W12 Flashcards
what are the type of distribution of phenotypes possible
A. discontinuous distribution of phenotypes
- one gene, eg. blood groups
- several genes + environment; therefore multifactorial
B: continuous distribution of phenotypes
- several genes and influenced by environment; quantitative traits
what are multifactorial traits
- discontinuous distribution of phenotype where the trait is either present or not
- if these multifactorial traits are influenced by the environment, identifying the environmental component and reducing it may reduce incidence
what is a threshold model and to what type of trait/distribution does it occur in
- multifactorial traits which come from discontinuous distribution
- where we can have different thresholds for makes and females so more common in the sex with the lower threshold
what happens?
- accumulate genetic predisposition from many genes until enough environmental factors present
what is a continuous distribution of phenotypes and what are the properties of its trait
quantitiative
- continuous distribution
- controlled by many genes- polygenic
- affected by environmental factors
what is polygenic inheritance, what are its assumptions
assumptions:
- many gene loci contributing to the phenotype of the quantitative trait
- each locus has two alleles, A1 and A2
- there is no dominant phenotype
- alleles are inherited in a similar way to any other trait
- assume Allele A1 adds to trait contributor
- assume Allele A2 does not add to trait, therefore, non contributor
- the effect of the alleles is cumulative
HOW DO WE TELL POLYGENIC INHERITANCE IS OCCURING?
- F1 can be explained by other gene interactions, but looking at F2, we see a difference ie. 2 genes, but 5 phenotypes
what is the effect of increasing number of loci influencing a quantitative trait
as the number of genes controlling a phenotypic character increases, the differences between phenotypic classes becomes less distinct
what is heritability and its application
a statistical measure of how strongly the phenotype of the offspring will resemble the phenotype fo the parents
breeding for improvement, leading to a new mean in the next population.
how can heritability be measured in humans
twin studies using MZ/DZ twins
- concordance: both twins with trait
- non concordance: one twin with and one without
if the difference in concordance between MZ and DZ twins is high, it suggests there is a large genetic component
there are many bewares for using this method tho, lets say for infectious disease; everyone will be affected, shciz; depends on paper definition of schiz,
we make the assumption there is a similar environment so the variable is in the genes
how do we work out how many polygenes there are and how much they contribute
- find number of offspring for one trait same as parent and divide this by total number of offpsring
- which ratio match (¼)^n, n=number of polygenes
- range of trait: max-min=base of trait
- how many contributing allies max=number of polygenes times 2
- base of trait/max contrib. of alleles
- what genotype? = how many contributors
what is epigenetic
changes in gene expression that do not involve the change in DNA sequence
for example:
- methylation can influence gene expression
- modifications of chromatin structure can influence gene expression without changing the dna
+dna methylation/demthylation
+histone acetylation/deacetylation
+rna interference
what is genomic imprinting
the sex of transmitting parent produces observable differences in phenotype
- this is due to specific genes being differentially marked during parental gametogenesis
what is the evidence of genomic imprinting
- triploids (3N) with 2N pat + 1N mat are different from 2N mat and 1N pat
- 2N pat + 1N mat: teratoma with teeth/hair evidence of embryo
- 2N mat and 1N pat: hydatidiform mole; mass of membrane, no evidence in embryo - 15q11-q13 mutation selection leads to different syndromes: mono allelic expression
- del of mat 15q11-q13: Angelmann syndrome: electrophoresis will show no A from mother
- del of pat 15q11-q13: Prader Willi syndrome where electrophoresis will show no A from father
how can electrophoresis be sued to differentiate prayer willi syndrome and angelmann syndrome
choose a region of dna with polymorphism, then PCR region and run on a gel
- no A from mat, angelmann
- no A from pat, prayer willi
what other causes are there for PWS/AS
uniparental disomy where both chromosomes of a pair come from same parent
A. PWS, UPD- mat 15
B. AS/UPD - pat 15
describe types of UPD
heterodisomy: non disjunction of meiosis 1
- electrophoresis will show 2 bands
isodisomy: non disjunction of meiosis 2
- electrophoresis will show 1 band (thick-ish)
what is process of inheritance of imprint
- due to methylation, if a male inherits a maternal imprint but passes to his daughter, the imprint changes to the paternal imprint
- this is because: early spermatogenesis or early oogenesis wipes imprints in primordial germ cells, but this is reestablished according to the sex of the parent
- new methylation patterns according to the sex of the parent (which may be different from what the parent had)
what are some facts about imprinting
approximately 60 human loci
has to be reversible
reverse in gametogenesis
methylation of cytosines
in population genetics how can variation be detected
- look for visible differences in phenotype
- chromosome differences eg. length of long arm of y
- immunological markers like blood groups
- protein gel electrophoresis, eg. drosophila enzymes (will have different weights)
- minisatellites
- microsatelites
singel nucleotide polymorphisms
what are variable regions often in non coding regions and describe them
minisatellitles of 15-100b, total length is 1-5kb
- tandem repeats of 18 bases
microsatellitles- STR - 2-9 bases
- can have several alleles according to how many repeats
single nucleotide polymorphism (SNP)
- 1 base can onus have two alleles at a locus
what is variable number of of tandem repeats
- minisaatellitles or micrositellites
- in non coding regions of genome
- repeat sequence between 15bp-100bp
- but varies between person to person
- THIS REPEAT IS INHERITED FROM PARENTS
what analyses can be done on variable regions in non-coding regions
- multilocus probe- found in more than one location
- use primers that locate several regions
- mini satellites - single locus probe- use primers that find one locus
- minisatellites
- micro satellites
what is the original source of variation?
mutation- original source of all variation
how is variation maintained?
- mutation- original source of all variation
- recombination at prophase 1 of meiosis
- segregation of alleles at anaphase 1 of meiosis
- independent assortment of chromosomes at anaphase 1 of meiosis
- fertilisation between genetically different gametes
how are SNIPS detected?
- dna sequencing
- restriction cutting sites where the change results in the loss or gain of a restriction enzyme recognition sequence
what are the applications of SNIPS
- identification
- parentage testing
- forensic analysis- crime
- victim identification in a mass disaster
- animal identification eg. racehorses
- conservation biology in evolutionary studies
how are SNIPS used? the process
extract dna sample
- make multiple copies via PCR
- run samples on gel
what is mendelian population
a community of interbreeding organisms
the set of genetic information carried by the population is called the gene pool ie. all the alleles carried by all members of the population
gene flow occurs when members of the population move
what is meant by polymorphism and monomorphism
poly: two or more forms of a trait exist in a mendelian population of an organism
- the frequency of the rarest form cannot be explained by recurrent mutation alone
mono: only one form is found in a mendelian population
what is HW equilibrium
- untrue: alleles for dominant traits always increased and recessive alleles decreased
- HW: genotypes in proportion of p=f(A) and q=f(a)
p^2+2pq+q^2=1
p^2=f(AA)
if heterozygote cannot be distinguished, allele frequency of recessive allele can be assumed as following HW equilibrium
this remains constant across generations
note: HBS can have constancy of allele freq. does not mean HW equilibrium
what are the assumptions of random mating?
- random mating
- no migration, no gene flow
- no selection acting
- no mutation
- infinite population size- large enough to prevent sampling errors and random effects
in what cases will HW equilibrium
if a population is not in HW equilibrium, one generation fo random mating will esrtabluhs HW equilibrium if HW assumptions apply and locus is autosomal
if a population is not in HW equilibrium for an x linked trait, one generation of (…) will (…)
if a population is not in HW equilibrium for an x linked trait, one generation of random mating will NOT restore equilibrium
