What is Mendel’s Law of Dominance?

Mendel’s Law of Dominance is a fundamental principle in genetics that was derived from the experiments conducted by Gregor Mendel on pea plants. This law provides insights into how certain traits are expressed in offspring when two homozygous organisms with contrasting characters are crossed.

Mendel’s Law of Dominance Explained:

1. Basic Understanding: At the core, Mendel’s Law of Dominance asserts that when two contrasting characters (traits) are present, only one gets expressed in the first generation. This expressed trait is termed as the ‘dominant’ trait, while the other, which remains unexpressed, is the ‘recessive’ trait.
2. Genetic Control: Every character in an organism is governed by distinct units termed as ‘factors’ or genes. These genes come in pairs, and each pair can either be homozygous (similar genes) or heterozygous (different genes). In heterozygous pairs, one gene dominates the other, leading to the expression of the dominant character.
3. Transmission to Offspring: Even though the recessive character remains latent in the first generation, it is passed on to the next generation in the same manner as the dominant character. The recessive trait becomes evident only when the offspring inherit two copies of the recessive allele, resulting in a homozygous recessive individual.
4. Fertilization Process: The two alleles responsible for a trait combine during fertilization. One allele is contributed by the maternal gamete, and the other comes from the paternal gamete.
5. Dominance in Genotype: It’s crucial to understand that the concept of dominance is applied strictly to genotypic characters and doesn’t necessarily depict the phenotype (observable characteristics) of the individual.
6. Exceptions and Evolutions: While Mendel’s Law of Dominance has been foundational in genetics, subsequent research has indicated that this law doesn’t always apply universally. There are other patterns of inheritance that have been discovered, suggesting the intricate nature of genetic inheritance.

Further Insights from Mendel’s Experiments: Upon analyzing results from thousands of pea plants, Mendel discerned that traits could be categorized into dominant and recessive. Dominant traits manifest in hybrid combinations, while recessive traits remain latent in hybrid offspring but reemerge in the subsequent generation. For instance, in Mendel’s experiments, violet-flower trait was dominant over the white-flower trait.

Dominance in Practice: A practical example can be seen in the case of albinism, a recessive trait. The principle of dominance elucidates that in a heterozygote (an organism with two different alleles for a trait), only the dominant allele gets expressed. The recessive allele, though latent, is passed on to the offspring in the same manner as the dominant allele. The recessive trait becomes evident only when the offspring inherit two copies of the recessive allele.

Examples of Dominant and Recessive Traits in Humans:

• Dominant Traits: Achondroplasia, Brachydactyly, Huntington’s disease, Marfan syndrome, Neurofibromatosis, Widow’s peak, Wooly hair.
• Recessive Traits: Albinism, Cystic fibrosis, Duchenne muscular dystrophy, Galactosemia, Phenylketonuria, Sickle-cell anemia, Tay-Sachs disease.

In summary, Mendel’s Law of Dominance offers a foundational understanding of how traits are inherited and expressed in organisms. However, as with many scientific principles, it’s essential to recognize its limitations and the complexities of genetic inheritance.

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Mendel’s Law of Dominance Definition

Mendel’s Law of Dominance is a fundamental principle in the field of genetics, formulated by Gregor Mendel, an Austrian scientist often referred to as the “father of modern genetics.” This law is one of Mendel’s three foundational principles, collectively known as Mendelian genetics, which laid the groundwork for our understanding of inheritance and the passing on of traits from one generation to the next.

Mendel’s Law of Dominance states that when two different alleles (alternative forms of a gene) are present in an individual’s genetic makeup, one allele (the dominant allele) will be expressed in the phenotype (observable traits) of the organism, while the other allele (the recessive allele) will not be visibly expressed in the presence of the dominant allele. In other words, the dominant allele masks the expression of the recessive allele.

For example, consider the case of flower color in pea plants. Mendel’s experiments showed that when a plant with a dominant allele for purple flower color (P) is crossed with a plant with a recessive allele for white flower color (p), the offspring (F1 generation) will all have purple flowers. This is because the dominant allele (P) masks the expression of the recessive allele (p).

