Dihybrid Cross of Mendel and Inheritance of Two Genes: Introduction, Examples, FAQ

A dihybrid cross is a kind of breeding experiment between two organisms that have alleles for two contrasting traits. It is a cross between two organisms that are heterozygous for two different traits. These traits are determined by the DNA segments on the chromosome called genes.

In a dihybrid cross, the parents have different pairs of alleles for each trait. One parent carries the homozygous dominant allele, while the other one carries the homozygous recessive allele. The offspring produced after the dihybrid cross in the F1 generation are all heterozygous for specific traits. The dihybrid cross is an important topic in biology subject.

This Story also Contains

1. What is a Dihybrid Cross?
2. Mendel's Experiment on Dihybrid Cross
3. The Dihybrid Cross Experiment
4. Explanation of Results through Punnett Square
5. Recommended video for "Dihybrid Cross"
6. MCQs on Dihybrid Cross

What is a Dihybrid Cross?

A dihybrid cross is a cross-breeding carried out for two different characters at a time to study their blended inheritance pattern. In this cross, organisms with two contrasting characters are being crossed to have the segregation and assortment mechanism of such traits analysed according to this law. In contrast,a monohybrid cross is when only one pair of contrasting characters is taken into account.

This setup is very important in genetics as it enables the researcher to observe the transmission of many characters and how genes on the different chromosomes behave during gamete formation.

The heredity concept was initiated in the mid-19th century by the Austrian monk Gregor Mendel, commonly referred to as the father of genetics. His findings regarding basic genetics, segregation, and independent assortment threw light on the science of biology owing to his progress in solving problems related to heredity.

Mendel's Experiment on Dihybrid Cross

The dihybrid cross is carried out between different organisms having two different pair of contrasting characters. Mendel’s experiments on the dihybrid cross are explained below-

Objective of the Experiment

The general significance of Mendel’s work is evidenced by the current application of genetics, also sometimes called Mendelian genetics, in various areas ranging from the breeding of crops for increased yield and disease resistance to that of gene therapy in medical sciences. It indicates how his work laid down the basis upon which people are gradually building their understanding of the principles of biological heredity. He also gave the three laws of inheritance, upon which today’s world of genetics stands.

Methodology

The specific purpose of Mendel’s dihybrid cross-experiment was to study the characteristics of two different factors together in one organism at a time, specifically in pea plants or Pisum sativum. In doing so, he hoped that he could disprove or prove his hypothesis of one of Mendel's Laws of Inheritance, the Law of Independent Assortment, by analysing the way genetic traits such as seed shape and seed colour are inherited. The other two laws are the Law of Dominance and the Law of Segregation.

The garden pea was thus chosen by Mendel for experiments because it has very distinguishable characteristics. Specifically, he chose plants that exhibited two traits: such factors as seed size (round or wrinkled), and seed colour (yellow or green).

Before performing his dihybrid cross, Mendel had to make the parental plants pure, which he achieved by letting the plants self-breed for generations, thus making sure that the parents were homozygous for both the traits – round yellow seeds (RRYY) and wrinkled green seeds (rryy).

The Dihybrid Cross Experiment

The cross that was carried out to understand the Law of Independent Assortment was the dihybrid cross. The dihybrid cross is explained below-

First Filial Generation (F1)

In Mendel's dihybrid cross experiment, the first filial generation (F1) resulted from crossing pea plants with different traits: Round yellow seeds (RRYY) and wrinkled green seeds (rryy), as the round yellow seeds are dominant over the wrinkled green seeds.

The F1 generation, as expected, was phenotypically uniform, with all the plants having round yellow seeds. This outcome indicated that one phenotype possessed superior characteristics of being round and yellow over the other phenotype, which was wrinkled and green in colour.

Phenotypic Ratio: Fully round with a bright yellow outer coat.

Genotypic Ratio: Let all the plants be RrYy type, for simplicity, the two characteristic features are round and yellow seeds.

Second Filial Generation (F2)

Crossing over the F1 generation plants gave Mendel the second filial generation, or the F2 generation. The F2 generation exhibited a phenotypic ratio of 9:3:3:1, which means sixteen young ones:

Nine of them possessed round yellow seeds, which are represented by RRYY and RrYy.

3 belonged to round green seeds (RRyy and Rryy).

3 had wrinkled yellow seeds (both rrYY and rrYy genotypes).

1 had wrinkled green seeds (rryy).

Explanation of Results through Punnett Square

The 9:3:3:1 phenotypic ratio is easily explained by observing the allele relationships with the help of a Punnett square that demonstrates all the possibilities in the F1 generation. Every cell in a Punnett square is an individual genotype, while phenotypes come from the blend of the mentioned genotypes. This ratio can be said to conform to Mendel’s Law of independent assortment, showing that alleles for seed shape, round or wrinkled, are independent of alleles for seed colour, yellow or green, during gamete formation.

