6.2- Patterns of Inheritance

gene

a sequence of bases on a DNA molecule that codes for a protein

allele

different versions of the same gene that code for variants of a characteristic

genotype

organism’s genetic makeup→ combination of alleles

phenotype

the characteristics expressed by the genotype

dominant

an allele that is always expressed it the phenotype→ only 1 copy present to express phenotype

recessive

allele that is only expressed when homozygous in genotype

locus

specific position of gene on chromosome

homozygous

2 identical alleles for a trait

heterozygous

two different alleles for a trait

ratio for heterozygous monohybrid cross

• genotypic ratio→ 1:2:1

• phenotypic ratio→ 3:1

codominance

• when two different alleles are equally expresses in an organisms phenotype→ phenotype is a mixture of both alleles rather than just one

• uppercase letter used to signify gene and superscript letters used to indicate alleles e.g. C^R and C^W for red red and white colour

multiple alleles

• genes that exist in more than two allelic forms

• organism can only have two alleles of a specific gene at once

• e.g. blood groups:

  • I^A, I^B, I^O alleles exist

inheritance of sex in humans

• 23rd pair of chromosomes are sex chromosomes:

  • X chromosome→ found in both male and females

  • Y chromosome→ only found in males

• Female body cells→ XX so gametes always contain X

• Male body cells→ XY so gametes contain either X or Y

sex linkage

• genes located on the X or Y chromosomes→ sex linked

• X chromosome carries most of the genes→ larger

• recessive alleles on X chromosome appear in phenotype more often in males- no corresponding allele on Y to mask them

haemophilia

• X-linked recessive disorder→ caused by a defective gene on the X chromosome

• allele alters DNA sequence coding for a blood clotting protein→ faulty allele= reduced blood clotting so excessive bleeding

inheritance of haemophilia

• mainly affects males→ no second X chromosome to mask faulty allele

• Doesn’t often affect females→ 2 alleles must be inherited for expression

• always inherited from mother in males→ males always inherit X chromosome from their mother

• mainly inherited from carrier mothers

• affected fathers can only pass it on to daughters→ only daughters inherit X chromosomes from their father

dihybrid cross

• shows the simultaneous inheritance of two genes controlling separate characteristics

• helps to:

  • determine if genes are linked

  • locate genes on specific chromosomes

  • calculate expected phenotypic ratios in subsequent generations

ratio for heterozygous dihybrid cross

9:3:3:1

autosome

chromosomes that do not determine the sex of an organism→ first 22 chromosome pairs

autosomal linkage

• when genes are linked on the same autosome

• inherited together in offspring rather than assorting independently

results of autosomal linkage

• non-random association alleles at different loci

• phenotypic ratios are different from those expected

• parental allele combination preserved across generations

how does crossing over affect autosomal linkage

• crossing over can separate linked genes

• when genes are linked, fewer offspring have different combinations of alleles from their parents due to crossing over (recombinant)

  • less genetic variation introduced from crossing over when genes are linked

• the closer the gene, the more likely they will be inherited together

calculating recombination frequency

• 50%= no linkage between genes

• less than 50% = some degree of autosomal linkage

• lower frequency= closer genes are located to each other on chromosome- less chance of being separated

epistasis

an interaction between genes where one gene affects/ masks the expression of another gene

hypostatic gene

gene whose expression is affected by another gene

epistatic gene

gene whose allele affects the expression of the hypostatic gene

phenotypic ratio in epistasis

9:4:3

recessive epistasis

• occurs when epistatic gene must be homozygous recessive to block expression of hypostatic gene

dominant epistasis

occurs when epistatic gene is dominant and actively modifies or blocks the expression of hypostatic gene

what is chi squared test used for

assessing whether outcomes of a genetic cross are significantly different from outcomes predicted by specific inheritance pattern.

criteria for chi squared test

• large sample size

• discrete data categories

• using raw counts

• comparison of experimental and theoretical results

steps in chi squared test

1. null hypothesis→ no significant difference between observed and expected, any difference due to chance

2. alt hypothesis→ significant difference between observed and expected results, difference due to factor other than chance

