6.2- Patterns of inheritance Flashcards
Describe phenotype
- the appearance of an organism
- influenced by both genotype and environment
Name Genetic factors that cause phenotypic variation
1) Gene Mutations
2) Chromosome mutations
3) Sexual reproduction
What us he name of agents that increase rate of mutation, name different categories
Mutagens:
- physical agents
- chemical agents
- biological agents
List physical agents (mutagens)
- X-rays
- Gamma rays
- UV light
List chemical agents (mutagens)
- Benzopyrene (found in tobacco smoke)
- Mustard gas
- Nitrous oxide
- aromatic amines (in some synthetic dyes)
- Reactive oxygen species- free radicals
- Colchicine
List biological agents (mutagens)
Biological agents
* Some viruses
* Transposons- jumping genes, remnants of viral nucleus acid that have become incorporated into our genomes
* Food contaminants such as mycotoxins from fungi, e.g. aflatoxins in contaminated nuts, chemicals in charred meat,, alcohol
Describe the effects of gene mutations
Can be harmful, advantageous, or neutral
Describe the characteristics of mutations that occur during gamete formation
- Persistent- can be transmitted through many generations without change
- Random- not directed by a need on the part of the organism in which they occur
When do chromosome mutations occur, name different types
Occur during meiosis. Include:
* Deletion
* Inversion
* Translocation
* Duplication
* Non-disjunction
* Aneuploidy
* Polyploidy
Outline deletion (chromosome mutation)
Part of a chromosome, containing genes and regulatory sequences, is lost
Outline inversion (chromosome mutation)
- A section of a chromosome may break off, turn through 180 degrees, and then join again
- Although all the genes are still present, some may now be too far way from their regulatory nucleotide (control site) to be properly expressed
Outline translocation (chromosome mutation)
- A piece of one chromosome breaks off and then becomes attached to another chromosome
- May also interfere with regulation of genes on the translocated chromosome
Outline duplication (chromosome mutation)
- a piece of a chromosome may be duplicated
- Overexpression of genes can be harmful, because too many of certain proteins or gene-regulating nucleic acids may disrupt metabolism
Outline Non-disjunction (chromosome mutation)
- One pair of chromosomes or chromatids fails to separate, leaving one gamete with an extra chromosome
- When fertilised by a normal haploid gamete, the resulting zygote has one extra chromosome- e.g. Down syndrome (Trisomy 21) is caused by non-disjunction
Outline aneuploidy (chromosome mutation)
- The chromosome number is not an exact number if the haploid number for that organism
- Sometimes chromosomes or chromatids fail to separate during meiosis (e.g. trisomy)
Outline polyploidy (chromosome mutation)
- If a diploid gamete is fertilised by a haploid gamete, the resulting zygote will be triploid (has 3 sets of chromosomes)
- The fusion of 2 diploid games can male a tetraploid zygote
- Doesn’t occur in animals
- Many cultivated plants are polyploid (they have more than 2 sets of chromosomes)
Describe how genetic variation occurs in sexual reproduction
Has contributed to evolution:
- allele shuffling during crossing over in P1
- independent assortment of chromosomes in M/A1
- independent assortment of chromatids in M/A2
- random fusion of genetically dissimilar haploid gametes (only contain one of each our of homologous chromosomes and one allele for every gene)
Describe examples of variation caused only by the environment
- Speaking regional dialect- offspring don’t inherit, but learnt by listening to others
- Losing a limb or scarring following injury
Describe an example of Variation caused by environment interacting with genes in plants
- If plants are kept in dim light after germination, or if the soil in which they are grown contains insufficient magnesium, then the leaves do not develop enough chlorophyll and are yellow or yellow-white
- The plant is described as chlorotic, or suffering from chlorosis
- The plant cannot photosynthesise
- Chlorotic plants have the genotype for making chlorophyll, but environmental factors are preventing the expression of these genes
Nam different causes of variation
- genetic
- environmental
- combination of the two
Briefly outline the history of artificial selection
- humans have been practicing animal and plant breeding via AS for around 10,000 years- since beginning of settled agriculture
What is the agent of selection in natural vs artificial selection
- natural- environment
- artificial- humans
Outline the process of artificial selection
- breeders select individuals with the desired traits and allow them to interbreed
- at the same time, they prevent those without the the desired characteristics from breeding
name examples of species humans have domesticated
- cereals
- potatoes
- vegetables and fruits
- cattle
- pigs
- sheep
- goats
- horses
- oxen
- dogs
- cats
- pigeons
- poultry
Describe desirable characteristics in plants (artificial selection)
- increased yield
- pest and disease resistance
Describe desirable characteristics in animals (artificial selection)
