Monohybrid Cross – Definition, Steps, Examples, Practice

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What is a Monohybrid Cross?

A monohybrid cross is a genetic cross between individuals who differ in only one trait or allele set. It is the study of inheritance patterns for a single trait, typically with a focus on a single gene with two opposing alleles.

In a monohybrid cross, two heterozygous (having two distinct alleles) individuals for a specific trait are crossed. One allele is inherited from each parent, and the progeny are observed to determine the inheritance and expression of the trait.

To illustrate a monohybrid cross, consider a simplified example of pea plant flower hue. Consider a flower-color gene with two alleles: “A” for purple flowers and “a” for white flowers. In this instance, a monohybrid cross would entail crossing two plants with the flower color phenotype “Aa” (heterozygous).

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The F1 generation will inherit one allele from each parent when these two plants are crossed. As the “A” allele is dominant over the “a” allele, the possible genotypes of the F1 generation will be “AA” (homozygous dominant) and “Aa” (heterozygous), while the phenotype will be purple flowers.

Based on the principles of Mendelian genetics, if the F1 generation plants are allowed to self-fertilize or are crossed with other plants of the same genotype, the resulting F2 generation will manifest a specific ratio of phenotypes and genotypes. In this situation, the phenotypic ratio will be 3:1, with three plants displaying purple flowers (AA or Aa) and one plant displaying white flowers (aa).

By analyzing the patterns of inheritance in monohybrid crosses, scientists can determine the underlying genetic principles involved in the transmission of traits from generation to generation. Monohybrid crosses are a fundamental tool for the study of genetics and can be used to predict the likelihood that certain traits will arise in future generations.

Monohybrid Cross Definition

A monohybrid cross is a genetic cross between two individuals that differ in only one trait or one set of alleles. It involves the study of inheritance patterns for a single characteristic, typically focusing on a single gene with two contrasting alleles. The purpose of a monohybrid cross is to observe how the trait is inherited and expressed in the offspring. By analyzing the patterns of inheritance in monohybrid crosses, scientists can understand how traits are passed from one generation to the next and determine the underlying genetic principles involved.

Steps of Monohybrid Cross

The steps involved in a monohybrid cross are as follows:

  1. Identify the traits: Determine the specific trait or characteristic of interest that you want to study. For example, consider flower color in pea plants.
  2. Define the alleles: Identify the two alleles that control the trait. Assign symbols to represent each allele. In our example, let’s use “A” to represent the dominant allele for purple flowers and “a” for the recessive allele for white flowers.
  3. Determine the parental generation: Select two individuals with different genotypes for the trait. For a monohybrid cross, choose parents that are heterozygous for the trait, meaning they have one dominant allele and one recessive allele. In our example, the parents would have the genotype “Aa.”
  4. Write the parental genotypes: Represent the genotypes of the parents using the allele symbols identified in step 2. In our example, write the genotypes of the parents as “Aa” and “Aa.”
  5. Determine possible gametes: Identify the possible types of gametes (sperm and egg cells) each parent can produce based on their genotype. In our example, both parents can produce two types of gametes: “A” and “a.”
  6. Create a Punnett square: Construct a Punnett square, which is a grid used to predict the possible genotypes and phenotypes of the offspring. Place the possible gametes of one parent along the top row of the grid and the possible gametes of the other parent along the left column.
  7. Fill in the Punnett square: Combine the gametes by matching each type of gamete from one parent with each type from the other parent. Fill in the squares of the Punnett square with the resulting combinations. This represents the possible genotypes of the offspring.
  8. Determine the genotypic and phenotypic ratios: Count the number of squares showing each genotype in the Punnett square. This will give you the genotypic ratio. Then, determine the phenotypes corresponding to each genotype and calculate the phenotypic ratio.
  9. Analyze the results: Interpret the genotypic and phenotypic ratios obtained from the Punnett square to understand how the trait is inherited and expressed in the offspring.

These steps help scientists predict the outcomes of a monohybrid cross and understand the principles of inheritance for a single characteristic controlled by a single gene.

