BIO 203 Q4

Euploid

The condition of having a complete set of chromosomes 

Aneuploidy

The condition of having an abnormal number of chromosomes, leading to an unbalanced chromosome complement 

Nondisjunction

A process where chromosomal separation does not occur 

• Can occur with chromosomes or sister chromatids

Nondisjunction in meiosis 1

failure of homologous chromosomes to separate

• Gametes are either n+1 or n-1

Nondisjunction in meiosis 2

failure of sister chromatids to separate normally

• Only two of the four gametes produced will be affected

• Affected gametes will be either n+1 or n-1

How cancer cells differ from normal cells

• Normal cells are highly specialized, but cancer cells dedifferentiate and can no longer produce correct proteins

• Cancer cells have higher rates of proliferation than normal cells

• Cancer cells are larger and have larger nuclei

• Cancer cells are poorly organized and so do not stay within boundaries (metastasis)

Proto-oncogene

normal genes stimulating cell division and progress through the cell cycle

• When mutated to be overactive, they form oncogenes

• Only need one mutation to become cancerous

Oncogene

A gene that promotes tumorigenesis by promoting inappropriate cell proliferation/growth. Cancer cells typically acquire a gain of function mutation in an oncogene

Tumor suppressor gene

A gene that prevents tumorigenesis

• The gene’s normal function is to put the “brakes” on a pathway that can lead to inappropriate growth (ie cell division). Active at cell cycle checkpoints

• Tumor suppressor gene function is lost in cancer cells (LOF mutation)

• Requires two mutations to be cancerous

• Ex. rb, p53

Two-hit hypothesis

loss of function in both copies of a tumor suppressor gene causes tumorigenesis

Genome instability

large number of chromosome duplications, deletions, rearrangements, and changes in number

• Common in cancer cells

Chromosome translocation

The relocation of a chromosome or chromosome segment to a non-homologous chromosome 

De novo

“from scratch”; term used to describe how a trait or characteristic arose in the population - meaning it did not come from a direct variation on another trait 

Differentiated

Process by which cells become restricted in their developmental potential and take on specialized morphologies and physiological activities

Totipotent

A cell with the potential to make any cell type from an organism. 

• Embryonic stem cells in vertebrates

• All genes have the potential to be expressed

Pluripotent

A cell with the potential to make most but not all cell types from an organism.

Biochemical vs biological pathways

• Biochemical pathways: the gene most upstream wins

  • Substrates generated are necessary for the next steps so the pathway becomes blocked by LOF mutations

• Biological pathways: the gene most downstream wins

Discontinuous variation

A phenotype distribution containing discrete or separable categories 

• Usually caused by few genes and little environmental effects

Continuous variation

In polygenic and multifactorial traits, the observation of phenotypic distribution over a continuous range. 

• Usually polygenetic

Additive genes

Genes contributing to a polygenic trait and producing their effect by their cumulative contributions that are approximately equal for each gene 

• As the number of additive genes increases, the number of possible phenotypic categories increases

Variance

Measure of the spread of distribution around the mean; sum of the square difference between each value and the mean, divided by the df (ie the number of independent variables) 

Latent variation

Phenotypes seen in the offspring not found in the population arise from new combinations of additive alleles 

• Alleles are already in the population

Norm of reaction

a curve that represents phenotype expressed by a given genotype as a function of environmental conditions 

• The gene by itself does not code for a trait. It codes for a trait in the context of a particular environmental condition

Variance

the measure of the spread of distribution around the mean

• Represents how much variance exists in the sample

• s² = Σ(value - mean) / df

• s = standard deviation

Phenotypic variance (Vp)

• Used to analyze quantitative trait variation

• Genetic variance: the proportion of phenotypic variance due to genotype differences

  • In the lab, Vg=0 because all alleles are controlled

• Environmental variance: proportion of the phenotypic variance due to environmental conditions

• Vp = Vg + Ve

Heritability

measures the proportion of phenotypic variation that is due to genetic variation

• High heritability is mostly a result of genetic variation - some environmental factors are still present

• Measure of the potential responsiveness of a trait to selection

• Measured as either broad or narrow sense

• Only accurate for the environment and population in which they are measured

• Subject to change

Broad sense heritability (H²)

The proportion of total phenotypic variance that is contributed by total genetic variance 

• H² = Vg/Vp

• Magnitude from 0.0 to 1.0

Narrow sense heritability (h²)

The proportion of total phenotypic variance that is contributed by additive genetic variance 

• h² = Va/Vp

• Vp = Va +Vd + Vi + Ve

• Magnitude from 0.0 to 1.0

Allelic effects of genetic variance

• Additive variance (Va ): the added effects of all alleles contributing to the trait

• Dominance variance (Vd): dominance relationships in which heterozygous individuals are not intermediate between the two homozygous states

• Interactive variance (Vi): epistatic interactions between alleles of genes contributing to a quantitative trait

Selection differential (S)

the difference between the means of the whole population and the breeding population

Response to selection (R)

the extent to which the difference between the population mean and the mean of the breeding population can be passed on to the progeny

• R = S * h²

• Greatest when h² = 1

Directional selection

occurs when the mean phenotypic value is shifted in one direction because one extreme of the phenotypic distribution is favored 

• Phenotypic range is narrowed and variance is reduced

• Change in mean

• Easier to do with higher h²

Stabilizing selection

favors an intermediate phenotype over extremes 

• Phenotypic range is narrowed and variance is reduced

• Same mean

Disruptive selection

occurs when the extreme phenotypes are favored over the intermediate phenotypes

• Phenotypic split may occur in the population

• Phenotypic variance is increased

• Same mean