However, in the F1 generation, the recessive allele (p) is not lost; it is simply hidden. When the F1 plants are allowed to self-pollinate or are crossed with each other, their offspring (F2 generation) will show a 3:1 ratio of purple to white flowers, indicating that the recessive allele has reappeared.

Mendel’s Law of Dominance is a key concept in genetics and inheritance, providing insights into how genetic traits are passed from one generation to the next and helping to explain the observed patterns of inheritance in various organisms.

Mendel’s Law of Dominance Characteristics

Mendel’s Law of Dominance is a foundational principle in genetics, elucidating how certain traits are inherited and expressed in offspring. This law provides insights into the behavior of genes and their role in determining the characteristics of organisms. Here are the key characteristics of Mendel’s Law of Dominance:

1. Origin of Dominance: The idea of dominance stems from the concept of factors, now known as genes, which are responsible for transmitting traits from parents to offspring.
2. Role of Genes: Genes serve as the primary units of inheritance. These genes, present within chromosomes, dictate the expression of various traits based on their interactions with other genes.
3. Diploid Cells and Chromosomes: In diploid organisms, cells contain paired chromosomes. Each chromosome in this pair is inherited from one of the two parents, ensuring a mix of genetic material in the offspring.
4. Alleles Represent Characters: Every trait or character is represented by an allele. Given that there are pairs of characters, there are consequently two alleles for each pair.
5. Homozygosity: Homozygous individuals possess two identical alleles for a particular trait. For instance, in pea plants, a homozygous tall plant would have the alleles TT, while a homozygous dwarf plant would have the alleles tt.
6. Gametogenesis and Allele Separation: During the process of gametogenesis, the homozygous chromosomes segregate, ensuring that each gamete receives only one allele, either T or t.
7. Fertilization and Trait Combination: As fertilization occurs, two gametes merge, giving rise to an offspring that inherits alleles from both parents. This offspring, having two different genes for a trait, is termed a heterozygote or hybrid.
8. Expression of Dominance: In the context of pea plants, even though the hybrid offspring inherits genes for both tallness and dwarfness, they exhibit only the tall trait. This observation led Mendel to conclude that tallness is a dominant trait, while dwarfness is recessive.

In essence, Mendel’s Law of Dominance offers a comprehensive understanding of how certain traits overshadow others in hybrid organisms. It underscores the intricate dance of genes and alleles, highlighting the dominant-recessive dynamics that shape the phenotypic expressions in organisms. This law, derived from meticulous observations and experiments, remains a cornerstone in the field of genetics.

Examples of Mendel’s Law of Dominance

Mendel’s Law of Dominance is exemplified through various instances of genetic inheritance observed in different organisms. Two classic examples that illustrate this law are Guinea Pigs and Pea Plants.

1. Guinea Pigs: In the case of guinea pigs, Mendel’s Law of Dominance is evident through the inheritance of coat color. When a homozygous black guinea pig (BB) is crossed with a homozygous white guinea pig (bb), the F1 generation comprises only black hybrid offspring. In this scenario, black is the dominant trait, while white is the recessive trait. When these black hybrids (Bb) are interbred, they give rise to both black and white offspring in a 3:1 ratio. This outcome reaffirms the dominance of the black color allele. Interestingly, a single white guinea pig emerges in the F2 generation, which is homozygous recessive (bb).

2. Pea Plants: Mendel’s experiments with pea plants offer another illustration of the Law of Dominance. Within this context, Mendel examined various traits, including stem length, seed color, and cotyledon shape. Focusing on stem length as an example, he worked with two opposing traits: tallness and dwarfness. The homozygous alleles for tallness were represented as TT, while those for dwarfness were represented as tt. Upon fertilization, the resulting hybrid plants could have genotypes TT, Tt, or tt. The phenotypic ratio of the hybrid offspring was observed to be 3:1, while the genotypic ratio was 1:2:1.

In this case, tallness is the dominant character, overriding the expression of dwarfness. While both alleles contribute to the genotype of the heterozygous individuals (Tt), the phenotype remains tall due to the dominance of the tallness allele. Dwarfness is only expressed when present in a homozygous condition (tt).

