TASK 2 Flashcards
Heritance
Blending inheritance
- Inheritance of traits from 2 parents (black and blond hair) produces offspring with characteristics that are intermediate
- -> a mixture between traits of the parents (brown hair)
Single-gene characteristic
- allelic variation in a single gene accounts for differences in the phenotype
Polygenic trait
- multiple genes have one effect
- each (dominant) allele “adds up” to the expression of the trait (if you have many alleles that code for height they add up and contribute to the overall big height)
Allele
- Variant form of a given gene
- Different alleles can result in different observable phenotypic traits
Homozygous
Having two copies of the same allele (AA or aa)
Heterozygous
Having two copies of different alleles (Aa)
Dominant allele
A gene that is expressed phenotypically in heterozygous or homozygous individuals
Recessive allele
A gene that is phenotypically expressed in the homozygous state
- has its expression masked in the presence of a dominant gene
Punnet square
- Square diagram
- used to predict an outcome of a particular cross or breeding experiment + determine the probability of an offspring having a particular genotype (and the resulting phenotype)
Degrees of dominance
- Co-dominance (fully red)
- Incomplete dominance
- Complete dominance
Co-dominance
- heterozygotes fully express the phenotype of both of their homozygous parents (i.e. red + blue)
Incomplete dominance
- Phenotype of the heterozygote is intermediate in form between those of the two homozygotes (i.e. red & blue = lila)
- third phenotype is a combination of the phenotypes of both alleles
Linkage
- Tendency of DNA sequences that are close together on a chromosome to be inherited together during meiosis.
- The closer together on a chromosome, the less the chance of recombination between them
Mendel’s laws
- Law of dominance
- Law of segregation
- Law of independent assortment
Law of dominance
- Recessive allele swill always be masked by dominant ones
- -> only the dominant phenotype will be expressed, while still having a heterozygote genotype
Law of segregation
- Allele pairs randomly separate / segregate during gamete formation and randomly unite at fertilization
- each gamete carries only one allele for each gene
Law of independent assortment
- When 2 or more characteristics are inherited, individual hereditary factors assort independently during gamete production
- giving different traits and equal opportunity of occurring together (wrinkles or smooth and yellow or green)
Quantitative genetics
Branch of population genetics that deals with continuously varying phenotypes as opposed to discretely identifiable phenotypes and gene products (predicting phenotypes which are not either/or)
Hardy-Weinberg equilibrium
Model of change in expected frequencies of alleles over generations, states that allele and genotype frequencies in a (perfect) population will remain constant from generation to generation in the absence of other evolutionary influences.
p + q = 1
p(^2) + 2pq + q(^2) = 1
Genetic drift
- Variation in the relative frequency of different genotypes in a small population
- due to the chance of disappearance of genes as individuals die or don’t reproduce
Heritability
- proportion of the observed phenotypic variation that can be accounted for by genetic variation
- -> calculate the phenotypic correlation between a particular pair of relatives
- 1 = all phenotypic variation explained by genotypic variation
- 0 = none of it is (= environmental effects account for it)
Coefficient of relatedness
- probability that 2 individuals share an allele due to common ancestry
- expressed by symbol r
- 0 = unrelated
- 1 = clones/ identical twins
- -> siblings: share no alleles, all alleles, or anywhere in between –> on average share half = r = ½
- -> other family members: easiest to draw a family tree –> count the number of steps between two people
Behavioural genetics
- Research that uses genetic methods to investigate the nature and origins of individual differences in behaviour
Twin studies
- compare MZ twins and DZ twins
- disentangle genetic, shared + non-shared environment
- MZ have r = 1/100 % same genes
- -> If there’s any difference in phenotypic correlation btw. MZ twins & DZ-twins = impact of the extra half in the coefficient of relatedness
Monozygotic twins
= Identical twins
- single fertilised egg splits
Dizygotic twins
= Fraternal twins
- Sibling produced by the same pregnancy –> developed from separate eggs (each fertilised by a different sperm cell)
Shared environmental influences
Factors that will influence both twins regardless of MZ or DZ like parental social class, parental behaviour, diet available, the school attended, or the house grown up in.
Non-shared environmental influences
Influences that are neither shared by MZ nor by DZ twins such as childhood diseases or accidents that affected one twin and not the other like a disease or an accident
Falconer’s estimate of heritability
- determine the genetic heritability of a trait based on the differenced between twin correlations (correlation of MZ twins being higher than the one of DZ twins)
2 * (r(monozygotic twins) – r (dizygotic twins) = estimated heritability
ACE models
- Epidemiological model
- states that individual differences in a phenotype are accounted by A (additive genetic factors), C (common environmental factors), and E (specific environmental factors)
Epistatic effects
- Epistasis = the phenotypic expression of a gene at one locus alters that of a gene at another locus
- -> gene that affects another is “epistatic” to that other gene (e.g. Labrador fur)
- Modifies 9:3:3:1 ratio
- ACE models = due to interactions between alleles at different loci
Narrow-sense heritabilities
Estimates of variance based on additive genetic effects (A) (study designs include only A, C, and E) = h2
Broad-sense heritabilities
Estimates of variance that include epistatic effects (all genetic factors) = H2
Complex traits
- Trait that does not follow Mendelian inheritance patterns
- likely derived from multiple genes and exhibits a large variety of phenotypes
Pleiotropy
One gene has multiple effects
Dominant effects
- due to interaction between pair alleles at one loci
Adoption studies
- disentangle genetic & environmental sources
- Kids get genetic contribution from one set of parents + shared environment from another
Mendelian diseases
- Lethal recessives = recessive alleles that cause death of an organism that carries it
- individual carrying a single recessive deleterious allele will be healthy and can easily pass the deleterious allele into the next generation (big influence individual level)
- -> Not a problem when population is large as then they’re rarely expressed (small influence population level)
- -> Reason to avoid inbreeding
Neutral theory of molecular evolution
- amount of divergence between the DNA sequences of any 2 populations/ species reflects the time since their common ancestor (= molecular clock)
Recombination
- during meiosis paired chromosomes line up next to one another –> exchange DNA –> sequence that was originally found on chromosome A is now found on chromosome B
= crossing over - Parental type offspring: inherit phenotype that matches either of the parents
- Recombinants: offspring with new combination
