Biology Topic 3 Genetics


Topic 3


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Sexual and Asexual Reproduction

Asexual Advantages

  • Reproduce quickly
  • No mate needed
  • Colonise new area quickly

Asexual Disadvantages

  • No genetic variation
  • Disease will affect whole population

Variation: Differences in the characteristics of organisms.

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Sexual and Asexual Reproduction 2

Sexual Advantages

  • Genetic Variation
  • Survival of the Fittest => Natural Selection => Evolution  => Become better adapted

Sexual Disadvantages

  • Fewer offspring 
  • Slower 
  • Mate needed
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Meiosis: A form of cell division in which one parent cells produces 4 haploid daughter cells.



  • Organelle replication.

Prophase 1

  • Chromosomes shorten and thicken.

Metaphase 1

  • Spindle fibres pull 1 chromosome from homologous pair to poles.

Anaphase 1

  • Chromosomes paire up and spindle fibres form.
  • Chromosomes line up in centre.
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Meiosis 2

Metaphase 2 

  • 2 new haploid cells form

Prophase 2

  • Spindle fibres attach to centromere

Telophase 1

  • Chromosome pulled to equator of the cell

Anaphase 2

  • Spindle fibres contract and 1 sister chromatid is pulled to each pole of the cell.

Telophase 2

  • 4 genetically-different haploid cells with one set of chromosomes.
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Meiosis 3

  • Only happens in gametes

Genome: Entire DNA of an organism.

  • Genes are found in a section of DNA

Genes: Section of DNA molecule that codes for a specific protein.

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DNA: A polymer made up of two strands coiled to form a double helix, linked by complementary base pairs joined together by weak hydrogen bonds and nucleotides (sugar and phospate group) which attach to one of the bases as the backbone.


Adenine    =   Guanine      (forms 2 Hydrogen bonds)

Cytosine   =  Thymine       (forms 3 Hydrogen bonds)

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DNA Extraction


1) Mash strawberries and put them in beaker                                                                        containing detergent and salt (3g to 100 cm3). Mix well.

2) Place beaker in water bath at 60ºC for 15 mins.

3) Filter the mixture to get the froth and big insoluble bits of cell out. 

4) Add 2 drops of protease enzyme.

5) Tilting the tube, gently add some ice-cold alcohol to the filtered mixture (same amount of each).

6) The DNA will appear as a stringy white precipitate that can be carefully fished out with a glass rod.




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Protein Synthesis

  • Proteins are made in ribosomes
  • Shape of the protein is determined by => order of bases=> order of amino acids => fold to make specifically shaped proteins

RNA Polymerase: An enzyme that creates mRNA from DNA.



  • RNA polymerase binds to non-coding DNA located in front of the gene
  • Seperates both DNA strands
  • Enzyme moves along the DNA strand (template strand).
  • Adds complementary RNA nucleotides.
  • Forms complementary mRNA from coding DNA of gene.
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Protein Synthesis 2


  • mRNA travels through nuclear pores and strands attach to ribosomes.
  • Ribosomes moves along mRNA three bases at a time (codon) for specific amino acids.
  • At each mRNA codon, molecule of tRNA (with complementary bases) line up.
  • Each molecule carries a specific amino acid.
  • As the ribosome moves along, joins amino acids in a polypeptide chain.
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Genetic Variants and Phenotypes

Mutations: A change to a gene caused by a mistake in copying the DNA base pairs during cell division, radiation or chemicals.

  • Some mutations cause changes in the phenotype (visible characteristics).

Alleles: Genetic variations caused by mutations.

Non-coding DNA

  • Mutations may cause => RNA polymerase not binding well                                                                                              => Reducing transcription                                                                                                              => Alters amount of protein produced

Coding DNA

  • Changes phenotype because of sequence of amino acids => activity of protein produced. 
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  • Crossed pea plants by moving pollen to flowers
  • Used the seeds from each flower to grow and observe plants.


