Learn about the concept of a dihybrid cross and how it helps understand the inheritance of two traits. Read on to learn the principles of Mendelian genetics and how they apply to dihybrid crosses.
The Principles of Mendelian Genetics
Austrian monk Gregor Mendel is regarded as the founder of modern genetics. Around the middle of the 1800s, he conducted multiple tests on pea plants to determine how features are transmitted from parents to offspring.
Mendel studied the inheritance in pea plants and suggested two important principles:
Law of Segregation:
According to the principle of segregation, each individual possesses two copies of each gene (one from each parent), which are segregated during gamete development. It indicates that each gamete (sperm or egg) contains a single copy of each gene. The offspring inherit two copies of each gene, one from each parent, during the fusion of gametes during fertilization.
Law of Independent Assortment:
According to this law, the alleles of distinct genes segregate independently during gamete formation. In other words, one character’s inheritance does not affect another characteristic’s inheritance.
These two principles are the basis of Mendelian genetics and apply to all genetic crosses, including dihybrid crosses.
What is a Dihybrid Cross?
A dihybrid cross is a form of a genetic cross that involves the inheritance of two distinct characteristics. It indicates that two specific genes situated on separate chromosomes are involved in determining the characteristics of the offspring.
Both parents in this type of cross are heterozygous for both traits. Regarding seed colour and seed form in pea plants, one parent may have yellow, round seeds (YYRR), while the other may have green, wrinkled seeds (yyrr).
The principles of Mendelian genetics, such as the law of segregation and independent assortment, help us understand the inheritance patterns in dihybrid crosses.
Steps of Dihybrid Cross
There are certain steps involved in conducting a dihybrid cross. These steps are:
Determine the parental genotypes:
The first stage in carrying out a dihybrid cross is identifying the genotypes of both parents. One parent in this example has the genotype YYRR, while the other has the yyrr.
Determine the possible gametes:
Next, determine each parent’s gametes. The possible gametes for the parent with the genotype YYRR are YR, whereas those for the parent with the genotype yyrr are yr.
Each allele for the two traits selected is designated with a specific letter. The dominant allele is indicated by an uppercase letter, while the recessive allele is indicated by a lowercase letter.
Construct a Punnett square:
The third step is to construct a Punnett square. The Punnett square is a genetics tool that predicts the probability of offspring acquiring specified qualities from their parents. It is a grid that displays all possible gamete combinations that can come from a genetic cross. The frequency of each genotype was estimated by counting the number of squares on the grid.
Determine the genotypes of the offspring:
The fourth stage is to identify the genotypes of the progeny of the cross. Examining the points at which the Punnett square rows cross with the square columns indicates information about the parents of the offspring.
Determination of the ratios:
The phenotypic ratio is the proportion of offspring with a specific trait, whereas the genotypic ratio is the proportion of offspring with a particular genotype. The ratios of phenotype to genotype can be predicted using the Punnett square approach.
Consider the dihybrid offspring of the previously described parental pea plants. Here is how the Punnett square for this cross is represented:
| R | r | |
| Y | YR | Yr |
| y | yR | yr |
The four possible gametes each parent produces are indicated in the Punnett square’s margins. The letters in the squares reflect the offspring genotypes. To define the phenotype of each offspring, we must use dominance and recessiveness factors.
Examples of Dihybrid Cross
Example 1: Seed Shape and Color in Peas
Gregor Mendel, the father of genetics, conducted various experiments on pea plants to understand the inheritance of traits. One of his experiments involved studying the inheritance of seed shape and colour.
He crossed round and yellow-seeded pea plants with wrinkled and green-seeded pea plants. The F1 generation resulted in all round and yellow-seeded plants, which showed dominance for both traits.
| Seed possibilities | P generation | F1 generation | F2 generation |
|---|---|---|---|
| round & yellow | X | all | 9 |
| round & green | — | — | 3 |
| wrinkled & yellow | — | — | 3 |
| wrinkled & green | X | none | 1 |
However, in the F2 generation, he observed a ratio of 9:3:3:1, where 9 plants had round and yellow seeds, 3 plants had round and green seeds, 3 plants had wrinkled and yellow seeds, and 1 plant had wrinkled and green seeds.
