genetic variation

Meiosis

A reduction division that produces haploid gametes with half the number of chromosomes of normal body (somatic) cells.

Mutation

The source of genetic variation.

Crossing over

The exchange of sections of DNA between homologous chromosomes.

Independent assortment

Describes the different possible combinations of maternal and paternal chromosomes.

Random fertilization

Introduces variation because the combination of gametes that fuses to form the zygote is random.

Allele

An alternative form of a gene.

Phenotype

Observable characteristics of an organism.

Genotype

The alleles present within cells of an organism.

Dominant

An allele that is always expressed in the phenotype, even if only one copy of the allele is present.

Recessive

An allele that is only expressed in the phenotype if two copies of the allele are present, i.e., homozygous recessive.

Homozygous

Two identical alleles of a gene (a homozygous organism is called a homozygote).

Heterozygous

Two different alleles of a gene (a heterozygous organism is called a heterozygote).

Codominance

Both alleles contribute to the phenotype without mixing, e.g., blood type.

Multiple Alleles

More than two possible versions of an allele.

Chi-Squared Test

A statistical test used to establish whether the difference between observed and expected results is significant.

Sample size for Chi-Squared Test

Must be sufficiently large (>20) and can only be used for discontinuous variation data.

Null hypothesis

The assumption that there is no significant difference between observed and expected results.

Critical value

The value obtained from the chi-squared test is compared to this to determine if the null hypothesis is accepted or rejected.

Significance in Chi-Squared Test

If the x² value is greater than the critical value, the null hypothesis is rejected, indicating a significant difference.

Autosomal gene linkage

Implied if the null hypothesis is rejected, e.g., color and wing length in Drosophila.

Linked Genes

Genes that are inherited together due to their proximity on the same chromosome.

Sex-linked Genes

Genes that appear on the X chromosome, often leading to disorders more common in men.

Haemophilia

A genetic disorder characterized by the absence of clotting factor VIII, resulting in improper blood clotting.

Red-green Colourblindness

A genetic condition affecting the ability to distinguish between red and green colors, often linked to the X chromosome.

Natural Selection

The process through which allele frequencies change over time due to selection pressures acting on the gene pool.

Stabilising Selection

A type of natural selection that reduces variation in a population by selecting against extreme phenotypes.

Disruptive Selection

A type of natural selection that increases diversity in a population by selecting for extreme phenotypes in diverse conditions.

Directional Selection

A type of natural selection that shows a change from one phenotype to another that is more advantageous to the environment.

Population Bottleneck

The effect of a catastrophic event that dramatically reduces population size by at least 50%, causing a severe decrease in genetic diversity.

Founder Effect

The loss of genetic variation that occurs when a small number of individuals form a new population with allele frequencies not representative of the original population.

Genetic Drift

Random changes in the gene pool of a population that occur by chance, not due to any selective advantage or disadvantage.

Hardy-Weinberg Equation

A mathematical formula used to estimate the frequency of alleles in a population, represented as p + q = 1.0.

p

The frequency of the dominant allele (represented by A) in a population.

q

The frequency of the recessive allele (represented by a) in a population.

p^2

The frequency of homozygous dominant individuals (AA) in a population.

2pq

The frequency of heterozygous individuals (Aa) in a population.

q^2

The frequency of homozygous recessive individuals (aa) in a population.

Genetic Equilibrium

A state in which allele frequencies in a population remain constant over generations.

Conditions for Hardy-Weinberg Equilibrium

No mutations, random mating, large population, no migration, and no selection pressure.