Mendel's Law of Dominance: Definition, Examples, diagram, diagram

Gregor Mendel, the father of genetics, laid the foundation of modern genetics through his experiments on pea plants. One of the Laws of Inheritance is the Law of Dominance. It explains how certain traits are expressed in offspring. Mendel discovered that when two different alleles are present for a trait, the dominant one masks the expression of the recessive one. This observation was crucial in understanding basic genetics and how traits are passed from one generation to the next.

This Story also Contains

1. Mendel’s Experiment And Laws
2. Mendel's Experiment With Pea Plants
3. The Law Of Dominance
4. The video recommended on Mendel's Law of Dominance
5. MCQs on Mendel’s Law of Dominance

Mendel’s experiments form the basis of the principles of inheritance and variation.His work provided a clear explanation of how traits are inherited and predicted patterns of inheritance. These principles remain central to genetics today, and form the basis for complex genetic concepts and helps to understand heredity, variation, and evolution.

Mendel’s Experiment And Laws

Gregor Mendel was born in 1822, in Heinzendorf, Austria. Mendel had a strong background in Science and Mathematics, which he applied in his practice during his work on pioneering genetics. It was through his rigorous research on pea plants (Pisum sativum), conducted from 1856 to 1863 in the Augustinian Abbey of St. Thomas at Brno, that laid the foundation of our understanding of heredity. His work contributed to the understanding that genes, located on chromosomes, are the units of inheritance.

Mendel's approach to experimentation was systematic and original. He chose pea plants because traits of pea plants are easily observable. Secondly, they are self-pollinating, making it possible to control breeding experiments. Mendel worked with seven distinct traits, including flower colour, flower position, seed shape, seed color, pod color, pod shape, and plant height. He crossed the plants with opposite traits and self-pollinated the progeny through several generations to develop some basic genetic principles of heredity. His use of statistical analysis to interpret the results was a breakthrough in biology at that time.

Although his findings were initially overlooked, it was rediscovered around the 20th century, which finally gave a major boost to the field of genetics. Mendel worked out the principles of inheritance: the Law of Dominance, the Law of Segregation, and the Law of Independent Assortment. His works were thus confirmed and eventually became the very foundations of modern-day genetic research and its application in medicine, agriculture, and biotechnology principles.

Mendel's Experiment With Pea Plants

Gregor Mendel conducted monohybrid cross experiments on pea plants (Pisum sativum) to determine the pattern of inheritance. He first carefully selected true-breeding plants expressing certain characteristics in their phenotype, such as the flower colour or seed shape. Mendel crossed the plants having two contrasting characteristics, like purple-flowered with white-flower plants, and observed the first generation (F₁) characteristics. He found that the F₁ generation consistently showed only one of the parental traits, which he termed the "dominant" trait, and the other, "recessive," apparently disappeared.

Mendel had chosen pea plants for his experiments for several reasons:

• First, generative time is relatively short, and secondly, they produce many offspring.

• He was able to carry out a statistical analysis of trait inheritance mainly because the plant had clear and distinct characteristics.

• Moreover, pea plants have several discrete and easily recognisable traits that occur in only two alternative forms, like round or wrinkled seeds, and tall or short plants. This dichotomy of the characteristics made the research much easier, and clear inferences could be drawn.

• Pea plants perform self-pollination; this helped Mendel manipulate their crossings and ensure the purity of the parental lines.

One important factor contributing to Mendel's success in the outcome of experiments was that he chose his work very wisely. He picked seven traits that displayed dominant-recessive relationships. Studying these characters through successive generations, Mendel formulated his principles of heredity. He found that the recessive trait reappears in the second generation in a constant 3:1 ratio and thus formulated Mendel’s Law of Segregation. This describes the formation of gametes, the pair of alleles separate so that each gamete gets only one allele for a particular trait.

The Law Of Dominance

The first law of Mendel, known as the Law of Dominance, is one of the central principles of genetics. It states that for any one trait when two different alleles are present in an organism, one allele will be expressed over another. An allele whose expression is seen is called "dominant," whereas one whose expression is masked is called "recessive." This was derived from Mendel's extended experiments on pea plants, where he had observed a certain regularity in the inheritance of traits of organisms.

Dominant and Recessive Alleles

• Dominant alleles: These are the alleles of a gene that express their trait when only one copy is present.

• Recessive alleles: This set of alleles is expressed only in a homozygous condition. The trait due to the recessive allele is not expressed in the presence of a dominant allele.

