Chapter 20 - Patterns of Inheritance and Variation Flashcards
gene
A length of DNA that codes for a single polypeptide or protein
locus
The position of a gene on a chromosome
alleles
Each gene can exist in two or more different forms
genotype
refers to the alleles of a gene possessed
e.g genotype of Aa = pink hair
homozygous
when the two allele copies are identical in an individual
heterozygous
when the two allele copies are different in an individual
phenotype
observable characteristics of an organism
e.g the red hair you see
Codominance
➜ when both alleles can be expressed in phenotype
➜ heterozygous will express both alleles
➜ Genotype for codominance represented by capital letter and allele by supeript
e.g blood type (gene for blood type is I)
AB blood has (IᴬIᴮ) - Alleles have A and B and both are exressed
F₁ F₂
➜ when a homo dom and homo rec crossed
↳ All F₁ = heterozygous
➜ when F₁ and F₁ crossed = F₂
Sex linkage
➜ 2 sex chromosomes: X and Y
➜ women have 2 copies of the X chromosome (XX)
➜ men have one X chromosome and one shorter Y chromosome (XY)
➜ some genes are found on a region of a sex chromosome that is not present on the other sex chromosome
➜ as the inheritance of these genes is dependent on the sex of the individual they are called sex-linked genes
➜ sex-linked genes are often on the longer X chromsome
e.g haemophilia where H = dom and h = rec
Female sufferer = xʰxʰ
Female non sufferer = xᴴxʰ OR xᴴxᴴ
Male sufferer = xʰy
Male non sufferer = xᴴy
therefore more common in males ha L
Autosomal linkage
➜ occurs on the autosomes (any chromosome that isn’t a sex chromosome)
➜ 2 or more genes on the same chromosome do not assort independently during meiosis
➜ these genes are linked and they stay together in the original parental combination
male sufferer of haemophilia X female carrier
Parent Phenotype:
Male sufferer X Female Carrier
Parent Genotype:
xʰy X xᴴxʰ
Then you draw a punnet square with xʰy on one side and xᴴxʰ the other
Alleles would be:
xᴴxʰ, xʰxʰ, xᴴy, xʰy
Phenotypic variation
difference in phenotypes between organisms of the same species
What is an example of phenotypic variation being explained by genetic factors?
the four different blood groups observed in human populations are due to different individuals within the population having two of three possible alleles for the single ABO gene
What makes up the phenotype?
Genotype + Environment
Different environments experience different conditions:
∘ Length of sunlight hours (which may be seasonal) - getting a tan or plants not getting enough light etc
∘ Supply of nutrients - nutrients to grow taller or become obese etc
∘ Availability of water
∘ Temperature range
∘ Oxygen levels
genetic variation
small differences in DNA base sequences between individual organisms within a species
Diet in animals
➜ fruit fly = grey but due to genetic mutant they become yellow regardless of environment
➜ if larvae of grey flies are given a diet of silver salts they also become yellow regardless of genotype
Growing conditions for plants
➜ plants grown in dark become yellow due to lack of magnesium
➜ this is chlorosis and occurs as chlorophyll synthesis slows down or stops
➜ plants grown in dark may also develop long stems with small curved leaves which is etiolation
genetic variation is caused by the following processes
- crossing over of non-sister chromatids during prophase I of meiosis
- independent assortment of homologous chromosomes during metaphase I of meiosis
- random fusion of gametes during fertilisation
Crossing over
➜ During meiosis I homologous chromosomes pair up
➜ non-sister chromatids can cross over and get entangled
➜ crossing points = chiasmata
➜ entanglement causes stress on DNA molecules
➜ As a result of this a section of chromatid from one chromosome may break and rejoin with the chromatid from the other chromosome
results in new combo of alleles on 2 cromosomes
Independent assortment
➜ random alignment of bivalents along equator of spindle forms diff combinations of alleles
➜ each pair of bivalent can be arranged with either chromosome on top which is random
➜ orientation of one homo pair is independant
➜ n.odiff possible chromosome combos is 2ⁿ where n = chromosomes in haploid
Random fusion of gametes
➜ each gamete has diff alleles due to crossing over blah blah
➜ happens during fertilization
➜ creates genetic variation between zygotes and therefore offspring will have an almost 0% chance of being genetically identical to parent
