GED Flashcards
Sympatric speciation that involves a hybrid between two related species is known as…?
allopolyploidy
Give examples of postzygotic reproductive barriers?
Hybrid infertility
Reduced hybrid viability
Hybrid breakdown (reproductive failure from F2)
What are hybrid zones?
Regions where two recently derived (or two incipient) species mate and give rise to hybrid offspring.
Alters gene flow -> reinforcement (loss of fitness), fusion or stability (fitness loss negligible)
Give examples of prezygotic reproductive barriers
Pre mating:
- habitat isolation
- temporal isolation
- behavioural isolation
Post mating:
- mechanical isolation
- gametic isolation
Define a species
Population of reproducing organisms that are isolated from other populations + cannot interbreed to produce fertile offspring
Genetic drift/bottleneck
random effects can cause major changes in small populations significantly altering the genetic variability
Fitness
Relative probability of survival + reproduction of a given genotype
- difficult to calculate precisely + both will vary depending on env conditions
Whydo some characters reduce fitness (maladaptive)?
Sexual selection - female choice, traits exaggerated until equilibrium when natural + sexual selection balanced
e.g. Peacock
Altruism
Behaviour of an animal that benefits another at its own expense
Bill Hamilton + his rule
Inclusive fitness is direct + indirect fitness (fitness of related offspring) -> ‘kin selection’
you get altruism as long as rb > c
Haplodiploidy in hymonoptera
Females diploid, males haploid so pass on all genetic info
-> more closely related to sisters than offspring
so genes more likely to be passed on if sisters reared than offspring
but many hymenoptera not eusocial so relatedness only precondition of evolution, not determining factor
What are transpsosons?
Sequences of DNA that can move in genome
- produce transposase which enbales them to ‘jump’
*retrotransposons produce RNA which is reverse transcribed + inserted into genome
Horizontal gene transfer
Movement of genetic info across normal mating barriers
- widespread amongst prokaryotes, not in multicellular organisms
- minor source of variation + can lead to adaptations
What is eDNA?
DNA extracted without an organism from soil, water or air
- non-invasive
- requires careful amplification + bioinformatic identification
- powerful in aquatic systems
How is genetic variation preserved?
Neutral theory - many alleles selectively neutral + their frequencies will fluctuate over time due to genetic drift
Balancing selection - heterozygote advantage + frequency dependent selection
Microevolution
changes in gene pool of a population of organisms over time
What is a gene pool?
All alleles of all genes of all individuals in a population – represents all genetic variation in a pop.
Macroevolution
Major evolutionary events above species level + speciation
Descriptive embryology
observations on morphology + cell development
-> can catalogue gene expression pattern during development
Fate maps can be used to assess fate of cell/groups of cells based on lineage labelling, does not disturb development
Experimental embryology
Determines how cells know what they should become using:
- asymmetric inheritance of cytoplasmic determinants
- communication between cells (induction)
Specification maps can assess what cell/group of cells will form if removed from embryonic environment
Asymmetric inheritance
Discovered using tunicates - mosaic development in some embryos
‘Red bit’ in cytoplasm forced into other parts of embryo, other tissues formed contained muscle cells (presence of cytoplasmic determinants)
e.g. mosaic development in insects
+ frog oocyte + egg has inherent asymmetry to protect nucleus + mRNA from radiation
Communication between cells
baby hair ligature experiments show embryo able to adjust development -> 2 embryos arise from one zygote
Organizer experiment
1924 Mangold + Spemann
Induction of muscle (somites) + neural tissue
- dorsal mesoderm determined by early gastrula stage
- ventral ectoderm + mesoderm competent to become neural + somitic tissue
Homeotic mutation
Transformation of one body part into another, normal structure may be in the wrong place
Caused by hox genes - evolutionary conserved transcription factors clustered in genome
Tandem + segmental gen duplication
Tandem - e.g. unequal crossover caused by chromosome mispairing, duplicated gene free to acquire new functions (subfunctionalisation) or is lost
Segmental - giant tandem duplication affecting whole chunk of chromosome