These examples vividly demonstrate Mendel’s Law of Dominance, which states that in a heterozygous individual carrying two different alleles for a trait, one allele (the dominant allele) will mask the expression of the other allele (the recessive allele) in determining the organism’s phenotype. This law underscores the fundamental principles of genetic inheritance and has played a pivotal role in shaping our understanding of genetics.

More Examples

1. Pea Plant Flower Color:
   • Pure red flower (RR) x Pure white flower (rr): When two pea plants, one with red flowers and the other with white flowers, are crossbred, all the offspring will have red flowers.
   • Dominance: The red color is dominant, meaning that even if an offspring inherits one red and one white gene (Rr), it will still display the red color.
2. Pea Plant Seed Shape:
   • Round seed (RR) x Wrinkled seed (rr): Crossing a plant with round seeds and a plant with wrinkled seeds results in all offspring having round seeds.
   • Dominance: The round shape is dominant over the wrinkled shape, so hybrids (Rr) will have round seeds.
3. Pea Plant Seed Color:
   • Yellow seed (YY) x Green seed (yy): When a yellow-seeded plant is crossed with a green-seeded plant, all the resulting offspring will have yellow seeds.
   • Dominance: Yellow is the dominant color, so even hybrids (Yy) will display yellow seeds.
4. Pea Plant Pod Shape:
   • Smooth pod (SS) x Constricted pod (ss): A cross between plants with smooth pods and those with constricted pods will produce offspring with only smooth pods.
   • Dominance: The smooth pod shape is dominant, so hybrids (Ss) will have smooth pods.
5. Pea Plant Pod Color:
   • Green pod (GG) x Yellow pod (gg): Crossing a green-podded plant with a yellow-podded plant results in offspring with green pods.
   • Dominance: Green is the dominant color, so hybrids (Gg) will have green pods.
6. Pea Plant Stem Length:
   • Tall stem (TT) x Short stem (tt): When a tall pea plant is crossed with a short one, all the offspring will be tall.
   • Dominance: Tallness is dominant over shortness, so hybrids (Tt) will be tall.
7. Tongue Rolling in Humans:
   • Some people can roll their tongues into a tube shape, while others can’t. This ability is determined by genetics.
   • Dominance: The ability to roll the tongue is dominant. So, individuals with either two dominant genes (RR) or one dominant and one recessive gene (Rr) can roll their tongues.
8. Earlobe Attachment in Humans:
   • Earlobes can be either freely hanging or attached directly to the side of the head.
   • Dominance: Having free-hanging earlobes is dominant over attached earlobes. Individuals with the genotype FF or Ff will have free-hanging earlobes.
9. Vision in Humans:
   • Colorblindness is a genetic condition where an individual cannot distinguish certain colors.
   • Dominance: Normal vision is dominant over colorblindness. Individuals with the genotype NN or Nn will have normal vision, while those with nn may be colorblind.
10. Hair Type in Humans:

• Hair can be naturally curly, wavy, or straight.
• Dominance: Curly hair is dominant over straight hair. So, individuals with the genotype CC or Cc will have curly hair, while those with cc will have straight hair.

In each of these examples, the dominant trait masks the presence of the recessive trait in the hybrid offspring. This is the essence of Mendel’s Law of Dominance.

Limitations of Mendel’s Law of Dominance

Mendel’s Law of Dominance is one of the foundational principles of genetics, derived from Gregor Mendel’s experiments with pea plants in the mid-19th century. According to this law, in a heterozygous organism (an organism with two different alleles for a trait), one allele can mask the expression of the other allele. The allele that masks the expression of the other is termed dominant, while the one that gets masked is termed recessive.