Mutations
➜ mutation occur now allele is different
➜ some new allele = advantageous or not or have no effect on phenotype due to degenerate (single amino acid may be coded for by more than one triplet code) gene code
Ratio for 2 Heterozygous parents
➜ do 4 X 4 dihybrid cross and the ratio for offspring with the alleles is
9 : 3 : 3 : 1
Epistasis
➜ two genes on different chromosomes affect the same feature
Dominant epistasis
➜ when the dom allele prevents the second gene from showing expression
e.g Say epistatic gene (USUALLY makes whatever white) is A/a = but its only dom
Second gene = B/b is purple or pink
AaBB should be purple due to 2 capital B but because of dominant epistasis the B doesn’t show up and hair colour becomes white
aaBb is purple as there the dominant epistatic gene is not present and so the capital B is purple
Ratio for dom epi = 12 : 3 : 1
Recessive epistasis
➜ when the recessive allele prevents the second gene from being expressed
e.g Say epistatic gene (USUALLY makes whatever white) is A/a = but its only rec
Second gene = B/b is purple or pink
aaBB would be white due to the recessive epistatic gene being present
Aabb would be pink due to only one recessive epistatic allele which is not enough to block the second gene (works same way as anything recessive)
Ratio for rec epi = 4 : 9 : 3
Compulsory epistasis
just know it exists and how it works
➜ genes work together so you need at least 1 dom allele of both genes to get one phenotype
X ———————–⟶ Y —————————⟶ Z
X = pre cursor Z = final colour
arrow from X to Y = A/a
arrow fromY to Z = B/b
AAbb = only goes from X to Y so does not get to final colour
AaBb = gets all the way to Z and will show the final colour
Chi squared test
X² = Σ(O-E)² DIVIDED by E
Intraspecific variation
variation among individuals of the same species
Interspecific species
variation between 2 or more species
Continuous variation
➜ differences that can be measured
∘ no distinct classes or categories
∘ characterisics can be measured and fall between a range of 2 extremes
➜ can be represented by a graph:
∘ mean = mode = median
∘ bell shaped distribution which is symmetrical to the mean
∘ 50% greater and 50% less than mean
due to both genetic and environmental factors
Polygenes
if a large number of genes have a combined effect on phenotype
Discontinuous variation
➜ differences that are categoric
∘ distinct classes or categories exist
∘ characterisics can not be measured
➜ can be represented by a bar graph!
only due to genetic factors
Natural selection
selection pressures produce a gradual change in allele frequencies over several generations
only due to genetic factors
Selection pressures
Environmental factors that affect the chance of survival of an organism
Stabilising selection
➜ natural selection that keeps allele frequencies relatively constant over generations
➜ things stay as they are unless there is a change in the environment
Directional selection
➜ natural selection that produces a gradual change in allele frequencies over several generations
➜ usually happens when there is a change in environment or a new allele has appeared in the population that is advantageous
Process of Directional selection
∘ there is always phenotypic variation in population
∘ selection pressure favours a specific phneotype
∘ individuals with favoured phenotype are fitter and more likely to pass on their advantageous alleles to their offspring
∘ those who don’t are less likely to survive and pass on their alleles
∘ over several generations the frequency of advantageous allele increases and the other decreases
Genetic drift
➜ when chance affects which individuals in a population survive to pass on their alleles
➜ In large populations, genetic drift is less likely to have n effect because any chance variations in allele frequencies usually even out across whole populations
Genetic bottleneck
➜ occurs when a previously large population suffers a dramatic fall in numbers
➜ major environmental event (e.g the end of the ice age or a fat earthquake) can massively reduce the number of individuals in a population which in turn reduces the genetic diversity in the population as alleles are lost
➜ surviving individuals end up breeding and reproducing with close relatives
Founder effect
➜ occurs when a small number of individuals from a large parent population start a new population
