Question

All humans are ~99.9% genetically identical, yet we do not appear identical. Even siblings from the same mother and father can look drastically different. What explains these differences? Use these vocab words in your answer: gene expression, homologous chromosomes, law of segregation, recombination, independent assortment, allele(s), mutations.

Answer 1

Human genome size = 3 X 10⁹ bp
The similarity between two individual genomes = 99.9%
Difference between two individuals = 0.01%

Even if the difference is 0.01%, it corresponds to
= 0.01 X 3000000000
= 30000000 bp

i.e. two individuals differ from each other at 30000000 bp. It is a big number to cause variation in phenotype even though most of these variations are located in the noncoding regions.

The phenotypic differences between two individuals are also attributed to epigenetic differences i.e. No change in the genomic sequence but differences in the epigenome that affects how individual genes are expressed.

During meiosis, homologous chromosomes pair with each other. Non-sister chromatids of a homologous pair undergo crossing over/recombination to produce new recombinant gametes. Genes/alleles that are distantly located exhibit independent assortment while genes that are located close together on the same chromosome exhibit close linkage and tend to assort together. Often, some mutations also arise during cell division. Hence each individual gets a unique set of genes from their parents which is responsible for their diverse phenotypes.

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