B1 topic 1: classification, variation and inheritance


  • Classification= grouping things by their features oor characteristics.
  • The more characteristics that are used to group similar organisms together, the more reliable is the classification.
  • Small groups that are similar can be grouped into larger groups. Kingdoms are the largest groups and species are the smallest.
  • Different Groups:
    • Kingdom
    • Phylum
    • Class
    • Order
    • Family
    • Genus
    • Species (Organisms that have the most characteristics in common.)

Keep Ponds Clean Or Frogs Get Sick

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The five kingdoms of organisms

  • Animalia (animals)
    • Multicellular (body made of many cells)
    • have no cell walls
    • no chlorophyll in cells.
    • feed heterotrophically (eat other organisms)
  • Plantae (Plants)
    • Multicellular
    • have cell walls
    • have chlorophyll
    • feed autotrophically (make own food)
  • Fungi
    • Multicellular
    • have cell walls
    • no chlorophyll
    • feed saprophytically (digest food outside the body)
  • Protoctista
    • Mostly unicellular (body is a single cell)
    • nucleus in cell.
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The five kingdoms of organisms continued

  • Prokaryotea
    • unicellular
    • no nucleus in cell

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Vertebrates and invertebrates

  • The phylum chordata contains animals that have a supporting rod running the length of their body.
  • Chordata are vertebrates (animals with backbones)

Vertebrate Groups

Scientists sort vertebrates into 5 main groups:

  • Oxygen absorbtion- gills/lungs
  • Fertilisation- internal/external
  • reproduction- viviparous (young born live)/ oviparous (lay eggs)
  • Thermoregulation- Homeotherm (control own body temperature)/ poikilotherm (temp varies with environment temp)
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  • A species is a group of organisms that can breed with each other and produce fertile offspring.
  • A few species can interbreed (breed with other species) to produce hybrids.
  • Hybrids are usually sterile (cannot produce offspring)

Constructing and using keys

  • A key based on obvious characteristics can help identify different species.
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Binomial Classification

  • Every species has a unique binomial name. (Genus name, species name) E.g Panthera leo.
  • Binomial names are useful becuse:
    • other people know exactly which species you mean.
    • you can see from the genus which species are closely related.
    • it helps us identify which environments contain the fewest species (low biodiversity) that are most at risk of extinction and need the most protection and conservation.

Classification complications

  • Classifying an organism within a particular species may not be easy. For example:
    • Dogs show a wide range of variatin but belong to one species.
    • Duck species occasionally interbreed to produce individuals with a range of charcteristics.

Ring species

  • Neighbouring 'species' interbreed frequently, producing a continuous range of characteristics from one end of the ring to the other. The begiining and end species cant interbreed.
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Reasons for variety

  • Different species are adapted to living in different environmets.
  • Organisms that live in extreme environments need special adaptations.


(see this for adaptations of deep-sea hydrothermal vent organisms and polar organisms)

Types of variation

  • Discontinuous variation- characteristics controlled by genes (genetic variation) e.g. blood group, gender.
  • An acquired characteristic is one that is changed by the environment.
  • Continuous variation is where a characteristic varies gradually and continuously between extremes, e.g. height or weight.
  • A normal distribution curve has a bell shape, showing that the most common variation lies between two etremes, with fewer individuals havig variations that are near to each extreme.
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Natural selection

  • Individuals of a species show variation.
  • Natural selection is where the environment (including climate and other organisms) selects which individuals pass on their genes to the next generation.
  • Stages of natural selection:
    • Overproduction
    • Stuggle for existence
    • Advantageous variations.
    • Individuals without varitions die.
    • Passing on variations.
    • Individuals with variations will increase.


  • Charles Darwin suggested that if the environment canges, natural selection will result in characteristics of a species changing gradually from generation to generation. 
  • New evidence supports Darwin's theory.
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Inside a cell

  • Most cells contain a nucleus in which there are chromosomes.
  • There are 2 copies of each chromosome in body cells- each copy has the same genes in the same order along its length (except chromosomes that determine sex).
  • A gene is a short piece of DNA at a particular point on a chromosome- a gene codes for characteristic, e.g. eye colour.
  • A gene may come in different forms, called alleles, that produce different variations of the characteristic, e.g. different eye colours.


  • Chromosomes of the same type are the same size and have the same genes in the same order.
  • Heterozygous- different alleles of the same gene.
  • Homozygous- the same alleles on both chromosomes.
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Genetic definitions

The gene for coat colour in rabbits has different alleles. The allele for brown colour (B) is dominant over the allele for black colou (b).

Genotype- shows the alleles (forms of the genes) in the inividual. 

Phenotype- the characteristics the are produced, including what the individual looks like.

BB (genotype) = brown coat (phenotype)

Bb (genotype) = brown coat (phenotype)

bb (genotype) = black coat (phenotype)

The effect of the recessive allele will only show when two copies are present in the genotype.

A dominant allele is not bigger or stronger than the recessive allele. It is dominant because you see its effect on the characteristic even if you only have one copy of it.

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Explaining inheritance

Monohybrid inheritance

  • Sometimes a characteristic is controlled by a single gene. This is called monohybrid inheritance. We can use a genetic diagram to help us understand how alleles are inherited.
  • Body cells contain two alleles for each gene. Both parent plants are are heterozygous- they have one allele for purple flower colour and one allele for white flower colour.
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Genetic disorders

  • Some genes have faulty alleles that caause health problems. These are genetic disorders.

Sickle cell disease:

  • Caused by having two copies of a recessive allele for the hemoglobin gene, which causes red blood cells to become sickle-shaped. 
  • People with the disease:
    • become short breathed and tire easily
    • have painful joints if red blood cells get stuck in capillaries
    • have reduced blood flow if red blood cells block a blood vessel, which may cause damage to body tissues, heart attack, stroke or even death.

Cystic fibrosis:

  • Caused by having two copies of a recessive allele for a cell membreane protein. This makes the mucus that lines tubes in the lungs and other parts of the body much thicker and stickier than normal. This can:
    • Increase the risk of lung infections.
    • Prevent enzymes getting into the digestive system to break down food, which can lead to weight loss.
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Family pedigrees

  • A family pedigree is a diagram that shows the inheritance of a characteristic in a family. 
  • Pedigree analysis can help us predict the chance that someone hase inherited a particular allele.
  • We can use pedigrees to evaluate the risk of inheriting a disease.
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