Inheritance

• Define genotype

  The alleles an organism has for a particular gene
• Define a phenotype

  The observable characteristics of an organism determined by its genotype and its interaction with the environment
• Define a dominant allele

  Always expressed in the phenotype
• Define a recessive allele

  Need two of the same allele to be expressed in the phenotype
• Define homozygous

  Both alleles present are the same
• Define heterozygous

  Two different alleles are present
• Define monohybrid inheritance

  The inheritance of a single gene
• Describe the ratio when two heterozygotes are crossed

  3:1

  Actual ratio may not fit this because:

  • Sample size is not big enough
  • Fertilisation is a random event so ratios can be out
  • Presence of lethal alleles that kill offspring won't be passed on
• Define co-dominant alleles

  Both alleles are expressed in the phenotype because they are equally dominant
• Define multiple alleles

  The presence of more than two alleles for a gene in a population
• Define dihybrid inheritance

  The inheritance of two different genes on two different chromosomes
• Describe Mendel's law of independent segregation

  Each member of a pair of alleles can combine randomly with either allele from another pair, if the genes are not on the same chromosome
• Define epistasis

  When one gene influences the expression of another gene
• Describe sex linkage

  A sex linked gene has its locus on the X or Y chromosome.

  Females have homologous X chromosomes

  Males have an X and a Y chromosome that are not homologous

  They contain different genes and, because the X chromosome has more genes than the Y chromosome, they are different lengths
• Describe why males are more likely to express recessive alleles on the X chromosome

  1. Males only have one copy of the X chromosome, but females have two
  2. Males only need one copy of the recessive allele for it to be expressed in the phenotype
  3. Females may have a dominant allele on the other X chromosome
• Define autosomal linkage

  When two genes are found on the same chromosome