Common Misconceptions-Principles of Inheritance and Variation

Alleles and Genes
Often the terms alleles and genes are misunderstood to be the same, but gene is the segment of the DNA which control the specific trait in organisms and an allele is the particular variant (alternative) of the gene. For example, the hair colour is a trait which is expressed by a gene while different hair colours are possible because of different alleles of the gene for hair colour.

Not so Common Heterogametic Males
What comes to your mind when you hear the word heterogametic? Males with XY chromosomes right? In humans, females have XX sex chromosomes and are homogametic and males have XY sex chromosomes and are heterogametic. But do all males are heterogametic with XY chromosomes? No, this is not the case. For example in grasshoppers, crickets, cockroaches and some other insects, the males have only one X chromosome, while females have XX chromosomes. Hence, in this system, though the males are heterogametic, but they have only one sex chromosome, X. In birds, males are actually homogametic with ZZ chromosomes while the females are heterogametic with ZW chromosomes.

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Non-Mendelian Inheritance
Mendel being entitled as the ‘father of genetics‘, we expect all genes to follow Mendel’s laws. But not all genes follow the Mendelian laws of inheritance. Some genes may disobey Mendelian laws of inheritance.This happens because not all traits segregate in accordance with Mendel’s laws. A non-mendelian trait does not pass on to successive generations. For e.g. blood types, hair colour, height, etc.

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Linkage and Recombination

Linkage and recombination are phenomena that describe the inheritance of genes. A linkage is a phenomenon where two or more linked genes are always inherited together in the same combination for more than two generations, where as recombination is a process by which pieces of DNA are broken and recombined to produce new combinations of alleles. The recombination frequency and linkage between the genes is inversely proportional. That means recombination frequency of the test cross progeny is always lower than 50%. Therefore, if any two genes are completely linked, their recombination frequency is almost 0%.

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Test cross and Back cross

We usually get confused between a  test cross and a back cross. But the main difference between test cross and back cross lies in their role of action.Test cross is a cross between F1 generation individual with recessive parent. It helps in the identification of the genotype of the F1 individual.

Back cross is a cross between F1 generation individual with one of its parent (dominant or recessive) and helps in the identification of the genotype by checking the segregation of genes at the time of gamete formation.

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