segregation
Summary
Segregation is Mendel's first law: the two alleles of a gene separate during gamete formation, so each gamete receives one. Its physical basis is homologous chromosomes separating in meiosis I. Most testable: Aa × Aa gives 1:2:1 genotypes, and 3:1 phenotypes with complete dominance.
Detail
Meaning: Segregation (Mendel's law of segregation) states that an individual carries two alleles of each gene and that these alleles separate during gamete formation, so each gamete receives only one. The physical basis is meiosis: homologous chromosomes, which carry the different alleles, separate in anaphase I. A heterozygote (Aa) therefore makes equal numbers of A and a gametes, and fertilization restores two alleles.
On the MCAT: Expect Punnett squares and pedigrees. Aa × Aa gives ¼ AA : ½ Aa : ¼ aa, which is a 3:1 phenotype ratio when one allele is completely dominant. A test cross (Aa × aa) gives 1:1 in the offspring. Nondisjunction is a failure of chromosomes to separate properly in meiosis, and it produces gametes with extra or missing chromosomes.
Common confusion: Segregation is not independent assortment. Segregation is about the two alleles of one gene separating from each other. Independent assortment is about alleles of different genes sorting separately, which holds for genes on different chromosomes but not for closely linked genes. Linked genes still segregate. They just do not assort independently.
Sources
- Campbell Biology
- Genetics: From Genes to Genomes
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.