9A: The Gene Pool

Gene

A specific sequence of nucleotides that codes for a particular protein or RNA molecule (or trait)

It is the unit of hereditary

Genotype

The genetic composition of an individual (its the combination of alleles in an organism)

Phenotype

The observable trait: expression of a genotype in an individual for a particular trait

The dominance of alleles and the environmental conditions influence the phenotype of an individual

Alleles

Different forms of a gene

Genetic Diversity

The variation in genes or alleles within a population or species

Population

All the individuals if a single species living in the same location at the same time

Gene Pool

The complete set of alleles present in a population

Variation

• Exists within populations and change over time

• Can be due to environmental factors or genetic factors in the populations gene pool

Continuous Variation

• Phenotype = genotype + environment

• Inheritance is polygenic (many genes)

Discontinuous (Discrete) Variation

• Phenotype = genotype

• Inheritance is monogenic (one gene)

Allele Frequency

The proportion of certain alleles in a gene pool

Qualities of Allele Frequency

• Can be calculated when each individuals genotype in a population is known

• Described on a scale of 0.0-1.0

Factors that can chance allele frequencies

MEGA

• Mutations

• Environmental selection pressure

• Genetic drift and gene flow

• Artificial selection pressure

The Hardy-Weinberg principle

MMCLS

• No Mutation

• Random Mating

• The population is Closed (no gene flow)

• The population is Large (no genetic drift)

• No Selection

Purpose of Mutations

• Responsible for introducing new alleles into a population (gene pool via changes to DNA)

• Increases genetic diversity

• Changes to an organisms DNA can occur at a range of levels

Types of Mutations

• Nucleotide: point mutations

• Gene: block changes

• Chromosome: aneuploidy

• Genome: polyploidy

Mutation

• A permanent change to a DNA sequence

• Most harmless and will not change the organisms phenotype

• Some will rest in a new allele (beneficial or deleterious)

• Can be spontaneous or induced by a mutagen

Mutagen

An agent that can cause mutations in DNA

Types of DNA Nucleotide Mutations

1. Point mutations

2. Frameshift mutations

3. Block mutations

Point Mutations

One nucleotide changed

• Silent mutation

• Missense mutation

• Nonsense mutation

Silent Mutation

• Nucleotide substituted, changing the codon, but still coding for the same amino acid

• Doesn’t affect protein structure

Missense Mutation

• Nucleotide substituted, changing the codon AND coding for a different amino acid

• Affects protein structure

Nonsense Mutation

• Nucleotide substituted, changing the codon to a STOP codon, ceasing transcription of the gene

• Affects protein structure

Frameshift mutation

A mutation that involves the insertion or deletion of one or two nucleotides, affecting the reading frame of all the following nucleotides

Block Mutations

• Mutations that affect a large chunk of DNA, or an entire gene

• They alter the structure of a chromosome and usually occur during the process of meiosis

Types of Block Mutations

1. Deletion mutation

2. Duplication mutation

3. Inversion mutation

4. Translocation mutation

Deletion Mutation

A section of DNA is removed from a chromosome, shortening the DNA

Duplication Mutation

When a section of DNA is replicated, lengthening the DNA

Inversion Mutation

When a section of DNA has its sequence reversed

Translocation Mutation

When two sections of DNA on different chromosomes switch places

Chromosomal Abnormalities

Genomes can also change by gaining or losing a chromosome or a set of chromosomes:

• Euploidy

• Aneuploidy

• Polyploidy

Euploidy

The usual number and sets of chromosomes

Aneuploidy

When a cell or organism varies from the usual number of chromosomes in its genome by the addition or loss of a chromosome

Polyploidy

When an organism contains additional sets of chromosomes

Types of Aneuploidy

Monosomy —> One less chromosome

Trisomy —> One additional chromosome

Types of Polyploidy

Triploidy —> Three sets of chromosomes

Tetraploidy —> Four sets of chromosomes

Effects of polyploidy on humans

Can be fatal

Effects of polyploidy on plants

• Increased fruit weight, fruit size seed size, crop load, etc.

• Reduction in fertility (in odd ploidy level individuals)

• Change in growth patterns

• Can be used to create higher diversity

• Greater ability to colonise new habitats than diploid ancestors

• Increase blooming period