Biology: Genetics, Populations, Evolution and Ecosystems

• What is a genotype?

  The genetic constitution (make-up) of an organism. It describes the alleles that an organism has.
• What is a phenotype?

  The observable or biochemical characteristics of an organism, resulting from the interaction between genotype and environment.
• What is a gene?

  A section of DNA that codes for an amino acid sequence of a polypeptide or functional RNA, determining a single characteristic of an organism.
• What is an allele?

  One of the alternative forms of a gene.
• How many alleles for a gene are present in a diploid organism?

  Two alleles, one at each homologous chromosome locus.
• What is the phenotype of pea plants with green pods?

  Green pods are dominant, so they are expressed when at least one dominant allele (G) is present.
• What is homozygous?


  When both alleles in a pair of homologous chromosomes are the same.
• What is heterozygous?


  When both alleles in a pair of homologous chromosomes are different.
• How is the dominant allele expressed in a heterozygous organism?

  Only the dominant allele is expressed in the phenotype.
• What is the difference between dominant and recessive alleles?

  Dominant alleles are always expressed, while recessive alleles are expressed only when two copies of the recessive allele are present.
• What is homozygous dominant?

  An organism that has two dominant alleles for a particular gene (e.g., GG).
• What is homozygous recessive?

  An organism that has two recessive alleles for a particular gene (e.g., gg).
• What does codominant mean?

  Two alleles that are equally dominant and can therefore both contribute to and/or expressed in the phenotype.
• Can you give an example of codominance?

  Heterochromia, where two different dominant alleles for eye color are both expressed in the phenotype.
• What are multiple alleles?

  When a gene has multiple alleles for a characteristic, for example, eye colour.
• How many alleles of a gene are found in an individual

  homologous chromosomes have two chromosomes and therefore two loci, only two alleles of a gene will be in existence.
• What is monohybrid inheritance?

  The inheritance of a single gene characteristic, for example of pea pod colour
• What is pure-breeding?


  When parents that are homozygous breed repeatedly to produce offspring with the same dominant or recessive alleles.
• What happens when two pure-breeding organisms with different alleles are crossed?

  The first filial (F1) generation will be heterozygous.
• What is the offspring produced by crossing

  two pure-bred organisms?

  It is known as the first filial (F1) generation and will always 

  have the dominant trait 
• What is the result of crossing pure-breeding green-pod plants (GG) with pure-breeding yellow-pod plants (gg)?


  The F1 offspring will have green pods (Gg).
• What is the phenotype ratio when two heterozygous organisms from the F1 generation are crossed?

  The offspring produced (F2 generation) will have a phenotype ratio of 3:1, three plants with green pods to one plant with yellow pods.
• What is a genetic ratio?

  A measure of the relative sizes of two classes that is expressed as a proportion.
• Can you simplify a genetic ratio?

  Yes, by dividing both values by their smallest common factor.
• Why might the observed results of genetic crosses differ from the expected results?

  Fertilization and gamete fusion are random, so the chance of getting the exact same expected results is not 100%.
• How does sample size affect genetic ratio results?

  •The larger the sample, the more likely the actual results are to come near to matching the expected ones

  •A small sample size means the expected results may not be the same as the actual results and will not be representative 
• What is dihybrid inheritance?

  The inheritance of two different genes that are located on two different chromosomes.
• How many gametes should parents produce in dihybrid inheritance?

  Parents should produce 4 gametes.
• in the pea pod example for dihybrid inheritance, what is the 1st step?

  1. List genes and alleles.

  Genes:

  • Seed colour (green and yellow)
  • Seed shape (round and wrinkled)

  Alleles:

  • Colour: Green, yellow

  (Green = G)   (Yellow = g)

  • Shape: Round, wrinkled

  (Round = R)   (Wrinkled = r)

  ➥  Because we are inheriting two genes at once, the parents genotype will be RrGg
• What is the second step?

  1. List gametes x4 with both recessive alleles 

  • rg
  • rg
  • rg
  • rg
• What is the third step?

  1. List the gametes so both alleles for each 

  gene appears twice 

  Parents genotype = RrGg

  • rg ➜ Rg
  • rg ➜ RG
  • rg ➜ rg
  • rg ➜ rG
• What is the fourth step?

  Place ALL four gametes into a punnett square

  and show the genetic crosses
• What is the genotype of the parent pea plant in the example?

  The genotype is RrGg.
• What are the gametes produced by the parent with genotype RrGg?

  The gametes are: Rg, RG, rg, rG.
• How do we derive the gametes from the parent RrGg?

  List all possible combinations of alleles: Rg, RG, rg, rG.
• How many gametes will be produced in a dihybrid cross?

  4 gametes will be produced.
• What is the Punnett square used for in dihybrid inheritance?

  The Punnett square shows the genetic crosses of all possible gamete combinations.
• What is the purpose of showing all 4 gametes in a dihybrid cross?

  To show all the possible combinations of alleles that can result from the genetic cross.
• How many genes are involved in dihybrid inheritance?

  Two genes are involved, each with two alleles.
• What is codominance?


  Codominance occurs when two alleles are equally dominant for a gene and both are expressed in the phenotype.