Inheritance

• gene - the DNA base sequence coding for the sequence of amino acids in a polypeptide or a functional RNA (tRNA or rRNA)
• allele - alternative versions of the same gene
• locus - the location of a gene on a chromosome
• homologous chromosomes - a pair of chromosomes coding for the same gene but different alleles.
• diploid - a cell with two copies of each chromosome
• haploid - a cell with a single copy of each chromosome
• genotype - the genetic constitution of an organism
• phenotype - the genetic constitution of an organism and interactions with its environment
• dominant - an allele which is always expressed when present.
• recessive - an allele which is only expressed when 2 copies are present
• codominance - alleles which are both expressed in the phenotype of the heterozygous
• If the alleles for a characteristic are the same - we say it is homozygous
• If the alleles for the characteristic are different - we say it is heterozygous
• Monohybrid Inheritance - the inheritance of a single gene
• With a monohybrid cross between two heterozygous we expect to get a 3:1 phenotypic ratio. This will not necessarily be observed, however as the sample size might be too small to represent the expected ratio
• Multiple Alleles - although, in a diploid organism, only 2 alleles can be present for a single gene - more than 2 alleles can exist.
• Dihybrid Inheritance - The inheritance of 2 genes, both of which are on different chromosomes
• A dihybrid cross between two heterozygotes would be expected to yield a phenotypic ratio of 9:3:3:1
• Reasons why the ration might not be 9:3:3:1

  • Sample size is too small
  • Random fusion of gametes
  • The gene might be linked (both on the same chromosome)