Recommended video for "Dihybrid Cross"

MCQs on Dihybrid Cross

Q1. Assertion : The phenotypic ratio of a dihybrid cross between two heterozygous parents is 9:3:3:1.

Reason: The ratio is a result of the independent segregation of alleles for each of the two genes being studied.

Option 1: Assertion and Reason are both true, and Reason is an accurate account of Assertion.

Option 2: Both Assertion and Reason are accurate, but Reason does not adequately explain Assertion.

Option 3: The assertion is correct, but the reasoning is incorrect.

Option 4: Both Assertion and Reason are incorrect.

Correct answer: (1) Assertion and Reason are both true, and Reason is an accurate account of Assertion.

Explanation:

In a dihybrid cross between two heterozygous parents (AaBb x AaBb), four possible gametes can be produced by each parent, which can combine in 16 possible ways to produce different genotypes and phenotypes in the offspring.

The Law of Independent Assortment states that during gamete formation, the segregation of alleles at one gene locus is independent of the segregation of alleles at another gene locus. This means that the alleles for each trait are sorted into gametes independently of one another, leading to a greater variety of offspring genotypes.

In this case, the two genes (A and B) are located on different chromosomes and are therefore unlinked, so they assort independently. As a result, the offspring can inherit any combination of alleles from the two parents, leading to a phenotypic ratio of 9:3:3:1 (dominant for both traits, dominant for one trait and recessive for the other trait, recessive for one trait and dominant for the other trait, recessive for both traits).

Hence, the correct answer is Option (1) Assertion and Reason are both true, and Reason is an accurate account of Assertion.

Q2. Assertion: In a dihybrid cross between two heterozygous parents, the expected phenotypic ratio is always 9:3:3:1.

Reason: The Law of Independent Assortment applies to all dihybrid crosses.

Option 1: Assertion and Reason are both true, and Reason is an accurate account of Assertion.

Option 2: Both Assertion and Reason are accurate, but Reason does not adequately explain Assertion.

Option 3: The assertion is correct, but the reasoning is incorrect.

Option 4: Both Assertion and Reason are incorrect.

Correct answer: (1) Assertion and Reason are both true, and Reason is an accurate account of Assertion.

Explanation:

The Law of Independent Assortment states that during gamete formation, the segregation of alleles at one gene locus is independent of the segregation of alleles at another gene locus. This means that the alleles for each trait are sorted into gametes independently of one another, leading to a greater variety of offspring genotypes.

In a dihybrid cross between two heterozygous parents, the expected genotypic ratio is 1:2:1:2:4:2:1:2:1 (YYRR, YYRr, YYrr, YyRR, YyRr, Yyrr, yyRR, yyRr, yyrr), which simplifies to a phenotypic ratio of 9:3:3:1 (dominant for both traits, dominant for one trait and recessive for the other trait, recessive for one trait and dominant for the other trait, recessive for both traits).

Therefore, the reason given in the assertion is correct, but the assertion is not entirely correct, as not all dihybrid crosses necessarily follow a 9:3:3:1 phenotypic ratio. This ratio is only expected if both genes are completely independent and unlinked, and if the heterozygous parents have the same alleles at both gene loci. If the two genes are linked or are not completely independent, then the expected phenotypic ratio may deviate from 9:3:3:1.

Hence, the correct answer is Option (1) Assertion and Reason are both true, and Reason is an accurate account of Assertion.

Q3. A Tall plant with Red seeds (both dominant traits) was crossed with a dwarf plant with white seeds. If the segregating progeny produced an equal number of tall red and dwarf white plants, what would be the genotype of the parents?

Option 1: TtRr × TtRR

Option 2: TtRr × ttrr

Option 3: TTRR X ttrr

Option 4: TTRR × TtRr

Correct answer: (2) TtRr × ttrr

Explanation:

To determine the genotypes of the parents, let's assign symbols to represent the alleles for the traits:

T = Tall (dominant)

t = Dwarf (recessive)

R = Red seeds (dominant)

r = White seeds (recessive)

Given that the segregating progeny produced an equal number of tall red and dwarf white plants, we can infer that the parents are heterozygous for both traits.

The genotype of the tall plant with red seeds (dominant traits) can be represented as TtRr, indicating that it carries one dominant allele for tallness (T) and one recessive allele for dwarfness (t), as well as one dominant allele for red seeds (R) and one recessive allele for white seeds (r).

The genotype of the dwarf plant with white seeds (both recessive traits) can be represented as ttrr, indicating that it carries two recessive alleles for both dwarfness (t) and white seeds (r).

Therefore, the genotypes of the parents are TtRr and ttrr.

Hence, the correct answer is option 2) TtRr × ttrr

Also Read:

• Practice MCQs on Dihybrid Cross

• Incomplete Dominance & Mendel's Experiment

• Codominance and Multiple Alleles