3.7.1 INHERITANCE

• Genotype - genetic constitution of an organism.

  Phenotype - the expression of an organism’s genetic constitution (genotype) and its interaction with the environment. (An organism’s visible characteristics as a result of its genotype and the impact of its environment).
• Gene - a sequence of DNA that codes for a polypeptide. Genes can exist in 2 or more different forms called alleles.
• In a diploid organism, chromosomes occur in pairs called homologous chromosomes - have the same genes at the same loci, although they may have different alleles. One chromosome in the pair is inherited from each parent, and they are similar in size, shape, and carry genes for the same traits.
• The alleles at a specific locus may be either homozygous (both alleles same eg. HH) or heterozygous (both alleles different eg. Hh).

  Alleles may be:

  • Dominant - Always expressed in the phenotype. If heterozygous, masks the expression of the recessive allele. 
  • Recessive - Only expressed in the phenotype when homozygous recessive (not expressed in phenotype of heterozygous individual).
  • Codominant - Both alleles expressed in the phenotype of a heterozygous individual. Neither allele is dominant over the other, and are both visible in phenotype.
• True/Pure Breeding - homozygous for that allele
• Monohybrid inheritance is the inheritance of a single gene.

  Monohybrid cross - genetic cross with 1 pair of alleles.

  • F1 Generation - Result of crossing homozygous dominant with homozygous recessive. (offspring of parents)
  • F2 Generation - Result of crossing 2 heterozygous. (offspring of individuals from F1 generation)
• Genetic Cross (Labelling):

  • State parents phenotype
  • State parents genotype
  • State possible alleles in the gametes (in circles)
  • Punnett square with offspring genotypes
  • Record offspring phenotypes as ratio - reference genotypes in punnett square
• Test Cross - used to work out the unknown genotypes of individual organisms.

  The unknown genotype is crossed with a homozygous recessive (for that trait) individual. 

  Find out whether organism showing dominant characteristic of trait is homozygous (dominant) or heterozygous:

  • If all offspring have dominant phenotype - the unknown genotype was homozygous dominant. 

  (If 1 parent is homozygous dominant, all children will display dominant trait, regardless of other parent)

  • If half the offspring have recessive phenotype - the unknown genotype was heterozygous.
• • In recessive inheritance (trait recessive so requires hom rec to be expressed) - if characteristic expressed in offspring phenotype with 2 parents not expressing characteristic = Parents heterozygous. 
  • In dominant inheritance (trait dominant so hom dom/het to be expressed) - if offspring not expressing trait in phenotype but has 2 parents who are = Parents heterozygous.
• Probability + Genetic Crosses

  Actual results of genetic crosses rarely the same as predicted results:

  • Individual fertilisations are independent of each other (don’t rely on results of last one) i.e. in genetic cross with 3:1 ratio, each time fertilisation occurs this ratio stands
  • The larger the sample, the more likely the actual results resemble the theoretical.
• Dihybrid inheritance involves the inheritance of 2 different characteristics at the same time.

  (Simultaneous examination of two different traits, each controlled by separate pairs of alleles).

  • ​During a dihybrid cross, alleles are independently assorted during gamete formation. 
  • In a dihybrid F1 generation cross, the phenotypic ratio for the F2 generation is always 9:3:3:1.
• Referencing offspring genotypes (dihybrid):

  Eg. 9 round, yellow (R-G-)

        3 round, green (R-gg)

        3 wrinkled, yellow (rrG-)

        1 wrinkled, green (rrgg)
• Crosses with Codominance + Multiple Alleles -

  Represented by 2 Capital letters superscript to a letter representing the gene.
• Multiple Alleles - Blood Group

  3 or more alleles for a particular gene.

  There may be a hierarchy of dominance with some alleles dominant over many others.

  • Capital letter for gene, superscript letters for alleles.

  An organism can only have 2 alleles as there is 1 pair of homologous chromosomes.

  Eg. Blood Group (ABO)

  I - Immunoglobulin gene.

  Allele IA - leads to the production of antigen A

  Allele I B - leads to the production of antigen B

  Allele I O - leads to the production of neither antigen A or B

  IA and I B are codominant, whereas I O is recessive to both of these.
• 1 pair of sex chromosomes.

  Male - XY, Female - XX (can be other way round for other organisms)
• Sex linkage

  • Occurs when there is a gene on the X chromosome, not present on the Y chromosome.  (When gene locus is on the long arm of X chromosome - which the Y doesn’t have).
  • Males inherit Y chromosome from father and X from mother (Females inherit X from both). Therefore males have higher chance of inheriting an X chromosome carrying an allele for something bad (disease).
  • (Males are more likely to exhibit recessive disorders like haemophilia).
  • Daughter will only suffer if father does.
• Autosomal linkage occurs if two or more genes are located on the same autosome (non-sex chromosome). 

  The two genes are less likely to be separated during crossing over, resulting in the alleles of the linked genes being inherited together.

  • 3:1 phenotypic ratio
  • Ratio may not be 3:1 due to crossing over in meiosis.
  • There is more chance of crossing over if the gene loci are far apart - greater distance between them.
• Epistasis - the interaction between 2 non-linked genes which causes one gene to mask the expression of the other in the phenotype.

  Eg. 2 genes in mice that control fur colour. 

  Genotypes AA or Aa have black bands in their fur, while genotype aa has solid black fur. 

  Gene B controls the expression of gene A: genotypes BB or Bb will allow expression of gene A, but genotype bb will not. Mice with genotype bb are all white (albino). 

  Black - aaBb, aaBB

  Agouti - AaBb, AaBB, AABb, AABB

  White - aabb, Aabb, AAbb
• • Recessive epistasis occurs when the presence of a recessive allele prevents the expression of another allele at a second locus. Recessive epistasis gives the ratio of 9:3:4. (mice example)
  • Dominant epistasis is when a dominant allele at one locus completely masks the alleles at a second locus. Dominant epistasis gives a ratio of 12:3:1.
• Chi-squared

  In inheritance it is used to test to compare the goodness of fit of observed phenotypic ratios with expected ratios.

  • The null hypothesis assumes that any difference that occurs between the expected and observed results is due to chance.