Monohybrid Cross example

Gregor Mendel’s Peas Experiemnt

  • In his groundbreaking experiments with peas, Gregor Mendel used monohybrid crosses to identify dominant and recessive traits, although he was unaware of this at the time.
  • Gregor Mendel was interested in numerous genetic characteristics, but we will focus on stem length. Consider two varieties of pea plants growing in a garden. The stems of one variety of pea plant are longer than those of the other. Assume, for the purposes of this example, that both varieties of pea plants are homozygous (LL and ll) and that long stems (LL) are dominant over short stems (ll).
  • A monohybrid cross, or breeding a long-stemmed pea plant with a short-stemmed pea plant, permits scientists such as Gregor Mendel to determine whether long stems or short stems are dominant. A monohybrid cross also enables researchers to assess how heterozygous offspring manifest the inherited genes.
  • As was previously stated, breeding a long-stemmed pea plant with a short-stemmed pea plant results in progeny with a heterozygous genotype (Ll). As long stems are the dominant phenotype, all progeny will have long stems. Observing the progeny of a monohybrid cross enables the determination of dominant genotypes and, by extension, dominant phenotypes, as demonstrated by Gregor Mendel’s iconic pea experiment.

Huntington’s Disease

  • Huntington’s Disease is a progressive degenerative disorder that affects between 4 and 15 individuals per 100,000 in the United States. Without a cure, it is a death sentence for those who are diagnosed. Geneticists are certain that this disease is inherited through a dominant gene, despite the fact that little is known about it.
  • A monohybrid cross was used to determine the genetic nature of Huntington’s disease at the most basic level. Everyone possesses the gene responsible for the complication, aptly named Huntingtin. Using this information, scientists paired the Huntingtin genes of a person who is homozygous dominant for the disorder (HH) with the Huntingtin genes of a person who is homozygous recessive for the disorder (hh).
  • All progeny from the cross carried the dominant allele for Huntington’s disease, despite the fact that this example has been greatly abbreviated. This experiment, if conducted on humans, would deliver bad news to both the parent and the child, but it would also reveal the predominance of the disease.

Other Examples

Example 1: Flower color in pea plants

Trait: Flower color

Alleles: “A” for purple flowers (dominant) and “a” for white flowers (recessive)

Parental generation: Parent 1: Homozygous dominant (AA) for flower color Parent 2: Homozygous recessive (aa) for flower color

Punnett square:

A A

a Aa Aa

a Aa Aa

Resulting offspring (F1 generation): All offspring will have the genotype Aa and the phenotype of purple flowers because the dominant allele A masks the recessive allele a.

Example 2: Seed shape in pea plants

Trait: Seed shape

Alleles: “R” for round seeds (dominant) and “r” for wrinkled seeds (recessive)

Parental generation: Parent 1: Homozygous dominant (RR) for seed shape Parent 2: Homozygous recessive (rr) for seed shape

Punnett square:

R R

r Rr Rr

r Rr Rr

Resulting offspring (F1 generation): All offspring will have the genotype Rr and the phenotype of round seeds because the dominant allele R masks the recessive allele r.

Example 3: Hairline shape in humans

Trait: Hairline shape

Alleles: “W” for widow’s peak (dominant) and “w” for straight hairline (recessive)

Parental generation: Parent 1: Heterozygous (Ww) for hairline shape Parent 2: Heterozygous (Ww) for hairline shape

Punnett square:

W W

W WW Ww

w Ww ww

Resulting offspring (F1 generation): The genotypic ratio of the offspring will be 1 WW : 2 Ww : 1 ww, while the phenotypic ratio will be 3 individuals with a widow’s peak (WW or Ww) : 1 individual with a straight hairline (ww).

These examples demonstrate how monohybrid crosses can be used to predict the genotypic and phenotypic ratios of offspring based on the inheritance patterns of a single trait controlled by a single gene.

Advantages of Monohybrid Cross

  • Simplicity: Monohybrid crosses involve the study of a single trait, making them relatively straightforward to understand and analyze compared to more complex crosses involving multiple traits.
  • Clear Mendelian Inheritance Patterns: Monohybrid crosses allow for the clear observation of Mendelian inheritance patterns, such as the segregation of alleles and the dominance of certain traits over others. This simplicity helps in establishing basic principles of inheritance.
  • Predictability: Monohybrid crosses enable the prediction of genotypic and phenotypic ratios in offspring, providing a level of predictability in understanding trait inheritance. This is particularly useful in selective breeding programs and genetic counseling.
  • Basic Building Block: Monohybrid crosses serve as a foundation for understanding more complex genetic concepts, such as dihybrid crosses and other patterns of inheritance. They provide a starting point for comprehending genetic principles and establishing a solid understanding of genetics.

