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6.4 Traits, Genes, and Alleles KEY CONCEPT Genes encode proteins that produce a diverse range of traits. 6.4 Traits, Genes, and Alleles The same gene can have many versions. • A gene is a piece of DNA that directs a cell to make a certain protein. • Each gene has a locus, a specific position on a pair of homologous chromosomes. 6.4 Traits, Genes, and Alleles • An allele is any alternative form of a gene occurring at a specific locus on a chromosome. – Each parent donates one allele for every gene. – Homozygous describes two alleles that are the same at a specific locus. – Heterozygous describes two alleles that are different at a specific locus. 6.4 Traits, Genes, and Alleles Question Time Distinguish between the terms locus and allele. An allele is an alternative form of a gene, which codes for a different form of the same trait. Alleles are found at the same location, or locus, on homologous chromosomes 6.4 Traits, Genes, and Alleles Genes influence the development of traits. • All of an organism’s genetic material is called the genome. • A genotype refers to the makeup of a specific set of genes. • A phenotype is the physical expression of a trait. 6.4 Traits, Genes, and Alleles • Alleles can be represented using letters. – A dominant allele is expressed as a phenotype when at least one allele is dominant. – A recessive allele is expressed as a phenotype only when two copies are present. – Dominant alleles are represented by uppercase letters; recessive alleles by lowercase letters. 6.4 Traits, Genes, and Alleles Polydactyly is the condition of having more than the typical number of fingers or toes. The allele for polydactyly is dominant. 6.4 Traits, Genes, and Alleles • Both homozygous dominant and heterozygous genotypes yield a dominant phenotype. • Most traits occur in a range and do not follow simple dominant-recessive patterns. 6.4 Traits, Genes, and Alleles Question Time What is the difference between genotype and phenotype? Genotype refers to the actual genes an organism carries. Phenotype typically refers to an organism’s physical appearance or the expression of a certain gene.