Patterns of Inheritance

• Phenotype

  Observable / physical characteristics that are determined by genotype and environment.
• How meiosis brings about genetic variation

  1. Random arrangement of chromosomes during lining up

  2. Crossing over of chromatids before the first division
• How random fertilisation brings about genetic variation

  Gametes are haploid cells, meaning they only contain half of a person's DNA. As this is determined by meiosis, every gamete contains different DNA. Therefore the same two individuals can produce genetically different offspring
• Monogenic inheritance

  Where one phenotypic characteristic is controlled by a single gene
• Genetic diagram for monogenic inheritance

  Parental phenotypes: Brown eyes, Blue eyes

  Parental genotypes: Bb, bb

  Gametes: B, b

  Offspring genotypes: Bb, Bb, bb, bb

  Offspring phenotypes: 2:2 brown eyes:blue eyes
• Dihybrid inheritance

  Where two phenotypic characteristics are determined by two different genes present on two different chromosomes at the same time
• Genetic diagram for dihybrid inheritance

  Parental phenotypes: Round yellow seeds, Wrinkled green seeds

  Parental genotypes: RRYY, rryy

  Gametes: RY, ry

  Offspring genotypes: RrYy

  Offspring phenotypes: Round yellow seeds
• Sex-linkage

  Where an allele is located on one of the sex chromosomes, meaning its expression depends on the sex of the individual
• Genetic diagram for sex-linked inheritance

  Parental phenotypes: Carrier female, Normal male

  Parental genotypes: XAXa, XAY

  Gametes: XA, Xa, Y

  Offspring genotypes: XAXA, XAXa, XAY, XaY

  Offspring phenotypes: Normal female, carrier female, normal male, colour-blind male
• Multiple alleles

  A gene with more than two alleles
• Genetic diagram for multiple allelic inheritance

  Parental phenotypes: Blood group A, Blood group B

  Parental genotypes: IAIO, IBIO

  Gametes: IA, IO, IB, IO

  Offspring genotypes: IAIB, IAIO, IBIO, IOIO

  Offspring phenotypes: Group AB, group A, group B, group O
• Codominant alleles


  Two dominant alleles that both contribute to the phenotype, either by showing a blend of both characteristics, or the characteristics appearing together
• Genetic diagram for codominant inheritance

  Parental phenotypes: Red flower, White flower

  Parental genotypes: CRCR, CWCW

  Gametes: CR, CW

  Offspring genotypes: CRCW

  Offspring phenotypes: Pink flower
• Autosomal linkage

  Where two or more genes are located on the same (non-sex) chromosome. In this case, only one homologous pair is needed for all four alleles to be present. For genes that aren't linked, two homologous pairs are needed
• Epistasis

  Where two non-linked genes interact, with one gene either masking or suppressing the other gene
• Chi-squared test

  A statistical test to find out whether the difference between observed and expected data is due to chance or a real effect. Can be used to compare expected phenotypic ratios with observed ratios
• How to perform a chi-squared test

  The formula results in a calculated value, which is then compared to a critical value (for the corresponding degrees of freedom). If the calculate value is greater than or equal to the critical value, we conclude there is significant difference and the results are not due to chance
• How the number of genes coding for a characteristic can influence variation

  Discontinuous variation= characteristic determined by one gene (monogenic inheritance)

  Continuous variation= characteristic determined by more than one gene (polygenic inheritance)
• Stabilising selection

  Occurs when environmental conditions stay the same. Individuals closest to the mean are favoured, and any new characteristics are selected against. Results in low diversity
• Discontinuous variation


  Characteristic determined by one gene (monogenic inheritance)
• Continuous variation


  Characteristic determined by more than one gene (polygenic inheritance)
• Stabilising selection

  Occurs when environmental conditions stay the same. Individuals closest to the mean are favoured, and any new characteristics are selected against. Results in low diversity.
• Directional selection

  Occurs when environmental conditions change. Individuals with phenotypes suited to the new conditions will survive and pass on their genes. Over time the mean of the population will move towards these characteristics.
• Genetic drift

  A change in a population's allele frequencies that occurs due to chance rather than selective pressures. In other words, it is caused by 'sampling error' during reproduction.
• Genetic bottleneck

  Where a catastrophic event dramatically reduces the size of a population, thereby decreasing the variety of alleles in the gene pool and causing large changes in allele frequencies.
• Founder effect

  When a small number of individuals become isolated, forming a new population with a limited gene pool, with allele frequencies not reflective of the original population.
• Hardy-Weinberg principle

  Allows us to estimate the frequency of alleles in a population, as well as if allele frequency is changing over time.
• Hardy-Weinberg equation for calculating allele frequency


  The frequencies of each allele for a characteristic must add up to 1.0. The equation is therefore; p + q = 1 Where p= frequency of the dominant allele, and q= frequency of the recessive allele.
• Hardy-Weinberg equation for calculating genotype frequency

  The frequencies of each genotype for a characteristics must add up to 1.0. The equation is therefore; p2 + 2pq + q2 = 1 Where p2= frequency of homozygous dominant, 2pq= frequency of heterozygous, and q2= frequency of homozygous recessive.
• Speciation

  Where a population is split and isolated, there are different selective pressures on the two groups. If the genetic makeup changes to the extent the two groups can not longer interbreed, they have become separate species.
• Allopatric speciation

  Speciation resulting from a physical barrier e.g. river, mountain range. The environments occupied by the two groups are different, and therefore different alleles are favoured.
• Sympatric speciation

  Speciation resulting from a non-physical barrier e.g. a mutation that no longer allows two organisms to produce fertile offspring. Any changes in anatomy or behaviour may also prevent breeding.
• Artificial selection

  Humans choose particular organisms to breed together in order to produce a desired characteristic in the offspring.
• Artificial selection in plants and animals

  • Plants= seeds used from plants that produce larger fruit and vegetables. Animals= cows with higher milk yield are chosen and selectively bred.
• It is important to keep a resource of genetic material when selective breeding to allow any traits that were accidentally bred out to be reintroduced, or to revert back to a point before any negative traits were introduced.
• Ethical issues around the use of artificial selection

  • Anatomical changes in animals e.g. respiratory issues in pugs
  • Higher susceptibility to disease in both plants and animals
• Speciation Question


  -Reproductive isolation

  -Change in allele frequency

  -Disruptive natural selection

  -Eventually different species can't interbreed to produce fertile offspring.