However, as genetics has advanced, several limitations and exceptions to Mendel’s Law of Dominance have been identified:

1. Incomplete Dominance: In some cases, neither allele is completely dominant over the other. Instead, the heterozygous phenotype is intermediate between the two homozygous phenotypes. For example, in snapdragons, a cross between a red-flowered plant and a white-flowered plant produces pink-flowered offspring.
2. Codominance: In codominance, both alleles are expressed equally in the phenotype of the heterozygote. A classic example is the ABO blood group system in humans, where individuals with both IA and IB alleles have type AB blood, which expresses both A and B antigens.
3. Multiple Alleles: Some genes have more than two alleles in a population. Again, the ABO blood group system is an example, with three alleles (IA, IB, and i) determining blood type.
4. Epistasis: Sometimes, the expression of one gene can mask or modify the expression of another gene. This phenomenon is called epistasis. For instance, in mice, a gene might determine fur color, but another gene can determine whether or not fur is actually produced. If the second gene prevents fur production, the effects of the first gene won’t be observed.
5. Polygenic Traits: Many traits are controlled by more than one gene, where each gene might have a small effect on the overall phenotype. These are called polygenic traits. Examples include height, skin color, and intelligence in humans.
6. Environmental Influence: The expression of genes can be influenced by environmental factors. For instance, the fur color of the Himalayan rabbit is determined by temperature. The colder parts of its body (ears, nose, feet) become darker.
7. Pleiotropy: This refers to a situation where a single gene affects more than one trait. For example, in humans, mutations in the PKD1 gene can cause polycystic kidney disease, but they can also affect other organs and lead to symptoms outside the kidneys.
8. Linkage: Mendel’s laws assume that all genes assort independently. However, genes located close together on the same chromosome tend to be inherited together, a phenomenon known as linkage. This can affect the expected outcomes of genetic crosses.
9. Gene Interaction: Sometimes, two or more genes can interact in a way that influences the phenotype. This can lead to non-Mendelian inheritance patterns.
10. Penetrance and Expressivity: Penetrance refers to the proportion of individuals with a particular genotype that actually displays the phenotype associated with that genotype. Expressivity is the degree to which a genotype is expressed in an individual. Both can vary, leading to differences in phenotypic expression even among individuals with the same genotype.

While Mendel’s laws laid the groundwork for the field of genetics, these exceptions and complexities show that inheritance patterns can be more intricate than what Mendel initially described.

Examples of dominant and recessive traits in humans

Human traits are determined by the combination of genes inherited from our parents. These genes can carry different forms, known as alleles, which can be dominant or recessive. Dominant traits are expressed when at least one copy of the dominant allele is present, while recessive traits are only expressed when two copies of the recessive allele are present.

Dominant Traits:

1. Achondroplasia: This is a type of dwarfism caused by a dominant allele. Individuals with one copy of the dominant allele will exhibit the trait, resulting in short stature and disproportionate limbs.
2. Albinism: Albinism is a condition where individuals lack pigment in their skin, hair, and eyes due to a faulty allele. Even a single copy of the dominant allele can lead to albinism.
3. Brachydactyly: People with this trait have shorter fingers and toes due to a dominant allele. It results in a characteristic appearance of the hands and feet.
4. Huntington’s Disease: This is a neurodegenerative disorder caused by a dominant allele. It leads to motor dysfunction, cognitive decline, and emotional disturbances later in life.
5. Marfan Syndrome: Marfan syndrome is caused by a dominant allele and affects the connective tissues in the body. It can result in tall stature, long limbs, and various cardiovascular issues.
6. Widow’s Peak: A dominant allele for a widow’s peak hairline creates a V-shaped point in the middle of the forehead.
7. Wooly Hair: This trait is characterized by tightly curled hair due to a dominant allele. Individuals with one copy of the allele will have this hair texture.

Recessive Traits:

1. Cystic Fibrosis: This is a life-threatening genetic disorder caused by two copies of a recessive allele. It affects the respiratory and digestive systems, leading to mucus buildup and related complications.
2. Duchenne Muscular Dystrophy: A recessive allele responsible for this disorder leads to progressive muscle weakening and loss. It predominantly affects males.
3. Galactosemia: Individuals with two recessive alleles for galactosemia are unable to metabolize galactose, a sugar present in milk. This can lead to severe health problems if not managed.
4. Neurofibromatosis: A person needs two copies of the recessive allele to develop this disorder characterized by tumors growing on nerve tissue.
5. Phenylketonuria (PKU): PKU is caused by two recessive alleles and affects the body’s ability to process the amino acid phenylalanine. If untreated, it can lead to intellectual disabilities.
6. Sickle-Cell Anemia: This recessive trait affects hemoglobin, causing red blood cells to become misshapen. It provides some resistance to malaria when inherited in one copy (known as sickle-cell trait).
7. Tay-Sachs Disease: Individuals with two recessive alleles for Tay-Sachs disease lack an enzyme necessary to break down certain fatty substances in the brain and nervous system, leading to severe neurological deterioration.

