# Exceptions to Mendelian Inheritance This section explores patterns of inheritance that do not follow the simple dominant-recessive relationship described by Mendel. These are known as **non-Mendelian inheritance** patterns and include incomplete dominance, co-dominance, and multiple alleles. These occur due to different ways contrasting alleles at the same *locus* interact in a *heterozygous* state. --- ## 1. Incomplete Dominance In incomplete dominance, the phenotype of the heterozygous offspring is a blend of the two parental phenotypes. **Mechanism:** Neither of the two alleles for a trait is completely dominant over the other. **Example:** Flower color in the Japanese four o'clock plant (*Mirabilis jalapa*). - A cross between a homozygous **red-flowered** plant ($R^1R^1$) and a homozygous **white-flowered** plant ($R^2R^2$) produces $F_1$ offspring that are all heterozygous ($R^1R^2$). - The $F_1$ generation has an intermediate phenotype: **pink flowers**. - When the $F_1$ generation (pink flowers) is self-crossed, the $F_2$ generation shows a phenotypic and genotypic ratio of **1:2:1** (1 red : 2 pink : 1 white). --- ## 2. Co-dominance In co-dominance, both alleles are expressed independently and equally in the phenotype of a heterozygote. There is no blending. **Mechanism:** The two contrasting alleles at the same locus express their traits independently without influencing each other. **Example:** The human MN blood group system. - This system is determined by two co-dominant alleles: $L^M$ and $L^N$. - These alleles code for M and N antigen molecules on the surface of red blood cells. - An individual can be homozygous for M ($L^M L^M$), homozygous for N ($L^N L^N$), or heterozygous ($L^M L^N$). - The heterozygous individual ($L^M L^N$) produces **both** M and N antigens on their red blood cells. | Blood Group Phenotype | Antigen Present | Genotype | | :--- | :--- | :--- | | M blood | M antigen | $L^M L^M$ | | N blood | N antigen | $L^N L^N$ | | MN blood | Both M and N antigens | $L^M L^N$ | --- ## Comparison: Incomplete Dominance vs. Co-dominance | Incomplete Dominance | Co-dominance | | :--- | :--- | | 1. In the heterozygous state, the alleles **blend** their phenotypic effects. | 1. In the heterozygous state, both alleles **independently express** their phenotypic effects. | | 2. The heterozygote shows an **intermediate phenotype** (e.g., pink flowers from red and white parents). | 2. The heterozygote shows **both parental phenotypes** at the same time (e.g., both M and N antigens are present). | | 3. **Example:** Flower color of four o'clock plant. | 3. **Example:** Human MN and AB blood groups. | --- ## 3. Multiple Alleles This occurs when a gene has more than two allelic forms in a population, often arising from gene mutations. **Mechanism:** While there may be many alleles for a gene in the population's gene pool, any diploid individual can only possess two of these alleles (one on each homologous chromosome). **Example:** The **ABO blood group** in humans. - This blood group is controlled by three alleles: $I^A$, $I^B$, and $i$. - $I^A$ and $I^B$ are co-dominant with each other and are both completely dominant over $i$. --- ## 4. Pleiotropy Pleiotropy occurs when a single gene influences multiple, seemingly unrelated phenotypic traits. --- ## 5. Epistasis Epistasis is a phenomenon where the effect of one gene (at one locus) is masked or modified by the presence of one or more other genes (at different loci). --- ## Possible Questions and Answers **Q:** What is the main difference between incomplete dominance and co-dominance? **A:** In incomplete dominance, the heterozygous phenotype is a *blend* of the two parental phenotypes (e.g., red + white = pink). In co-dominance, both parental phenotypes are *fully and separately expressed* (e.g., MN blood type has both M and N molecules). **Q:** What is the characteristic phenotypic ratio for the $F_2$ generation in a monohybrid cross involving incomplete dominance? **A:** The ratio is 1:2:1, which mirrors the genotypic ratio. For example, 1 red-flowered : 2 pink-flowered : 1 white-flowered plant. **Q:** Can an individual have three alleles for a single gene? **A:** No. A diploid individual can only have two alleles for any given gene, one inherited from each parent. However, more than two alleles for that gene can exist within the larger population (multiple alleles).