Inheritance and genes

Key definitions

• Inheritance is the transmission of genetic information from generation to generation.
• Genotype is the genetic make-up of an organism.
  • It is also the combination of alleles an organism possesses for a particular characteristic.
• Phenotype is the observable features of an organism.
• Homozygous is having two identical alleles of a particular gene.
• Heterozygous is having two different alleles of a particular gene.
  • A heterozygous individual will not be pure-breeding.
• A dominant allele is an allele that is expressed if it is present in the genotype.
• A recessive allele is an allele that is only expressed when there is no dominant allele of the gene present in the genotype.
• Codominance is a situation in which both alleles in heterozygous organisms contribute to the phenotype.
• A sex-linked characteristic is a feature in which the gene responsible is located on a sex chromosome and that this makes the characteristic more common in one sex than in the other.
  • Red-green colorblindness is an example of sex-linkage.

Genes and alleles

• Remember that a gene is a length of DNA that codes for a protein.
• Each gene is always located in the same place on one of the chromosomes.
• Each species has its own genes, and every individual from the same species has the same genes.
  • However, genes vary between individuals within each species.
• The different versions of each gene are known as alleles.
  • There may be two alleles of a gene, but usually there are many more.
• In Figure 16.6.2, you can see three genes on one pair of chromosomes.
  • The alleles are A/a, B/B and d/d.
    • These two alleles, A/a, are different, therefore the individual is heterozygous for this gene. Heteozygous means having two different alleles of a particular gene. The dominant allele (A) (an allele expressed if present in the genotype, the genetic make-up of an organism) will be expressed in the phenotype, the observable features of an organism.
    • B/B are the same, therefore the individual is homozygous dominant. Homozygous means that the two alleles of a particular gene are identical. It is dominant because it is present in the genotype.
    • d/d are the same, therefore the individual is homozygous recessive. It is recessive because there is no dominant allele present in the genotype. The recessive allele (d) will be expressed in the phenotype.
  • It is important to notice that a dominant allele uses CAPITAL LETTERS and a recessive allele uses lowercase letters.

Note

A gene controls a particular feature in the phenotype. A gene may have two or more alleles (different versions of the same gene). The dominant allele produces a different effect on the phenotype than the recessive allele (e.g., hair length in cats). It is important to never write that dominant alleles are ‘strong’ or that recessive alleles are ‘weak’.