For example, Mendel crossed the true-breeding tall pea plants with true-breeding dwarf pea plants. The produced F1 were all tall and of genotype Tt, thus explaining the concept of dominance. In this scenario, the tall allele was considered dominant over the dwarf. However, in F₂, the ratio became 3:1, thus indicating that both alleles are present in a genotype but only the dominant trait is demonstrated in a phenotype.

Examples of Dominant-Recessive Trait Pairs in Pea Plants:

• Seed shape: Round (dominant) vs. wrinkled (recessive)

• Flower colour: Purple (dominant) vs. white (recessive)

• Pod colour: Green (dominant) vs. yellow (recessive)

• Plant height: Tall (dominant) vs. short (recessive)

Table: Examples of Dominant and Recessive Traits in Pea Plants

Understanding Mendel's Law of Dominance is crucial for grasping the basics of genetic inheritance. This principle helps explain why certain traits appear in offspring while others do not. By observing the patterns of dominant and recessive traits, Mendel was able to predict the outcomes of genetic crosses, laying the groundwork for modern genetics.

The video recommended on Mendel's Law of Dominance

MCQs on Mendel’s Law of Dominance

Q1. In a monohybrid cross, Aa is crossed with Aa, which would most likely result in the first generation?

1. All plants will exhibit a dominant phenotype

2. All plans will exhibit recessive phenotypes.

3. Dominant and Recessive phenotypes will each be 50%

4. All plants will exhibit a 75% dominant phenotype.

Correct Answer: 3) Dominant and Recessive phenotypes will each be 50%

Explanation:

In a monohybrid cross, we consider the genotypes Aa and aa. A represents the dominant allele, while a denotes the recessive one. An Aa individual is heterozygous, having both alleles and will exhibit the dominant trait. Conversely, an aa individual is homozygous recessive and will express the recessive trait.

Employing a Punnett square, we assess the genetic outcomes:
- Aa offspring are heterozygous, showing the dominant trait.
- aa offspring are homozygous recessive, displaying the recessive trait.

The genotypic ratio simplifies to 1:1 (Aa: aa).

If A is completely dominant:
- The phenotypic ratio is 1:1 for dominant to recessive traits.
- Half the offspring will exhibit the dominant trait (Aa).
- The other half will show the recessive trait (aa).

In incomplete dominance or codominance cases, the phenotypic ratio varies according to trait expression. However, the first generation generally yields a 50:50 split of dominant and recessive phenotypes when A is dominant.

Hence, the correct answer is option 3) Dominant and Recessive phenotypes will each be 50%.

Q2. According to Mendel's Law of Dominance

1. Characters are controlled by factors

2. Factors occur in pairs

3. In a dissimilar pair of factors, one factor dominates the other

4. All of these

Correct Answer: 4) All of these

Explanation:

Based on monohybrid crosses, Mendel proposed the law of dominance.

1. Characters are controlled by factors.

2. Factors occur in pairs.

3. In a dissimilar pair of factors, one factor dominates the other

Hence, the correct answer is option 4)All of these.

Q3. In mice, Y is the dominant allele for yellow fur and y is the recessive allele for grey fur. Since Y is lethal when homozygous, the result of cross Yy x Yy will be

1. 3 yellow: 1 grey

2. 2 yellow: 1 grey

3. 1 yellow: 1 grey

4. 1 yellow: 2 grey

Correct Answer: 2) 2 yellow: 1 grey

Explanation:

In this genetic scenario, allele Y is dominant for yellow fur, while homozygous YY is detrimental, leading to the death of the embryo or organism. A cross between two heterozygous mice, each bearing one yellow fur allele and one recessive allele (Yy × Yy), generates three possible genotypes through a Punnett square:

1. YY: Lethal, resulting in non-surviving individuals.
2. Yy: Heterozygous, displaying yellow fur (viable).
3. yy: Homozygous recessive, presenting grey fur (also viable).

The phenotypic ratio derived from such a cross is:
- Yellow fur (Yy): 2 outcomes
- Grey fur (yy): 1 outcome

Since the homozygous dominant (YY) does not survive, it does not contribute to the final phenotypic ratio of living offspring. Thus, the actual phenotypic ratio is:
- Yellow: 2
- Grey: 1

This situation exemplifies a typical case involving a lethal allele.

Hence, the correct answer is option 2) 2 yellow: 1 grey

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