Monohybrid Cross – Inheritance of One Gene

Monohybrid cross is a fundamental concept in Mendelian genetics that explores the inheritance of a single gene with two alleles. It follows the laws set by Gregor Mendel, the father of genetics. Scientists can predict the probability of dominant and recessive traits in offspring through monohybrid inheritance.

Monohybrid and dihybrid inheritance is crucial knowledge in genetics because it helps in explaining how inherited traits are passed from parents to offspring. The monohybrid cross and dihybrid cross are addressed in this article, providing their principal differences and example of monohybrid cross problems with answers to increase your knowledge.

What is a Monohybrid Cross?

A monohybrid cross tests the inheritance of one gene between two parents that have varying alleles for such a characteristic. Such a genetic cross follows the principles of Mendelian genetics, whereby characteristics are inherited in line with dominant and recessive alleles.

Monohybrid pattern of inheritance is generally illustrated through the use of Punnett squares and can be applied to make predictions of offspring ratio. Scientists generally compare monohybrid and dihybrid cross problems and solutions to differentiate between single-trait and two-trait patterns of inheritance. Monohybrid crosses can be studied to learn genetics problems monohybrid crosses and how specific traits are inherited.

Gregor Mendel’s Peas and the Origin of Monohybrid Crosses

Austrian monk Gregor Mendel has also been referred to as the "father of modern genetics" since he was the first person to conduct experiments regarding the heritability of characteristics. Austrian monk Gregor Mendel used to conduct experiments on pea plants (Pisum sativum) in the 1860s to identify how characteristics like seed color, flower color, and height were inherited from one generation to another.

Mendel's input was in the aspect that he worked with a single character at a time, therefore what today is referred to as a monohybrid cross. He crossed the pea plants of varying characters and found that their offspring followed normal patterns of inheritance. For example, after crossing a yellow-seed-bearing plant (dominant) and a plant bearing green seeds (recessive), he observed that the offspring had the proportion of 3:1 for yellow seeds over the green ones. This led him to devise the fundamental rules of inheritance such as the Law of Segregation.

Mendel's experiments with pea plants transformed the fact that they gave empirical evidence for the theory that characteristics are inherited in the form of fixed units, now referred to as genes, and that alleles of genes for genes segregate during reproduction. This observation would be the foundation of the science of genetics problems monohybrid crosses  and the basis of monohybrid crosses.

Definition and Key Features of Monohybrid Cross

A monohybrid cross is the investigation of a single trait where a single gene determines the trait under investigation. Both parents give one allele for the gene in question, and offspring receive a combination of these alleles. The most important characteristic of a monohybrid cross is that it investigates how alleles of a single gene interact, often one dominant and one recessive allele. This cross allows us to make predictions about the probability of offspring showing particular traits depending on their parents' genotypes.

Suppose, in Mendel's pea plant experiment, the trait was seed color. Dominant allele Y of yellow seeds and recessive allele y of green seeds. Monohybrid cross between two pea plants of genotype "Yy" (heterozygous) would give rise to children of different genotypes and phenotypes, i.e., yellow and green seeds.

How to Carry Out a Monohybrid Cross?

It is very simple to perform a monohybrid cross and has only a couple of simple steps. The first would be being aware of the genotype of the parents because it will tell us what the parents will transfer to their offspring as alleles. The second would be finding out the possible genetic constitution of the offspring based on the parents' alleles. Lastly, through the use of programs like Punnett squares, the probability of different genotypes and phenotypes can be determined.

Steps to Conduct a Monohybrid Cross

1. Select the Traits to Investigate: Initial choice of the trait to be studied, i.e., color of the flower, seed, or height of the plant.

2. Make Parental Genotypes: Determine parental genotypes (genetic makeup). For instance, one is homozygous recessive (yy) and the other is homozygous dominant (YY).

3. Gamete Formation: Every parent will produce gametes (sperm or egg) with either of the two alleles of the gene. A parent who has the genotype "YY" will produce gametes containing only the "Y" allele and a parent with "yy" will produce gametes with only the "y" allele.

4. Fertilization: When both parents are undergoing fertilization, the two parent alleles come together and form the offspring genotype. Offspring from both parents receive one allele and a unique set of alleles.

5. Predict Offspring Genotypes and Phenotypes: Utilize a Punnett square to study the likelihood of offspring genotypes and phenotypes. It foresees the expected genetic results.