Limitations of Monohybrid Cross

  • Limited Scope: Monohybrid crosses focus on the inheritance of a single trait, which may not capture the complexity of genetic interactions and inheritance patterns that occur in real-world scenarios. Many traits are influenced by multiple genes and can exhibit more intricate inheritance patterns.
  • Lack of Consideration for Linked Genes: Monohybrid crosses do not account for the phenomenon of genetic linkage, where genes located close to each other on the same chromosome tend to be inherited together. This limitation overlooks the possibility of linked genes affecting trait inheritance.
  • Oversimplification of Genetics: Monohybrid crosses provide a simplified view of genetics and may not fully represent the complexities of genetic inheritance and gene interactions. Real-world genetics involves the interplay of multiple genes, environmental factors, and epigenetic influences that are not accounted for in monohybrid crosses.
  • Incomplete Understanding of Phenotypic Variations: Monohybrid crosses do not address the full range of phenotypic variations observed in nature. Many traits are influenced by a combination of genetic and environmental factors, and monohybrid crosses alone may not provide a comprehensive understanding of these variations.
  • Limited Application to Complex Traits: Monohybrid crosses may not be suitable for studying complex traits that are controlled by multiple genes and exhibit polygenic inheritance. Complex traits, such as height or intelligence, require the consideration of multiple genes and their interactions, which monohybrid crosses cannot adequately address.

Uses of Monohybrid Cross

  • Understanding Mendelian Inheritance: Understanding Mendelian Inheritance Gregor Mendel used monohybrid crosses to establish the principles of inheritance in his experiments with pea plants. Mendel formulated his laws of segregation and dominance by observing the patterns of trait transmission across generations.
  • Determining Dominant and Recessive Traits: Monohybrid crosses are used to determine whether a trait is dominant or recessive. By crossing individuals with known traits and observing the phenotypes of their progeny, scientists can determine whether a trait is dominant or recessive.
  • Predicting Genotypic and Phenotypic Ratios: Monohybrid crosses enable for the prediction of the genotypic and phenotypic ratios of the offspring. Scientists can estimate the likelihood of particular genotypes and phenotypes arising in the offspring of a cross using Punnett squares or mathematical calculations.
  • Selective Breeding: In selective breeding programs, monohybrid crosses are utilized to produce progeny with desired traits. Breeders can increase the frequency of desirable traits in subsequent generations by selectively mating individuals with particular phenotypes.
  • Genetic Counseling: Monohybrid crosses can be utilized in genetic counseling to determine the likelihood of inheriting particular genetic disorders or traits. By analyzing the genotypes of parents and employing monohybrid crosses, genetic counselors can determine the likelihood of passing on particular traits to progeny.
  • Plant and Animal Breeding: In plant and animal breeding programs, monohybrid crosses are utilized to enhance the quality and characteristics of crops and livestock. Breeders can create offspring with enhanced traits, such as increased yield, disease resistance, or specific physical characteristics, by meticulously selecting parent individuals with desired traits and conducting monohybrid crosses.
  • Evolutionary Studies: Monohybrid crosses can provide insights into the genetic diversity and evolutionary processes of populations for evolutionary studies. By analyzing the distribution of traits within a population and examining inheritance patterns, scientists can obtain a better understanding of how traits are transmitted and how they may evolve over time.

Monohybrid Cross vs Dihybrid Cross – What is the differences between Cross and Dihybrid Cross?

DifferencesMonohybrid CrossDihybrid Cross
Number of TraitsInvolves the study of a single traitInvolves the study of two different traits simultaneously
Parental GenerationInvolves two purebred parents for one traitInvolves two purebred parents for two different traits
GametesOnly one type of gamete is producedTwo different types of gametes are produced
Punnett SquareRequires a 2×2 Punnett squareRequires a 4×4 Punnett square
Genotype PossibilitiesProduces three possible genotypesProduces nine possible genotypes
Phenotype PossibilitiesProduces two possible phenotypesProduces four possible phenotypes
AllelesDeals with one pair of allelesDeals with two pairs of alleles
Law of SegregationDemonstrates the segregation of alleles during gamete formationDemonstrates the segregation of alleles for two different traits during gamete formation
Law of Independent AssortmentNot applicableDemonstrates the independent assortment of alleles for two different traits during gamete formation
Inheritance PatternsShows patterns of dominance, recessiveness, or co-dominance for a single traitShows patterns of dominance, recessiveness, or co-dominance for two different traits
ExampleCrossing two purebred tall and short pea plantsCrossing two purebred yellow and green pea plants for seed color and roundness
Genetic RatioProduces a 1:2:1 genotypic ratioProduces a 1:2:2:4:1:2:1:2:1 genotypic ratio
Phenotypic RatioProduces a 3:1 phenotypic ratioProduces a 9:3:3:1 phenotypic ratio
ApplicationUsed to study inheritance patterns of a single traitUsed to study inheritance patterns of multiple traits simultaneously
Mendel’s ExperimentsMendel’s experiments on pea plants were primarily monohybrid crossesMendel’s experiments on pea plants involved both monohybrid and dihybrid crosses

