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

Keyword	Definition
Gene	A small section of DNA that codes for a specific characteristic.
Allele	Different versions of the same gene.
Genotype	The combination of alleles of a gene that are inherited.
Phenotype	The physical appearance of the inherited alleles.
Homozygous dominant	Two dominant alleles of a gene are inherited. This results in the dominant characteristic being expressed in the phenotype.
Homozygous recessive	Two recessive alleles of a gene are inherited. This results in the recessive characteristic being expressed in the phenotype.
Heterozygous	Two different alleles of a gene are inherited.

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.

Biology; Genetics; KS4 Year 10; Inheritance and genetic diagrams

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

Genotype	Alleles	Chance	Phenotype
*Homozygous dominant*	BB	$25\%$	Brown hair
*Heterozygous*	Bb	$50\%$	Brown hair
*Homozygous recessive*	bb	$25\%$	Blonde hair

We can also find out the possibilities of phenotypes:

Phenotype	Chance
Brown hair	$75\%$
Blonde hair	$25\%$

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.

Biology; Genetics; KS4 Year 10; Inheritance and genetic diagrams

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

Genotype	Chance	Phenotype
XX	$50\%$	Female
XY	$50\%$	Male

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

1.	Mendel bred together pea plants that would always produce offspring that are identical to the parents. He produced some pea plants that were tall, and some that were short. These offspring formed the first generation of peas in the experiment. One of these characteristics was also the dominant characteristic.
2.	Mendel then bred the tall plants with the short plants from the first generation. He found that all the offspring in this second generation were tall. This meant that the inherited characteristic of being tall was dominant, and the characteristic of being short was recessive.
3.	These offspring of the second generation were bred with each other. The cross of heterozygous plants with each other resulted in a mix of characteristics, with three quarters of the plants being tall and a quarter being short.

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.