Chapter 4 - Section 4.4 - Study Guide - Assess Your Learning Outcomes - Page 137: 8

Codominance and incomplete dominance are two patterns of inheritance that differ from simple dominance in how alleles interact and affect the phenotype of an organism. Here's how they differ: **1. Simple Dominance:** - In simple dominance, one allele (the dominant allele) masks the expression of the other allele (the recessive allele) at a particular gene locus. - The heterozygous genotype (Aa) shows the same phenotype as the homozygous dominant genotype (AA). - The heterozygous individual appears phenotypically identical to the homozygous dominant individual because the dominant allele completely overpowers the recessive allele. - Example: In Mendel's pea plants, the allele for yellow seed color (Y) is dominant over the allele for green seed color (y). A plant with the genotype YY or Yy will have yellow seeds because the dominant Y allele masks the expression of the recessive y allele. **2. Codominance:** - In codominance, both alleles at a particular gene locus are fully expressed in the phenotype of a heterozygous individual. - The heterozygous genotype (AB) results in a phenotype that shows characteristics of both alleles equally. - Codominance typically involves multiple alleles for a gene, and each allele contributes to a distinct and observable trait. - Example: In the ABO blood group system, the A and B alleles are codominant. If an individual inherits one A allele and one B allele (genotype AB), their blood type will express both A and B antigens, resulting in the AB blood type. **3. Incomplete Dominance:** - In incomplete dominance, the heterozygous genotype (Aa) results in a phenotype that is intermediate between the two homozygous phenotypes. - The alleles do not fully dominate or mask each other; instead, they blend to produce a third, unique phenotype. - Incomplete dominance typically occurs when the alleles produce proteins or traits that interact quantitatively, leading to a range of possible expressions. - Example: In snapdragon flowers, the allele for red flower color (R) is incompletely dominant over the allele for white flower color (r). A heterozygous plant (Rr) will have pink flowers, which is an intermediate phenotype between the red (RR) and white (rr) homozygous phenotypes. In summary, simple dominance involves one allele fully masking the expression of another allele, resulting in a clear dominant-recessive relationship. Codominance involves both alleles being fully expressed and contributing to distinct traits in a heterozygous individual. Incomplete dominance results in an intermediate phenotype in heterozygotes, where neither allele fully dominates the other, leading to a blending or mixing of traits.

Codominance and incomplete dominance are two patterns of inheritance that differ from simple dominance in how alleles interact and affect the phenotype of an organism. Here's how they differ: **1. Simple Dominance:** - In simple dominance, one allele (the dominant allele) masks the expression of the other allele (the recessive allele) at a particular gene locus. - The heterozygous genotype (Aa) shows the same phenotype as the homozygous dominant genotype (AA). - The heterozygous individual appears phenotypically identical to the homozygous dominant individual because the dominant allele completely overpowers the recessive allele. - Example: In Mendel's pea plants, the allele for yellow seed color (Y) is dominant over the allele for green seed color (y). A plant with the genotype YY or Yy will have yellow seeds because the dominant Y allele masks the expression of the recessive y allele. **2. Codominance:** - In codominance, both alleles at a particular gene locus are fully expressed in the phenotype of a heterozygous individual. - The heterozygous genotype (AB) results in a phenotype that shows characteristics of both alleles equally. - Codominance typically involves multiple alleles for a gene, and each allele contributes to a distinct and observable trait. - Example: In the ABO blood group system, the A and B alleles are codominant. If an individual inherits one A allele and one B allele (genotype AB), their blood type will express both A and B antigens, resulting in the AB blood type. **3. Incomplete Dominance:** - In incomplete dominance, the heterozygous genotype (Aa) results in a phenotype that is intermediate between the two homozygous phenotypes. - The alleles do not fully dominate or mask each other; instead, they blend to produce a third, unique phenotype. - Incomplete dominance typically occurs when the alleles produce proteins or traits that interact quantitatively, leading to a range of possible expressions. - Example: In snapdragon flowers, the allele for red flower color (R) is incompletely dominant over the allele for white flower color (r). A heterozygous plant (Rr) will have pink flowers, which is an intermediate phenotype between the red (RR) and white (rr) homozygous phenotypes. In summary, simple dominance involves one allele fully masking the expression of another allele, resulting in a clear dominant-recessive relationship. Codominance involves both alleles being fully expressed and contributing to distinct traits in a heterozygous individual. Incomplete dominance results in an intermediate phenotype in heterozygotes, where neither allele fully dominates the other, leading to a blending or mixing of traits.