Monohybrid cross Practice

In pea plants, the gene for flower color has two alleles, P (purple) and p (white). If a plant with the genotype Pp is crossed with a plant with the genotype pp, what will be the phenotypic ratio of the offspring?
a) 1:1 purple to white
b) 3:1 purple to white
c) 1:2:1 purple to white
d) 1:3 purple to white

In cats, the gene for fur color has two alleles, B (black) and b (brown). If a heterozygous black cat (Bb) is crossed with a brown cat (bb), what is the probability of the offspring being black?
a) 0%
b) 25%
c) 50%
d) 75%

In corn plants, the gene for kernel texture has two alleles, S (starchy) and s (sweet). If a homozygous starchy corn plant (SS) is crossed with a homozygous sweet corn plant (ss), what will be the genotypic ratio of the offspring?
a) 1:0 starchy to sweet
b) 1:1 starchy to sweet
c) 3:1 starchy to sweet
d) 1:2:1 starchy to sweet

In mice, the gene for coat color has two alleles, C (black) and c (white). If a heterozygous black mouse (Cc) is crossed with a white mouse (cc), what will be the phenotypic ratio of the offspring?
a) 1:0 black to white
b) 1:1 black to white
c) 3:1 black to white
d) 1:2:1 black to white

In chickens, the gene for comb shape has two alleles, R (rose comb) and r (single comb). If two heterozygous chickens (Rr) are crossed, what is the probability of the offspring having a rose comb?
a) 0%
b) 25%
c) 50%
d) 75%

In fruit flies, the gene for eye color has two alleles, B (red eyes) and b (white eyes). If a heterozygous fruit fly (Bb) is crossed with a white-eyed fruit fly (bb), what will be the genotypic ratio of the offspring?
a) 1:0 red eyes to white eyes
b) 1:1 red eyes to white eyes
c) 3:1 red eyes to white eyes
d) 1:2:1 red eyes to white eyes

In rabbits, the gene for fur texture has two alleles, F (long fur) and f (short fur). If a homozygous long-furred rabbit (FF) is crossed with a homozygous short-furred rabbit (ff), what will be the phenotypic ratio of the offspring?
a) 1:0 long fur to short fur
b) 1:1 long fur to short fur
c) 3:1 long fur to short fur
d) 1:2:1 long fur to short fur

In roses, the gene for flower scent has two alleles, S (fragrant) and s (non-fragrant). If a homozygous fragrant rose (SS) is crossed with a non-fragrant rose (ss), what is the probability of the offspring being fragrant?
a) 0%
b) 25%
c) 50%
d) 100%

In horses, the gene for coat color has two alleles, C (chestnut) and c (black). If a homozygous chestnut horse (CC) is crossed with a black horse (cc), what will be the genotypic ratio of the offspring?
a) 1:0 chestnut to black
b) 1:1 chestnut to black
c) 3:1 chestnut to black
d) 1:2:1 chestnut to black

In dogs, the gene for tail length has two alleles, T (long tail) and t (short tail). If a heterozygous long-tailed dog (Tt) is crossed with a short-tailed dog (tt), what will be the phenotypic ratio of the offspring?
a) 1:0 long tail to short tail
b) 1:1 long tail to short tail
c) 3:1 long tail to short tail
d) 1:2:1 long tail to short tail

In sunflowers, the gene for petal color has two alleles, Y (yellow) and y (red). If a homozygous yellow sunflower (YY) is crossed with a heterozygous yellow sunflower (Yy), what will be the phenotypic ratio of the offspring?
a) 1:0 yellow to red
b) 1:1 yellow to red
c) 3:1 yellow to red
d) 1:2:1 yellow to red

In tomatoes, the gene for fruit shape has two alleles, R (round) and r (oval). If a heterozygous round tomato plant (Rr) is crossed with an oval tomato plant (rr), what is the probability of the offspring having round fruits?
a) 0%
b) 25%
c) 50%
d) 75%

In butterflies, the gene for wing pattern has two alleles, W (striped) and w (spotted). If a homozygous striped butterfly (WW) is crossed with a heterozygous striped butterfly (Ww), what will be the genotypic ratio of the offspring?
a) 1:0 striped to spotted
b) 1:1 striped to spotted
c) 3:1 striped to spotted
d) 1:2:1 striped to spotted

