Alleles and Genotype

 Alleles, Genotype & Phenotype

Alleles and genotypes are important foundations of genetics. An allele is a particular form of a gene and they are passed from parents to their offspring. A genotype is the combination of two alleles, one received from each parent. The expression of a genotype is called the phenotype and the specific combination of the two alleles (the genotype) influences the physical expression (the phenotype) of the physical trait that the alleles carry information for.



An allele is a particular form of one specific gene. When Gregor Mendel completed his experiments on peas he was crossing different traits of one characteristic, such as flower color. Genetically, the variation in traits, e.g. purple flowers or white flowers, is caused by different alleles. In most cases in the plant and animal world, individuals have two alleles for each gene; one allele is inherited from their father and the second from their mother.

Depending on which alleles an individual has received will determine how their genes are expressed. For example, if two parents have blue eyes and pass the blue-eyed alleles onto their children, their children will also possess the alleles for blue eyes. Certain alleles have the ability to dominate the expression of a particular gene. For example, if a child has received a blue-eye allele from their father and a brown-eye allele from their mother, the child will have brown eyes because the brown-eye allele is dominant over the blue eye allele. In this case the brown-eye allele is known as the dominant allele and the blue-eye allele is known as the recessive allele.



The genotype is the genetic combination of two alleles. If, for example, a child has received one brown-eye allele – represented by ‘B’ – and one blue-eye allele – represented by ‘b’ – then their genotype would be ‘Bb’. If, however, the child received two brown-eye alleles their genotype would be ‘BB’; and a child with two blue-eye alleles ‘bb’.

As previously mentioned, the brown-eye allele is dominant over the blue-eye allele so a child with the genotype ‘Bb’ would in theory have brown eyes, rather than blue or a mix between the two. Genotypes with two alleles that are the same, i.e. ‘BB’ and ‘bb’, are known as homozygous genotypes and genotypes with two different alleles are known as heterozygous genotypes.



The physical appearance of the genotype is called the phenotype. For example, children with the genotypes ‘BB’ and ‘Bb’ have brown-eye phenotypes, whereas a child with two blue-eye alleles and the genotype ‘bb’ has blue eyes and a blue-eye phenotype. The phenotype can also be influenced by the environment and sometimes certain alleles will be expressed in some environments but not in others. Therefore two individuals with the same genotype can sometimes have different phenotypes in they live in different environments.



  • Gene – a section of DNA that provides the genetic material for one characteristic
  • Allele – a particular form of a gene. One allele is received from each parent
  • Genotype – the combination of the two alleles that are received from an individual’s parents
  • Phenotype – the physical expression of the gene which is determined by both the genotype and the environment
  • Heterozygous – a genotype with two different alleles
  • Homozygous – a genotype with two of the same alleles


Punnet Squares

Punnet squares are used to identify the possible genotypes and phenotypes of offspring of two adults. They are a useful tool for recognizing the chance of offspring expressing certain traits. The punnet square to the right shows the potential genotypes of offspring when a homozygous dominant (BB) adult breeds with a homozygous recessive (bb) adult. In this instance all the offspring will heterozygous (Bb) for this characteristic and only the dominant trait will be expressed. In terms of genotypes and phenotypes, if the ‘BB’ genotype coded for the dominant brown eye trait and the ‘bb’ genotype coded for recessive blue eye trait, all the offspring will have the genotype ‘Bb’ and the expressed phenotype will be the dominant brown eye trait.

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