Genes Topic 3.1 BIology SL
➢ A gene is a hereditary factor that consists of a length of DNA and influences a
specific characteristic
○ Hereditary = passes on from parent to offspring
○ Characteristic = genetic traits such as hair colour, blood type
➢ Humans have approximately 21 000 genes
➢ A gene for a specific trait occurs in a specific locus on a chromosome
○ Locus (pl. loki) = a specific region on a chromosome
➢ Normal body cells carry 2 chromosomes of each pair, one from the mother and one
from the father
Alleles
➢ Alleles are different variations or versions of a gene
○ A specific form of a gene differing from other alleles by one or a few bases
○ Example: attached or detached earlobes
➢ They are also hereditary
○ New alleles can occur through mutation
Mutations
➢ A mutation is a random, rare change in the genetic material
➢ One type involves change of the base sequence
○ Does not happen if DNA replication works correctly
○ A base substitution mutation results in a single letter being changed
■ A different amino acid is placed in the polypeptide chain
➢ Can be positive or negative for an organism
➢ A mutation that provides an individual or a species with a better chance for survival
is considered to be a beneficial mutation
○ Good chance to be passed on
○ Historically humans have not been able to digest lactose as adults
○ The last 10 000 years humans have held cattle which produce dairy products
○ Another mutation enabled the digestion of lactose
■ Good source of energy = survival
■ Most common among people from N. Europe and N. America
➢ A mutation that causes disease or death is detrimental mutation , therefore is less
likely to be passed on
○ Because it decreases chance for survival
➢ There are neutral mutations that do not have any effect on a species survival
1
,Genes Topic 3.1 BIology SL
○ The gene ABCCII codes for the consistency of ear wax
■ Wet or dry => 2 different alleles
■ This gene is also connected to
● Smell of underarm sweat
● Amount of produced breast milk
● Potentially an indicator of the risk of breast cancer
○ A single base substitution in a gene on chromosome 16
■ A G variant codes for dry
■ The A variant codes for wet
○ Wet is more common in Europe and Africa and dry is more common in Asia
➢ When a mutation is passed on from one generation to the next, it becomes a new
allele
Sickle cell anaemia
➢ The sickle cell disease is a mutation sometimes found in the gene that codes for
haemoglobin in red blood cells
○ The 6th codon in the sequence of haemoglobin, GAG, becomes GTG
○ Instead of glutamic acid, valine is added
■ Valine has different shape and properties so the protein is modified
■ Oxygen cannot be carried as efficiently
➢ People with the disease are resistant to malaria infection
○ Occurs in tropical regions, because of a parasite called Plasmodium, and it is
transferred by infected Anopheles mosquito
○ High fever, chills, even death
➢ People born with 2 copies for standard disc-shaped cells have only disc-shaped red
blood cells
○ Highly susceptible to malaria infection
➢ People born with one gene for disc-shaped cells and one gene for sickle shaped
cells have what is called sickle cell trait
○ They have some disc-shaped and some sickle shaped red blood cells
○ In most cases do not suffer from anaemia
■ The result of low red blood cell levels and is characterised by a
paleness of skin and low energy levels
○ Have better resistance to malaria, due to chemical imbalances
■ The survival of Plasmodium becomes difficult, due to little
potassium in sickle cells
2
, Genes Topic 3.1 BIology SL
➢ Those with 2 genes for sickle cells have only sickle-shaped red blood cells
○ They suffer from severe anaemia
○ Have the highest resistance to malaria
Sequencing DNA
➢ Genome is the complete set of an organism's base sequence
➢ Human Genome Project’s goal was to determine the order of all the bases the
genome possesses
○ In 2003 it achieved its goal
➢ Now, scientists are working on figuring out which sequence represents genes and
which gene does what
➢ Sanger technique
○ We take a sample of DNA, cut it into fragments and copy these fragments
○ We add a primer to start the process
○ A DNA polymerase enzyme starts to add free nucleotides to the fragments
■ Some of the nucleotides are ddNTPs and are terminators (stop
elongation) and have been previously marked with fluorescent
markers so they can be identified
○ We get strands of various lengths and using electrophoresis we place them
from longest to shortest
○ A laser activates the fluorescent markers on the nucleotides during
electrophoresis in order to see on what base the elongation has stopped
■ A computer analyses the wavelength of the light and determines if it
is A, C, T or G
■ Replicated many times
○ When they are done with fragment 1, they progress to fragment 2, 3 & so on
○ The fragments get mixed up and using computers, scientists can put them
back in the order they were originally