In summary, dominant and recessive traits in humans are determined by the presence of specific alleles. Dominant traits are expressed even when just one copy of the allele is present, while recessive traits require both copies to be present for expression. These traits contribute to the rich diversity seen in the human population and play a vital role in genetics and inheritance.

Dominant Traits	Recessive Traits
Achondroplasia	Cystic Fibrosis
Albinism	Duchenne Muscular Dystrophy
Brachydactyly	Galactosemia
Huntington’s Disease	Neurofibromatosis
Marfan Syndrome	Phenylketonuria (PKU)
Widow’s Peak	Sickle-Cell Anemia
Wooly Hair	Tay-Sachs Disease

Facts on Mendel’s Law of Dominance

1. Pioneer of Genetics: Gregor Mendel, an Austrian monk, is known as the “Father of Genetics.” He conducted his experiments on pea plants in the mid-19th century and formulated the laws of inheritance.
2. Basic Principle: The Law of Dominance states that in a cross of parents that are pure for contrasting traits, only one form of the trait will appear in the next generation. This dominant trait will mask the presence of the other, recessive trait.
3. Pea Plant Experiments: Mendel chose pea plants for his experiments because they have easily distinguishable characteristics, such as tall or short height, yellow or green seeds, and so on.
4. Homozygous and Heterozygous: If an individual has two identical alleles for a trait, they are homozygous for that trait. If they have two different alleles, they are heterozygous. A heterozygous individual will express the dominant trait.
5. Recessive Traits: Recessive traits are not expressed in the presence of a dominant allele. They only appear when an individual has two copies of the recessive allele (homozygous recessive).
6. Genotypic and Phenotypic Ratios: When Mendel crossed two heterozygous individuals (for a particular trait), the resulting offspring had a genotypic ratio of 1:2:1 (homozygous dominant: heterozygous: homozygous recessive) and a phenotypic ratio of 3:1 (dominant trait: recessive trait).
7. Not Always Simple: While Mendel’s experiments with pea plants showed clear dominant and recessive traits, many traits in nature are influenced by multiple genes and can have incomplete dominance or codominance.
8. Mendel’s Three Laws: The Law of Dominance is just one of Mendel’s three laws of inheritance. The other two are the Law of Segregation and the Law of Independent Assortment.
9. Rediscovery: Mendel’s work was largely ignored during his lifetime. It was only around the turn of the 20th century that scientists rediscovered his research and recognized its significance.
10. Modern Genetics: Mendel’s laws laid the groundwork for the field of genetics. Today, with advances in DNA sequencing and molecular biology, scientists have a much deeper understanding of inheritance, but Mendel’s principles still hold true for many simple genetic traits.

Latest Research Work on Mendel’s Law of Dominance

1. Evolution of Mendelian dominance in gene regulatory networks associated with phenotypic robustness
   • Authors: Kenji Okubo, Kunihiko Kaneko
   • Publication Year: 2021
   • Abstract: Mendelian inheritance is a fundamental law of genetics. This research discusses the evolution of dominance in gene regulatory networks, especially in cases beyond the classical one-to-one mapping. The study reveals that dominance can be achieved even in complex gene regulatory networks and is associated with increased robustness against genome mixing during meiosis.
   • Link to the paper
2. Important heterosis on hybridization
   • Publication Year: 2021
   • Abstract: This paper discusses the genetic understanding of heterosis following the rediscovery of Mendel’s laws in 1900. It delves into the genetic explanations for the increased heterozygosity in hybrids and the dominance hypothesis, which suggests that hybrids are stronger because recessive genes from one parent are usually concealed by dominant genes from the other.
   • Link to the paper
3. The genetic architecture of the maize progenitor, teosinte, and how it was altered during maize domestication
   • Authors: Qiuyue Chen, Luis Fernando Samayoa, Chin Jian Yang, and others.
   • Publication Year: 2020
   • Abstract: This research studies the genetics of domestication, especially focusing on maize and its ancestor, teosinte. The study observed a reduced number of QTL in maize compared to teosinte and a higher dominance in maize, likely due to the selective removal of certain variants during domestication.
   • Link to the paper

