Inheritance Flashcards
homozygote
gene with two identical alleles (PP, pp)
heterozygote
gene with two different alleles ( one of them is dominant -> visible and the other recessive (Pp)
First Mendelian Law :
Law of Segregation
The two alleles for the heritable characteristic segregate during gamete formation (meiosis) and end up in different gametes
-> If two hetereozygote individuals are crossed with one another ( F1 genertaion)
allele
alternative form of gene that accounts for variations in inherited characters
example
gene : hair colour
allele1 blond; allele2 brown
Punnet Square
predicts allele composition
true-breeding
p generation are both homozygote (PP and pp)
ratio in f1 and f2 generation after true breeding
f1 : Pp Pp Pp Pp (4:0)
f2. PP Pp Pp pp (3:1)
Law of Independent Assortment
- Each pair of alleles segregats independently of each other pair during gamete formation
- each gene has an independent biologically effect
Law of Dominance
recessive allele will be always masked by dominant allele
Mendelian Disease -> lethal recessives
Lethal recessives - recessive allele that causes death of an organism that carries it
- an individual carrying a aingle recessive deleterious allele will be healthy and can easily pass the deleterious allele into next generation as long as population is large as then -> rarely expressed
REASON TO AVOID INBREEDING
co-dominant genes
fully express the phenotype of both of homozygous parents
Complete dominance
dominant allele completely masks the recessive one
- one copy of the dominant allele is enough to produce some enzyme
Incomplete dominance
heterozygous phenotype intermediate between the two homozygous phenotypes
Co-dominance
two allels each affect the phenotype in seperate, distinguishable way , both phenotypes are fully expressed
eg blood group
Pleiotropy
one gene affects multiple phenotypic characteristics
-> causes multiple disesase eg albinism
Epistasis
the phenotypic expression of a gene at one locus alters the that of a gene at another locus
- > the gene that affects another is ‘epistatic’ to that other gene
- > modyfies 9:3:3:1 ratio
Quantitative characters
classification is impossible as they vary along a continuum ( eg skin colour)
Polygenic Inheritance
additive effect of multiple genes on a single phenotypic character
Eg AABBCC -> darkest
AaBbCc -> middle
aabbcc-> lightest ( majority of the population is in the middle)
Norm of reaction for a genotype
genotype is generally not associated with one definite phenotype but rather with a range of phenotypic possibilities ( broadest for polygenic characteristics)
Multifactorial characters
refers to both genetic and environmental factors ; collectively influence phenotype
Linkage
when two genes lie on the same chromosome
closer together -> greater linkage
Variation
- provides raw material for natural selection
- interplay between environment and genotype will determine which genetic combinations persist over time
- if new arised traits better suit for a given environment , organisms possessing those will thrive and leave more offspring -> next generation: generation again shuffled
Hardy Weinberg - conditions
1 no mutation in gene pool 2 random mating 3 extremely large population size 4 no gene flow ( from and to other populations) -> no migration 5 no selection
Hardy Weinberg Principle
the frequencies of alleles and genotypes in a population will remain constant from generation to generation provided that only Mendelian segregation adn recombination of alleles are at work -> equilibrium
HW - theorem
p = frequency of one allele q= frequency of the other allele
p+q=1
p2+2pq+q2=1
HW and natural selection
NS occurs when the frquencies change a
Equilibrium
- if expected frequencies are same as actual frequencies
- some genes can be in equilibrium while other at same time arent
Genetic Drift
- the smaller a population the more frequencies will fluctuate
- new mutations can fluctuate all the way till point where everyone has it -> muations becomes fixed
- the number of new mutation arising is greater the greater the population
Neutral Theory of Molecular Evolution
amount of divergence between the dna sequences of two populations reflects their time since their common ancestor -> molecular clock
- accounts for much of the functionless molecular change not phenotype
Heritability (h2)
proportion of the observed phenotypic variation between individuals that can be accounted for by genetic variation
-> soil example demonstrates this
misunderstanding of heritable
Heritable does not mean ‘‘genetically’’ determined
Environmental variation
Environmental variation changes the heritability -> as environmental variation becomes higher heritability goes down
NECESSARY for evolutionary change
Schizophrenia
risk of developing schizophrenia increases systematically as a function of the degree of genetic similarity -> pattern of affected individuals does not conform to Mendelian proportions
genes with additive gene effect ( neither dominant nor recessive)
leads to normal distribution in population
phenotypic variation
genetic variation + environmental variation
Coefficient of relatedness (r)
probability that any particular allele in individual A is identical by descent ( derived from same immediate source) as the allele in individual B
r = 0 -> unrelated r = 1 -> identical twins
coefficient of relatedness between siblings
humans are diploid organisms ( receive one 1/2 maternal and one 1/2 paternal gene)
siblings could share no alleles ( in case they got in both case the opposite of the half gene) ; all allels ( both siblings got same set of genes from parents) or anywhere inbetween , thus average 0.5
Monozygotic Twins
identical r = 1
Dizygotic Twins
no more related than normal siblings r=0.5
Non-shared environmental influences
any phenotypic difference bw Monozygotic Twins that must be due to NSE eg accident, disease -> thus
estimate of impact = 1-rmz
Shared environmental influences
eg parental social class, behavior, scool, household
estimate of its effect = rmz-heritability of the trait
Falconers Estimate of Heritability
equiation to estimate the heritability of traits ( mainly used in twin studies)
h*2 = 2 x rmz -rdz
ACE model components
A - heritability
C - effect of shared environment
E - effect of non-shared environment
-> C tends to be much less important than E but it is often underestimated
ACE Model
it relies on assumption that genetic effects accumulate in an additive manner -> r =1 is twice the effect of r = 0.5
Adaptation studies
helps distangle genetic and environmentalsources
- kids get genetic contribution from one set of parents and the shared environment from another
- > test correlation between kids and their biological parents and adopted parents ; or biological siblings adopted into different families
selection vs evolution
selection : individuals that are well adapted on average survive longer
evolution : over time selected characteristics will increase in frequency in the population
mechanisms of evolution and HW
correspond to violations of different Hardy-Weinberg assumptions. They are: mutation, non-random mating, gene flow, finite population size (genetic drift), and natural selection.
genetic hitchhiking
when an allele changes frequency not because it itself is under natural selection, but because it is near another gene that is undergoing selective pressure ( in linkage with that gene)