  • He got tall plants and bred them together.
  • Produced tall plants in the first generation.
  • Took best offspring and rebred them together. 
  • Second generation has few short plants.
  • Mendel discovered alleles 
  • Difficult to understand inheritance as scientists could not see how 'factors' (genes) could explain many variations in eye colour or how it explained Darwin's Theory of Evolution.
  • 2 factors for each characteristics.
  • If factors could not change, then species could not evolve.
  • Ideas accepted after discovery of chromosomes.
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  • Genes for the same characteristic can have variations caused by alleles
  • Alleles are genotype
  • Phenotype are visible characteristics
  • If allele shows its phenotype => dominant
  • If not => recessive
  • If allele for 1 gene is same => homozygous
  • If different => heterozygous

Chromosome: Thread-like structure structure found in the nuclei of cells. Each chromosome contains one enormously long DNA molecule packed with proteins.

Dominant: Describes an allele that will always affect a phenotype as opposed to a recessive allele, whose effect will not be seen if a dominant allele is present. 

Recessive: Describes an allele that will only affect the phenotype if the other allele is also recessive. It has no effect if the other allele is dominant.

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Alleles 2

Homozygous: When both the alleles for a gene are the same in an organism.

Heterozygous: When both the alleles for a gene are different in an organism.

Genotype: The alleles for a certain characteristic that are found in an organism.

Phenotype: The characteristics produced by a certain set of alleles.

Gamete: A haploid cell produced by meiosis used for sexual reproduction.

Zygote: A fertilised egg cell.

Monohybrid Inheritance

Monohybrid Inheritance: The study of how alleles of just one gene are passed from parents to offspring.

  • Genetic diagrams can show a recessive characteristic (only if both alleles are recessive).
  • Genetic diagrams can show possible combinations when 2 organisms breed => used to predict ratio of phenotypes.
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Alleles 3

Punnet Diagrams

  • Punnett diagrams also showinheriatnce
  • 50% chance boy or girl
  • Punnet Squares work out theoretical probability of inheriting certain genotypes.

Family Pedigrees

  • Shows genotype and resulting phenotypes inherited in families. 
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Determining sex

  • Female is **
  • Male is XY
  • Sperm cell determines sex 
  • 50% chance for male / female

Punnett Square: Diagram is used to predict the characteristics of offspring produced by 2 organisms with known combinations of alleles.

Family Pedigree Chart: A chart showing the phenotypes and sexes of several generations of the same family, to track how characteristics have been inherited.

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Multiple and Missing Alleles

Blood Groups


Phenotype         O           A           B           AB

Genotype         OO        Ao          Bo          AB

                                      AA          BB

  • A, B & O are proteins on the surface of a RBC => immunoglobins
  • If blood for patient does not match then RBC will clump together => kills
  • ABO blood group system classifies blood groups

ABO Blood Group System: System of sorting human blood into 1 of 4 phenotypes (A, B, AB, O) on the basis of immunoglobins on RBC.

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Multiple and Missing Alleles 2

Alleles for the Blood Groups

Alleles                 Diagram                    Group

Iᴬ                                                              A

Iᴮ                                                              B

Iᴼ                                                             O              <= Recessive

IᴬIᴮ                                                           AB            <= Codominant

  • Gene responsible for immunoglobins has 3 alleles
  • Can be homozygous or heterozygous
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Multiple and Missing Alleles 3

Codominant: When 2 alleles for a gene both affect the phenotype, eg. for blood group AB.

Sex-linked Genetic Disorders

  • Human Y chromosome misses some genes from X chromsome => men only have one allele for some genes
  • If allele for X chromosome causes a genetic disorder, then a man develops that disorder.
  • For women=> if disease is recessive, then because she is ** => she will not get the disorder 
  • Only when both alleles are recessive on **.
  • Probability of woman getting disorder is smaller than that of a man.
  • Sex-lined genetic disorders show different paterns of inheritancce on men and woman.
  • Examples are red-green colour blindness => cone cells see red and green as same
  • 8% of men and 0.5% of women

Sex-linked Genetic Disorders: Disorders caused by genes that is inherited differently in males and females such a RG blindness which is more common in men than women.

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Gene Mutation

  • Most phenotypic features are the result of multiple genes rather than single gene inheritance.

Variation: Differences in the characteristics of organisms.

