Unit 3: Inheritance
Language of Genetics
Gene
A gene is a section of DNA or RNA which codes for a function (or a protein). An example of gene is TP53, which codes for a tumour suppressor to prevent cancer.
Allele
An allele is a different form of a gene, which decides the type of trait. One gene may have different alleles, at the same chromosome location.
Let's take the gene called ABO, which controls our blood group type, as example. Of that gene, there are different types of alleles, determining our blood group type.
Genotype
A certain trait’s genotype is the genetic information of the trait.
Phenotype
A certain trait’s phenotype is the physical characteristic the trait.
Dominant
A dominant allele masks a recessive allele, thus the dominant trait will always show.
Recessive
A recessive allele will only be shown if both allele is recessive.
The below example explains the dominant and recessive relationship by demonstrating the result of different allele combinations in a gene that controls the color of the flower, where R is for red allele and r is for white allele:
As seen in the example, the flower with both alleles is red in color. This shows that the R allele is dominant because it masked the r allele, making the flower red.
Laws of Inheritance
Law of segregation
Given that each gene in a normal cell has 2 alleles that maybe the same or different, the law of segregation suggests that:
- Allele pairs separate during gamete formation, so gametes only have 1 allele per gene
- If the two alleles is different, the dominant allele will always be the observable trait, while the recessive allele will be masked by the dominant one.
Alleles separate during gamete formation
Law of independent assortment
The law of independent assortment applies when a cross is carried out between two individuals that are hybrid for two traits on separate chromosomes. This low states that during gamete formation, the genes for one trait are not inherited along with the gene for another trait.
In the following example, we will take the inheritance of a plant seed’s height (T for tall and t for dwarf) and color (Y for yellow and y for green).
As you can see, each gene’s allele inheritance would not affect others.
Drawing Diagrams
Punnett square
Punnett square is used to predict the outcome of a cross, while the parent’s genotype is know. Below are the steps to draw a punnett square, taking flower color inheritance (a monohybrid cross) as example:
- Write down the label of alleles
R - red allele
r - white allele
2. Draw the boxes (leave space on top and left of the box)
3. Write down parent genotype, one parent on one side
4. Write down the combination
5. If possible or needed, write the phenotype in the box
Genetic diagram
A genetic diagram shows all possible allele combinations of the offspring. You can think it as the step-by-step version of a punnett square.
- Write down the label of alleles
2. Write down parent’s phenotype and genotype
3. Write down the gametes and draw a circle around (Note that if a parent’s genotype is homologous, you only need to draw one gamete carrying on of the allele)
4. Draw the lines of fertilization and write down the offspring’s genotype and phenotype
5. Write down the phenotype ratio if needed
Mutations
Mutations can be classified into chromosomal mutations and gene mutations, the table below compares both.
Chromosomal mutations
Chromosomal mutations are mutations or abnormalities in the chromosome structure or number. Below are different types of chromosomal mutations.
- Deletion
2. Duplication
3. Inversion
4. Insertion
5. Translocation
6. Aneuploidy
Aneuploidy results in an extra or missing chromosome. A commons example of aneuploidy is an extra chromosome 21, which results in Down syndrome.
Gene mutations
1. Deletion
2. Insertion
3. Substitution
A specific type of substitution is called nonsense mutation, which results in a substitution that creates a stop codon. This causes the protein to be truncated, incomplete and most likely nonfunctional.
4. Inversion
