18. Genetics and inheritance

• What is meant by genotype 


  Genetic constitution of an organisms based on combination of alleles
• What is meant by phenotype 


  Expression of genotype and its interaction with the environment
• What must an organism be for it to be studied in inheritance

  Reproduces sexually

  short life cycle

  small size
• What is a mono hybrid cross


  When inheritance of ONE gene is studied
• What is a test cross or back cross

  When an organism displaying the dominant trait could be either Hetero or homo so it is crossed with a recessive phenotype.
• What is codominance 


  When both alleles are expressed in the phenotype of a heterozygous organism
• What is multiple alleles

  Some genes have more than 2 alleles. combinations are much more varied - example blood type
• Genotype of females 


  XX
• Genotype of males


  XY
• What is sex linkage

  When certain genes are carried on the sex chromosome and are inherited differently in males and females
• Many genes on the X chromosome won’t have a ……… one on the Y

  equivalent
• Why are males more likely to have a recessive phenotype

  Only one recessive allele needs to be present on the X chromosome
• Males always inherit what from mothers

  X
• Males always pass what to daughters

  X
• What is evidence for sex linkage in pedigree diagrams

  Greater numbers of affected males
• Evidence against sex linkage in a diagram

  When a recessive mother has a dominant son or a father.
• What is a dihybrid cross

  Considers how two different genes can be inherited.
• What gametes are produced by AaBb

  AB , Ab , ab, aB
• What is epistasis

  Arises in a dihybrid cross when there is interaction between two genes.
• What is the normal ration you would expect in a heterozygous dihybrid cross

  9:3:3:1
• What is autosomal linkage

  When non sex chromosomes (autosomes) have genes on the same chromosome
• What happens to genes that are autosomally linked

  They will stay together during independent assortment