UNIT 14 Flashcards
(21 cards)
Define gene, allele, locus, genotype and phenotype.
A gene is the heritable factor made of DNA that influences a specific characteristic;
An allele is a version of a gene;
Locus is the specific location of a gene;
Genotype is the genetic makeup of an organism;
Phenotype is the observable traits of an organism;
Explain how a mutation can affect a characteristic.
Mutation is a change in the base sequence of a gene;
Base sequence determines sequence of amino acids in a protein;
Change in base sequence may cause a change in the sequence of amino acids;
Proteins may not function correctly/cause harm/some mutations cause cancer;
E.g. allele for sickle cell anaemia cause by single base substitution from codon GAG to GTG;
Causes valine to be produced instead of glutamic acid
Outline how polygenic traits can show a normal distribution.
Polygenic traits cause by two or more genes;
The greater the number of genes that are responsible for a trait, the greater the phenotype;
Polygenic traits often show continuous variation;
Continuous range of phenotypes rather than distinct categories;
Large range of phenotypes, where the majority of organisms are in the middle of the normal distribution curve;
Often influenced by the environment;
Explain the role of inheritance in genetic diseases.
Genetic disease cause by genes/alleles;
Alleles passed from parents to offspring / gametes have one allele for each characteristic from each parent;
During fertilisation the alleles from both parents come together / combination of alleles determines the characteristics in offspring;
Diseases caused by dominant alleles only need allele from one parent to cause disease in offspring;
Huntington’s is an autosomal dominant condition;
Disease caused by recessive alleles need alleles from both parents to cause disease in offspring;
PKU is an autosomal recessive condition;
PKU caused by mutation of gene coding for PAH enzyme converting phenylalanine to tyrosine;
Most sex-linked characteristics are controlled by alleles found on X chromosome;
Haemophilia is a sex-linked recessive condition;
Sex-linked genetic diseases are more common in males than females;
If a mother is affected by sex-linked characteristic all of her sons will have the condition;
Outline the differences and similarities between homologous chromosomes.
Similarities
Length;
Centromeres are in the same position;
Sequence of genes;
Gene loci;
Differences
Alleles;
One chromosome is inherited maternally and one chromosome is inherited paternally;
Explain why expected and observed Mendelian ratios may differ.
Mendelian ratios as the proportion of genotypes that would be expected from a punnett square;
Mutations;
Genes are not inherited as expected;
Interactions between genes and the environment;
Phenotypic plasticity;
Autosomal linkage/linked genes;
Sex-linkage/sex-linked genes;
Hence do not assort independently during meiosis;
When linked genes are far apart on the same chromosome, crossing over can occur making them linked;
Organisms inheriting diseases that are fatal to embryos disrupt phenotypic ratios;
Polygenic inheritance;
Incorrect assumption that a trait is determined by one gene;
If inheritance is monogenic but two different genes result in the same phenotype;
Distinguish between sex-linked inheritance and autosomal inheritance in humans.
Sex-linked genes usually found on X chromosome from 23rd chromosome pair while autosomal genes found on 1st-22nd pair;
Inheritance is dependent on biological sex of offspring in sex-linked but not in autosomal;
Biological males receive one allele for sex-linked characteristics, females receive two but both biological males and females receive two alleles for each characteristic for autosomal;
Higher chance for biological males to have sex-linked recessive characteristic than females but no difference in autosomal;
Biological female affected by recessive sex-linked trait means all her sons will be affected but not necessarily for autosomal;
Biological females can be carriers in sex-linked and biological males cannot but both sexes can be carriers for autosomal;
Offspring with autosomal recessive: parents usually do not have disease
Phenylketonuria (PKU)
Autosomal recessive disease
Phenylalanine → PAH (phenylalaninehydroxylase) → tyrosine
Mutation in gene coding for PAH → cannot convert to tyrosine
High conc of phenylalanine → poor development of the brain/body = mental/physical retardation
Diet low in protein, low in phenylalanine
Functioning PAH is caused by dominant allele
Recessive allele cannot produce PAH (homozygous recessive) → PKU
Phenotypic plasticity is the effect the environment has on the phenotype by changing gene expression
Mutation in clotting factor IX
X-linked recessive allele
Female = homozygous recessive
Male = recessive = haemophilia
Haemophilia = blood cannot clot properly
Linked gene = 2+ alleles on the same homologous pair of chromosomes, can be sex linked
Polygenic inheritance = multiple genes coding for the same characteristic
Define gene pool.
All the genes and alleles in an interbreeding population;
Outline the possible causes of changes to a gene pool.
Gene pool as all the genes and alleles in an interbreeding population;
Changes in allele frequency;
Reproductive/temporal/behavioural/geographical isolation;
Natural selection/evolution;
Accumulation of mutations;
Crossing over during meiosis;
Mixing with individuals from a different population;
Outline natural selection.
