Alleles
Alleles are alternate forms of a gene. Alleles control the contrasting characteristics of each trait. Alleles can either be dominant or recessive.
Dominant alleles
Dominant alleles are symbolised by capital letters. A dominant allele will always express (show) itself.
Recessive alleles
Recessive alleles will only express themselves in the absence of dominant allele. Recessive alleles are symbolised by lower case letters.
Genotype
The genetic makeup (combination of alleles) of an organism.
Phenotype
- The physical features/appearance of an organism
Homozygous
Homozygous means an organism has two identical alleles for a trait. Example : BB, bb
Heterozygous
Heterozygous means an organism has two different alleles for a trait. Example: Bb
Inheritance of Sex
females have two X chromosomes. Each ovum produced through meiosis will contain one X chromosome. males have one X and one Y chromosome. When sperm are produced through meiosis, they will contain either an X chromosome or a Y chromosome.
Genetic Traits Carried on Sex-Chromosomes
The Y chromosome is shorter than the X chromosome so it has fewer genes. Some genes on the X chromosome do not have a corresponding gene on the Y chromosome. Due to this, some genetic traits are more common in males than in females.
Pedigrees
A pedigree is a pictorial representation of the pattern of a genetic trait or condition in a family. Males are drawn as squares. Females are drawn as circles. Individuals with the trait are shaded.
Co-dominance
- Co-dominance is a form of inheritance where two or more alleles are equally dominant. This means that if both alleles are present, they will both be expressed in the resulting phenotype.
Genetic Mutation
A mutation is a permanent change in the DNA sequence. Mutations can occur in one or more genes.
Mutagens
Mutations occur naturally, but some factors can increase the chance of mutations occurring. Such as, Radiation, Chemicals, UV light
Negative Mutations from Radiation
Exposure to radiation can cause cancers and birth defects.
Variation
Some naturally occurring mutations lead to the production of new alleles. These new alleles lead to a more diverse gene pool and variation within a species.
Adaptations
A small number of these mutations may lead to adaptations, Animals that develop adaptations more suited to survival are more likely to live, and therefore more likely to pass on their genes
Species
the largest group of organisms in which any two individuals can produce fertile offspring
Natural Selection
Natural selection is the process by which individuals that are best suited to their environment pass on their genes. This theory was proposed by Charles Darwin
Gene Pools
All species have limited alleles that exist in a population. The gene pool includes all the alleles for all the genes in that population. When new mutations arise, alleles are introduced to this gene pool and the gene pool will increase.
Allele Frequency
Allele frequency is how common an allele is in a particular population.
Speciation
Speciation is the process by which one species splits into two or more separate species. The process of speciation occurs in three steps: Variation, Isolation, Selection.
Step 1: Variation
A few genetic mutations produce new alleles, which result in new traits. These new traits make the population more diverse. This is called variation.
Step 2: Isolation
Isolation happens when different groups within a population are somehow prevented from interbreeding. There are two main ways in which this occurs: Geographical barriers and Climatic barriers.
Step 3: Selection
Each group is exposed to selection pressures, such as: Food / shelter availability, Environmental conditions, Biological factors (predators, diseases etc.) Over time, the groups become less and less alike. Eventually, the two groups become so different that they cannot mate any more and the two are considered different species.