  • Early humans had brown eyes
  • Ancestors moved from Africa, changes in alleles for melanin production => blue eyes and fair skin
  • This is a mutation => occurs during cell division

Mutation: A change to a gene caused by a mistake in copying the DNA base pairs during cell division, or by the effect of radiation or certain chemicals.

  • Happens when there is a mistake in copying DNA during cell division.
  • Some mutations are very small & cause a change in the amino acid in the final protein structure
  • Other mutations can cause the protein not to made at all or function properly.
  • 'Silent' mutations have no effect
  • Mutations are rare => specific enzymes repair DNA and ensure mistakes are not made.
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Gene Mutation 2

Point Mutations

  • Changes in individual gene => miscopying of nucleotides
  • Deletion or insertion of nucleotide => frame shift
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Gene Mutation 3

Substitution Mutations

  • Change of 1 base
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Gene Mutation 4

Chromosomal Mutations

  • Gene Deletion
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Gene Mutation 5

  • Gene Duplication
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Gene Mutation 6

  • Inversion
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Gene Mutation 7

  • Translocation
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Gene Mutation 8

Whole-Chromosome Mutations

  • Entire chromosome is lost / repeated during cell division
  • Down's syndrome is caused by extra chromsome 21

Effect of Mutation

  • Production of new & superior protein => gain of reproductive advantage
  • Neutral mutation => no change
  • Production of inferior/ no protein => Fatal / Disease-causing
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Gene Mutation 9

When Does It Happen?

  • Just by chance => DNA being copied
  • UV Radiation (the Sun)
  • X-Ray
  • Carcinogens (some chemicals => cancer-causing)
  • Mutations in somatic (body) cells may lead to cancer
  • Only mutations in gametes that are passed onto future generations.
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Gene Mutation 10

Human Genome Project

  • In 2003, first human genome was fully decoded.
  • This was result of the Human Genome Project

Human Genome Project: The international project that mapped the base pairs in the human genome.

  • Produced map of 3.3 billion base complementary base pairs in one set of 46 human chromosomes.
  • This has shown there are variations between people, but that >99% of DNA bases in different people are the same. 
  • Mapping the genome can indicate risks of developing diseases that are caused by different alleles of genes. 
  • Can also help identify which medicines might be best to treat a person's illness, because alleles affect how medecines work in our bodies.
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Gene Mutation 11



  • Drug used to prevent blood clots for people with risk of a heart attack or stroke. 
  • Some people have alleles that means the drug will not protect them.
  • Alleles reduce or increase effect of the drug


  • Drug used to reduce high levels of cholesterol in the blood
  • Some people suffer a side effect of myopathy (weak muscles) if they take the drug.
  • Alleles increase or decrease the risk of side effects.


  • Drug used to reduce the risk of blood clots.
  • Some people are more sensitive to the drug than others, due to their alleles, and so need to take smaller doses => can cause dangerous bleeding.
  • Alleles increase or decrease sensitivity.
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  • Some of the variation between individuals of the same species is the result of variation in their genes.

Genetic Variation: Differences between organisms caused by differences in the alleles they inherit from their parents, or differences in genes caused by mutation. Also called inherited variation.

  • Genetic variation shows differences in phenotypes

Environmental Variation: Differences between organisms caused by environmental factors such as the amount of heat, light and damage by other organisms. These differences are called acquired characteristics.

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Variation 2

  • In some cases, characteristics only show environmental variation => a loss of a limb through an accident.
  • This is an example of an acquired characteristic.
  • Variation can be grouped into 2 types:
  • Discontinuous (Data can only take a limited set of values)  
  • Continuous (Data can be any value in a range)

Discontinuous Variation: Data values that can have only one of a set number of options are discontinuous. An example is blood group, where there is a difference in characteristic.

Continuous Variation: Continuous data can take any value between 2 limits. Examples include length, mass and time, where there are differences in characteristics.  

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Variation 3

DIscontinuous Graphs

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Variation 4

Continuous Graphs

  • Continuous data for variation often forms a bell-shaped curve, known as a normal distribution.
  • It is called this because expected for large amount of data for a characteristic where:            
  • Mode is the median in the for the whole range                                                                        
  • Further the value from the median => fewer the individuals that have that value.                     
  • Mean, mode and median are the same value.
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