Patterns of Inheritance

• Define phenotype 


  the expression of an organism's genetic constitution e.g. low metabolism combined with its interaction with the enviorment e.g. diet
• How can meiosis bring about genetic variation?

  • random arrangement of chromosomes during lining up.
  • crossing over of chromatids before the first division
• How does random fertilisation bring 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.
• Define monogenic inheritance

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

  Parental Phenotypes Brown eyes Blue eyes

  Parental genotypes Bb bb

  Gametes B b b b

  Offspring genotypes Bb, Bb, bb, bb

  Offspring phenotypes 2: brown eyes: blue eyes
• Define dihybrid inheritance

  where two phenotypic characteristics are determined by two different genes present on two different chromosomes at the same time.
• Draw a genetic diagram for dihybrid inheritance

  Parental phenotypes Round yellow Wrinkled green

  Parental genotypes RRYY rryy

  Gametes RY RY ry ry

  Offspring genotypes RrYy

  Offspring phenotypes round yellow seeds
• what is meant by sex-linkage?

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

  Parental phenotypes Carrier female Normal male

  Parental genotypes XA Xa XA Y

  gametes XA XA, XA Xa, XA Y, Xa Y

  Offspring genotypes XA XA, XA Xa, XA Y, Xa Y

  Offspring phenotypes Normal female, carrier female

  normal male, colour-blind male
• What is meant by multiple alleles?

  A gene with more than two alleles
• Draw a genetic diagram for multiple allelic inheritance

  Parental phenotypes Blood group A Group B

  Parental genotypes IA IO IB IO

  Gametes IA IO IB IO

  Offspring genotypes IA IB, IA IO, IB IO, IO IO

  Offspring phenotypes Group AB, Group A

  group B, group O
• What is meant by codominant alleles?


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

  Parental phenotypes Red flower White flower

  Parental genotypes CR CR CW CW

  gametes CR CR CW CW

  Offspring genotypes CRCW

  Offspring phenotypes pink flower
• what is meant by autosomal linkage 


  where two or more genes are located on the same (non-sex) chromosome. this is the 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
• what is meant by epistasis?

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

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

  The formula result in number, which is then compared to a critical value (for the corresponding degrees of freedom)

  • if number > or equal to the critical value we conclude there is no significant difference and the results occurred due to chance
• How can the number of genes coding for a characteristic influence variation?

  • discontinuous variation = characteristics determined by one gene
  • continuous variation = characteristic determined by more than one gene
• what is 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
• What is 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
• what is 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.
• What is meant by a genetic bottleneck?

  Where a catastrophic event dramatically reduces the size population, thereby decreasing the variety of alleles in the gene pool and causing large changes in allele frequencies.
• what is meant by the 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
• what is the 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 alleles for a characteristic must add up to 1.0 the equation is therefore p + q = 1

  p = frequency of dominant allele

  q = frequency of the recessive alleles
• Hardy-Weinberg equation for calculating genotype frequency

  Frequencies of each genotype for a characteristics must add up to 1.0. The equation = p2 + 2pq + q2 = 1

  p2 = frequency of homozygous dominant

  2pq = frequency of heterozygous

  q2 = frequency of homozygous recessive
• Define speciation 


  Where a population is split and isolted 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 become separate species
• what is meant by allopatric speciation?

  speciation resulting from a physical barrier e.g. river

  the environment occupied by the two groups are different and therefore different, and therefore different alleles are favoured
• what is meant by 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
• what is artificial selection?


  Humans choose particular organism to breed together in order to produce a desired characteristic in the offspring
• give examples of 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
• why is it important to keep a resource of gene material when selective breeding allows selective breeding?

  Allows any traits that were accidentally bred out to be reintroduced, or to revert back to point before any negative traits were introduced
• give some 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