*gave rise to 2 hox clusters in drosophila
Whole genome duplication events
Allotetraploidy - hybridisation between two separate species
Autotetraploidy - duplication of genome through improper meiosis
*can have significant effect on phenotype
Paralogous genes
Duplicated genes within a single genome
Orthologous genes
Same gene in different organism
39 hox genes in human + mouse, 4 clusters on 4 chromosomes, evolved from set of 13 paralogous groups
- involved in anterior-posterior patterning + positioning
Evidence for hox gene involvement in anterior-posterior patterning
Expression patterning, comparative embryology, gene knockout experiments
Hans Driesch (1891) + Spemann
First to show each cell of a 2 cell or 4 cell stage sea urchin is totipotent - so can give rise to an embryo
Spemann showed no loss in totipotency up to 16 cell stage - 1st ever cloning experiment
1952 Briggs and Kings
1st nuclear transplantation - able to clone frog embryos from blastula nuclei
Waddington landscape
Potency decreases w/ time
John Gurdon
Took differentiated cells from frog, cloned nuclei -> developed into fully formed frogs
so nuclei can be reprogrammed + differentiation not final
challenged Waddington landscape
1996 Dolly Sheep first mammal cloned by Campbell + Wilmut
Embryonic stem cells
Formed from epiblast cells from blastocyst embryos - pluripotent
Can be maintained indefinitely in culture
- led to chimera + generation of teratomas
- field of organoids spawned
- can use them to knockout targeted gene sin mice
iPS cells
Differentiated cells reprogrammed back to pluripotent embryonic state without use of nuclear transplantation
Klf-4 + Sox-2 + Oct-4 + Myc -> fibroblasts -> iPSC colonies
Progenitor vs Stem cells
Progenitor - some capacity to proliferate + potency so can differentiate into 1 or more cell types
Stem - capable of indefinite self renewal + potency
How can embryonic and iPS cells be used?
Age related mascular degeneration treatment
Neurodegenerative diseases e.g. Parkinsons
Mitochondrial genome
- maternal + cytoplasmic inheritance
- DNA coated w/ non histone proteins
- DNA circular + much smaller than nuclear
Mitosis
Ensures both daughter cells inherit one copy of duplicated genome
Asymmetric division in stem cells leads to production of differentiated cell + stem cell (muscle repair)
Karyotypes: condensed chromosomes during metaphase
Meiosis
Produces haploid genomes to enable sexual reproduction + increases genetic diversity
Follows G2
1: recombination occurs, hom chroms separated -> 2 haploid cells w/ 2 chromatids
2: sister chromatids divide, equational division
-> haploid gametes w/ 1 chromatid produced
Incomplete dominance
Heterozygote phenotype is intermediate btw 2 homozygote phenotypes
Pleiotropy
Gene that can influence more than one trait
Lethal allele
Can cause skewed phenotypic ratios -> certain homozygous allele combinations are lethal
e.g. achondroplasia -> infant mortality
Why don’t all individuals with same genotype display same phenotype?
Penetrance - measure % of people w/ given genotype who exhibit expected phenotype
e.g. incomplete penetrance - breast cancer susceptibility
Expressivity - measures extent given geneotype is expressed at phenotypic level
e.g. variable expressivity
Epistasis
Interaction between two or more genes that control a single phenotype
- One mutation affects phenotype of another mutation
- Either mutation presents no phenotype, but double mutant has phenotype
- Either mutation has same phenotype but double mutant has different phenotype
Epigenetics
heritable changes in gene expression that do not involve alterations to DNA sequence of genome
e.g. DNA methylation of histones, binds to cytosine bases -> heterochromatin tightly packed so transcription unable to take place
Genomic imprinting
Alleles can be imprinted w/ epigenetic tags, so other allele preferentially expressed
e.g. maternal imprinting pf lgf2 gene - paternal copy expressed (if paternal allele mutated then results in dwarf mouse)
Human genomic imprinting
Prader-Willi + Angelman syndrome result from microdeletions in region of chromosome 15
Prader-Willi: deletion paternal origin + maternal imprinted by methylation
Angelman: deletion maternal origin + paternal imprinted by non-coding RNA acting as anti-sense
How is c allele epistatic to all other coat genes in mice?
Albino allele is recessive loss of function mutation in enzyme tyrosinase required for melanin synthesis
Why is W allele epistatic to all other coat genes in mice?
