Theme 5: Gene*c Varia*on
15.1 Genotype and Phenotype
15.1.1 Genotype is the gene3c makeup of a cell or organism;
phenotype is its observed characteris3cs
• Genotype- Gene'c makeup of a cell or organism
• Polymorphism- Refers to any gene'c difference among individuals that
is sufficiently common that it would almost certainly be present in a
group of 50 randomly chosen individuals; Results from a muta'on
◦ I.e. One of 2/+ alterna've forms (alleles) at a chromosomal region
(or locus) that differs in either a single nucleo'de base, or have
variable numbers of tandem nucleo'de repeats in a given
popula'on of individuals
◦ In humans, polymorphisms reside mainly in non-coding regions of
DNA
• Polymorphisms allow for assembly of high-density gene'c
maps and so, are oPen called DNA markers
• Phenotype- Individual's observable characteris'cs or traits; May be
visible or seen in development, physiology or behaviour (such as colour
blindness and lactose intolerance)
◦ Results from genotype-environment interac'ons
15.1.2 The effect of a genotype oDen depends on several
factors
• Example: Gene'c varia'on in the gene for beta-globin on chromosome
11
◦
• The 3 forms of the gene are quite common in African
popula'ons and those of African descent
• Alleles- The different forms of a gene; Correspond to
different DNA sequences (i.e. polymorphisms) in the genes
, • Most common is A allele
• S allele is associated with sickle-cell anemia
• Homozygous- An individual who inherits an allele of the same type from
each parent
◦ Example: AA, SS, CC (from above example)
• Heterozygous- Individuals who inherit different types of alleles from
their parents
◦ Example: AS, AC, SC
• Homozygous SS- Individual has sickle-cell anemia
◦ Note: Sickle shape of cell is caused by the aggrega'on of the
abnormal beta-hemoglobin protein, resul'ng in long rod-like
structures within the cell
◦ Sickle-cell anemia can also cause acute pain due to the damage
that can occur at vital organs and 'ssues due to the sharp shape
of the beta-globin cells
• Heterozygous SA- Individual has only a mild form of blood disease;
Beneficial in areas with malaria
◦ Note: Both forms of the cell (regular and sickle) are made BUT,
enough of the regular beta-globin cells are produced to allow for
the proper transport of oxygen and also, prevent the exhibi'on of
sickle-cell anemia symptoms
• Is a muta'on harmful or beneficial?
◦ Depends on whether muta'on is homozygous or heterozygous
◦ Depends on the environment (i.e. Is there malaria in this
environment?)
• Heterozygous AC- Par'al protec'on against malaria
• Homozygous CC- More protected from malaria; Very mild anemia that
requires no medical treatment
• Note: Sickle-cell muta'on is found in many popula'ons with different
alleles or haplotypes emerging independently across various
popula'ons
◦ Example: Among na'ons with highest prevalence of sickle-cell
anemia, found that there may be 5 dis'nct beta-globin haplotypes
15.1 Genotype and Phenotype
15.1.1 Genotype is the gene3c makeup of a cell or organism;
phenotype is its observed characteris3cs
• Genotype- Gene'c makeup of a cell or organism
• Polymorphism- Refers to any gene'c difference among individuals that
is sufficiently common that it would almost certainly be present in a
group of 50 randomly chosen individuals; Results from a muta'on
◦ I.e. One of 2/+ alterna've forms (alleles) at a chromosomal region
(or locus) that differs in either a single nucleo'de base, or have
variable numbers of tandem nucleo'de repeats in a given
popula'on of individuals
◦ In humans, polymorphisms reside mainly in non-coding regions of
DNA
• Polymorphisms allow for assembly of high-density gene'c
maps and so, are oPen called DNA markers
• Phenotype- Individual's observable characteris'cs or traits; May be
visible or seen in development, physiology or behaviour (such as colour
blindness and lactose intolerance)
◦ Results from genotype-environment interac'ons
15.1.2 The effect of a genotype oDen depends on several
factors
• Example: Gene'c varia'on in the gene for beta-globin on chromosome
11
◦
• The 3 forms of the gene are quite common in African
popula'ons and those of African descent
• Alleles- The different forms of a gene; Correspond to
different DNA sequences (i.e. polymorphisms) in the genes
, • Most common is A allele
• S allele is associated with sickle-cell anemia
• Homozygous- An individual who inherits an allele of the same type from
each parent
◦ Example: AA, SS, CC (from above example)
• Heterozygous- Individuals who inherit different types of alleles from
their parents
◦ Example: AS, AC, SC
• Homozygous SS- Individual has sickle-cell anemia
◦ Note: Sickle shape of cell is caused by the aggrega'on of the
abnormal beta-hemoglobin protein, resul'ng in long rod-like
structures within the cell
◦ Sickle-cell anemia can also cause acute pain due to the damage
that can occur at vital organs and 'ssues due to the sharp shape
of the beta-globin cells
• Heterozygous SA- Individual has only a mild form of blood disease;
Beneficial in areas with malaria
◦ Note: Both forms of the cell (regular and sickle) are made BUT,
enough of the regular beta-globin cells are produced to allow for
the proper transport of oxygen and also, prevent the exhibi'on of
sickle-cell anemia symptoms
• Is a muta'on harmful or beneficial?
◦ Depends on whether muta'on is homozygous or heterozygous
◦ Depends on the environment (i.e. Is there malaria in this
environment?)
• Heterozygous AC- Par'al protec'on against malaria
• Homozygous CC- More protected from malaria; Very mild anemia that
requires no medical treatment
• Note: Sickle-cell muta'on is found in many popula'ons with different
alleles or haplotypes emerging independently across various
popula'ons
◦ Example: Among na'ons with highest prevalence of sickle-cell
anemia, found that there may be 5 dis'nct beta-globin haplotypes
