Inheritance of Two Genes: Introduction, Dihybrid Cross, Examples & Significance

Inheritance of Two Genes: Certain people have a strong resemblance to their parents. The genetic material of the parent is passed on to the child due to which they have a striking resemblance to them. But what exactly are Inheritance and Variation Principles? In sexual reproduction, inheritance is highly visible. This is because, in this process, the variation of inherited characteristics is high. Let us learn about the inheritance of two genes in this article.

Mendel and his Experiments

Gregor Johann Mendel was the first to establish the basic principles of heredity in the mid-nineteenth century. As a result of his contributions, he is known as the “Father of Modern Genetics.” In his garden, Mendel examined the rules of pea plants, following the inheritance pattern from generation to generation. He began by using a monohybrid cross to investigate the inheritance of one gene in the plant. On pairs of pea plants with one opposing trait, he looked at only one character (plant height). Later, he investigated the inheritance of two genes in the plant using a dihybrid cross.

Fig: Seven Characters Chosen by Mendel

Mendel studied the following seven characters, each with its own set of characteristics:
1. Stem height: Tall/dwarf
2. Seed shape: Round/wrinkled
3. Seed colour: Yellow/green
4. Pod colour: Green/yellow
5. Pod shape: Inflated/constricted
6. Flower colour: Violet/white
7. Flower position: Axial/terminal

Study Inheritance Of One Gene

Laws of Inheritance

In the subject of genetics, Mendel created the groundwork and eventually proposed the laws of inheritance. Gregor Mendel proposed three laws of inheritance: the Law of Segregation, the Law of Independent Assortment, and the Law of Dominance. His trials on pea plants with a variety of traits resulted in these laws.

Mendel’s Laws of Heredity are usually stated as:
1. The Law of Segregation: Each inherited trait is defined by a gene pair. Parental genes are randomly separated into the sex cells, so that sex cells contain only one gene of the pair. Offspring, therefore, inherit one genetic allele from each parent when sex cells unite in fertilisation.
2. The Law of Independent Assortment: Genes for different traits are sorted separately from one another so that the inheritance of one trait is not dependent on the inheritance of another.
3. The Law of Dominance: An organism with alternate forms of a gene will express the dominant form.

Fig: Summary of Mendel’s Three Laws of Inheritance

Inheritance of Two Genes Definition

Inheritance of two genes is defined as the study of the inheritance of two genes controlling two different characters. A dihybrid cross is a cross between two individuals who differ in two traits controlled by two separate genes. If both parents are homozygous for both genes, the offspring of the F1 generation will be uniformly heterozygous for both genes and will have the dominant phenotype for both characteristics.

When homozygous parents with two pairs of contrasting characteristics cross, one pair’s inheritance is independent of the others. In other words, when a dihybrid produces gametes, the assortment (distribution) of alleles or traits is unaffected by the parents’ initial pairings. The dihybrid cross or dihybrid ratio can be used to explain this law.

Inheritance of Two Genes or Dihybrid Cross

Fig: Dihybrid cross using a Punnett square

Mendel crossed a pair of contradictory features at a time, such as seed colour and shape. He took the wrinkled-green seed and crossed it with the round-yellow seed. In the F1 generation, he only got round-yellow seeds. This showed that seeds with a round shape and a yellow colour are dominant.

Seeds’ wrinkled shape and green colour, on the other hand, are recessive traits. The F1 offspring were then self-hybridised. In the F2 generation, this resulted in four possible seed combinations. In the phenotypic ratio of 9:3:3:1, there were round-yellow, round-green, wrinkled-yellow, and wrinkled-green seeds.

Seed colour and seed shape are the two traits being studied in this dihybrid cross. The dominant characteristics of yellow seed colour (YY) and round seed shape (RR) are homozygous in one plant (YYRR). In contrast, the recessive traits of green seed colour and wrinkled seed shape are homozygous in the other (yyrr).

Consider the letters “Y” for yellow seeds and “y” for green seeds, “R” for round seeds and “r” for wrinkled seeds. As a result, the parental genotype will be “YYRR” (yellow-round seeds) and “yyrr” (yellow-round seeds) (green-wrinkled seeds).

Dihybrid Cross: F1 Generation

When a yellow and round true-breeding plant (organism with identical alleles) is hybridised with a true-breeding plant with green and wrinkled seeds (yyrr), the subsequent F1 generation will all be heterozygous for yellow seed colour and round seed shape (YyRr). This F1 generation is represented by the single-round yellow seed in the illustration. This indicated that the round shape and yellow colour of seeds are dominant. Meanwhile, the wrinkled shape and green colour of seeds are recessive traits.

Fig: F1 Generation of Dihybrid Cross

Dihybrid Cross: F2 Generation

Self-hybridisation of these F1 generation plants produces offspring with a 9:3:3:1 phenotypic ratio in seed colour and shape variations, known as the F2 generation. This is depicted in the diagram. A Punnett square can depict the probable consequences of a genetic cross.

Fig: F2 Generation of Dihybrid Cross

Around 9 out of 16 of F2 plants will have round, yellow seeds, 3 out of 16 will have round, green seeds, 3 out of 16 will have wrinkled, yellow seeds, and 1 out of 16 will have wrinkled, green seeds in the subsequent F2 generation. The F2 offspring have four distinct phenotypes and nine distinct genotypes.

