patterns of inheritance

• gene - a length of dna that codes for a polypeptide
• allels - form of a gene
• locus - the position of a gene on a chromosome
• dominant - an allele that when present always expresses itself in the phenotype
• recessive - an allele that only expresses itself when no dominant allele is present
• codominant - where two alleles of the same gene are expressed in the phenotype of the heterozygote
• homozygous - the alleles of a gene in a cell are both the same
• heterozygous - the alleles of a gene in a cell are different
• genotype - all the alleles present for a particular trait
• phenotype - the way the genotype is expressed, the characteristics of the organism
• sex linkage - when a gene is present on one of the sex chromosomes but not the other, the recessive phenotype is more common in the mixed sex
• autosomal linkage - two genes on the same chromosome that are inherited together
• epistasis - the interaction between two genes a different loci, one gene may mask the expression of the other
• test cross - to show the homozygous recessive phenotype

  • ratio 3:1
  • if there are no offspring showing recessive phenotype, homozygous partner
  • if offspring show recessive phenotype, heterozygous parents
• codominance

  standard ratio, 1:2:1

  to represent the gene use a capital letter and superscript capital letter to represent the allele

  • 3 characteristics, 1 intermediate
• if a female inherits a recessive diseased allele of a gene on an X chromosome, she will likely have a dominant version on the other X

  • if a male inherits a recessive allele on his X he will have the disease
• conditions under which gene frequencies wouldn't change

  • large populations - no genetic drift
  • no migration - population is isolated
  • no mutation - alleles won't change
  • random mating
  • no favoured alleles - no natural selection
• p = frequency of dominant allele in population
• q = frequency recessive allele in the population
• p^2= frequency of homozygous dominant individuals
• q^2= frequency of homozygous recessive individuals
• 2pq= frequency of heterozygous individuals
• dihybrid cross - two genes inherited simultaneously

  • 9:3:3:1
• autosomal linkage

  • if two genes are on the same chromosome they will not show independent assortment in meiosis
  • alleles will be inherited together and are linked
  • ratio will not be dihybrid cross
  • may not always be inherited if chiasmata form creating crossing over
• epistatic gene = gene that prevents the expression of alleles of the gene at a 2nd locus
• hypostatic gene = genes phenotype is masked by the epistatic gene
• recessive epistasis

  • 9:3:4 ratio
• dominant epistasis - 12:3:1 ratio

  • when gene 1 is dominant, it may code for an inhibitor that prevents transcription of hypostatic gene
  • or may code for a protein that inhibits enzyme activity