3
➢ A gene is a hereditary factor that consists of a length of DNA and influences a
specific characteristic
○ Hereditary = passes on from parent to offspring
○ Characteristic = genetic traits such as hair colour, blood type
➢ Humans have approximately 21 000 genes
➢ A gene for a specific trait occurs in a specific locus on a chromosome
○ Locus (pl. loki) = a specific region on a chromosome
➢ Normal body cells carry 2 chromosomes of each pair, one from the mother and one
from the father
Alleles
➢ Alleles are different variations or versions of a gene
○ A specific form of a gene differing from other alleles by one or a few bases
○ Example: attached or detached earlobes
➢ They are also hereditary
○ New alleles can occur through mutation
Mutations
➢ A mutation is a random, rare change in the genetic material
➢ One type involves change of the base sequence
○ Does not happen if DNA replication works correctly
○ A base substitution mutation results in a single letter being changed
■ A different amino acid is placed in the polypeptide chain
➢ Can be positive or negative for an organism
➢ A mutation that provides an individual or a species with a better chance for survival
is considered to be a beneficial mutation
○ Good chance to be passed on
○ Historically humans have not been able to digest lactose as adults
○ The last 10 000 years humans have held cattle which produce dairy products
○ Another mutation enabled the digestion of lactose
■ Good source of energy = survival
■ Most common among people from N. Europe and N. America
➢ A mutation that causes disease or death is detrimental mutation , therefore is less
likely to be passed on
○ Because it decreases chance for survival
➢ There are neutral mutations that do not have any effect on a species survival
1
,Genes Topic 3.1 BIology SL
○ The gene ABCCII codes for the consistency of ear wax
■ Wet or dry => 2 different alleles
■ This gene is also connected to
● Smell of underarm sweat
● Amount of produced breast milk
● Potentially an indicator of the risk of breast cancer
○ A single base substitution in a gene on chromosome 16
■ A G variant codes for dry
■ The A variant codes for wet
○ Wet is more common in Europe and Africa and dry is more common in Asia
➢ When a mutation is passed on from one generation to the next, it becomes a new
allele
Sickle cell anaemia
➢ The sickle cell disease is a mutation sometimes found in the gene that codes for
haemoglobin in red blood cells
○ The 6th codon in the sequence of haemoglobin, GAG, becomes GTG
○ Instead of glutamic acid, valine is added
■ Valine has different shape and properties so the protein is modified
■ Oxygen cannot be carried as efficiently
➢ People with the disease are resistant to malaria infection
○ Occurs in tropical regions, because of a parasite called Plasmodium, and it is
transferred by infected Anopheles mosquito
○ High fever, chills, even death
➢ People born with 2 copies for standard disc-shaped cells have only disc-shaped red
blood cells
○ Highly susceptible to malaria infection
➢ People born with one gene for disc-shaped cells and one gene for sickle shaped
cells have what is called sickle cell trait
○ They have some disc-shaped and some sickle shaped red blood cells
○ In most cases do not suffer from anaemia
■ The result of low red blood cell levels and is characterised by a
paleness of skin and low energy levels
○ Have better resistance to malaria, due to chemical imbalances
■ The survival of Plasmodium becomes difficult, due to little
potassium in sickle cells
2
, Genes Topic 3.1 BIology SL
➢ Those with 2 genes for sickle cells have only sickle-shaped red blood cells
○ They suffer from severe anaemia
○ Have the highest resistance to malaria
Sequencing DNA
➢ Genome is the complete set of an organism's base sequence
➢ Human Genome Project’s goal was to determine the order of all the bases the
genome possesses
○ In 2003 it achieved its goal
➢ Now, scientists are working on figuring out which sequence represents genes and
which gene does what
➢ Sanger technique
○ We take a sample of DNA, cut it into fragments and copy these fragments
○ We add a primer to start the process
○ A DNA polymerase enzyme starts to add free nucleotides to the fragments
■ Some of the nucleotides are ddNTPs and are terminators (stop
elongation) and have been previously marked with fluorescent
markers so they can be identified
○ We get strands of various lengths and using electrophoresis we place them
from longest to shortest
○ A laser activates the fluorescent markers on the nucleotides during
electrophoresis in order to see on what base the elongation has stopped
■ A computer analyses the wavelength of the light and determines if it
is A, C, T or G
■ Replicated many times
○ When they are done with fragment 1, they progress to fragment 2, 3 & so on
○ The fragments get mixed up and using computers, scientists can put them
back in the order they were originally
3
