Question

1. How is it that we all have the same genes, yet we can have different genotypes for a given gene that results in the expression of a different phenotype? Again, I am looking for a description of what is happening on a molecular level to result in different phenotypes.

Answer 1

1. A section of DNA that encodes a trait or specific character is called a gene. The part of a DNA in which a gene is present is called locus. A gene can exist in alternating forms called alleles. An organism can inherit two copies of same allele or one copy of two different alleles from their parents.If an individual pocess two identical alleles, their genotype is said to be homozygous at that locus, if not then it is called hetrozygous genotype. The subsequent combination of alleles that an individual possesses for a specific gene is called it's genotype.  

Let's consider an example of a gene coding for eye color in humans ; Brown (BB or Bb) and blue (bb).

The brown allele is dominant (B), and the blue allele is recessive (b). If the child inherits two different alleles from their parents then they will have brown eyes (Bb). For the child to have blue eyes, they must be homozygous for the blue eye allele (bb).

A cross between hetrozygous brown (Bb) and Homozygous blue (bb) will produce offsprings with brown eye (Bb) or with blue eye ( bb) but a cross between Homozygous brown (BB) and Homozygous blue (bb) will yeild genotype of hetrozygous brown (Bb) only.

Alleles of the same gene can be autosomal dominant or recessive. An autosomal dominant allele (B) is expressed over a recessive allele(b).

The sum total of observable character in an organism is called the phenotype ( Brown eye or blue eye) of that organism. A phenotype is influenced the genotype. The phenotype is also influences by environmental factors and epigenetic modifications (acetylation, Methylation etc).

Eg. Our genes control the amount of melanin that we produce, however, exposure to UV light encourages increased melanocytosis and as a result the skin become darker.