2.5 - Genetic Variation

DNA

A double-stranded nucleic acid that stores the organism's unique genetic code.

complementary base pairing

C pairs with G, A pairs with T (in DNA), A pairs with U (in RNA).

gene

A sequence of DNA that carries the genetic code for the production of a protein.

chromosome

A thread-like structure of tightly packed DNA and protein found in the cell nucleus.

trait

A characteristic or condition determined by genes.

gamete (sex cell)

A reproductive cell having the haploid number of chromosomes. Sperm, egg, pollen, ovule.

somatic cell

Any cell of the body that is not a sexually reproductive cell.

autosome

Any chromosome that is not a sex chromosome.

allosome (or sex chromosome)

Chromosomes that determine the sex of the individual.

homologous pairs of chromosomes

Chromosome pairs that are similar in length, gene position, and centromere location.

homologue

One of the two homologous chromosome pairs.

sister chromatids

Pair of replicated chromosomes. One is the original chromosome, the other is an identical copy.

centromere

A structure in a chromosome that holds sister chromatids together.

haploid

A cell that contains a single set of unpaired chromosomes.

diploid

A cell that contains two complete sets of chromosomes, one from each parent.

karyotype

A diagram of all the chromosomes from one cell arranged as homologous pairs in descending size order.

genotype

Combination of alleles an individual possesses.

phenotype

Observable characteristics or physical features of an individual.

allele

Alternative form(s) of a gene.

genetic variation

The difference in DNA sequences between individuals within a population.

continuous variation

Variation in which a series of intermediate phenotypes fall between the extremes.

discontinuous variation

Variation in which no intermediate phenotypes fall between the extremes.

locus (loci plural)

The position that a given gene occupies on a chromosome.

protein

Large, complex molecules that have critical roles in the cell and body. Encoded for by genes.

chiasma (chiasmata plural)

The point at which homologous pairs contact during crossing over.

crossing over (recombination)

Two homologous pairs of chromosomes exchange segments of their genetic material → increases genetic variation.

independent assortment

Homologous pairs of chromosomes line up in random combinations along the equator, independent of other chromosomes → increases genetic variation.

segregation

Sister chromatids line up in random combinations along the equator, independent of other sister chromatids → increases genetic variation.

mitosis

Type of cell division for somatic cell growth and repair. Daughter cells are diploid and identical.

meiosis

Type of cell division that reduces the chromosome number from diploid to haploid and produces genetically unique gametes.

mutation

A permanent change in the base sequence of DNA.

mutagen

An agent, such as a chemical, UV light, or a radioactive element that can induce or increase the frequency of mutation in an organism.

fertilisation

The event where a male gamete (sperm, pollen) and female gamete (egg, ovule) fuses to form a zygote.

zygote

A fertilised egg/diploid cell created immediately after fertilisation.

centrosome (centriole also ok)

Organelle important for cell division as it produces spindle fibres.

spindle fibres

String-like structures produced by centrosomes that attach to the centromere of a chromosome during cell division.

daughter cells

General term for cells produced by cell division.

complete dominance

Pattern of inheritance where the dominant allele will always be expressed.

incomplete dominance

Pattern of inheritance where neither allele is fully expressed, resulting in an intermediate or blended phenotype.

codominance

Pattern of inheritance where both alleles in a heterozygous organism contributes independently and equally to the phenotype.

multiple alleles

Alleles of which there are more than two alternatives available for one gene.

lethal allele

An allele that causes the death of an organism at any stage of life. Caused by mutations in an essential gene.

recessive lethal allele

An allele that causes death in homozygotes only.

dominant lethal allele

An allele that causes death in both homozygotes and heterozygotes.

dihybrid cross

Mating between two organisms where the inheritance patterns of TWO genes are studied.

monohybrid cross

Mating between two organisms where the inheritance patterns of ONE gene is studied.

linked genes

Genes that are located on the same chromosome, and are inherited together.

filial

Term used to refer to the generation number of offspring

test cross

Mating between a homozygous recessive organism and a phenotypically dominant organism with an unknown genotype.

recombinant

chromosomes

Chromosomes that have segments from both parents due to crossing over.

homozygous

Both alleles of a gene are the same. (E.g. AA or aa)

homozygous recessive

Both alleles of a gene are recessive. (E.g. aa)

homozygous dominant

Both alleles of a gene are dominant. (E.g. AA)

heterozygous

Both alleles of a gene are different. (E.g. Aa)

homozygote

An organism with a homozygous genotype.

heterozygote

An organism with a heterozygous genotype.

gene pool

All the alleles present amongst the individuals in a population.

genetic diversity

Variation in the genotypes in a population or gene pool.

allele frequency

The percentage of each allele in a gene pool.

allele fixation

When the frequency of an allele goes from <100% to 100%, thus decreasing genetic diversity.

natural selection

The process where the alleles best adapted to the environment will survive and reproduce.

selection pressure

An environmental factor that can drastically change the frequency of alleles in a gene pool.

migration

The process of individuals moving into or out of a population.

gene flow

When immigrating/emigrating individuals interbreed with individuals in the new population, thus adding their alleles to the gene pool. (Also applies to emigration).

genetic drift

Random change in allele frequency of a population due to chance.

founder effect

When a new gene pool of a small population is established from a larger population.

bottleneck effect

When a species is suddenly reduced to a few individuals. As the numbers in the population recovers, there is low genetic diversity.

Purine

2 ringed structure (guanine and adenine)

Pyrimidine

1 ringed structure (cytosine and thymine)