7.1 inheritance

• what is meant by the term genotype

  genetic constitution of an organism
• what is meant by the term phenotype

  the expression of this genetic constitution (genotype) and its interaction with the environment
• what are alleles and how do they arise

  variations of a particular gene (same locus)

  arise by mutation (changes in DNA base sequence
• how many alleles of a gene can be found in diploid organisms

  2 as diploid organisms have 2 sets of chromosomes (chromosomes are found in homologous pairs)

  but there may be many (more than 2) alleles of a single gene in a population
• dominant allele

  always expressed (shown in the phenotype)
• recessive allele

  only expressed when 2 copies present (homozygous recessive)/ not expressed when dominant allele present (heterozygous)
• codominant alleles

  both alleles expressed/contribute to phenotype (if inherited together)
• homozygous

  alleles at a specific locus (on each homologous chromosome) are the same
• heterozygous

  alleles at a specific locus (on each homologous chromosome) are different
• what do monohybrid crosses show

  inheritance of one phenotypic characteristic coded for by a single gene
• what do dihybrid crosses show

  inheritance of two phenotypic characteristics coded for by two different genes
• explain the evidence from a pedigree diagram which should show that the allele for (named phenotype) is dominant

  (named phenotype) parents (n&n) have a child (n) without (named phenotype)

  so both parents (n&n) must be heterozygous/carriers of recessive allele

  if it were recessive all offspring would have (named phenotype)
• explain the evidence from a pedigree diagram which would show that the allele for (named phenotype) is recessive

  parents (n&n) without (named phenotype) had a child (n) with (named phenotype)

  so both parents (n&n) must be heterozygous/carriers of recessive allele
• what is a sex-linked gene

  a gene with a locus on a sex-chromosome (normally X)
• explain why males are more likely to express a recessive X-linked allele

  females (XX) have 2 alleles so only express recessive allele if homozygous recessive/can be carriers

  males (XY) have 1 allele (inherited from mother) so recessive allele always expressed
• explain the evidence from a pedigree diagram which would show that the allele for (named phenotype) on the X-chromosome is recessive

  mother (n) without (named phenotype) has child (n) with (named phenotype)

  so mother (n) must be heterozygous/carrier of recessive allele
• explain the evidence from a pedigree diagram which would suggest that (named recessive phenotype) is caused by a gene on the X-chromosome

  only males tend to have (named recessive phenotype)
• explain the evidence from a pedigree diagram which would show that the gene for (named phenotype) is not on the X-chromosome

  (named phenotype) father (n) has daughter (n) without (named phenotype)

  father (n) would pass on allele for (named phenotype) on X-chromosome so daughter (n) would have (named phenotype)

  OR

  (named phenotype) mother (n) has a son (n) without (named phenotype)

  mother (n) would pass on allele for (named phenotype) on X-chromosome so son (n) would have (named phenotype)
• explain how autosomal linkage affects inheritance of alleles

  two genes located on same autosome (non-sex chromosome)

  so alleles on same chromosome inherited together

  stay together during independent segregation of homologous chromosomes during meiosis

  but crossing over between homologous chromosomes can create new combinations of alleles

  if the genes are closer together on an autosome they are less likely to be split by crossing over
• what is epistasis

  interaction of (products of) non-linked genes where one masks/suppresses the expression of the other
• describe when a chi-squared test can be used

  when determining if observed results are significantly different from expected results (frequencies)

  e.g comparing the goodness of fit of observed phenotypic ratios with expected ratios

  data is categorical (can be divided in to groups e.g phenotypes)
• suggest why in genetic crosses, the observed phenotypic ratios obtained in the offspring are often not the same as expected ratios

  fusion/fertilisation of gametes is random

  autosomal linkage/epistasis/sex-linkage

  small sample size so not representative of whole population

  some genotypes may be lethal (cause death)