Patterns of inheritance

• Genotype 


  the genetic makeup of an organism
• Phenotype

  the visible characteristics of an organism
• What leads to genetic variation

  mutations, sexual reproduction and environmental factors
• Mutagens - agents that can increase the rate of mutation

  Physical: x-rays, gamma rays, UV light

  Chemical: nitrous gas, mustard gas, benzopyrene

  Biological: some viruses, food contaminants (mycotoxins)
• Mutations that occur during gamete formation

  > persistent - can be transmitted through generations without change

  > random - not directed by a need on the part of the organism in which they occur
• Chromosome mutations

  > fusion of two diploid gametes make a tetraploid zygote

  > when a diploid gamete is fertilised by a haploid gamete resulting gamete will be triploid (has 3 sets of chromosomes)

  • many cultivated plants are polyploidy
• Sexual reproduction

  Meiosis produces genetically different gametes. During meiosis genetic variation may result from

  • allele shuffling - swapping of alleles between non-sister chromatids during crossing over in prophase 1
  • independent assortment of chromosomes during metaphase 1
  • independent assortment of chromatids during metaphase 2

  > Gametes produced by meiosis are individual and genetically dissimilar. They contain only one allele for every gene and one of each pair of homologous chromosomes.

  > Random fusion of gametes increases genetic diversity
• Environmental factors

  Phenotypic variation caused by...

  • speaking in dialect - don't inherit through genes
  • diet
  • losing a limb or gaining a scar
• Variation caused by environment interacting with genes

  > Plants kept in dim light after germination or placed in soil that has insufficient magnesium then leaves don't develop enough chlorophyll.

  > Plant cannot photosynthesise

  > Chlorotic plants have genotype for making chlorophyll but environmental factors are preventing expression of these genes

  • E.g: Chlorosis in plants causes yellow leaves due to lack of chlorophyll because of a mineral deficiency in the soil
• Gene - base sequence of DNA that codes for the amino acid sequence of a polypeptide
• Allele - different versions of a gene

  • an individual inherits two alleles one from each parents at the gene locus
• Population genetics

  > studies variation in the alleles and genotypes within the gene pool and how their frequencies vary over time.

  > factors affecting allele frequencies and genetic diversity within a gene pool:

  • population size
  • natural selection
  • migration
  • non-random mating
  • mutation rates
  • genetic drift
• Hardy-Weinberg Principle

  > predicts allele frequency in a population

  > it assumes that

  • the population is large enough to justify sampling errors
  • mating is random within population
  • there is no mutations
  • no migration or genetic drift
  • no selective advantage for any genotype so no selection
• Artificial selection

  > selective breeding of organisms

  > humans choose the desired phenotypes by selecting the genotypes that contribute to the gene pool of the next generation (allowing those with desirable traits interbreed)

  > desirable characteristics in

  • plants: increased yield, pest and disease resistance
  • livestock: docility and ability to be trained in herds

  breeders may select those individuals that grow best under certain desired conditions and cross-pollinate them collecting offspring and repeat process over many generations
• Inbreeding depression


  > At each stage of selective breeding, the genetic diversity in the gene pool of the selected breed is reduced

  > if related individuals are crossed, inbreeding depression can result

  > chances of an individual inheriting homozygous recessive allele defects are then increased
• Hybrid vigour

  Breeders outcross individuals belonging to 2 different varieties to obtain individuals that are heterozygous at many gene loci
• > Commercial varieties of crops have reduced causing them to become genetically similar.

  > If a pathogen is introduced plants succumb to the infection

  > breeders may have to outcross cultivated varieties with varieties like their wild ancestors to increase hybrid vigour

  > samples of these types need to be conserved in gene banks
• Gene banks store genomes in their organisms

  • rare breed farms
  • seed banks
  • frozen embryos
  • sperm banks
  • botanic gardens and zoos
• Artificial selection in dogs

  > traits in dogs considered desirable by humans may put dogs at a selective disadvantage in the wild

  > through inbreeding some pedigree dogs are susceptible to disease

  > some coat colours selected for aesthetics would fail to camouflage the animals

  dogs have been domesticated for hunting, herding, aid for the disabled, protection and aesthetics
• Arguments FOR selective breeding - useful traits

  • increased drought resistance in plants
  • increased milk yield in cattle
  • reduced calf mortality
  • good length and angle of teats for ease of milking
  • additional nutritious qualities:
  • high fat and protein content in milk
  • high muscle to fat ratio
• Arguments AGAINST selective breeding

  • artificial selection is for benefit of humans not organism
  • crops/animals become less genetically diverse leading to fewer alleles in gene pool
  • loss of fertility
  • harmful recessive alleles may be expressed
  • increased homozygosity
  • genetic erosion
  • crops and animals susceptible to diseases as they are genetically uniform
• Chi 2 test

  tests if there is a significant difference between our expected and observed values

  • if x value is higher than critical value then we reject null hypothesis as there IS a significant difference between expected and observed values
  • to work out critical value: number of categories - 1
• Epistasis

  when the expression of one gene masks or suppresses the expression of another gene which reduces phenotypic ratio

  • Normal 9:3:3:1 ratio is reduced to....
  • 9:3:4 in recessive epistasis
  • 12:3:1 in dominant epistasis

  > gives an unexpected phenotypic ratio

  involves interaction of genes at different loci

  + or - gene expression of gene 'B' as a result of gene 'A' can result in...