Is a dominant loss of function mutation in transmembrane growth factor receptor (c-kit) that is needed for proliferation of melanocytes
- melanin still produced but in wrong place
What is needed to be defined as a male?
Y chromosome (even XXY still male - aneuploidy)
-> one gene (SRY) - sex determining region on Y chromosome needed
SRY - TF recognises sequences in promoters, activates expression, target genes crucial for gonad development
Gynandromorph
Sexual mosaic - cells on right have female chromosome set while cells on left have male chromosome set
Sex linked inheritance
- usually involves genes located on X, fewer linked on Y
- males hemizygous for genes on X chromosome
nearly all X-linked traits recessive + most in males
e.g. haemophilia, Duchenne muscular dystrophy, red-green colour deficiency
Dosage compensation by X chromosome inactivation (XCI)
1 X chromosome in each female becomes inactivated -> highly condensed/not expressed + seen as Barr body
Mitosis means female mammals are mosaic
e.g. tortoiseshell cats
What do gene maps show us and what different types are there?
Show relative order of genes on chromosome + distance between genes
linkage - uses crossing over to determine distance between genes
physical - use restriction enzymes to cut DNA at specific sites e.g. Human genome project
cytogenetic - uses dye stains to create banding pattern, genes assigned to p (small) and q (large) arm either side of centromere
How does linkage mapping work?
Frequency of crossing over between 2 genes on same chromosome is proportional to distance between them
If 50% are recombinant gametes, then genes on diff chromosome or far apart on same one
If less than 50% recombinant gametes, genes linked on same chromosome + smaller the RF, closer the genes
*can use testcross to determine genotypes of gametes -> cross double heterozygote w/ tester strain (aabb)
What is genetic distance?
Equal to recombination frequency (RF)
RF = (no. of recombinant progeny * 100) / total no. of progeny
units are centimorgans (cM)
Pedigree analysis
standardised family tree can help determine type of disease mutation
Linkage analysis
evidence of genetic linkage between disease gene + genetic markers
commonly used DNA markers are short tandem repeats (STRs) and single nucleotide polymorphisms (SNPs)
STRs/microsatellites 2-4 nucleotides whereas minisatellites are >10 nucleotides
Most SNPs in non-coding DNA (use crossing over between mutation + SNPs)
Chromosome abnormalities
Aneuploidy is variation in number of chromosomes, mainly caused by non-disjunction
- monosomy (2n -1) -> turner syndrome (XO)
- trisomy (2n + 1) -> Klinefelter syndrome, Triplo-X, XXY syndrome, Down syndrome
- tetrasomy (2n + 2)
Causes of Down Syndrome
Autosomal trisomy (21)
Robertsonian translocation (14-21)
Genetic mosaicism - individuals have mix of normal + trisomy 21 cells
Maternal inheritance of mitochondrial genome
Mammals - paternal mitochondria + sperm components (except nucleus) destroyed after fertilisation
Plants + algae, maternal inheritance most common, paternal + biparental also found
mt and cp genomes often transmitted in uniparental pattern - females give trait to all of children
Heteroplasmy
passive segregation of mt during cell division, causes variation of high + low levels of mutated/diseased mt in oocyte
Characteristics of complex traits
- polygenic w/ low penetrance
- familial clustering but inheritance not predictable
- often strongly influenced by environment
Heritability
The total phenotypic variance due to genes
H^2 = Vg / Vp
high heritability means genetic differences in population explain high proportion of phenotypic variability + easier to identify genetic variants associated w/ that trait
Can be estimated:
- monozygotic twins share env + all alleles so relatedness =1
- dizygotic twins share env + half their alleles so relatedness is 0.5
Concordance
Probability that if one twin affected, other twin also affected
Difference in concordance between MZ and DZ can be used to estimate heritability
Genome wide associated studies (GWAS)
Population level approach to gene mapping
Odds ratio can be used:
OR > 1.0 allele gives higher risk of disease
OR < 1.0 allele is protective
-> can be used to calculate probability association between each SNP + the disease
Can be displayed using Manhattan plot
Haplotype blocks
Simplify GWAS - closely linked SNPs on same chromosome often inherited together on these blocks
No crossing over takes place, each block defined by small no. tag SNPs
