What is the key difference between a genotype and a phenotype?

A genotype refers to the specific combination of alleles an organism carries in its DNA (e.g., $Bb$). In contrast, a phenotype is the actual physical characteristic or trait that is expressed and observable (e.g., blue eyes).

How does a homozygous genotype differ from a heterozygous genotype?

A homozygous genotype consists of two identical alleles for a particular gene, such as $AA$ or $aa$. A heterozygous genotype contains two different versions of the gene, represented as $Aa$.

When comparing dominant and recessive alleles, how many copies are needed for each to be expressed?

A dominant allele only requires one copy in the genotype (e.g., $Aa$) to be visible in the phenotype. A recessive allele requires two copies ($aa$) because it is masked by the presence of a dominant allele.

Is it true that dominant alleles are always more common or 'stronger' in a population?

No, this is a common misconception. 'Dominant' only refers to the relationship between alleles in how they are expressed; it does not mean the allele is more frequent, 'stronger', or more beneficial for survival.

Why is it incorrect to say an individual has a 'dominant phenotype' if they only have one dominant allele?

It is not technically incorrect to say they have the dominant phenotype, but the error lies in assuming the genotype must be homozygous. A 'dominant trait' appears the same whether the organism is homozygous dominant ($AA$) or heterozygous ($Aa$).

What is the danger of using different letters (like $T$ and $s$) for the same gene in a genetic diagram?

Using different letters can lead to confusion and incorrect results in Punnett squares. Standard practice uses the same letter (Capital and lowercase) to clearly indicate they are variants of the same gene at the same locus.

What is the definition of a 'recessive allele'?

A recessive allele is a version of a gene that is only expressed in the phenotype when no dominant allele is present. It must be inherited from both parents (homozygous recessive) to be visible.

Define the term 'Allele'.

An allele is an alternative version of a single gene. Different alleles for the same gene have small variations in their DNA sequence, leading to different variations of an inherited characteristic.

What does the term 'Homozygous Dominant' mean?

This term describes a genotype consisting of two identical dominant alleles (e.g., $TT$). In this state, the organism will always express the dominant trait and pass that allele to all offspring.

Under what genetic condition will a recessive phenotype be expressed?

A recessive phenotype is only expressed when an organism is homozygous recessive for that gene. This means the organism inherited a recessive allele from both parents and lacks any dominant allele that would mask it.

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3. Reproduction & Inheritance

Key Terms in Genetics

Summary

The study of inheritance relies on a precise set of terms that distinguish between the genetic code an organism carries and the physical traits it expresses. Understanding the relationships between alleles, genotypes, and phenotypes is fundamental to predicting how characteristics are passed from one generation to the next.

1. Definition & Core Concepts

Gene: A section of a DNA molecule that serves as the instruction for building a specific protein, which in turn influences a physical characteristic. It is the fundamental unit of heredity passed from parents to offspring.
Allele: Alternative versions or forms of the same gene that arise due to small variations in the DNA base sequence. For example, a gene for eye color might have one allele for brown and another for blue.
Genotype: The specific combination of alleles that an individual possesses for a particular gene. It is the internal 'blueprint' represented by letter pairs such as $BB$ or $Bb$ .
Phenotype: The observable physical or biochemical characteristics of an organism, which are determined by the interaction of its genotype with the environment. Examples include hair color, height, or blood type.

A diagram showing a pair of homologous chromosomes with specific loci labeled as homozygous and heterozygous.

2. Underlying Principles

Diploidy and Pairs: Most organisms carry two copies of every chromosome, one inherited from each biological parent. Consequently, they possess two alleles for every gene, which constitutes their unique genotype.
Allelic Interaction: When an individual has two different alleles (a heterozygous state), the interaction between them determines the phenotype. In many cases, one allele is 'dominant' and prevents the expression of the other 'recessive' allele.
Inheritance Patterns: The specific combination of alleles is random during gamete formation, but the resulting pairings follow predictable logical rules that allow scientists to calculate the probability of specific traits appearing in offspring.

3. Methods & Techniques

4. Key Distinctions

5. Exam Strategy & Tips

6. Common Pitfalls & Misconceptions

7. Connections & Extensions