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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




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

1. Law of segregation

2. Law of dominance

3. 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 ?

1. Novel phenotypic combinations can arise through sexual reproduction

2. 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

1. Alleles will become neither more common nor more rare over time

2. 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



Results from fluctuation, (Genetic drift) and ends with everybody in the population having the same allele


Neutral theory of molecular evolution


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

  1.  small population: high probability of fixation
  2.  big population: high probability of mutation


=> both cancel each other out



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


1. Additive genetic variance (Heritability)

2. Common environmental factors (Shared environment)

3. 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



Refers to a group of individuals of the same species that live in the same area and interbreed, producing fertile offspring


Gene pool

Refers to the genetic makeup consisting of all copies of every type of allele at every locus in all members of the population


True breeding

Refers to an organism that always passes down certain phenotypic traits to its offspring of many generations

ex.: purple flower will only produce purple flowers in the F generations

--> homozygous



Refers to the crossing of 2 true-breeding varieties

--> heterozygous



With co-dominance, a cross between organisms with two different phenotypes produces an offspring with a third phenotype in which both of the parental traits appear together

ex.: white cow + red cow = white + red cow 


Incomplete Dominance

Refers to a form of intermediate inheritance in which one allele for a specific trait is not completely expressed over its paired allele

--> results in a third phenotype in which the expressed physical trait is a combination of the phenotypes of both alleles

ex.: white + red rose --> pink rose


Do only dominant and recessive alleles determine an organisms appearance ?


there is incomplete + codominance which contradicts the law of dominance