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

**1. Dominant and Recessive Alleles:** - **Dominant Allele:** A dominant allele is a genetic variant that, when present in an individual's genotype, will determine the observable trait, or phenotype, even if only one copy of the allele is present in a heterozygous individual. Dominant alleles are typically represented by uppercase letters (e.g., "A"). - **Recessive Allele:** A recessive allele is a genetic variant that will only determine the observable trait if an individual has two copies of the recessive allele (homozygous recessive). Recessive alleles are expressed in the phenotype when there are no dominant alleles present. Recessive alleles are typically represented by lowercase letters (e.g., "a"). **2. Homozygous and Heterozygous Individuals:** - **Homozygous:** Homozygous individuals have two identical alleles at a specific gene locus (position on a chromosome). They can be homozygous dominant (two dominant alleles, such as "AA") or homozygous recessive (two recessive alleles, such as "aa"). Homozygous individuals will express the corresponding trait determined by the alleles they carry. - **Heterozygous:** Heterozygous individuals have two different alleles at a specific gene locus, one dominant and one recessive (e.g., "Aa"). In heterozygous individuals, the dominant allele typically masks the expression of the recessive allele, and the trait associated with the dominant allele is observed in the phenotype. **3. Why a Person Can Be Homozygous for Some Alleles and Heterozygous for Others:** Genetic inheritance is complex, and individuals inherit one set of alleles from each parent. This means that a person can inherit different combinations of alleles for various genes. Here's why a person can be homozygous for some alleles and heterozygous for others: - **Independent Assortment:** Genes located on different chromosomes assort independently during meiosis, the process of cell division that produces gametes (sperm and egg cells). As a result, an individual can inherit different combinations of alleles for different genes from their parents. - **Crossing Over:** During meiosis, genetic material can exchange between homologous chromosomes in a process called crossing over. This introduces additional genetic diversity in the offspring, as it can lead to novel combinations of alleles. - **Multiple Alleles:** Some genes have multiple alleles, meaning there are more than two variations of the gene in the population. In these cases, individuals can inherit any combination of alleles for that gene, leading to various possible genotypes and phenotypes. - **Incomplete Dominance and Codominance:** In some cases, alleles may not exhibit simple dominant-recessive relationships. Instead, alleles may show incomplete dominance (where heterozygotes have an intermediate phenotype) or codominance (where both alleles are fully expressed). In such instances, individuals can have a range of phenotypes depending on their specific genotype. In summary, a person can be homozygous for some alleles and heterozygous for others due to the independent assortment of genes, genetic recombination through crossing over, the presence of multiple alleles for some genes, and variations in dominance relationships and expression patterns among different genes. These factors contribute to the genetic diversity and complexity observed within populations.

**1. Dominant and Recessive Alleles:** - **Dominant Allele:** A dominant allele is a genetic variant that, when present in an individual's genotype, will determine the observable trait, or phenotype, even if only one copy of the allele is present in a heterozygous individual. Dominant alleles are typically represented by uppercase letters (e.g., "A"). - **Recessive Allele:** A recessive allele is a genetic variant that will only determine the observable trait if an individual has two copies of the recessive allele (homozygous recessive). Recessive alleles are expressed in the phenotype when there are no dominant alleles present. Recessive alleles are typically represented by lowercase letters (e.g., "a"). **2. Homozygous and Heterozygous Individuals:** - **Homozygous:** Homozygous individuals have two identical alleles at a specific gene locus (position on a chromosome). They can be homozygous dominant (two dominant alleles, such as "AA") or homozygous recessive (two recessive alleles, such as "aa"). Homozygous individuals will express the corresponding trait determined by the alleles they carry. - **Heterozygous:** Heterozygous individuals have two different alleles at a specific gene locus, one dominant and one recessive (e.g., "Aa"). In heterozygous individuals, the dominant allele typically masks the expression of the recessive allele, and the trait associated with the dominant allele is observed in the phenotype. **3. Why a Person Can Be Homozygous for Some Alleles and Heterozygous for Others:** Genetic inheritance is complex, and individuals inherit one set of alleles from each parent. This means that a person can inherit different combinations of alleles for various genes. Here's why a person can be homozygous for some alleles and heterozygous for others: - **Independent Assortment:** Genes located on different chromosomes assort independently during meiosis, the process of cell division that produces gametes (sperm and egg cells). As a result, an individual can inherit different combinations of alleles for different genes from their parents. - **Crossing Over:** During meiosis, genetic material can exchange between homologous chromosomes in a process called crossing over. This introduces additional genetic diversity in the offspring, as it can lead to novel combinations of alleles. - **Multiple Alleles:** Some genes have multiple alleles, meaning there are more than two variations of the gene in the population. In these cases, individuals can inherit any combination of alleles for that gene, leading to various possible genotypes and phenotypes. - **Incomplete Dominance and Codominance:** In some cases, alleles may not exhibit simple dominant-recessive relationships. Instead, alleles may show incomplete dominance (where heterozygotes have an intermediate phenotype) or codominance (where both alleles are fully expressed). In such instances, individuals can have a range of phenotypes depending on their specific genotype. In summary, a person can be homozygous for some alleles and heterozygous for others due to the independent assortment of genes, genetic recombination through crossing over, the presence of multiple alleles for some genes, and variations in dominance relationships and expression patterns among different genes. These factors contribute to the genetic diversity and complexity observed within populations.