were pollinated among themselves or were allowed to undergo self-pollination. The flowers of plant are covered with aseptic bags to avoid the interaction of unwanted pollen grains with flowers of stigma. F1 plants were allowed to mate, and seeds were produced from them. The seeds produced by the F1 plants were allowed to grow and plants were raised from those seeds constituted the F2 generation. The plants of F2 generation were tall and dwarf. Their  ratio is 3:1 means three tall and one dwarf plant. This indicates that the F1 plants had a trait for tallness as well as a trait for dwarfness. Such genetically mixed offspring produced by crossing two pure parents having contrasting traits are called hybrids

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Phenotypic Ratio For The Offspring: F2 generation of Monohybrid cross

F3 Generation:

Mendel allowed each F2 plant to form seeds by self-pollination. Seeds were produced by the F2 generation plants, collected and were sown, and  plants raised from the.  The plants so raised formed F3 generation. Mendel observed that the tall and dwarf plants behaved differently. The breeding of dwarf plants was pure, i.e., they produced dwarf plants on self-pollination. Evidently they did not have the trait for tallness. One of the tall plants, only one third plants bred true and were, thus, pure for tallness. The plants produced from the F3 generations were found to be plants of F1 generation means plants having ratio of 3 tall to 1 dwarf.  The indicated that they had the trait for dwarfness also, i.e, they were hybrids.

Dihybrid Cross

Mendel also studied the inheritance of two characters simultaneously. Mendel considered form of the seed and color of the cotyledons. He crossed the parental generation of pea plants having phenotype of Round and Yellow seeds and phenotype of green and wrinkled seeds. The phenotype of both plants can be written as RRYY x rryy.  Gametes are produced from them and allowed to fuse. The seeds were produced and all the plants raised from them constituted F1 generation. All the plants of F1 generation are found to be having Round and Yellow seeds which is dominant. The recessive characters are never expressed in F1 generation.

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Phenotypic Ratio For The Offspring :F1 generation having phenotype round and yellow seeds

Following are the points given below to explain dihybrid cross:

  • F1 plants allowed to self pollinate to produce the F2 generation plants. The gametes formed from F1 generation are RY, Ry, rY and ry. The gametes were allowed to fuse and formed the F2 generation. We can determine the phenotype by crossing the gametes or genotypes using Punnet Square.
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Phenotypic Ratio For The Offspring : F2 generation of Dihybrid cross
  • The plants so produced from the F1 generation forms four types of different plants in the ratio of 9:3:3:1, which shows 9 plants are dominant having round and yellow seeds. 3 plants showing the dominant Round  and recessive characters and other three showed recessive and dominant character in seeds and one plant showed all recessive character seeds.
  • The phenotypic ratio of a dihybrid cross is simply the combination of the monohybrid ratios. Due to the dominance of both the round and yellow features, their F2 monohybrid ratios would be 3 round to 1 wrinkled and 3 yellow to 1 green. If we multiply the monohybrid ratios we get the dihybrid ratio of F2 generation which is 9:3:3:1.
  • The plants formed by the self pollination of F1 generation of dihybrid cross were hybrids having two unlike alleles for seed shape and seed color which is RrYy.
  • Hybrids of F1 generation plants were crossed with each other. .During gamete production, the alleles controlling seed shape and cotyledon colour segregate or separate once more. Four different types of gametes  are formed from each hybrid. The four gametes formed are RY, Ry, rY, ry.
  • On crossing the gametes, we get the four different types of plants having different types of seeds quality or phenotypic character. Total 16 plants were obtained by the fusion of gametes.
  • Two plant shows the character of seed having Heterozygous dominant character (RRYY) and heterozygous recessive character (rryy) and are pure. Rest of all plants are heterozygous dominant -recessive hybrids.
  • In F2 generation, crossing of F1 hybrids produced the heterozygous dominant and recessive traits in the 3:1 ratio.

This proves that the alleles of different characters do not interfere with each other’s behaviour, but are sorted out independently of one another.

Frequently Asked Questions

Q1. What is the difference between Phenotype and Genotype?

PhenotypeGenotype
It is the expression of a trait. e.g., tall pea plant. It is the gene equipment that determines a trait,e.g., TT or Tt for a tall pea plant.
Direct observation of a person can reveal the phenotype.It can be ascertained from the ancestry or progeny of an individual.
It is determined by genotype. It is not influenced by phenotype.

Q2. Define Homozygous individuals.

Individuals having similar alleles for a specific trait (TT or tt ) is known as homozygous individuals. They possesses dominant or recessive alleles but never both. They produce only one kind of gametes (T, T or t, t).

Q3. Define Law of Dominance.

It states that characters are controlled by factors and factors are always present in pair. These factors later called alleles or genes. The factor that expressed itself is described as dominant factor and the unexpressed one as recessive factor.

Q4. Define Law of Segregation.

It states that alleles which contain pair of factors, according to this law , the two alleles are get separated from each other during gamete formation at meiosis I and again during meiosis II , two copies of chromosome separate from each other, which shows alleles do not mixes and gametes are always pure for each character.

Q5. What do you mean by allele?

A pair of genes that controls the two alternative expressions of the same character and have the same site in the homologous chromosomes is called alleles or allelomorphs.

Q6. Define Law of Independent assortment.

This law is applicable to having more than one characters. During gametogenesis, two characters will not influence or depend on each other characters. Both the characters will segregate independently without influencing others characters or trait.

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