genetics

• monohybrid cross: genetic cross that studies the inheritance of 1 characteristic controlled by 1 gene (dom or rec)
• dihybrid cross: genetic cross that studies the inheritance of 2 characteristic controlled by 2 gene (linked genes or inherited independently)

  9:3:3:1 inherited indep

  3:1 linked, no CO

  deviate from 9:3:3:1 linked, CO
• test cross: reveals genotype of an organism exhibiting dominant traits (AA or Aa?) test cross results 1:1 for monohybrid, 1:1:1:1 for dihybrid
• reciprocal cross: used to determine if characteristic is sex-linked. 2 separate crosses are conducted using the same characteristics but sexes are reversed

  diff pheno: sex linked

  same pheno: not sex linked
• lethal genes: causes death is organism is homozygous for that allele (not a phenotype)
• linked genes:

  • located on same chr, usually located physically close enough together on a chr so that alleles of 2 genes are inherited tgt
• sex-linkage: genes carried on sex chr, resulting in characteristic being mainly expressed in one sex

  eg. haemophilia, colour blindness
• XX: homogametic XY:heterogametic
• why can different characteristics can be inherited independently in dihybrid inheritance?

  • 9:3:3:1 phenotypic ratio indicated alleles of each characteristics are found on diff chr
  • alleles of diff genes assort independently of eo, bc the alignment of each homologous pair at metaphase plate is independent of the other chr pair
• why are mutations usually recessive?

  • mutation may result in alleles that usually code for non-functional proteins
  • in heterozygous, the functional allele codes for a functional protein & its production masks the effect of the recessive mutant allele, resulting in normal phenotype
• mendelian cross: not linked genes
• explain why there's a greater than expected number of parental phenotypes?

  • parental genes coding for fur & hair texture are on same chr
  • alleles coding for white fur and straight hair are linked on same chr
  • alleles coding for black fur and wavy hair are linked on same chr
  • thus greater chance for alleles to be inherited together, resulting in greater number of phenotypes
  • number of recombinants is smaller, as CO is based on chance, frequency of CO is dependent on the distance between 2 genes
• describe how BbHh can be derived from BBHH x bbhh


  (linked genes)

  • during P1 of M1, CO occurs btwn non sister chromatids of homo chr
  • at chiasma, portion of chromatid containing B breaks and rejoins to portion of chromatid containing h, resulting in new linkage group being formed, where B that codes for X and h that codes for Y are linked on the same chr
  • gametes that contains chr that contains allele B linked to allele h fuses with gamete that contains chr that contains allele B linked to allele h
• differences between continuous and discontinuous:

  • phenotypes vary along a continuum VS few distinct & discrete phenotypes
  • varies between limits or extremes VS no intermediates
  • normal distribution curve VS bar graph
  • quantitative VS qualitative
• epistasis: a form of gene interaction in which a gene at one locus alters the phenotype expression of another gene at a 2nd locus
• recessive epistasis: since genotype aa masks phenotype expression of B/b locus, genotype aa is epistatic over B/b locus
• 9:7 ratio (flower colour)

  • gene C codes for functional enzyme C which converts the clc precursor into a clc intermediate, the homozygous recessive alleles cc result in absence of functional enzyme C
  • gene P codes for enzyme P, which converts clc intermediate into purple pigment. homozygous recessive allele pp results in absence of functional enzyme P
• 9:3:4 ratio (labrador colour)

  • allele B codes for enz B which produces black pigmentation
  • allele b codes for enzyme which produces brown pigmentation
  • a separate gene, C allows deposition in the coat and cc prevents depositio, resulting in gold phenotype
• 15:1 (kernel colour)

  • functional enzyme A or B can produce a common precursor
  • this product give colour to kernel, thus only 1 dominant allele at either of loci is required to generate product
  • duplicate dominance
• 12:3:1 ratio (squash colour)

  • allele W codes for functional enzyme that inhibites expression of G/g locus
• 13:3 ratio (white leghorn chicken)

  • C allele allows for production of coloured feathers
  • I allele allows for production of functional inhibitor