Using Punnett Squares to Predict Outcomes

A Punnett square is a genetic tool used in Mendelian genetics monohybrid cross studies. It helps predict possible genetic combinations in offspring. Punnett squares provide an easy means of showing and calculating the possible genetic results of a monohybrid cross. One parent's alleles are put along the top of a Punnett square, and the other parent's alleles are put along the side. Each offspring's genotype is created by the interaction of alleles within each box.

For instance, a cross of two heterozygous pea plants (Yy) would be this Punnett square:

The Punnett square given indicates that the offspring would have a 25% chance of being homozygous dominant (YY), a 50% chance of being heterozygous (Yy), and a 25% chance of being homozygous recessive (yy). This method is also applied in monohybrid and dihybrid cross problems with answers to determine inheritance probabilities.

Monohybrid Cross Example

Solved Example: Pea Plant Height

Let's attempt a simple example of monohybrid cross problems with answers with pea plant height. Tall (T) is dominant over short (t). Two heterozygous tall plants are mated (Tt). A Punnett square can be used to make predictions about the genotype and phenotype outcome in offspring.

From this cross, we can deduce the following probabilities:

• 50% of the offspring will be heterozygous (Tt) and tall.

• 25% will be homozygous dominant (TT) and tall.

• 25% will be homozygous recessive (tt) and short.

This cross gives a 75% chance for tall plants and a 25% chance for short plants.

Huntington’s Disease as a Monohybrid Inheritance Example

Huntington type of human genetic disease shows monohybrid inheritance. The disease is caused by the presence of dominant allele (H). Any individual that carries one dominant allele will be diseased, but anybody carrying two recessive alleles (hh) will not be diseased.

For instance, if a person with genotype "Hh" (heterozygous) marries another person with genotype "hh" (homozygous recessive), then the genotypes of their offspring will be:

The cross monohybrid inheritance  is 50% that the offspring inherit the Huntington's allele and become affected (Hh), and 50% that they inherit two recessive alleles and are unaffected (hh).

Dominant and Recessive Alleles in Monohybrid Crosses

There is a single dominant allele and a single recessive allele in a monohybrid cross. The dominant alleles express themselves in the phenotype with only one copy, whereas recessive alleles express themselves only if two copies are present.

Confirming Dominant Traits through Test Crosses

Test cross helps one determine the genotype of an organism whose phenotype is dominant. Cross the organism with another organism whose genotype is homozygous recessive (e.g., "tt" if one is working with pea plant height) to obtain this. Genotypes of offspring will help determine whether the dominant organism is heterozygous (Tt) or homozygous dominant (TT).

For instance, if a tall plant (T) of indeterminate genotype is crossed with a short plant (tt), and all the offspring are tall, then the tall parent will be homozygous dominant (TT). If offspring are as tall as and as short as, then the tall parent will be heterozygous (Tt).

Understanding Genotype vs. Phenotype

Genotype in genetics problems monohybrid crosses  refers to the genetic makeup of an organism (the two alleles one has), and phenotype refers to the external manifestation of the genes (the traits or characteristics one has). Phenotype in a monohybrid cross is the outcome of the interaction between alleles one has. For instance, in pea plants, genotype "YY" or "Yy" produces yellow seeds (phenotype), but genotype "yy" produces green seeds.

Dihybrid Cross vs. Monohybrid Cross

Dihybrid cross is an examination of two opposite characters at once, while monohybrid cross is an examination of a single character. Dihybrid cross is an examination of two independent genes and the cross produces a more complex pattern of inheritance.

Key Differences Between Monohybrid and Dihybrid Crosses

The only variation that exists between a mendelian genetics monohybrid and dihybrid crosses is the number of traits being viewed. A monohybrid cross is observing one trait being passed down, while a dihybrid cross is observing two traits being passed down.

A monohybrid cross will typically be 3:1 in phenotypes, and a dihybrid cross will typically be 9:3:3:1. This is due to the fact that, in a dihybrid cross, both of the two genes for each of the two traits separate independently of the other such that there are more combinations of traits among offspring.

Example from Pea Plants

In mendelian genetics monohybrid and dihybrid crosses,  The studies says two inherited characters —color (yellow and green) and shape (round and wrinkled) of seeds. Two heterozygotes of the two characters were crossed (YyRr), and it produced nine combinations of the color and shape of seeds in the ratio of 9:3:3:1. This kind of heredity indicates that genes for two characters can sort independently.

On the other hand, though, a monohybrid cross would be with one characteristic, say plant height or seed color, and would yield a less complex 3:1 dominant to recessive allele ratio.