- docility
- placidity
- ability to be trained
- animals that normally live in social groups (herds) with a dominance hierarchy may also be able to be trained to accept a man as the pack leader, and to tolerate being penned with other animals
What does artificial selection produce
new breeds of organisms
Table of organisms and their desirable traits/examples of use by humans
Describe how new breeds can be produced (artificial selection)
- selective breeding programmes
- e.g. breeders may grow many plants of a particular type under the conditions they wish these plants to withstand e.g. low temperatures, high salinity
- they will then select those individuals that grow best under these conditions and cross pollinate them, collect and sow the seeds, and repeat this process over many generations
- selective breeding programme takes around 20 years
Describe the term for the issue with inbreeding
Inbreeding depression
Outline inbreeding depression
- at each stage of selective breeding, the individuals with the desirable characteristics and no/few desirable characteristics are selected
- inevitably, the genetic diversity ins the gene pool of the selective breed is reduced
- if related individuals are crossed, inbreeding depression can result- the chances of an individual inheriting 2 copies of a recessive harmful allele are increased
Describe how growing plants in specific desirable conditions aids in artificial selection
- it is random mutations giving rise to new phenotypes - happens independently of the selection process
- growing one environment that being made to withstand doesn’t directly contribute to this- just allows us to see which plants have a mutation that makes them able to tolerate the conditions
Describe hybrid vigour
When breeders outcross individuals belonging to 2 different varieties to obtain individuals that are heterozygous at many gene loci
How do breeders obtain individuals that are heterozygous at many gene loci
Hybrid vigour
Describe what has happened to the number of commercially grown varieties of crops over the last 50-100 years, why
greatly reduced:
- selective breeding reduces the organisms genetic diversity
- all commercial varieties are genetically similar- if a pathogen was introduced, most plants would succumb to the infection
How do breeders overcome the issue of commercial plants being genetically similar
- outcross the cultivated varieties with varieties more like their wild ancestors to increase hybrid vigour
- samples of such wild ancestor types need to be conserved- often in gene banks
What are gene banks, name examples
Store genomes, but in their organisms:
- rare breed farms
- wild populations of organisms
- crops in cultivation
- botanic gardens and zoos
- seed banks
- sperm banks
- cells in tissue culture
- frozen embryos
Describe an example of the necessity of gene banks in the UK
- Much of the wheat grown in the UK has a dwarfing allele introduced from a Japanese variety of wheat
- If given extra fertiliser, the wheat does not grow taller and fall over in the wind, but uses the extra nutrients to increase seed size and yield
- However, if the environmental temperature rises above 30 °C, the effect of this allele is changed and yield is decreased
- If climate change is likely to produce higher temperatures during the British summer, a new breed of wheat will have to be developed
- Wheat breeders are looking, in a gene bank, for different dwarfing alleles
Describe ethical considerations of artificial selection
- domesticated animals may retain many juvenile characteristics, making them friendly, docile and playful BUT makes them less able to defend themselves- loss of nervous disposition may make them easy prey
- livestock animals e.g. pigs selected to have more lean meat and less fat BUT might succumb to low environmental temperatures during winter off they were not housed
- dogs have been domesticated for many thousands of years and used by humans for hunting, companionship, protection, herding , transport, guide dogs, and for aesthetic qualities BUT may put dogs at selective disadvantage if they had to survive in the wild, some breeds through inbreeding from a limited number of pedigree dogs have susceptibility to disease, and some coat colours would camouflage the animals
Conditions to which different dog breeds are susceptible to table
Who first investigated genetics
Gregor Mandel
What type of organisms did Mendel study
- easy to grow
- naturally self fertilising, but easy to cross-fertilise artificially
What is the monohybrid cross
- involves only 1 characteristic with 1 pair of contrasting crates
- mates 2 parent strains (P1 generation) with different phenotypes- produces F1(first filial) generation
- then breed F1 1to produce F2
Describe what is meant by a chacteristic that is monogenic
- governed by one gene
= the one gene has 2 distinct alleles
Describe different types of allele
- dominant- when present in homozygous or heterozygous individuals, produces the dominant phenotype
- recessive- will only produce recessive phenotype if homozygous- two recessive alleles with no dominant allele present
