Alleles and Inheritance

Genetic information for human is stored in 23 chromosomes, which come in pairs - one from the mother, the other from the father. Thus every individual has two copies of each gene. Alleles are different forms of a gene located in exactly the same position on homologous chromosomes as shown in the left of the diagram.

The sequence of the DNA base pairs for the brown eye and blue eye alleles (the bey2 gene) as shown in the right of the diagram is essentially similar except the one base pair A-T in the former is replaced by G-C in the latter. Homozygous refers to the case where the two alleles are identical; while heterozygous signifies the alleles have different forms. The allele expressed in the heterozygote is dominant; while the one not expressed is recessive. In the bey2 example, brown is dominant over blue unless it is damaged. In that case, the recessive allele takes over as the functional gene.

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Genotype is the genetic make-up of an allele. The allele responsible for a trait is usually indicated by a capital letter for the dominant factor (such as B for brown eye) and by a lowercase letter for the recessive factor (such as b for blue eye). Phenotype refers to the physical characteristics (such as brown eye or blue eye). The transmission of traits is usually plotted on the Punnett square. Examples in the followings trace the eye colour transmission to progeny from different combination of male/female genotypes.

The above example for eye colour genotype is over simplified. Actually, at least three genes are involved in eye colour determination.