1. Principles of Inheritance
Mendel's Laws of Inheritance
Gregor Mendel, through his meticulous experiments with pea plants, established fundamental principles governing how traits are passed from parents to offspring. These principles are universally known as Mendel's Laws of Inheritance.
a. Law of Dominance
The Law of Dominance states that in a heterozygote, one allele will conceal the presence of another allele for the same trait. The allele that is expressed is called the dominant allele, and the allele that is masked is called the recessive allele.
Example: In pea plants, the allele for tallness (T) is dominant over the allele for shortness (t). A plant with genotype Tt will exhibit the tall phenotype because the dominant tall allele masks the expression of the recessive short allele.
b. Law of Segregation
The Law of Segregation states that during the formation of gametes, the two alleles for a heritable character separate (segregate) from each other so that each gamete carries only one allele for that character.
Explanation: This means that an individual inherits one allele from each parent for a particular gene. When that individual produces gametes, these two alleles separate, and each gamete receives only one of them. This ensures that offspring inherit one allele from each parent, restoring the diploid condition.
Example: A parent with genotype Tt for height will produce two types of gametes: 50% carrying the T allele and 50% carrying the t allele.
c. Law of Independent Assortment
The Law of Independent Assortment states that alleles for different genes assort independently of each other during gamete formation, provided they are located on different chromosomes or are far apart on the same chromosome.
Explanation: This law applies to the inheritance of two or more genes. It signifies that the inheritance pattern of one trait does not influence the inheritance pattern of another trait, as long as the genes are not linked on the same chromosome or are far enough apart to undergo recombination frequently.
Example: Consider a dihybrid cross involving pea plants with two traits: seed color (Yellow Y, green y) and seed shape (Round R, wrinkled r). A parent with genotype YyRr will produce four types of gametes in equal proportions: YR, Yr, yR, and yr.
2. Incomplete Dominance and Co-dominance
Mendel's principles predominantly describe situations of complete dominance, where one allele completely masks the expression of the other. However, geneticists have discovered variations to this rule.
a. Incomplete Dominance
Incomplete dominance is a form of intermediate inheritance in which one allele for a specific trait is not completely expressed over the other allele. This results in a phenotypic expression that is a mixture or blend of the phenotypes of the two parents.
Characteristics:
- Heterozygotes express a phenotype that is intermediate between the two homozygous phenotypes.
- No allele is truly dominant or recessive; rather, there is partial expression.
- The genotypic ratio and phenotypic ratio are identical in the F2 generation (typically 1:2:1).
Example: In snapdragon flowers (Antirrhinum majus), when a homozygous red flower (RR) is crossed with a homozygous white flower (WW), the F1 generation (RW) produces pink flowers. If these F1 pink flowers are self-pollinated, the F2 generation will consist of red, pink, and white flowers in a 1:2:1 ratio, respectively.
b. Co-dominance
Co-dominance is a form of inheritance wherein the alleles of a gene pair in a heterozygote are both fully expressed. This results in offspring with a phenotype that simultaneously exhibits characteristics of both parental phenotypes, without blending.
Characteristics:
- Both alleles contribute to the phenotype without any blending of traits.
- The heterozygote displays distinct characteristics of both homozygous phenotypes simultaneously.
- Similar to incomplete dominance, the genotypic ratio and phenotypic ratio are the same in the F2 generation (1:2:1).
Example: The ABO blood group system in humans is a classic example. Individuals with AB blood type (genotype IAIB) express both A and B antigens on the surface of their red blood cells. Here, the alleles IA and IB are co-dominant, and both are fully expressed.
Another example is roan cattle, where a cross between red (RR) and white (WW) parents produces roan (RW) offspring with distinct red and white hairs interspersed throughout their coat.
Comparison of Incomplete Dominance and Co-dominance
| Feature | Incomplete Dominance | Co-dominance |
|---|---|---|
| Phenotype of Heterozygote | Intermediate or blended phenotype | Both parental phenotypes expressed simultaneously and distinctly |
| Expression of Alleles | Neither allele is completely dominant; partial expression leads to an intermediate trait | Both alleles are fully and equally expressed, leading to a combined trait |
| Example | Pink snapdragon flowers (Red × White → Pink) | ABO blood group (IAIB blood type), Roan cattle (Red × White → Roan) |
3. Allele Concept
An allele is an alternative form of a gene that arises by mutation and is found at the same place (locus) on a chromosome. Diploid organisms typically possess two alleles for each gene, one inherited from each parent.
a. Multiple Alleles
Multiple alleles refer to the existence of more than two allelic forms for a specific gene within a population. While an individual diploid organism can only possess two alleles for a given gene, there can be many different alleles present across the entire population.
Characteristics:
- A single gene has three or more alleles.
- These alleles occupy the same specific locus on homologous chromosomes.
- Multiple alleles can exhibit various dominance relationships among themselves (e.g., complete dominance, incomplete dominance, co-dominance).