➜ new population is made up of only a few individuals from the original population so only some of the total alleles from the parent population will be present
➜ not all of gene pool is present in this smaller population
➜ which alleles end up in new founding population is up to chance
p + q = 1
p = frequency of the dominant allele
q = frequency of the recessive allele
p² + q² + 2pq = 1
p² = chance of an individual being homo dom
q² = chance of individual being homozygous recessive
Genetic
When two populations of the same species become reproductively isolated from each other, they can eventually become genetically isolated
Speciaton
the formation of new species from pre-existing species over time, as a result of changes to gene pools from generation to generation
➜ if 2 populations no longer reproduce with each other then there is no interchange of genes so they evolve independently which can form 2 populations that can no longer successfully able to interbreed
➜ when the genetic differences lead to an inability of members of the populations to interbreed and produce fertile offspring, speciation has occurred
Allopatric speciation
➜ separated from each other by a geographical barrier
➜ very common
➜ they can not physically get to one another
➜ e.g earthquakes, body of water, mountain
e.g lemurs in madagascar & east africa due to heavy rain
reliant on mutations as no mutation = no new alleles for gene pool
Sympatric speciation
➜ due to a reproductive barrier
➜ via ecological separation
↳ populations in same area but diff environment
↳ e.g different soil pH, eating at diff places
OR
➜ via behavioural separation
↳ populations have diff behaviours
↳ e.g breeding season diff, courting methods, feeding methods
reliant on mutations as no mutation = no new alleles for gene pool
Artificial Selection
process by which humans choose organisms with desirable traits and selectively breed to enhance the expression of these traits over many generations
Process of artificial selection via selective breeding
➜ populaion shows phenotypic variation
➜ A breeder (human) selects individual with desired phenotype
➜ Another individual with desired phenotype is selected (they should not be closely related)
↳ can be progeny tested (estimating breeding val of individual based on performance of its offspring)
↳ check data base of statistics
➜ selected individuals are bred (can be artificially)
➜ After offspring has reached maturity, it is tested for desirable trait and selected again for further breeding
➜ process repeated untill all offspring show desired trait
E.g of animals selectively bred
Cows for milk/mea
↳ cow and bull picked via progeny testing
↳ Artificial insemination as bulls are aggressive
↳ cows are more prone to ailemts such as mastitis (inflammation of udder) and lameness
Horses to be faster for races
↳ fastest female and male horse picked
↳ looked at statistics on data base
Dogs for domestication/cuteness
↳ pugs = serious respiratory problems (Brachycephalic dogs samira u dont need to know that - just ignore)
↳ sometimes individuals bred too closely and have health issues
E.g of plants selectively bred
➜ for disease resistance
➜ increased crop yield
➜ better tasing fruits
↳ wheat plant - to be fungal disease resistant
↳ rice - bacteria resistant
importance of maintaining a resource of genetic material
➜ maintain a resource of genetic material that includes types that are close to original
➜ ensures gene pool doesn’t become too small
➜ prevent inbreeding depression which increases chances of 2 harmful recessive alleles combining
➜ promotion of hybrid vigour (the increase in certain characteristics which leads to increased growth and survivability) which increases frequency of hetero alleles
➜ source of replacement if population is in danger like some disease
➜ unknown future requirements (medicine etc)
➜ prevent monoculture (cultivation of a single crop in an area)
Ethical Considerations Surrounding Artificial Selection
➜ can lead to inbreeding
➜ when the best are always bred the gene pool decreases so = inbreeding depression
➜ increased chance of organisms inheriting harmful genetic defects due to harmful recessive alleles combining (decrease quality of life)
➜ organisms vulnerable to disease as resistant alleles is reduced from gene pool (knowingly putting them in harms way)
➜ e.g bulldogs with breeding problems (achycephalic againignore that samira xx)
e.g cattle being susceptible to disease
