Question

Why does a neuron and a liver cell look and behave so differently?

By what mechanisms do new alleles arise?

What are the sources of genetic variation in a sexually reproducing population?

Answer 1

1. All cells in the body of a multicellular organism such as humans are derived from a single cell known as the zygote by mitotic divisions. The mitotic division is an equational division and produces two identical cells.

So, all cells in the human body contain the same genome. However, each cell is exposed to different environmental and internal signals so that it exhibits a variable differentiation status. i.e. each cell is terminally differentiated in different aspects such as morphology and function due to differential gene expression patterns.
A neuron and a liver cell appear different in their morphology and function as they express different sets of genes (Tissue-specific expression).

2. Mutation = A sudden heritable change in the genome
Allele = Alternative form of a gene = Any sequence variant of a gene
The mutation is the ultimate source of allelic variation.

3. Genetic variation in sexually reproducing organisms is derived by two mechanisms
a. Independent assortment of chromosomes during gametogenesis
b. Crossing over between non-sister chromatids of homologous pair of chromosomes