What is the primary difference between a dominant and a recessive allele?

A dominant allele is one that expresses its phenotypic effect even when only one copy is present in a heterozygous individual. It masks the presence of the recessive allele. A recessive allele, on the other hand, only expresses its phenotypic effect when two copies are present (i.e., in a homozygous recessive individual). In a heterozygous state, its expression is completely suppressed by the dominant allele. The presence of a dominant allele means the trait it codes for will be visible, while a recessive allele's trait will only be visible if no dominant allele is present.

Does the Law of Dominance apply to all traits and all organisms?

No, the Law of Dominance, while fundamental, does not apply universally to all traits or all organisms. It describes complete dominance, where one allele completely masks the other. However, there are well-documented exceptions like incomplete dominance (where the heterozygote shows an intermediate phenotype, e.g., pink flowers from red and white parents) and co-dominance (where both alleles are fully expressed in the heterozygote, e.g., AB blood type in humans). These deviations highlight the complexity of genetic interactions beyond simple dominance.

How does the Law of Dominance explain the 3:1 phenotypic ratio in the F2 generation of a monohybrid cross?

In a monohybrid cross, the F1 generation consists entirely of heterozygotes (e.g., Tt for tall pea plants). When these F1 individuals self-pollinate or cross, their gametes (T and t) combine randomly. The possible genotypes in F2 are TT, Tt, and tt in a 1:2:1 ratio. According to the Law of Dominance, both TT and Tt genotypes will express the dominant phenotype (tall), while only the tt genotype will express the recessive phenotype (dwarf). This leads to three parts expressing the dominant trait and one part expressing the recessive trait, hence the 3:1 phenotypic ratio.

What are 'factors' as described by Mendel, and how do they relate to modern genetics?

Mendel, unaware of DNA or chromosomes, used the term 'factors' to describe the discrete units of heredity responsible for transmitting traits from parents to offspring. He inferred their existence based on the consistent patterns of inheritance he observed. In modern genetics, these 'factors' are known as genes. Each gene occupies a specific locus on a chromosome and exists in different forms called alleles. Mendel's 'factors' are essentially the alleles of genes that determine specific characteristics.

Can a recessive trait ever be more common in a population than a dominant trait?

Yes, absolutely. The terms 'dominant' and 'recessive' refer to how alleles express themselves phenotypically in a heterozygous individual, not to their frequency in a population. A recessive allele can be much more common than a dominant allele. For example, polydactyly (extra fingers or toes) in humans is caused by a dominant allele, but it is relatively rare in the population. Conversely, having five fingers (the 'normal' trait) is recessive to polydactyly, yet it is far more common. Population frequency is determined by factors like natural selection, genetic drift, and mutation rates, not by the dominance relationship itself.

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Law of Dominance

Biology

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The Law of Dominance, one of Gregor Mendel's fundamental principles of inheritance, states that in a cross between two pure breeding organisms (homozygous) for contrasting traits, only one form of the trait (the dominant trait) will appear in the first filial (F1) generation. The other trait (the recessive trait) remains hidden or unexpressed in the F1 generation but reappears in the subsequent se…

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The Law of Dominance, proposed by Gregor Mendel, is a fundamental principle of heredity. It states that when two pure-breeding parents with contrasting traits are crossed, only one trait, the dominant one, will appear in the first filial (F1) generation.

The other trait, the recessive one, remains hidden. This occurs because traits are controlled by discrete units called factors (now known as genes), which exist in pairs. When an individual inherits two different forms (alleles) of a gene, the dominant allele masks the expression of the recessive allele.

For example, in pea plants, tallness (T) is dominant over dwarfness (t). A cross between pure tall (TT) and pure dwarf (tt) plants yields all tall (Tt) F1 offspring. If these F1 plants are self-pollinated, the F2 generation will show a 3:1 phenotypic ratio of dominant to recessive traits, demonstrating the reappearance of the masked recessive trait.

This law is crucial for understanding basic inheritance patterns but has exceptions like incomplete dominance and co-dominance.

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Key Concepts

Monohybrid Cross and F1 Generation

A monohybrid cross involves studying the inheritance of a single pair of contrasting traits. When Mendel…

Dominant vs. Recessive Allele Expression

The core of the Law of Dominance lies in how different alleles interact. A dominant allele produces a…

Homozygous vs. Heterozygous States

Understanding these terms is crucial for predicting inheritance patterns. A homozygous individual has two…

Law of Dominance: — One of Mendel's three laws.

Postulates:

1. Characters controlled by discrete units (genes/factors). 2. Factors occur in pairs (alleles). 3. In dissimilar pairs, one factor (dominant) masks the other (recessive).

Monohybrid Cross (P: Pure Dominant x Pure Recessive):

- F1 Generation: All heterozygous, express dominant phenotype. - F2 Generation (from F1 x F1): - Genotypic Ratio: 1 (Homozygous Dominant) : 2 (Heterozygous) : 1 (Homozygous Recessive) - Phenotypic Ratio: 3 (Dominant) : 1 (Recessive)

Key Terms: — Gene, Allele, Dominant, Recessive, Homozygous, Heterozygous, Phenotype, Genotype.

Exceptions: — Incomplete Dominance (intermediate F1, 1:2:1 F2 phenotypic ratio), Co-dominance (both expressed in F1, 1:2:1 F2 phenotypic ratio).

Example: — Pea plant height (Tall 'T' dominant over dwarf 't').

Dominant Always Masks Recessive In F1.

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Biology

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Law of Segregation and Independent Assortment