Principles of Mendelian Genetics
Long before the discovery of DNA or chromosomes, a 19th-century Augustinian friar named Gregor Mendel deduced the fundamental principles of heredity. By meticulously cross-breeding pea plants (Pisum sativum) and statistically analyzing their traits over multiple generations, Mendel established the core rules of how biological information is passed from parent to offspring.
These rules, now known as Mendelian genetics, remain the foundation of classical genetics.
1. Key Terminology
[edit]Before exploring Mendel’s laws, it is essential to define the vocabulary used to describe genetic inheritance:
- Gene: A distinct sequence of DNA that contains the instructions for producing a specific protein, which in turn influences a physical trait.
- Allele: A variant form of a gene. For example, a gene for flower color might have a "purple" allele and a "white" allele.
- Locus: The specific physical location of a gene on a chromosome.
- Genotype: The exact genetic makeup of an organism regarding a specific trait (the specific alleles it possesses).
- Phenotype: The observable, physical manifestation of the genotype (what the trait actually looks like).
- Homozygous: Possessing two identical alleles for a specific gene (e.g., AA or aa).
- Heterozygous: Possessing two different alleles for a specific gene (e.g., Aa).
2. Dominant and Recessive Traits
[edit]In classic Mendelian inheritance, traits are governed by simple dominance. When an organism is heterozygous (possessing two different alleles), one allele often masks the expression of the other.
- Dominant Allele: The allele that is expressed in the phenotype even if only one copy is present. It is conventionally represented by a capital letter (e.g., 'A').
- Recessive Allele: The allele that is masked and only expressed in the phenotype if the organism possesses two copies (homozygous recessive). It is conventionally represented by a lowercase letter (e.g., 'a').
Example: If purple flower color ('P') is dominant over white flower color ('p'), then both 'PP' and 'Pp' genotypes will result in purple flowers. Only the 'pp' genotype will result in white flowers.
3. Mendel's Laws of Inheritance
[edit]Mendel's experimental observations led him to formulate three core principles that describe how alleles behave during reproduction.
The Law of Segregation (First Law)
[edit]This law states that during the formation of gametes (sperm and egg cells), the two alleles for a given trait separate (segregate) from each other. Consequently, each gamete carries only one allele for each gene. When fertilization occurs, the offspring receives one allele from each parent, restoring the pair.
The Law of Independent Assortment (Second Law)
[edit]Mendel observed that the inheritance of one trait does not affect the inheritance of another. This law states that alleles for different genes sort into gametes independently of one another.
- Modern caveat: We now know this is only strictly true for genes located on different chromosomes or situated very far apart on the same chromosome. Genes located close together are "linked" and tend to be inherited together.
The Law of Dominance (Third Law)
[edit]As described above, this law asserts that in a heterozygote, one trait will conceal the presence of another trait for the same characteristic. The dominant allele will be expressed exclusively.
4. Tools for Predicting Inheritance: The Punnett Square
[edit]A Punnett square is a simple graphical tool used to predict the probability of an offspring inheriting a particular genotype.
To construct a basic monohybrid cross (analyzing one trait): 1. Determine the genotypes of the two parents. 2. Place the alleles from one parent along the top of a 2x2 grid. 3. Place the alleles from the other parent along the side of the grid. 4. Fill in the intersecting boxes by combining the alleles from the corresponding row and column.
Example: A Heterozygous Cross (Aa x Aa)
| A | a | |
|---|---|---|
| A | AA | Aa |
| a | Aa | aa |
Analysis of the Results:
- Genotypic Ratio: The ratio of the different genetic combinations. In this cross, it is 1 AA : 2 Aa : 1 aa (or 1:2:1).
- Phenotypic Ratio: The ratio of the observable traits. Because both AA and Aa express the dominant trait, the ratio is 3 Dominant : 1 Recessive (or 3:1).
5. Beyond Simple Dominance (Genetic Variance)
[edit]While Mendel’s laws provide the foundation, real-world genetics often involves more complex interactions that lead to greater genetic variance:
- Incomplete Dominance: Neither allele is completely dominant. The heterozygous phenotype is a blend of the two homozygous phenotypes (e.g., a red flower crossed with a white flower produces pink offspring).
- Codominance: Both alleles are simultaneously and fully expressed in the heterozygote (e.g., AB blood type in humans, where both A and B antigens are present).
- Polygenic Inheritance: A single trait is controlled by the interaction of multiple genes (e.g., human height, skin color), resulting in a continuous spectrum of phenotypes rather than distinct categories.