Inheritance: monohybrid, dihybrid and co-dominance

In a nutshell

Inheritance is the process in which genes are given to the offspring from parents. Is it possible to predict the phenotype of offspring using the parental genotypes and Punnet squares. Scientists then compare their observed results with the results expected using a chi-squared test.

Definitions

Key word	Definition	Example
Gene	The sequences of bases in a DNA molecule that encode for a protein that results in a characteristic.	A gene that encodes for pea pod colour.
Allele	Different versions of a gene.	Pea pods can be yellow or green.
Genotype	The genetic makeup of an organism.	Peas can be GG, Gg or gg for pod colour.
Phenotype	The expression of an organism's genetic makeup and the interaction it has with the environment.	Green pods.
Dominant	An allele whose characteristic always appears in the phenotype if it is present.	The allele for green pods (G) is dominant so having the genotype GG or Gg will produce green pods.
Recessive	An allele whose characteristic only appears when two copies are present.	The allele for yellow pods (g) is recessive, so if the pea has the genotype gg it will be yellow.
Codominant	Alleles that are both expressed in the phenotype.	The alleles for haemoglobin are codominant.
Locus	A fixed position on a chromosome where a specific gene is located.	The locus for pea pod colour is located on chromosome $5$ in peas.
Homozygote	An organism that carried two copies of the same allele.	GG or gg.
Heterozygote	An organism that carries two different alleles.	Gg.
Carrier	A person who carries an allele that is not expressed in their phenotype but can be passed on to their offspring.	A person may be a carrier of cystic fibrosis but not have the disease.

Monohybrid inheritance

Humans are diploid organisms, this means we have two sets of chromosomes and two alleles for each gene. One is inherited from the mother and the other is inherited from the father.

Monohybrid inheritance is the inheritance of a characteristic that is controlled by a single gene. We use monohybrid crosses to show the likelihood of different alleles of that gene being inherited.