Homologous Chromosomes Notes and Key Terms
Homologous Chromosomes:
 Come in pairs
o One is maternal (from mom)
o One is paternal (from dad)
 They have the same types of genes in the same place on the chromosome
Allele:
 Allele = specific variant / version of a gene
 Example:
o Gene = abdomen color
o Alleles (options) for abdomen color
 Black (B) - Dominant
 Orange (b) - Recessive
 There are always two alleles for each gene because there is one on each
homologous chromosome
Genotype vs. Phenotype
 Genotype: the alleles present for a gene/trait
o We use 2 letters for genotypes, one for each allele on the different
homologous chromosomes
o Example:
 BB, bb, or Bb
o If the alleles are the same, this is called homozygous
 Example: BB or bb
o If the alleles are different, heterozygous
 Example: Bb
 Phenotype: the version of the trait that is shows up (expressed)
o Example
 Trait is abdomen color
 Phenotype Options:
 Brown
 Orange
o Dominant phenotype shows up with either one or two dominant alleles
 Example: Bb = Brown or BB = Brown
o Recessive phenotype shows up only with two recessive alleles
 Example: bb = Orange