Genes and Chromosomes Genes and Chromosomes Tools A B genes located on chromosomes linked genes do not undergo independent assortment recombinants individuals with new combinations of genes incomplete dominance the active allele does not compensate for the inactive allelle gamtes contain a single copy of each gene zygote fertilized egg genetics the branch of biology that studies heredity alleles different forms of a gene gametes reproductive cells genotype genetic makeup homozygous organisms that have two identical alleles for a particular trait heterozygous organisms that have two different alleles for the same trait test cross the cross of an organism of unknown genotype and a homozygous recessive individual diploid a cell that contains both sets of hmologous chromosomes haploid cells that contain a single set of chromosomes polar bodies the other three cells produced in the female during meiosis sex-linked a gene located on one of the chromosomes Y-chromosome small and hook-shaped codominance a condition in which both alleles of a gene are expressed incomplete dominance the active allele does not compensate for the inactive allele, and the heterozygous phenotype is somewhere in between the hompzygous phenotypes