Monohybrid Cross - Inheritance Of One Gene: Definition & Example

What Is Monohybrid Cross?

A monohybrid cross is a type of genetic breeding experiment aimed at studying the inheritance of one trait controlled by a single gene locus. This identifies the segregating and recombining nature of alleles into offspring, hence giving fundamental ideas about genetic inheritance patterns at the core of modern genetics.

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This Story also Contains

1. What Is Monohybrid Cross?
2. Monohybrid Cross Definition
3. Mendel's Laws Of Inheritance
4. How To Carry Out A Monohybrid Cross?
5. Monohybrid Cross Example
6. Genotype And Phenotype Determination
7. Video Recommended On Monohybrid Cross - Inheritance Of One Gene

Gregor Mendel laid the foundation of genetics through experiments with pea plants in the mid-19th century. He carefully did monohybrid crossings, observing traits such as seed shape, flower colour, and plant height for generations to outline laws essential for genetic heredity.

Mendel's approach was quite systematic and meticulous. He took up traits one at a time and used plants that were breeding true for particular traits—that is, he had plants that consistently produced offspring with the same trait when they were self-pollinated. This means he crossed plants that were consistently tall with consistently short ones, thus assuring himself of the results obtained from each cross.

From the experiments, Mendel derived two fundamental laws of heredity: the Law of Segregation and the Law of Independent Assortment. The Law of Segregation says gene pair will randomise during gamete formation, with one allele ultimately incorporated into each gamete. In essence, the Law of Independent Assortment accounts for the basis of genetic diversity by the Independent assortment of alleles of different genes during gamete formation.

Monohybrid Cross Definition

A monohybrid cross is a genetic experiment whereby the inheritance of one trait controlled by one gene locus is investigated. The cross will involve the breeding of organisms that are homozygous for different alleles—one dominant and one recessive—for the trait under investigation. In this way, such a cross will allow segregation and recombination of these alleles in the offspring to occur and give rise to various classical patterns of inheritance, which include dominant or recessive traits. The methodology behind this experiment is necessary for an understanding of genetic inheritance and brings insight into how characteristics are passed from one generation to another.

Mendel's Laws Of Inheritance

Mendel formulated two basic principles of heredity: the Law of Segregation and the Law of Independent Assortment. The first law states that alleles segregate randomly during gamete formation, while the second one indicates that alleles of different genes assort independently during gamete formation. These two laws lie at the heart of monohybrid crosses, which predict genotypic and phenotypic ratios in offspring.

How To Carry Out A Monohybrid Cross?

A monohybrid cross would require the steps that fully explain how traits are passed down from one generation to the next. The following are the steps for carrying out a monohybrid cross:

Steps for Carrying Out a Monohybrid Cross

Identification Of Parental Traits

Select organisms that differ only in one characteristic of interest, such as the colour of the flower in plants or the colour of fur in animals. For example, you can select pea plants with yellow and green coloured seeds. YY for Yellow, yy for green.

Determine Parents' Genotype

Firstly, research data on the genetic content of organisms chosen for the experiment. See if they are homozygous, with the same alleles, say YY or yy for the trait, or heterozygous having different alleles, for instance, Yy.

Perform The Cross

Cross a homozygous dominant (YY) with a homozygous recessive (yy) parent, or cross two heterozygous parents (Yy) with one another to view both the dominant and recessive alleles in the offspring.

View Offspring in F1 Generation

Allow the parents to undergo natural self-pollination, or mating depending on the organism. Collect the seeds or offspring from the cross for genetic analysis.

Genotype And Phenotype Ratios

Use Punnett squares or probability to predict the genotype ratio and phenotype ratio among offspring. Punnett square is a method of showing all of the possible combinations of alleles that each parent can contribute, along with the probabilities.

Offspring Phenotype Analysis

Observe the phenotypes of the offspring for different traits they express. Record the number of offspring that have the dominant and recessive traits.

Interpret Results

Compare observed ratios of phenotypes to the predicted ratios in the Punnett square. Compare whether the traits are according to Mendelian inheritance (e.g. 3:1 ratio of dominant: recessive trait in the F1 generation). Make Inferences: Describe how alleles segregate and combine in the offspring based on these results. Explain what this means for both dominant and recessive alleles and the resulting phenotypes.