Mendel’s experiment proved that seed shape and colour inheritance are distinct and that the traits are distributed independently during gamete production. The law of independent assortment states that the inheritance of one trait does not affect the inheritance of the other trait.
Example 2: Coat Color and Tail Length in Mice
The study of mice’s coat colour and tail length is another example of a dihybrid cross. Assume a white and long-tailed mouse crosses with a black and short-tailed mouse. We will get all black and long-tailed mice in the F1 generation, suggesting dominance for both features.
Yet, in the F2 generation, we have a 9:3:3:1 ratio comparable to Mendel’s experiment. This ratio demonstrates that coat colour and tail length inheritance are independent of one another and follow the law of independent assortment.
Example 3: Eye Color and Wing Shape in Drosophila
Fruit flies are a common model organism used in genetics research. One of the experiments involving a dihybrid cross in fruit flies was conducted to understand the inheritance of eye colour and wing shape.
The wild-type fruit fly has red eyes and normal wings. When crossed with a mutant fruit fly with sepia eyes and vestigial wings, the F1 generation resulted in all red-eyed and normal-winged flies.
However, in the F2 generation, the ratio observed was 9:3:3:1, similar to the previous examples. This experiment also showed that the inheritance of eye colour and wing shape is independent and follows the law of independent assortment.
Example 4: Hair Texture and Hair Color in Rabbits
Rabbits are frequently used in genetics studies due to their short gestation time and the ease with which their progeny may be studied. The purpose of one of the rabbit dihybrid cross experiments aimed to investigate the inheritance of hair texture and colour.
A homozygous rough-coated and black rabbit was crossed with a homozygous smooth-coated and white rabbit to produce the offspring. The F1 generation produced only black, rough-coated rabbits, demonstrating the dominance of both features.
Yet, a ratio of 9:3:3:1 was recorded in the F2 generation, showing again that the inheritance of hair texture and hair colour are independent and obey the law of independent assortment.
FAQs
Q: What is a dihybrid cross in genetics?
Ans: A dihybrid cross is a genetic cross between two parents with two qualities controlled by two pairs of alleles each. It enables us to comprehend the inheritance of two qualities simultaneously and forecast the offspring’s phenotypic proportion.
Q: What are the principles of Mendelian genetics?
Ans: The principles of Mendelian genetics are the law of segregation and independent assortment. The law of segregation states that there are two alleles of a gene in an individual, and these alleles segregate from each other during the formation of gametes. The law of independent assortment states that the alleles of different genes segregate independently during gamete formation.
Q: What is a Punnett square, and how is it used in genetics?
Ans: A Punnett square is a grid used to calculate the probability of different genotypes and phenotypes in the offspring of a genetic cross. It helps us predict the outcome of a cross and understand the inheritance of different traits.
Q: Why do scientists use dihybrid crosses?
Ans: A dihybrid cross allows us to examine the inheritance pattern of two separate traits simultaneously. Let’s imagine we’re crossing two pea plants. The seed colour and form are the two characteristics we’re looking at.
Q: Who is the father of the dihybrid cross?
Ans: Mendel performed dihybrid crosses on pea plants and discovered the law of independent assortment, a fundamental law of genetics. Mendel began his research by crossing two homozygous parental organisms with different characteristics.
Read Also
References and Sources
- Verma PS and Agarwal VK (3005). Cell Biology, Genetics, Molecular Biology, Evolution, and Ecology. Multicoloured Edition.
- https://www.studysmarter.us/explanations/biology/heredity/mendelian-genetics/
- https://mindmapcharts.com/index.php/quiz-mcqs/biology/5692-mendel-s-law-of-independent-assortment
- https://www.coursehero.com/file/p51938kh/the-dominant-allele-is-indicated-by-an-uppercase-letter-and-the-recessive-allele/
- https://socratic.org/questions/562a93b211ef6b1a79823009
- https://courses.lumenlearning.com%2Fwm-biology1%2Fchapter%2Freading-laws-of-inheritance