Overproduction of offspring/more offspring produced than the environment can support;
Struggle for survival/competition for resources;
Variation between individuals from crossing over and independent assortment during meiosis will make some more successful than others/better adapted;
Better adapted individuals reproduce and pass on alleles to offspring;
Allele frequency increase over time;
Results in evolution;
Describe the selection pressures acting on guppies using John Endler’s experiments as an example.
Male guppies express genes for brighter colours;
Hypothesised brighter colours under less predation/less colour with predation;
Observed wild guppies and those in artificial ponds;
Independent variable as predation;
Guppies from wild with varying colours introduced to tanks, allowed to breed;
One set of guppies in pond with predation, one set without;
Ponds with different sized gravel;
Brightly coloured males tend to be eaten, less likely to mate and pass on alleles to next generation;
Predatory selection pressure leads to decrease in allele frequency for bright colours over time;
Brighter coloured guppies in ponds with no predators;
Brightly coloured guppies (under no predation) likely to mate and pass on alleles to next generation;
Sexual selection leads to increase in allele frequency for bright colours over time;
Show evolution of guppy colouration involves dynamic interplay between natural and sexual selection;
Natural selection as the process where individuals better adapted to environment more likely to survive and reproduce;
Sexual selection affects mating and reproductive success often at the cost of survival;
Outline the conditions for Hardy-Weinberg principle.
Large population;
No migration;
No mutation;
Random mating;
No selection;
Define evolution.
Gradual change in the heritable characteristics of a species over time;
Explain the different types of selection pressures.
Directional selection: one extreme is selected for, the other is selected against;
Increase frequency of favoured allele;
Stabilising selection: extremes are selected against, intermediates are selected for;
Decreases variation within population;
Disruptive selection: intermediates are selected against, extremes are selected for;
Increases variation within population;
Explain how the misuse of antibiotics can lead to the development of antibiotic resistant bacteria.
Antibiotics as a selective pressure;
Some bacteria are resistant to some antibiotics/possess a mutation/gene for antibiotic resistance;
Bacteria (with antibiotic resistance) are not killed by antibiotics/antibiotics are ineffective;
Transfers of the gene for antibiotic resistance to another bacterium through conjugation/using plasmids;
Resistance gene can pass to next generation/offspring, through cell division/binary fission;
Resistant bacteria have no competition and proliferate in the absence of other bacteria/proportion of resistance bacteria in a population increase;
MRSA (methicillin resistant Staphylococcus aureus) is an example of antibiotic resistant bacteria/any other valid example;
Example of misuses such as over prescription of antibiotics/failure to complete the full course;
Outline how polyploidy results in reproductive isolation.
Polyloids possess more than two sets of chromosomes;
Can be caused by hybridisation between two closely related species;
Can result from failed spindle formation/non-disjunction/failed chromosome separation/chromosomes replicate and cell does not divide/diploid gamete fuses with haploid gamete/two sperm fertilise one egg;
New tetraploid and original diploid cannot interbreed to produce fertile offspring;
Results in reproductive isolation from original population;
Once isolated, polyploid diverges genetically from original population;
Polyploid organism is a new species;
Outline how reproductive isolation can result in changes in allele frequencies between populations.
Allopatric speciation is caused by geographical isolation and, therefore reproductive isolation;
Sympatric speciation takes place in the same habitat due to different niches/differences in feeding behaviour/behavioural differences/courtship differences;
Both processes lead to isolation of populations;
Genetic variation occurs due to genetic drift/mutation;
Reproductive isolation prevents or limits gene flow between populations;
Leads to changes in allele frequencies within those populations;
Over time this leads to formation of a new species due to inability to breed with the original population of the species;
Adaptive radiation results in a single ancestral species evolving into multiple, closely related new species which may live in close proximity to each other. Outline 3 mechanisms by which closely related species living in close proximity may be reproductively isolated.
Behavioural isolation;
Temporal isolation;
Geographic isolation;
Post-zygotic barriers;
Hybrid offspring are sterile;
Mechanical isolation;
Two closely related species have different chromosome numbers;
Incompatibility of gametes/inability of sperm to fertilise egg;
Outline the process by which two species may be formed from an ancestral population through the process of allopatric speciation.
Geographic isolation;
Different mutations in the two different populations;
Different phenotypes produced in the two different populations;
Different selection pressures in the two different areas;
Different traits selected for in the two different areas;
Genetic drift;
Reproductive/behavioural/temporal isolation / post-zygotic barriers;
Gene flow cannot occur;
Changes accumulate over many generations;
Explain what polyploidy is and how it can lead to speciation.
Polyploidy = more than two sets of chromosomes e.g. tetraploid
Hybridisation = two diff species
Reproductive Geographical isolation = both experience diff selection pressure → speciation
Adaptive radiation = eg same species become diff species due to adapting to different niches e.g. darwin’s finches beak size/shape
Outline the principal taxonomic ranks in the modern classification system.
Domain is the largest rank;
Domains include Archaea, Eubacteria, and Eukarya;
Domain Eukarya is split into kingdoms;
From largest to smallest, kingdoms are divided into phyla/phylum, classes, orders, families, genera/genuses, and species;
Species are sometimes divided into subspecies;
