We naturally inherit genetics directly from our parents and the allocation of the genes we inherit is completely random. However, in genetics, predicting inheritance is a skill that can help us determine the potential outcomes of reproduction.
There are countless different genes and genetic sequences in our DNA, which are inherited from our parents at the time of conception. During conception, we receive a random selection of chromosomes from each parent. These chromosomes will determine our genetics and physical characteristics down the line.
Each gene always comes with different alleles, which are different variants of the gene. For example, the gene for eye colour in humans. We have two different alleles for each gene; one allele comes from our mother and one from our father.
These alleles work together to determine an individual’s final phenotype (observable characteristics) and genotype (genetic makeup of the person).
Alleles can be either dominant or recessive. For example, the allele for brown eyes is dominant whereas the allele for blue eyes is recessive. A dominant gene will always override recessive genes. So, if an individual has a copy of both a recessive blue eye allele and a dominant, brown eye allele, they will always have brown eyes.
Monohybrid inheritance is the inheritance of characteristics that are controlled by a single gene. We can investigate monohybrid crosses using punnet squares.
A Punnett square is a genetic diagram that shows the possible combination of alleles that the offspring could have.
The allele for brown eyes is dominant (B) and the allele for blue eyes is recessive (b).
Two parents with brown eyes are expecting a child. The father is homozygous (BB), so he has two identical alleles of the gene for eye colour. Whereas, the mother is heterozygous (Bb), so she has two different alleles of the gene for eye colour.
So, the genetic cross between the two parents can result in any of the following allele combinations:
|B||BB (Homozygous Brown)||BB (Homozygous Brown)|
|b||Bb (Heterozygous Brown)||Bb (Heterozygous Brown)|
As you can see in the punnet square above, any offspring from this pairing will always have brown eyes. However, there is a 50% chance (2 out of 4) that the offspring will carry the blue-eyed gene.
If this child grew up and had a child with a blue-eyed partner (bb), the Punnett Square for the genetic cross will be:
|b||Bb (Heterozygous Brown)||bb (Homozygous Blue)|
|b||Bb (Homozygous Blue||bb (Homozygous Blue|
In this genetic cross, there is a 50% chance (2 out of 4) that the child would be born with blue eyes, and a 50% chance that the child would have brown eyes. So, the ratio of Bb to bb is 1:1
These examples are single gene combinations. However, it is important to note that most phenotypes are controlled by multiple genes.
Family tree diagrams (also known as pedigree charts) show the presence of phenotypes of a particular gene, within a family, over several generations. The diagram uses different symbols and colours to show male, female, healthy and affected individuals.
For example, 3 (healthy male) and 4 (affected female) had four children (10, 12, 13 and 14)
We know that the affected mother (2) must have had a healthy allele too because she only passed on the disease to some of her children. This means that she is heterozygous (e.g. Bb). It also means that the healthy offspring of 1 and 2 have a homozygous recessive phenotype.
Family tree diagrams can be used to work out the genotype of each individual and the probability of inheriting a genetic disorder. However, we can now use blood tests to determine the presence of genes.