Diagram: An Example Of a Monohybrid Cross

Monohybrid Cross Example

An example of a monohybrid cross is:

Gregor Mendel’s Peas

Gregor Mendel, in the mid-19th century, conducted scientific experiments regarding pea plants that turned out to be the very foundations of our present understanding of monohybrid crosses and genetic inheritance. Mendel followed the inheritances of individual traits, such as seed colour and seed texture, through generations and presented principles of genetic inheritance in a very lucid and systematic way.

One of Mendel's famous experiments was conducted to analyse seed colour in peas. Mendel did controlled breeding experiments with the plants having homozygous yellow seeds (YY) crossing with plants having homozygous green seeds (yy). All of the first generation, F1, ended up with offspring producing yellow seeds. Thus, the yellow allele seemed to have dominance over the green allele and even did a nice job at thoroughly masking it.

In subsequent crosses, Mendel crossed heterozygous yellow-seeded plants from the F1 generation with one another. The next generation of plants, or the F2 generation, grew with three yellow-seeded plants to one green-seeded plant in a ratio of 3:1, or YY or Yy and yy, respectively. This 3:1 ratio thus confirmed Mendel's hypothesis about the inheritance of flower colour and gave evidence of a dominant and recessive allele.

Mendel's work on seed texture in peas was similar. He noted that some pea plants always gave smooth seeds; and others wrinkled seeds. In performing monohybrid crosses, Mendel showed that alleles for seed texture segregated and recombined independently of alleles for seed colour, thus proving his Law of Independent Assortment.

The diagram given below shows the different characteristics of peas used by Mendel in his experiments

Huntington's Disease

Huntington's disease is a case of monohybrid inheritance occurring in humans, and that example is used to illustrate the interaction between dominant and recessive alleles. It is caused by a mutation in the HTT gene on chromosome 4. The child has a 50% chance of having the disease if one of the parents contributes a single copy of the mutated gene (Hh). Inheritance of two copies, HH causes a more severe illness, while the normal gene, hh does not result in Huntington's disease at all.

The fact it is dominant means a single copy of the mutated gene is enough to cause the disease, thus allowing monohybrid crosses to predict the probability of inheritance. This only purely genetic disorder, therefore, stresses the need to understand the genetic risk factor and the associated ethics relevant to the genetic counselling and testing of a hereditary disease.

Confirming Dominant Traits

Monohybrid crosses confirm the claims of dominant traits among characters because one can see the trait steadily within the offspring, thus proving dominance in one allele over another.

Genotype And Phenotype Determination

Genotype refers to the makeup of an organism genetically, for instance, AA, Aa, or aa. The phenotype refers to the expression of genes that can be identified, such as tall and short. Homozygous is having two identical alleles of a certain gene and is represented by AA or aa, while heterozygous has two different alleles of a certain gene, for instance, Aa.

Genotype and phenotype ratios can be determined using monohybrid crosses. This can be explained by the fact that crossing two heterozygous tall pea plants, Tt x Tt, gives the ratio 3:1 for tall, TT or Tt, to short, tt.

Dihybrid Cross

A dihybrid cross is a type of genetic experiment where two different traits are studied, being controlled by two gene loci. So, some of the important points that should be realized about dihybrid crosses are:

Two Traits: The dihybrid cross involves crossing entities that have a difference in two traits—for example, seed colour and seed texture in peas.

Law of Independent Assortment: Mendel's Law of Independent Assortment expresses that alleles of different genes isolate independently during the formulation of gamete rolls, making way for all possible combinations of traits in offspring.

Punnett square: It permits the calculation of the possible genotypic and phenotypic ratios of offspring from a dihybrid cross.

Genetic Diversity: Dihybrid crosses illustrate one of the ways the genes for several different features assort independently the genetic diversity within a population.

Dihybrid crosses are needed to understand the workability of genetic diversity and the inheritance pattern of multiple traits that detail the complexity of genetic interactions.

Test Cross

A test cross is a genetic cross designed to determine what an organism's genotype is when it expresses a dominant phenotype but its genotype is not known. When crossed with another plant homozygous recessive for the same character, the phenotypic ratio of the offspring produced will be either homozygous or heterozygous. It is a method mainly used in establishing homozygous dominance and heterozygosity of an organism with dominant phenotypes.

Test crosses are essential tools in either a genetic research or breeding program. They provide the genetic makeup of organisms but at the same time, help predict what will come out from further crossings. To add to this, they are instrumental in pinpointing carriers of genetic disorders and learning the pattern of inheritance relating to some characteristics in populations.

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