  • dominant epistasis: AA, Aa
  • recessive epistasis: aa
• Co-dominance

  both alleles of one gene are dominant

  they show 3 different phenotypes including a mixed ones

  • DONT write genes as RW as you will be representing epistasis (2 different genes)
  • Have large 'C' representing gene and smaller allele above
• q - recessive allele

  p - dominant allele

  q^2 -Homozygous recessive genotype

  P^2 - Homozygous dominant genotype

  2pq - Heterozygous
• Multiple alleles

  more than 2 alleles for one gene

  e.g: ABO blood groups

  • Type o is a universal donor
  • Type AB is co-dominant and is a universal receiver
• Sex-linkage

  genes found on sex chromosomes in the genome

  • females have XX less likely to be affected but can be carriers
  • males have XY so more easily affected by alleles on X as there are no alleles on the Y to counteract it
• Dihybrid inheritance

  shows inheritance pattern of 2 genes

  each gamete carries one allele of both genes

  4 possible genotypes

  • e.g: pea crossing
• There may be unexpected ratios due to:

  • random fertilisation
  • autosomal linkage

  and

  • no crossing over occurring to separate these genes

  the further away the genes are on a gene locus the more likely they are to be separated

  to test if there is linkage or epistasis:

  x2 test which finds the significant difference between expected and observed ratios of offspring

  • if x2 > CV at P= 0.05 there is a significant difference between the frequencies and linkage/epistasis occurred
• Phenotypic ratios 


  expected

  9:3:3:1 - dihybrid inheritance

  shows 2 genes on different chromosomes

  both inherited as separate units

  
• Autosomal linkage

  > when two gene loci found on same autosomal (non-sex) chromosome are inherited together

  > gene loci can impact the combination of the 2 genes and how they are inherited:

  influencing independent assortment

  
• How many different gamete possibilities would a haploid number of 8 give?

  = 256

  $2^8$
• Genetic variation is typically a product of

  • variety of alleles - large gene pool
  • random fertilisation
  • meiosis which produces genetically unique gametes
  • offspring have alleles from more than 1 parents
  • crossing over in prophase 1 between non-sister chromatids
  • independent assortment in metaphase 1 and 2
• Describe how the phenotypic ratio 9:3:3:1 might be different if two genes were autosomally linked

  a higher proportion would be heterozygous like their parents
• Suggest and explain one reason, other than epistasis, why the phenotype ratio might not be 9:3:3:1

  > autosomal linkage

  > both alleles occur on same chromosome so no independent assortment takes place

  > so alleles are inherited together in same gamete

  (unless crossing over occurs/ a chiasmata forms between gene loci)
• How can genetic diversity be measured?

  genetic polymorphism
• Explain why a genetic bottleneck can lead to low genetic diversity.

  > many alleles are lost when a population drops

  > modern populations descended from few survivors with a limited gene pool
• With reference to Fig. 20.2, explain the effect of stabilising selection on beak size on Daphne Island.

  • Genetic variation as a result of sexual reproduction, mutation and meiosis
  • Finches with extreme beak lengths less likely to survive- being selected against
  • birds with very small and large beaks do not survive (beaks less than 7.4mm or more than 11.6mm)
  • survivors possess alleles for average beak depth - 11.2mm
  • alleles for for average beak depth more likely to be inherited by offspring
  • results in increase of these alleles from generations
  • bell-shaped curve skewed to right

  
• Explain, with reference to selective breeding, why it is important to maintain viable wild populations of crop plant species (6 marker)

  • genetic variation - populations less susceptible to new diseases and changing climates
  • gene banks are a source of useful alleles
  • source of replacement if cultivated population of crops is vulnerable
  • introducing different alleles increasing gene pool
  • preventing inbreeding depression and promoting hybrid vigour
• Definitions

  A) genotype

  B) phenotype

  C) homologous

  D) heterozygous

  E) recessive allele

  F) dominant allele

  G) codominant

  H) multiple alleles

  I) sex-linkage

  J) autosomal linkage

  K) epistasis

  L) monohybrid

  M) dihybrid