In peas, the gene for seed shape has two alleles, S (smooth) and s (wrinkled). If a smooth-seeded pea plant (SS) is crossed with a wrinkled-seeded pea plant (ss), what will be the phenotypic ratio of the offspring?
a) 1:0 smooth to wrinkled
b) 1:1 smooth to wrinkled
c) 3:1 smooth to wrinkled
d) 1:2:1 smooth to wrinkled

In mice, the gene for coat length has two alleles, L (long) and l (short). If a heterozygous long-haired mouse (Ll) is crossed with a short-haired mouse (ll), what is the probability of the offspring having long hair?
a) 0%
b) 25%
c) 50%
d) 75%

Answers:

b) 3:1 purple to white
c) 50%
c) 3:1 starchy to sweet
c) 3:1 black to white
c) 50%

b) 1:1 red eyes to white eyes
b) 1:1 long fur to short fur
c) 50%
b) 1:1 chestnut to black
c) 3:1 long tail to short tail
c) 3:1 yellow to red
b) 25%
c) 3:1 striped to spotted
c) 3:1 smooth to wrinkled
b) 25%

FAQ

What is a monohybrid cross?

A monohybrid cross is a genetic cross between two individuals that differ in a single trait. It involves studying the inheritance of one specific trait or gene.

What is the purpose of a monohybrid cross?

The purpose of a monohybrid cross is to understand the inheritance pattern of a single trait and predict the genotypic and phenotypic ratios of the offspring.

How is a monohybrid cross represented?

A monohybrid cross is often represented using a Punnett square, where the possible combinations of alleles from the parental generation are used to determine the genotypes and phenotypes of the offspring.

What are the expected genotypic and phenotypic ratios in a monohybrid cross?

In a monohybrid cross between two heterozygous individuals, the expected genotypic ratio is 1:2:1 (homozygous dominant:heterozygous:homozygous recessive). The expected phenotypic ratio is 3:1 (dominant phenotype: recessive phenotype).

How are dominant and recessive traits determined in a monohybrid cross?

Observing the phenotypes of the offspring in a monohybrid cross can help determine whether a trait is dominant or recessive. The presence of the dominant phenotype indicates the dominance of that trait.

Can monohybrid crosses be used to study human traits?

Yes, monohybrid crosses can be used to study human traits, particularly those that follow Mendelian patterns of inheritance. However, ethical considerations and practical limitations restrict direct experimentation in humans, so human traits are often studied indirectly through pedigree analysis.

Are monohybrid crosses limited to only two alleles?

No, monohybrid crosses can involve more than two alleles, but the simplest cases involve only two alleles for a given trait.

Can monohybrid crosses be used to study traits influenced by multiple genes?

Monohybrid crosses are not suitable for studying traits influenced by multiple genes, as they focus on the inheritance of a single gene. Traits influenced by multiple genes require more complex genetic analyses.

How are monohybrid crosses useful in selective breeding?

Monohybrid crosses help breeders predict the inheritance of specific traits in offspring, enabling them to selectively breed individuals with desired characteristics. This allows for the amplification of desirable traits in subsequent generations.

Can monohybrid crosses explain all types of genetic inheritance?

No, monohybrid crosses provide a simplified model for understanding basic patterns of inheritance. More complex forms of inheritance, such as codominance, incomplete dominance, and polygenic traits, require more advanced genetic analyses beyond monohybrid crosses.

References

  • Monohybrid Crosses and Segregation. (2021, August 12). University of Arkansas at Little Rock. https://bio.libretexts.org/@go/page/25731
  • https://www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/monohybrid-cross
  • https://biologydictionary.net/monohybrid-cross/
  • https://www.aakash.ac.in/important-concepts/biology/monohybrid-cross-inheritance-one-gene
  • https://www.thoughtco.com/monohybrid-cross-a-genetics-definition-373473
  • http://www.biology.arizona.edu/mendelian_genetics/problem_sets/monohybrid_cross/01q.html
  • https://thebiologynotes.com/monohybrid-cross/
  • http://www.biology.arizona.edu/mendelian_genetics/problem_sets/monohybrid_cross/01t.html
  • https://www.khanacademy.org/science/in-in-class-10-biology/in-in-heredity-and-evolution/in-in-heredity-mendels-experiment/e/monohybrid-cross
  • https://study.com/academy/lesson/monohybrid-cross-definition-example-quiz.html
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