Topic 7: Genetics, populations, evolution and ecosystems

• What is the definition of genotype?


  All of the alleles that an organism carries on its chromosomes.
• What is the definition of phenotype?


  Observable characteristics of an organism resulting from genotype and environmental factors.
• Why do most organisms carry two alleles for each gene?


  Because they are diploid organisms.
• What is a dominant allele?


  Only a single allele is required for the characteristic to be expressed in the phenotype.
• What does it mean if an organism is homozygous dominant?


  It has two dominant alleles for a trait.
• What is a recessive allele?


  The characteristic is only expressed if there are two recessive alleles present.
• What is codominance?


  Both alleles are expressed equally and contribute to the phenotype.
• What does the term pure breeding refer to?


  A homozygous pair of alleles.
• What is an allele?


  An alternative form of a gene.
• What is a locus?


  The specific position of a gene on the chromosome.
• What is monohybrid inheritance?


  • Controlled by a single gene
  • Example: cystic fibrosis
  • Affected individuals have a doubly recessive phenotype
• What is the expected phenotype ratio in a monohybrid cross?


  A 3:1 ratio is typically observed.
• Why is the 3:1 ratio in monohybrid inheritance not exact in real situations?


  Because the joining of male and female gametes is random, leading to probability variations.
• What is dihybrid inheritance?


  • Involves two characteristics
  • Determined by two different genes
  • Present on two different chromosomes
• What is codominance and give an example?


  • Both phenotypes are expressed equally
  • Example: black and white feathers in birds
  • Example: red and white flowers in roses
• What are the three alleles associated with human ABO blood groups?

  IA, IB, and Io.
• What does allele IA produce?


  Antigen A.
• What does allele IB produce?


  Antigen B.
• What does allele Io produce?


  Neither antigen A nor B.
• What is sex linkage?


  The expression of an allele dependent on the gender of the individual.
• How many pairs of sex chromosomes do humans have?


  One pair of sex chromosomes.
• What are the sex chromosomes for males and females?


  Males have XY and females have XX.
• Why do males only carry one allele for sex-linked traits?


  Because they have only one X chromosome.
• What is an example of a sex-linked disease?


  Haemophilia.
• How is haemophilia inherited in males?


  It is inherited from the mother since the gene is on the X chromosome.
• What is autosomal linkage?


  • Two or more genes on the same autosomal chromosome
  • Fewer combinations of alleles if on the same chromosome
  • More combinations if on different chromosomes,a non-sex chromosome
• What is epistasis?


  • Interaction of different loci on a gene
  • One gene locus affects another
  • Can mask or suppress expression of another gene locus
• What is recessive epistasis?


  When the presence of a recessive allele prevents the expression of another allele at a second locus.
• What ratio is associated with recessive epistasis?


  1. 3:4.
• What is dominant epistasis?


  When a dominant allele at one locus completely masks the alleles at a second locus.
• What ratio is associated with dominant epistasis?


  12:3:1.
• What is the chi-squared test used for?


  • Establish whether the difference between observed and expected results is due to chance
  • Tests the null hypothesis
  • Determines statistical significance
• What is the null hypothesis?


  It states that there is no statistical significance in the results.
• What are the criteria for the chi-squared test?


  • Sample size must be over 20
  • Data must fall into discrete categories
  • Use raw counts, not percentages
• What is the formula for the chi-squared test?


  The value obtained is compared to the critical value.
• What is the critical value in chi-squared tests?


  In chi-square, the critical value is p=0.05.
• What happens if the value obtained is equal to or greater than the critical value?


  The null hypothesis is accepted as the difference is not significant.
• What happens if the value obtained is less than the critical value?


  The null hypothesis is rejected, indicating a significant difference.
• What is a population in genetics?


  • A group of organisms of the same species
  • Occupying a particular space at a particular time
  • Can potentially interbreed
• What is the gene pool?


  The total number of alleles present in a population.