Inheritance and genetic diagrams

In a nutshell

We can inherit genetic characteristics from our parents. However, there is a vast number of combinations of genetic material we inherit that can result in unique characteristics. Genetic diagrams may help us understand the chances of inheriting certain characteristics. 

Definitions

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Inheriting characteristics

Genes can be passed from parents to offspring. The combination of genes inherited can mean that some characteristics come from the mother and come from the father. Variation in inherited characteristics arises from alleles. One allele of a gene is inherited from the mother and one is inherited from the father. This results in a unique combination of physical characteristics. 

Some allele copies are dominant and some are recessive, which affects both the genotype and the phenotype. The genotype can contain two dominant alleles and this is called a homozygous dominant genotype. It can also contain two recessive alleles, which is called a homozygous recessive genotype. It can also contain two different alleles, which is called a heterozygous genotype. 

If a dominant allele is present in a person's genotype, it will always be expressed in the phenotype. Therefore, a homozygous dominant or heterozygous genotype will always express the dominant characteristic. Recessive alleles will only be expressed in the phenotype when the dominant allele is absent. Therefore, we always require two recessive alleles to express a recessive characteristic in the phenotype. 

Genetic diagrams

The combination of alleles from monohybrid inheritance in a genotype and its resulting phenotype can be presented in a genetic cross diagram. 

Example

Let us consider alleles for hair colour. One allele can code for brown hair; this allele is dominant. We can call this allele B. Another allele codes for blonde hair; this allele is recessive. We can call this allele b. 

A couple are considering having a child and are curious to know what their child's hair colour could be. The mother has a phenotype of brown hair. She has a genotype with alleles B and b. The father has a phenotype of brown hair. He also has a genotype with alleles B and b. 

Their child will inherit one allele from each parent, resulting in a new combination of alleles for their genotype. This will impact their phenotype. 

From the genetic diagram, we can see that the child has the following possibilities of genotypes: 

We can also find out the possibilities of phenotypes:

Determining sex

We can also use genetic crosses to determine the chances that a future child is a male or female. However, sex is not determined by dominant and recessive alleles but it is determined by the combined expression of two chromosomes. 

Sex is encoded by the X and Y chromosomes. A female will have the genotype of XX and a male will have the genotype of XY. 

The mother will have the genotype XX and the father will have the genotype XY. 

From the genetic diagram, we can see that the child has the following possibilities of genotypes and phenotypes: 

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Gregor Mendel

Genetic diagrams came about due to experiments run by Gregor Mendel. In these experiments, Mendel bred together peas with different characteristics and observed the phenotypes of the offspring produced. He then used these phenotypes to determine which characteristics were dominant and which were recessive. This set up the premise that characteristics could be inherited in specific patterns. 

Mendel's experiment

Mendel's work is important as these experiments were conducted before DNA was discovered; inadvertently, Mendel had discovered the way in which genes worked. His experiments also produced repeatable results, which increased their reliability. Without it, we would not have been able to understand inheritance and the importance of genes in developing characteristics. 

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