Describe the meaning of heterozygous
- having different alleles ay a particular gene locus on a pair oh homologous chromosomes
- not true breeding
Describe the meaning of homozygous
- having identical alleles at a particular gene locus on a pair of homologous chromosomes
- true breeding
What is a punnet square
- all possible gametes are assigned to a row
- those of the female parent go in the vertical column, male goes in horizontal row
- genotypes of the next generation are predicted by combining the male and female genotypes- represents all possible random fertilisation events
Punnet square example- monogenic inheritance
What is used to work out the genotypes of phenotypically similar individuals
The test cross
Describe the test cross for monogenic inheritance
- used to work out whether e.g. tall plants are TT or Tt- tall (T) is dominant so both genotypes could produce tall phenotype
- the organism exhibiting the dominant pheonotype (but of unknown genotype) are crossed with an individual showing the recessive phenotype (homozygous recessive genotype)
- if any of the individuals have the recessive phenotype, the dominant phenotype organism Is heterozygous (Tt)
Monogenic inheritance tests cross example
Co-dominance example cross
Describe human blood groups
- exhibits dominance, codominance, and multiple alleles
- Groups include A, B, AB, and O
- determined by 3 alleles of a single gene on chromosome 9- gene codes for an isoagglutinogen (I) on the surface of erythrocytes
- Ia and Ib are dominant (also codominant- if both present, will both contribute to phenotype)
- Io is recessive
Blood group genetic cross example
haemophilia example genetic cross
Cat fur colour genetic cross example
Diagram of the diffident possible gametes on 2 pairs of homologous chromosomes
Independent assortments leads to 4 possible gametes when both characteristics are heterozygous:
Dihybrid cross example
What is the combined probability of occurrence when 2 independent events occur simultaneously
The product of individual probabilities (NB- different when genes are on the same chromosome)
Autosomal linkage example
Recessive epistasis example cross
Dominant epistasis example cross
Describe complementary action in genes
- e.g. one gene locus determines whether the coat will have colour, and the next determines the type of colour
- if both c’s are recessive (homozygous), the coat will be alibono
- if there is a dominant C but no dominant A, the coat will remain black
- if there is both a dominant C and a dominant A, the coat will be agouti- the black pigment is deposited and then develops a yellow band
Example of complementary gene action cross
Chi-squared example
Outline stabilising selection
- favours intermediate phenotypes
- normally occurs when organisms environment remains unchanged
- e.g. robin eggs- larger clutches lead to malnourishment, whereas smaller clutches may result in no viable offspring
Outline disruptive selection
- favours both extreme phenotypes, intermediate phenotype is selected against
- e.g. rabbits with black, white and brown fur- if move to area of black/white rocks, brown rabbits would be eaten before reproduction
Describe directional selection
- if an environment changes, e.g. by becoming colder, there may now be advantage for become larger- larger mass will be the ideal and will be selected for
- more will survive and reproduces- ore likely to pass genes/alleles for larger size onto offspring
- over several generations, there is gradual shift in the optimum value for that trait
Describe genetic bottleneck
- when a population size shrinks and then increases again, it is said to have gone through a genetic bottleneck
- after an event the genetic diversity within that population will be reduced
- there may be loss of some advantageous alleles or a disproportionate frequency of harmful alleles (deleterious)
- puts populations’ chances of long-term survival at risk
- sometimes, population shrinks to such a small size that its fertility is affected- leading its species becoming endangered then extinct
- however, if the ones that survive are those that have a particular advantage, e.g. resistance to a pathogen, then a bottleneck could improve the chances of the gene pool while also shrinking genetic diversity
- species that have been selectively bred for certain traits have also been through a genetic bottleneck
Describe the founder effect
- if a new population is established by a very small number of individuals who originate from a larger parent population, the new population is likely to exhibit loss of genetic variation
- some groups of migrating humans, not fully representative of the parent population, have set up populations in new areas
- if they have remained isolated from other human populations because of religious/cultural differences due to geographic isolation- means the new population will have a small gene pool
- happened in iceland, the Faroe island, Pitcairn Island, Easter island and among the Amish people of North America
equation of the Hardy-Weinberg principle
Hardy-Weinberg example- reccessive trait
Hardy-Weinberg example- dominant trait
