TOPIC 7 -

  • MEIOSIS - a reduction division, the main role of meiosis is the production of haploid gametes as cells produced by meiosis have half the number of chromosomes of normal body (somatic) cells

  • mutation is the source of genetic variation, meiosis introduces variation via the production of gametes with new combinations of alleles

  • this happens by:

    • CROSSING OVER: where sections of DNA are exchanged between homologous chromosomes

    • INDEPENDANT ASSORTMENT: which describes the different possible combinations of maternal and paternal chromosomes that are inherited

    • RANDOM FERTILISATION: also introduces variation because the combination of gametes that fuses to form the zygote is random

  • KEY TERMS:

    • ALLELE: an alternative version of a gene ( EG blue, green or brown eyes

    • PHENOTYPE: the observable characteristics of an organism

    • GENOTYPE: the alleles present within cells of an organism

    • DOMINANT: an allele that is always expressed in the phenotype, even if only one copy of the allele is present

    • RECESSIVE: an allele that is only expressed in the phenotype if two copies of the allele

    • HOMOZYGOUS: two identical alleles of a gene EG RR (called a homozygote)

    • HETEROZYGOUS: two different alleles of a gene EG Rr (called a heterozygote)

    • CODOMINANCE: both alleles contribute to the phenotype without mixing EG blood type - you can inherit A and B alleles which gives you the blood group AB

    • MULTIPLE ALLELES: more than two possible versions of an allele EG fur colour of a cat could be yellow, black or white

  • CHI - SQUARED TEST:

    • the chi squared test is a statistical test which can be used to establish whether the difference between observed and expected results is significant

    • it can only be used if the sample size is sufficiently large ( above 20 ) and it can only be used for discontinuous variation data

    • the chi squared test can be used to determine whether the null hypothesis is correct or not

    • the null hypothesis is the assumption that there is no significant difference between observed and expected results

    • the value obtained is compared to the critical value

    • in a case where the calculated value obtained is less than the critical value, the null hypothesis is accepted as the difference is not significant

    • in a case where the X² ( calculated value ) value is greater than the critical value, the null hypothesis rejected meaning that the difference between observed and expected results is very unlikely to occur due to chance and is significant

    • the calculation is:



      x² = the sum of (observed - expected)²

      —————————————

      expected


  • INHERITANCE:

  • if the null hypothesis is rejected this implies autosomal gene linkage EG colour and wing length in drosphilia

  • this results from the presence of alleles on the same chromosome, they are linked and so are inherited to a greater or lesser extent, inherited as is they are the same gene

  • the closer together the genes are on the chromosome, the more closely linked they are, and the less likely they are to be separated by recombination