Example: The ABO blood group system in humans, mentioned earlier, is controlled by three alleles: IA, IB, and i. IA and IB are co-dominant to each other, and both are completely dominant over the recessive allele i. This system allows for four possible blood phenotypes: A, B, AB, and O.
b. Pseudo Allele
A pseudo allele is a gene that initially appears to be a single functional unit based on recombination studies (where no recombination is typically observed between them), but it is actually composed of two or more closely linked genes (cistrons) that behave as a single gene in terms of inheritance.
Explanation: Pseudo alleles are distinct but tightly linked genes located so close together on a chromosome that crossing over between them is a rare event. Consequently, they are usually inherited as a single unit, mimicking the behavior of a single allele. They represent a functional gene complex rather than a single, indivisible allele.
Example: The Bar eye locus in Drosophila melanogaster, which influences eye shape, was initially thought to be a single gene with multiple alleles. Later, it was discovered to be a tandem duplication of a specific chromosomal segment, where the duplicated segments behave as pseudo alleles.
c. Lethal Allele
A lethal allele is an allele that, when present in a homozygous (and sometimes heterozygous) state, results in the death of an organism. The death can occur at any stage of development, ranging from an early embryo to an adult.
Characteristics:
- Lethal alleles are most commonly recessive, meaning two copies are required for the lethal effect (e.g., in a homozygous recessive genotype).
- Some lethal alleles can be dominant, causing death even in heterozygotes.
- Lethal alleles significantly modify the expected Mendelian ratios. For instance, in a monohybrid cross, the F2 phenotypic ratio might become 2:1 instead of the classic 3:1 if the homozygous recessive genotype is inviable.
Example: The yellow coat color gene (AY) in mice. The allele AY is dominant for yellow coat color but acts as a recessive lethal allele. When two heterozygous yellow mice (AYA) are crossed, the expected genotypic ratio is 1 AYAY : 2 AYA : 1 AA. However, the AYAY individuals die during embryonic development, resulting in a modified phenotypic ratio of 2 yellow : 1 agouti (normal gray-brown) mice among the viable offspring.
4. Epistasis, Hypostasis and Pleiotrophy
These concepts describe more complex interactions between different genes or the multifaceted effects of a single gene on an organism's phenotype.
a. Epistasis
Epistasis is a phenomenon where the expression of one gene (the epistatic gene) masks or modifies the phenotypic expression of another gene (the hypostatic gene) at a different locus.
Explanation: Unlike dominance, which describes interactions between alleles at the same gene locus, epistasis describes interactions between genes located at different loci (i.e., different chromosomes or different positions on the same chromosome). The epistatic gene effectively prevents or alters the phenotypic expression of the hypostatic gene.
Types of Epistasis: There are various types, each resulting in different modified Mendelian ratios, such as recessive epistasis (e.g., 9:3:4 F2 ratio), dominant epistasis (e.g., 12:3:1 F2 ratio), duplicate recessive epistasis (e.g., 9:7 F2 ratio), etc.
Example (Recessive Epistasis): Coat color in Labrador retrievers. Two main genes determine coat color. One gene (B/b) determines the pigment color: B for black and b for chocolate. Another gene (E/e) controls the deposition of pigment in the fur. If a dog has at least one dominant E allele, pigment is deposited. However, if a dog has the homozygous recessive ee genotype, no pigment is deposited, resulting in a yellow dog, regardless of the B/b alleles it possesses. Here, the 'e' allele (when homozygous ee) is epistatic to the 'B/b' alleles, as it masks their expression.
b. Hypostasis
Hypostasis refers to the gene whose phenotypic expression is masked or suppressed by another gene (the epistatic gene) at a different locus.
Explanation: Hypostasis is the complementary term to epistasis. If gene A is epistatic to gene B, then gene B is considered hypostatic to gene A. It is not an independent genetic interaction but rather the term used to describe the gene whose effect is subordinate in an epistatic relationship.
Example: Continuing with the Labrador retriever example, the B/b gene for pigment color is hypostatic to the E/e gene for pigment deposition, especially when the 'ee' genotype is present. The expression of B/b is suppressed by ee.
c. Pleiotrophy
Pleiotrophy is the phenomenon where a single gene affects two or more distinct, seemingly unrelated phenotypic traits.
Explanation: Instead of influencing only one trait, a pleiotropic gene has multiple, diverse effects on an organism's phenotype. This often occurs because the gene codes for a product (e.g., an enzyme, a hormone, or a structural protein) that is involved in multiple metabolic pathways, physiological processes, or developmental stages.
Examples:
- Sickle cell anemia in humans: A single point mutation in the gene coding for the beta-globin chain of hemoglobin leads to a change in the shape of red blood cells (sickle shape). This single genetic alteration causes a cascade of seemingly unrelated effects, including anemia, increased resistance to malaria, periodic pain crises, organ damage, and an increased susceptibility to infections.
- Marfan Syndrome: This is a genetic disorder caused by a defect in the FBN1 gene, which codes for fibrillin-1, a protein vital for connective tissue formation. A single defect in this gene affects multiple body systems, leading to a tall and slender build, abnormally long limbs and fingers, heart problems (such as aortic aneurysm), and eye abnormalities (like lens dislocation).