Chapter 20 - Patterns of Inheritance and Variation

Question 1

gene

Answer 1

A length of DNA that codes for a single polypeptide or protein

Question 2

locus

Answer 2

The position of a gene on a chromosome

Question 3

alleles

Answer 3

Each gene can exist in two or more different forms

Question 4

genotype

Answer 4

refers to the alleles of a gene possessed

e.g genotype of Aa = pink hair

Question 5

homozygous

Answer 5

when the two allele copies are identical in an individual

Question 6

heterozygous

Answer 6

when the two allele copies are different in an individual

Question 7

phenotype

Answer 7

observable characteristics of an organism

e.g the red hair you see

Question 8

Codominance

Answer 8

➜ 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

Question 9

F₁ F₂

Answer 9

➜ when a homo dom and homo rec crossed
↳ All F₁ = heterozygous
➜ when F₁ and F₁ crossed = F₂

Question 10

Sex linkage

Answer 10

➜ 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

Question 11

Autosomal linkage

Answer 11

➜ 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

Question 12

male sufferer of haemophilia X female carrier

Answer 12

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

Question 13

Phenotypic variation

Answer 13

difference in phenotypes between organisms of the same species

Question 14

What is an example of phenotypic variation being explained by genetic factors?

Answer 14

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

Question 15

What makes up the phenotype?

Answer 15

Genotype + Environment

Question 16

Different environments experience different conditions:

Answer 16

∘ 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

Question 17

genetic variation

Answer 17

small differences in DNA base sequences between individual organisms within a species

Question 18

Diet in animals

Answer 18

➜ 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

Question 19

Growing conditions for plants

Answer 19

➜ 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

Question 20

genetic variation is caused by the following processes

Answer 20

• 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

Question 21

Crossing over

Answer 21

➜ 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

Question 22

Independent assortment

Answer 22

➜ 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

Question 23

Random fusion of gametes

Answer 23

➜ 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

Question 24

Mutations

Answer 24

➜ 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

Question 25

Ratio for 2 Heterozygous parents

Answer 25

➜ do 4 X 4 dihybrid cross and the ratio for offspring with the alleles is

9 : 3 : 3 : 1

Question 26

Epistasis

Answer 26

➜ two genes on different chromosomes affect the same feature

Question 27

Dominant epistasis

Answer 27

➜ 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

Question 28

Recessive epistasis

Answer 28

➜ 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

Question 29

Compulsory epistasis

just know it exists and how it works

Answer 29

➜ 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

Question 30

Chi squared test

Answer 30

X² = Σ(O-E)² DIVIDED by E

Question 31

Intraspecific variation

Answer 31

variation among individuals of the same species

Question 32

Interspecific species

Answer 32

variation between 2 or more species

Question 33

Continuous variation

Answer 33

➜ 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

Question 34

Polygenes

Answer 34

if a large number of genes have a combined effect on phenotype

Question 35

Discontinuous variation

Answer 35

➜ 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

Question 36

Natural selection

Answer 36

selection pressures produce a gradual change in allele frequencies over several generations

only due to genetic factors

Question 37

Selection pressures

Answer 37

Environmental factors that affect the chance of survival of an organism

Question 38

Stabilising selection

Answer 38

➜ natural selection that keeps allele frequencies relatively constant over generations
➜ things stay as they are unless there is a change in the environment

Question 39

Directional selection

Answer 39

➜ 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

Question 40

Process of Directional selection

Answer 40

∘ 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

Question 41

Genetic drift

Answer 41

➜ 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

Question 42

Genetic bottleneck

Answer 42

➜ 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

Question 43

Founder effect

Answer 43

➜ 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

Question 44

p + q = 1

Answer 44

p = frequency of the dominant allele
q = frequency of the recessive allele

Question 45

p² + q² + 2pq = 1

Answer 45

p² = chance of an individual being homo dom
q² = chance of individual being homozygous recessive

Question 46

Genetic

Answer 46

When two populations of the same species become reproductively isolated from each other, they can eventually become genetically isolated

Question 47

Speciaton

Answer 47

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

Question 48

Allopatric speciation

Answer 48

➜ 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

Question 49

Sympatric speciation

Answer 49

➜ 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

Question 50

Artificial Selection

Answer 50

process by which humans choose organisms with desirable traits and selectively breed to enhance the expression of these traits over many generations

Question 51

Process of artificial selection via selective breeding

Answer 51

➜ 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

Question 52

E.g of animals selectively bred

Answer 52

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

Question 53

E.g of plants selectively bred

Answer 53

➜ for disease resistance
➜ increased crop yield
➜ better tasing fruits
↳ wheat plant - to be fungal disease resistant
↳ rice - bacteria resistant

Question 54

importance of maintaining a resource of genetic material

Answer 54

➜ 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)

Question 55

Ethical Considerations Surrounding Artificial Selection

Answer 55

➜ 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