Mendel’s Law of Dominance Worksheet

1. Definitions: a. Dominant Allele: _________________ b. Recessive Allele: _________________
2. True or False: a. A dominant allele will always be expressed in the phenotype. ( ) b. Recessive alleles can only be expressed when two copies are present. ( ) c. If an organism has two dominant alleles for a trait, it will not express that trait. ( )
3. Fill in the Blanks: a. In pea plants, the allele for tall plants (T) is _________ over the allele for short plants (t). b. An organism with the genotype Tt will have a _________ phenotype. c. An organism with the genotype tt will have a _________ phenotype.
4. Match the Genotype to the Phenotype: a. TT – _________ b. Tt – _________ c. tt – _________Options: i. Tall ii. Short
5. Scenario: In a population of flowers, the red color (R) is dominant over the white color (r).a. What is the phenotype of a flower with the genotype Rr? _________________ b. What is the phenotype of a flower with the genotype rr? _________________ c. If two Rr flowers are crossed, what is the probability that their offspring will be white? _________________

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Answers:

1. a. An allele that expresses itself even when only one copy is present. b. An allele that can only express itself when two copies are present.
2. a. True b. True c. False
3. a. Dominant b. Tall c. Short
4. a. Tall b. Tall c. Short
5. a. Red b. White c. 25%

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FAQ

References

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   • Authors: V. A. McKusick
   • Publication Year: 1998
   • Abstract: This paper provides a comprehensive catalog of human genes and genetic disorders, emphasizing the Mendelian inheritance patterns.
   • Link to the paper
2. Mendel’s legacy: The origin of classical genetics.
   • Authors: J. Sapp
   • Publication Year: 2002
   • Abstract: This paper delves into the origins of classical genetics, tracing back to Mendel’s foundational work.
   • Link to the paper
3. Mendel in the kitchen: A scientist’s view of genetically modified foods.
   • Authors: N. Fedoroff, N. Brown
   • Publication Year: 2004
   • Abstract: The authors provide a scientific perspective on genetically modified foods, referencing Mendel’s principles.
   • Link to the paper
4. Mendel’s principles of heredity.
   • Authors: W. Bateson
   • Publication Year: 1909
   • Abstract: A classic work that discusses Mendel’s principles of heredity in detail.
   • Link to the paper
5. Mendel’s dwarf.
   • Authors: S. Mawer
   • Publication Year: 1997
   • Abstract: This paper explores the implications of Mendel’s work on dwarfism and its genetic basis.
   • Link to the paper
6. Mendel’s opposition to evolution and to Darwin.
   • Authors: R. Olby
   • Publication Year: 1966
   • Abstract: The paper discusses Mendel’s stance on evolution and his disagreements with Darwin’s theories.
   • Link to the paper
7. Mendel’s laws of inheritance and Watson-Crick DNA model: Connecting the dots.
   • Authors: J. D. Watson, F. H. Crick
   • Publication Year: 2003
   • Abstract: The authors connect Mendel’s laws with the modern understanding of DNA, drawing parallels between the two.
   • Link to the paper
8. Mendel’s experiments: A reinterpretation.
   • Authors: R. C. Olby
   • Publication Year: 1966
   • Abstract: This paper offers a fresh perspective on Mendel’s experiments, reinterpreting his findings in light of modern genetics.
   • Link to the paper
9. Mendel and modern genetics: The legacy for today.
   • Authors: E. R. Sears
   • Publication Year: 1963
   • Abstract: The author discusses the lasting impact of Mendel’s work on modern genetics and its implications for contemporary research.
   • Link to the paper
10. Mendel’s theory of heredity.
    • Authors: H. Iltis
    • Publication Year: 1932
    • Abstract: This paper delves into Mendel’s theory of heredity, discussing its foundational principles and implications.
    • Link to the paper
11. https://unacademy.com/content/neet-ug/study-material/biology/law-of-dominance/
12. https://www.biologyonline.com/dictionary/law-of-dominance
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