Introduction
Mendel’s basic model of inheritance explains the characteristics of inheritance and how they are inherited in all organisms, including human beings.
Key points
- Alleles are the different versions of genes. Usually, an organism has two different alleles – one recessive allele, which is suppressed, and the other is the dominant allele.
- Law of independent assortment – as the name suggests, inheritance is independent in this law. It states that the inheritance of genes of different traits is independent of each other.
- Genes are the heritable factors that are determined by heritable traits. Pairs of genes are present in an organism, i.e. there is the presence of two genes in an organism.
- Law of segregation – each sperm cell or each egg only receives one copy of the genes present in an organism during the process of gamete production. A random copy is allocated to each gamete.
These rules are mandated for our understanding of inheritance – how the phenotype (observable features) of an organism is determined by the genotype (set of alleles). To fully explain the inheritance model, some extensions, exceptions and Deviations from Mendelism need to be understood.
Deviations from Mendelism involving single genes
Some of the variations in single genes in Mendel’s model are –
1) Codominance – codominance refers to the relationship between the two versions of a gene. Each individual receives it from his or her parents. If the alleles are different, dominant alleles would be present. When the alleles are present in the same phenotype, it is known as codominance.
2) Pleiotropy – pleiotropy affects only one characteristic, but many characteristics are affected by the genes.
3) Multiple alleles – real populations usually have multiple alleles of a gene, while Mendel’s law studies just two alleles of his pea genes.
4) Deviations from Mendelism – incomplete dominance – in incomplete dominance, heterozygous offspring show their intermediate traits, and every genotype will have its phenotype. By having the same ratio in genotype and phenotype, they will always be the same. In traits of independent dominance, two traits of a particular trait are integrated so that neither dominates the other, and a new phenotype is created.
5) Sex linkage – genes carry some sex chromosomes such as the X chromosome and Y chromosome in humans, showing different inheritance patterns compared to autosomal (non-sex) chromosomes of genes.
6) Lethal alleles –Alleles that cause the death of the organism that carries them.
Variations involving multiple genes
Interactions between different pairs of genes involve other variations of Mendel’s laws. More than one gene affects or controls many characteristics, and when two genes affect a similar process, they can interact in many ways. Just like:
1) Epistasis – The alleles of one gene conceal or mask the other gene of an allele.
2) Complementary genes – the same phenotype may be given by two different types of recessive alleles.
There are three exceptions to Mendel’s observations: incomplete dominance, codominance, and pleiotropy. However, the exception to Mendel’s law of segregation is Linkage. In the law of segregation, a group of genes is inherited together, generation after generation and they are not segregated.
Incomplete dominance – it is a kind of intermediate inheritance. In this type of dominance, one allele of a particular feature is not wholly expressed over other alleles. It is also known as a partial allele. If we take the example of roses, the allele of the white colour is dominated by red. But the heterozygous flowers in which both the alleles are present would be of pink colour. When neither of the two alleles is dominant over the other, incomplete dominance occurs. This results in a combination of both types of phenotype. Gregor Mendel also proves this based on the pea plants. He studied the contrasting traits of seven characters, and all the characters show similar patterns of inheritance.
Key takeaways
1) Genes – genes contain some instructions for making proteins, and these are small units of DNA. Many genes are included in the chromosome.
2) Mutations – while some mutations do not impact health, other mutations cause diseases. These are the permanent changes to the DNA.
3) Recombination – it is one of the important processes. Sometimes a DNA is broken, and recombinations help us repair the broken DNA. When making sperms and eggs, it helps shuffle the DNA.
4) Complex diseases – complex diseases are caused by many different types of genes and environmental influences.
Conclusion
We have learnt about Deviations from Mendelism – Incomplete dominance classification, Mendel’s law of inheritance, Mendelian inheritance, Mendel’s basic model and its exceptions. Variations involving single genes are – codominance, pleiotropy, multiple alleles, incomplete dominance, sex – Linkage and lethal alleles. Variations involving multiple genes are epistasis and complementary genes.