A diploid organism (e.g. a human) has two alleles for each gene in their chromosomes. Alleles are alternative forms of a gene, so an organism could have two of the same allele (homozygous) or two different alleles (heterozygous). The alleles an organism has (the genotype) will determine what characteristics it has (the phenotype). There is quite a lot of terminology in this topic, but gradually it will become second nature. In this article we are starting with the simplest version of inheritance: monohybrid inheritance.
Dominant and recessive alleles
Alleles can either be dominant or recessive. Some can show codominance, but we will cover that in the next article. If an allele is dominant, only one copy is needed for the characteristic to be expressed in the phenotype. If an allele is recessive, two copies are needed for the characteristic to be expressed in the phenotype. Therefore if an organism is heterozygous (has one dominant and one recessive allele), the dominant characteristic will be expressed in the phenotype but the recessive allele is part of the organism’s genotype and it can be passed on to offspring. The organism is said to be a carrier of the recessive allele.
Don’t forget that gametes are formed in meiosis and only contain one allele. Head over to the meiosis article to revise how gametes are formed.
Monohybrid inheritance is the inheritance of a characteristic controlled by a single gene. You can easily work out the possible outcomes of a monohybrid cross by drawing a genetic diagram or Punnett square. From these, you can work out the probability of offspring having a particular genotype or phenotype.
In the example below, two beetles are crossed. The gene we are looking at is the gene for wing colour. The allele for blue wings (B) is dominant, and the allele for white wings (b) is recessive. If two homozygous parents are crossed, there is a 100% chance that the first generation (F1) offspring will have the heterozygous genotype and the blue phenotype. If two of the F1 generation are crossed, there is a 75% chance that the second generation (F2) offspring will have blue wings and 25% chance they will have white wings. The phenotypic ratio is 3:1.
Genetic crosses get a little more complicated when looking at more than one gene, or genes that are linked. These will be the subjects of the next few articles.
- Genotype is the alleles an organism has, phenotype is the characteristic which is expressed.
- Alleles can be dominant or recessive. The recessive phenotype is only expressed if the organism is homozygous for the recessive allele.
- Monohybrid inheritance looks at inheritance of characteristics coded for by one gene.
- Phenotypic ratio can be calculated from genetic diagrams and Punnett squares.