3. predict expected phenotypic ratios among offspring

4. conduct crosses and record observed ratios

5. calculate chi squared statistic

6. compare to critical value at chosen probability level→ if χ² is greater than critical value, reject H₀

degrees of freedom

number of phenotypes-1

factors affecting evolution

• mutation

• gene flow

• genetic drift

• natural selection

• sexual selection

gene flow

the transfer of alleles within the population

genetic drift

• random changes in allele frequencies within a population’s gene pool due to chance events

• does not occur due to natural selection

• can accelerate development of new species in small isolated populations

categories of factors limiting population size

• density dependent factors→ depend on size of population e.g. competition, predation, disease

• density independent factors→ impact pop. regardless of size e.g. natural disasters, climate change

bottleneck effect

• when pop. size reduces suddenly and drastically and reduction lasts for at least 1 generation

• can lead to reduced gene pool and genetic diversity→ causes issues related to inbreeding and reduced fertility

• may also allow beneficial mutation to become more prevalent

founder effect

• when a small group splits from a larger population and small new population is established by this small number of individuals

• can lead to reduced gene pool and decreased genetic diversity

• rare alleles may become more common in new population

how does variation drive selection

• generates range of phenotypes within population, enhancing likelihood that some individuals have alleles for advantageous traits

• individuals with beneficial traits are more likely to survive and reproduce under changing conditions

• natural selection occurs

types of selection

• directional

• stabilising

• disruptive

directional selection

• selects for one extreme phenotype over others

• increases allele frequency for one extreme phenotype

• shifts curve in direction of favoured extreme

• e.g. antibiotic resistance in bacteria

stabilising selection

• selects for the average phenotype and selects against extreme phenotypes

• increases allele frequency for average phenotype, decreases frequency for extremes

• narrows curve

• e.g. human birth weights

disruptive selection

• increases allele frequency for multiple extreme phenotypes, decreases allele frequency for intermediates

• curve shifts into multiple peaks either side of average peak

• e.g. bird beaks adapting to become larger and smaller when there are different food sources

hardy weinberg principle

helps calculate allele frequency for a gene in a population

assumptions made in hardy weinberg principle

• no mutations

• no migration into or out of population

• mating is random

• large population size

• no natural selection pressures

variables in hardy weinberg

• p → frequency of dominant allele

• q → frequency of recessive allele

• p² → frequency of homozygous dominant

• 2pq→ frequency of heterozygous

• q² → frequency of homozygous recessive

hardy weinberg principle equations

• p+q=1

• p²+2pq+q² =1

reproductive isolation

• when population cannot interbreed successfully to produce fertile offspring

• results in genetic isolation

prezygotic reproductive barriers

• prevent fertilisation and formation of a zygote

• e.g. habitat isolation, variations in mating rituals

postzygotic reproductive barriers

• often a result of hybridisation between different species

• produce infertile offspring, reducing reproductive potential

allopatric speciation

• some members of population are geographically isolated by a physical barrier e.g. mountain, river, sea

• geographical separation exposes distinct parts of population to different environmental pressures

• prezygotic reproductive barriers= reproductive isolation

• reproductive isolation prevents gene flow and physical separation= genetic divergence

• causes populations to evolve separately and form separate species

sympatric speciation

• takes place within same geographical location

• ecological or behavioural separation mechanisms e.g. different habitat preference lead to groups becoming reproductively isolated

• reproductive isolation prevents gene flow and leads to genetic divergence

• causes populations to evolve separately and form separate species

what does speciation occur due to

• reproductive isolation of populations

• genetic divergence→ natural selection and genetic drift

adaptive radiation

• organisms diversify rapidly from ancestral species into wide array of new forms→ each adapted to specific ecological niche

• more likely to occur when change in environment makes new resources available

artificial selection

• when humans breed organisms selectively for specific genetic traits and determine which individuals reproduce

process of artificial selection

• select population that displays variation

• select individuals with desired traits

• selectively breed individuals together that display desired traits

• grow and test offspring for desired traits

• repeat selection process across many generations

issues with artificial selection

• inbreeding:

  • decreases genetic diversity

  • can lead to expression of harmful recessive alleles

  • can lead to inbreeding depression→ loss of ability to survive and grow

  • reduces fitness and stability

outbreeding

• breeding unrelated individuals

• increases genetic diversity

• reduces expression of harmful recessive alleles

• can lead to hybrid vigour→ increased ability to survive and grow well

• increases fitness and adaptability