Single gene
When a certain gene is known to cause a disease, we refer to it as a single gene disorder or a Mendelian disorder
–> they follow his laws of heredity
Polygenic gene
Refers to a trait whose phenotype is influenced by more than one gene
–> offspring phenotype is predicted from parental phenotype using quantitative genetic models
ex.: height, skin color
Homozygous
(Reinerbig)
A diploid organism that has two copies of the same allele
ex.: AA or aa
Heterozygot (Mischerbig)
A diploid organism that has one copy of two different alleles
–> only the dominant allele will be visible in the phenotype
ex.: Aa
Mendels theory of heredity
Inheritance involves the passing of discrete units of inheritance, or genes, from parents to offspring
Mendels Principles of heredity
- Law of segregation
- Law of dominance
- Law of independent assortment
Law of dominance
The dominant allele determines the appearance/phenotype of the organism
Law of segregation
Each individual carries 2 alleles, one of each parent
Law of independent assortment
Phenotypic traits controlled by different genes can be separated from each other through generations
What does the law of independent assortment suggest ?
- Novel phenotypic combinations can arise through sexual reproduction
- Natural selection can change the frequencies of one phenotypic characteristic, without changing the frequency of another
Why does independent segregation occur ?
Because at meiosis, the selection between which of the 2 copies goes forward to the gamete is done INDEPENDENTLY for each chromosome
Mendelian diseases
Follow his principles of heredity + apply to all single gene characteristics
ex.: Huntingtons disease, phenotypic effects develop after the individual has reproduced
Hardy-Weinberg equilibrium
Model that predicts what will happen in a sexually reproducing population where there is no natural selection
- Alleles will become neither more common nor more rare over time
- The relative proportions of homo- and heterozygotes will be constant
FORMULA: P^2 + 2pg + g^2
Genetic drift
Refers to a variation in the relative frequency of different genotypes in a small population, leading to a disappearance of particular genes as individuals die or do not reproduce, all due to CHANCE
–> there will be a certain amount of change even in the absence of natural selection
Fixation
Results from fluctuation, (Genetic drift) and ends with everybody in the population having the same allele
Neutral theory of molecular evolution
(Kimura)
States that most evolutionary changes at the molecular level, and most of the variation within and between species, are due to random Genetic drift/Fluctuation of mutant alleles that are selectively neutral
- small population: high probability of fixation
- big population: high probability of mutation
=> both cancel each other out
Heritability
Refers to the proportion of the observed phenotypic variation that can be accounted for by genetic variation within a particular population at a particular time
–> mainly estimated using twin + adoption studies
Coefficient of relatedness
Refers to the probability that any particular allele in one individual is identical (=derived from the same immediate source) as the allele in individual B
–> the higher the level of inbreeding the closer the coefficient of relationship between the parents approaches a value of 1
ex.: the probability that my brother has the same allele as me is 50%, me and my grandma 25%
Dizygotic twins vs Monozygotic Twins
DZ: result from the implantation of 2 fertilized eggs in the same cycle
–> genetically different
MZ: result from the implantation of 1 fertilized egg
–> genetically the same
In which way do shared + non-shared environments influence twins ?
Shared environments will influence both twins, regardless if MZ or DZ
ex.: parental social class, parental behavior, schools
Non-shared environments won’t influence them both, but affect them individually
ex.: diseases, accidents
ACE model (Twin studies)
Refers to a statistical model used to analyze the results of twin studies
–> aims to decompose sources of phenotypic variation into three categories
- Additive genetic variance (Heritability)
- Common environmental factors (Shared environment)
- Specific Environmental factors (Non shared environment)
Dominant allele
Refers to an allele that is expressed even though only one copy is present
Recessive allele
Refers to an allele for which both copies are required for phenotypic expression
Population
Refers to a group of individuals of the same species that live in the same area and interbreed, producing fertile offspring
