HL 10. Genetics and Evolution

10.1 Meiosis

Important questions: How is meiosis different from mitosis? Where does meiosis happen? Why is meiosis so important for natural selection?

Introduction: Fill in this summary table comparing mitosis and meiosis


Cell division location Number of cells produced Chromosome number Type of cells

Meiosis is linked with natural selection because it promotes ‘variation’. One of the necessary conditions for natural selection to operate is that individuals in a population are different from one another.

There are three ways that meiosis promotes variation:

  • Crossing over (also called recombination)
  • Independent assortment (new mixes of chromosomes)
  • Fertilisation (which brings together DNA from two individuals).

The third way ‘fertilisation’ happens after meiosis. Meiosis creates the sex cells, which carry out fertilisation.

1) Crossing over, or ‘recombination’.

First a quick recap of the stages of meiosis

During Prophase 1 of meiosis, exchanges occur between homologous chromosomes pairs. This is called ‘crossing over‘. The homologous chromosome pairs become physically entangled, this is called synapsis. The points where the chromosomes are entangled are called ‘chiasmata‘. This process is important as it can separate alleles which normally do not get separated because they are on the same chromosome, or ‘linked’.

Because the alleles which are normally on the same chromosome are separated from each other, new combinations of alleles are formed, hence ‘recombination’.

10.2 Inheritance

Objectives: To carry out a dihybrid cross using punnet squares.


  • Phenotype: the expression of the alleles eg. brown eyes
  • Genotype: the alleles which are present for a specific trait
  • Unlinked genes: Genes which are carried on separate chromosomes
  • Linked genes: Genes which are carried on the same chromosome, and therefore are inherited together (unless they are separated by recombination).
  • Punnet square: the calculation of possible combinations of gametes
  • Homozygous: the phenotype where the alleles are the same as each other
  • Heterozygous: the genotype where the alleles are different to each other
  • Parent genotypes: the genotypes of the original parents (in classic genetics experiments usually true-breeding)
  • First filial generation: the generation after the first cross. (Often heterozygous in classic genetics experiments)
  • Second filial generation: the generation which comes from the crossing of F1 (in classic genetics self-pollination is often used)

Required knowledge: Pre-IB work on monohybrid crosses.

1) http://slideplayer.com/slide/8703611/ Slides 1-16

2) If you don’t recall this, watch the following teacher-led video of a monohybrid cross

Gregory Mendel’s work on inheritance included two laws:

1) There are two alleles present for every gene eg. Bb for eye colour.

2) The law of independent assortment states that genes are inherited separately, and this holds true if the genes are carried on separate chromosomes.

It has been established that Mendel’s second law only holds true for genes which are carried on separate chromosomes are called ‘unlinked genes’.

Task: Number each member of the class, find out if they can roll their tongues. Using the information in the image below, pick two anonymous individuals and predict what would be the result of their progeny, with respect to tongue rolling. Summarise any assumptions made

Screen Shot 2017-10-23 at 10.22.58

Di-hybrid crosses.

A di-hybrid cross works the same way as a monohybrid cross, however two genes are investigate instead of one. Try the examples in the slideshow below, working on your own and then checking your answer against the given results.

Follow the five steps of a Mendelian cross:

  1. Identify parent genotypes
  2. Deduce possible gametes
  3. Punnet square
  4. Filial generation Genotype ratio
  5. Filial generation Phenotype ratio



 Actitivy: See page 446 in the textbook attempt to work out a dihybrid cross also see answer on this website


 10.3 Gene pools and speciation

Big questions: How do new species from existing species? Can we observe speciation occurring in the world today?

Lesson one: Stabilising, directional and disruptive selection

Objective: To recognise the influences of stabilising, directional and disruptive selection on natural populations.


  • Gene pool: all the genes and their different alleles, present in an interbreeding population
  • Speciation: the process of forming new species
  • Divergence: when the individuals in one species become different to each other and begin speciation
  • Allele frequency: the occurrence of a specific allele in a population (or populations)
  • Reproductive isolation: when the individuals of two populations cannot for some reason, interbreed. -This may be geographical (because of a natural barrier like a lake), temporal (to do with time like flowers opening at different times), or behavioural (animals choosing not to breed with another population).


A gene pool

The concept of a gene pool is used to study evolution in species. It is recognised that a species is a group of potentially interbreeding populations, with a common gene pool. These means that they are reproductively isolated from other species. Individuals that reproduce are contributing to the gene pool of the next generation.

If two populations of a same species become reproductively isolated, then they are no longer sharing the same gene pool!


image credit: premedhq.com

image credit: premedhq.com


Evolution is defined as the cumulative change in the heritable characteristics of a population over time. That means that the gene pool is changing.

Natural selection has the effect of varying the frequency of specific alleles in a gene pool. Mutations tend to add new alleles to the gene pool. Barriers to gene flow which emerge between populations may divide gene pools.

Effect of natural selection on the gene pool.

There are three distinct kinds of effect that natural selection may have on a gene pool:

  1. Directional selection: the population changes in a specific direction (eg. giraffes evolving longer necks).
  2. Stabilising selection: the population is becoming more similar to each other. This means that the extremes of a characteristic are becoming less common, and more moderate expression of a characteristic is becoming more common.
  3. Disruptional selection: the population is becoming divided, as the extremes are favoured and the intermediate varieties are selected against. This can lead to divergence.


image credit: Brian McCormick

image credit: Brian McCormick


Activity: databased questions: 456, 457 

Activity Online lab: Changes in the rock pocket mouse

click on the link, and download the student materials. There is a short film to watch. You will be asked to draw bar graphs, you may wish to use excel but graph paper is available.


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