6.1.2 - patterns of inheritance

• the chi squared test is a statistical test to find out whether the difference between observed and expected data is due to chance or a real effect. it can be used to compare expected phenotypic ratios with observed ratios.
• The chi squared test works by:

  the formula results in a number.

  the degrees of freedom are calculates (number of categories - 1)

  Value Fromm formula compared to critical value

  if the number is less than the critical value we accept null hypothesis that results occurred due to chance
• The formula for chi squared test is: (O-E)^2 / E
• Phenotype: observable characteristics of an organism, such as height, hair color, and eye color
• Meiosis can bring about genetic variation by

  • random arrangement of chromosomes during lining up
  • crossing over of chromatids before first division
• random fertilisation brings about genetic variation as gametes are haploid cells. every gamete contains different DNA as this is determined by meiosis. therefore the same two individuals can produce genetically different offspring.
• Monogenic inheritance is where one phenotypic characteristic is controlled by a single gene.
• in genetic diagrams, you need to show:

  • parent genotypes
  • parent phenotypes
  • gametes
  • offspring genotypes (draw a punnet square)
  • offspring phenotypes and ratios
• Dihybrid inheritance is when two phenotypic characteristic are determined by two different genes present on two different chromosomes at the same time.
• sex linkage is where an allele is located on one of the sex chromosomes, meaning its expression depends on the sex of the individual
• multiple alleles is when a gene has more than two alleles
• Codominant alleles are two dominant alleles that both contribute to the phenotype, either by showing a blend of both characteristics or the characteristics appearing together.
• Autosomal linkage is 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.
• Interspecific variation: variation between different species.
• Intraspecific variation: variation between members of the same species.
• sex linkage is due to genes being carried on the X chromosome
• somatic chromosomes: body cell chromosomes
• Epistasis is where two non-linked genes interact, where one will either mask or suppress the other gene.
• discontinuous variation is determined by one gene (monogenic inheritance )
• continuous variation is determined by more than one gene (polygenic inheritance)
• Recessive Epistasis: the presence of homozygous recessive allele prevents the expression of alleles at another loci.
• Recessive epistasis usually produces the phenotypic ratio

  9:4:3
• dominant Epistasis is where the dominant allele at one gene locus masks the expression of the allele at the second locus.
• Dominant Epistasis usually produces phenotypic ratio 12:3:1
• complementary fashion is where you need at least one dominant allele at each loci in order for a colour to be expressed.
• Complementary fashion usually produces a phenotypic ratio 9:7
• 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.
• gene pool: set of all the alleles of the population of a species
• allele frequency: probability of an allele appearing in the gene pool
• Frequency of each allele for a characteristic must add up to 1.0.

  P + q = 1

  where p = frequency of dominant allele

  q = frequency of recessive allele.
• probability of a homozygous dominant pair = p^2
• probability of a homozygous recessive pair = q^2
• To calculate frequency of allele pairs in a population we use:

  P^2 + 2pq + q^2 = 1
• probability of a dominant allele phenotype = p^2 + 2pq
• 2pq = probability of heterozygous individual
• q^2 = probability of homozygous recessive individual
• p^2 = probability of homozygous dominant individual
• hardy Weinberg principle is that allele frequencies remain constant from generation to generation so can be used to predict future alleles.
• hardy Weinberg principle for future generations only applies if:

  • no mutations
  • population isolation and no migration
  • no natural selection
  • very large population
  • random mating
• Speciation is where a population is split and isolated, there are different selective pressures on the two groups. If the genetic makeup changes to the extent two groups can no longer interbreed, they have become separate species.