Science, Technology and the Environment (SC NATS 1840A) - Tuesday
November 13 , 2012 th
Readings: 5.2, 5.3, 5.4 Genetics and Natural Selection
The Big Picture:
22 Amino Acids (monomers) Polymer called a polypeptide Protein (one or more polypeptides)
5 Nitrogen Bases (pieces of monomers) 5 Nucleotides (monomers) Polynucleotide Nucleic Acids
– DNA, RNA (one or more polynucleotides) Definition of Traits Manufacturing Instructions
Realization of Traits Protein (one or more polypeptides) Polymer called a polypeptide 22 Amino
Traits & Their Chemical Basis:
Traits: Any single feature or quantifiable characteristic of an organism. Result of many
biochemical processes involving specific proteins.
Eg. Hair/eye/skin, colour, gender, ability to break down lactose, etc.
Nucleus of each human cell contains 48 chromosomes which work in pairs to set traits
Single highly-coiled molecule of DNA;
Each has a counterpart that carries information about the same traits (one from each
Slight variations in the Genes on each member of a pair (different alleles) possible,
corresponding to differences in the relevant trait (eg. Brown eyes vs. Blue eyes)
Sometimes, DNA replication goes away resulting in changes in a gene sequence. Such a change
is called a mutation
Changes in DNA lead to changes in RNA and thus in the proteins encoded.
Most mutations are fatal to the affected cells. But sometimes a mutation occurs that creates a
new viable allele of a gene, i.e. a new version of the associated protein.
A viable mutation in a sex cell of an organism may be passed on to future generations. This
phenomenon drives evolution.
Not all alleles of a gene are equal (biochemically).
Suppose gene for some trait has only two alleles. One of them is dominant (D) - will be
expressed – and the other is recessive (r) when they are combined in a cell. When chromosomes of mother and father are mixed there are three possible genotypes for any
offspring with regard to this trait: DD, Dr, rD, rr
First D Inherited from mother. Second D inherited from father
Dominant Gene: If individual’s genotype includes at least one dominant allele of the gene, then the
dominant version of the associated trait results. The expressed trait is the organism’s phenotype.
The recessive form of the trait is only expressed if both genes are of the recessive allele.
Eg. Mom’s Genotype: DD D (phenotypes)