Inheritance of Two Genes Conclusion

1. Mendel discovered seeds of four different sorts in the ratio 9:3:3:1 for round-yellow, round-green, wrinkled-yellow, and wrinkled-green seeds.
2. There were two parental combinations, yellow round and green wrinkled, and two new combinations, yellow wrinkled and green round, among these four varieties.
3. The ratio of four different phenotypes observed in all mendelian dihybrid crossings was 9:3:3:1. The dihybrid ratio is the name given to this ratio.
4. In F2 generation, yellow round, yellow wrinkled, green round, and green wrinkled seeds were observed in the ratio 9:3:3:1.

Gregor Mendel developed his law of independent assortment after conducting a dihybrid cross. This law states that alleles are transmitted to offspring independently of one another. Alleles separate during meiosis, leaving each gamete with one allele for a single trait. These alleles are randomly united upon fertilisation.

Inheritance of Two Genes Examples

Dihybrid Cross in Pea plants

The cross between a homozygous pea plant with round yellow seeds and green wrinkled seeds is an example of a dihybrid cross. As discussed earlier, RY, Ry, rY, and ry are four possible combinations of the four alleles. The four alleles are mixed up at random to produce four different types of gametes. Self-hybridisation of these F1 generation plants produces offspring with a 9:3:3:1 phenotypic ratio in seed colour and shape variations.

Dihybrid Cross in Drosophila

Fig: Dihybrid cross in Drosophila

1. The homozygous long-winged and black-bodied Drosophila and vestigial-winged and grey-bodied flies are other examples of a dihybrid cross.
2. Long-winged grey-bodied Drosophila flies were obtained in the F1 generation. After mating with each other, the F2 generation is produced.
3. Nine long-winged grey-bodied, three long-winged black-bodied, three vestigial-winged grey bodies, and one vestigial-winged black-bodied fly were obtained in the F2 generation.

Inheritance of Two Genes Significance

Significance of Mendel’s Laws of Inheritance:

1. It explains that individuals of the F₁ generation express the dominant traits.
2. It proves that alleles are transmitted to offspring independently of one another.
3. These rules can easily explain the appearance of new features.
4. Plant traits can be carefully selected to create new kinds with desirable characteristics.

Dihybrid Cross Vs. Monohybrid Cross

A dihybrid cross focuses on differences in two traits, whereas a monohybrid cross focuses on a single trait. In a monohybrid cross, the parent organisms have homozygous genotypes for the trait being studied but distinct alleles for those traits, resulting in different phenotypes. One parent is homozygous dominant, while the other is homozygous recessive.

The F1 generation plants produced from a monohybrid cross are heterozygous, and only the dominant trait is observed, as in a dihybrid cross.

Summary

Mendel studied the laws of pea plants in his garden, observing the inheritance pattern from one generation to the next. A dihybrid cross is a breeding experiment between two organisms that are identical hybrids for two traits. When an organism with identical alleles is hybridised with a true-breeding plan, the subsequent F1 generation will all be heterozygous. Seed colour and seed shape are the two traits being studied in this dihybrid cross. The dominant characteristics of yellow seed colour (YY) and round seed shape (RR) are homozygous in one plant (YYRR). In contrast, the recessive traits of green seed colour and wrinkled seed shape are homozygous in the other (yyrr). In the dihybrid cross, the phenotypic ratio is 3:1 of yellow and green colour and round and wrinkled seed shape in the F1 generation. Self-hybridisation of these F1 generation plants produces offspring with a 9:3:3:1 phenotypic ratio in seed colour and shape variations, known as the F2 generation.

Frequently Asked Questions (FAQs) on Inheritance Of Two Genes

Q.1. What are the two laws of inheritance?
Ans: Mendel’s Laws of Heredity are usually stated as:
1) The Law of Segregation: Each inherited trait is defined by a gene pair. Parental genes are randomly separated into the sex cells, so that sex cells contain only one gene of the pair. Offspring, therefore, inherit one genetic allele from each parent when sex cells unite in fertilisation.
2) The Law of Independent Assortment: Genes for different traits are sorted separately from one another so that the inheritance of one trait is not dependent on the inheritance of another.

Q.2. What are the three principles of inheritance?
Ans: Law of inheritance is made up of three laws: Law of segregation, the law of independent assortment, and the law of dominance.

Q.3. Who is known as the father of modern genetics?
Ans: Gregor Mendel is known as the father of modern genetics. This honour was bestowed upon him in recognition of his pioneering experiments in genetics and inheritance.

Q.4. What is an example of a Dihybrid cross?
Ans: A classic example of a dihybrid cross is Mendel’s experiment with peas. The goal of the experiment was to see if there was a link between different pairs of alleles.

Q.5. How to find the genotype of a Dihybrid cross?
Ans: A. Creating a parental cross would be the first step (P).
B. Make a 4×4 (or 16 square) Punnett Square to cross the attributes you’ve chosen.
C. Determine the parents’ genotype and assign letters to the alleles – lower case letters for recessive traits, upper case letters for dominant traits.
D. According to logic, arranging the traits on the square — recessive traits only occur if both parents have recessive traits. For instance, if both parents carry the recessive trait “f,” the emerging trait will be (“ff”). If one of the parents has an “F,” the offspring will have the trait “Ff,” but never “fF.”

Study Mendel’s Law Of Inheritance Here

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