Revision:Genetics and evolution


  • It means the differences in characteristic (phenotype) within a species.
  • Source of variation: Genetic (due to different alleles) and Environment.

Discrete Variation


  • Have distinct categories into which individuals can be placed.
  • Tend to be qualitative, with no overlap between categories
  • Controlled by one gene, or small amount of genes.
  • Usually unaffected by environment.
  • More common in plants.
  • Eg. Human blood group, detached ear lobes, flower colour, seed colour.

Continuous Variation


  • Have no distinct categories into which individuals can be placed.
  • Tends to be quantitative, with overlaps between categories.
  • Controlled by large amount of genes(polygenic).
  • Affected by environment.
  • Common in human and other animals.
  • Eg. Height, hair colour, heart rate, muscle efficiency, intelligence, growth rate, rate of photosynthesis.

Sex Determination

  • SEX is determined by the sex chromosomes (X and Y).
  • They are called heterosomes, while the other 22 pairs are called autosomes.
  • In humans the sex chromosomes are homologous in females (XX) and non-homologous in males (XY).
  • Using monohybrid-cross to show that there will always be a 1:1 ratio of males to females.
  • Sex is determined solely by the sperm.
  • There are techniques for separating X and Y sperm, and this is used for planned sex determination in farm animals using IVF.

Sex Linkage

  • The X and Y chromosomes don’t just determine sex, but also contain many other genes that have nothing to do with sex determination.
  • Inheritance of these genes is different for males and females, so they are called sex linked characteristics.

[Notes below might be a bit complicated, read carefully n make sure u understand them, if not, do scream,"HELP!!!] The first example of sex linked genes-eye colour in Drosophila fruit flies. 1. Red eyes (R) are dominant to white eyes (r)

  • when a red-eyed female is crossed with a white-eyed male, the offspring all have red eyes, as expected for a dominant characteristic.

However, when the opposite cross was done (a white-eye male with a red-eyedfemale) all the male offspring had white eyes .

  • It could be explained if the gene for eye colour was located on the X chromosome.
  • Note that in these crosses the alleles are written in the form XR (red eyes) and Xr (white eyes) to show that they are on the X chromosome.
  • Males always inherit their X chromosome from their mothers, and always pass on their X chromosome to their daughters.

Other examples of sex linkage include colour blindness, haemophilia, premature balding and muscular dystrophy


  • In most situations one allele is completely dominant over the other, so there are just two phenotypes.
  • But in some cases there are three phenotypes, because neither allele is dominant over the other.
  • So the heterozygous genotype has its own phenotype.
  • This situation is called codominance.

A good example of codominance is flower colour in snapdragon (Antirrhinum) plants. The flower colour gene C has two alleles: CR (red) and CW (white). The three genotypes and their phenotypes are: homozygous RR-red

homozygous WW-white

heterozygous (RW)-pink

  • Another example of codominance is sickle cell haemoglobin in humans.
  • The gene for haemoglobin Hb has two codominant alleles: HbA (the normal gene) and HbS (the mutated gene).
  • There are three phenotypes:


  • Normal. All haemoglobin is normal, with normal red blood cells.


  • Sickle cell trait. 50% of the haemoglobin in every red blood cell is normal, and 50% is abnormal.
  • The red blood cells are slightly distorted, but can carry oxygen, so this condition is viable.
  • However these red blood cells cannot support the malaria parasite, so this phenotype confers immunity to malaria.


  • Sickle cell anaemia.
  • All haemoglobin is abnormal, and molecules stick together to form chains, distorting the red blood cells into sickle shapes.
  • These sickle red blood cells are destroyed by the spleen, so this phenotype is fatal.

Other examples of codominance include coat colour in cattle (red/white/roan), and coat colour in cats (black/orange/tortoiseshell).

Multiple Alleles

  • An individual has two copies of each gene, so can only have two alleles of any gene, but there can be more than two alleles of a gene in a population.
  • An example of this is blood group in humans.
  • The red blood cell antigen is coded for by the gene I (for isohaemaglutinogen), which has three alleles IA, IB and IO.
  • IA and IB are codominant, while IO is recessive.

Other examples of multiple alleles are: eye colour in fruit flies, with over 100 alleles; human leukocyte antigen (HLA) genes, with 47 known alleles.

Multiple Genes

  • the inheritance of characteristics involving two genes.
  • look at three situations:

2 independent genes, controlling 2 characteristics (the dihybrid cross).

2 independent genes controlling 1 characteristic (polygenes)

2 interacting genes controlling 1 characteristic (epistasis)

The Dihybrid Cross

Mendel also studied the inheritance of two different characteristics at a time in pea plants, so we’ll look at one of his dihybrid crosses. The two traits are seed shape and seed colour. Round seeds (R) are dominant to wrinkled seeds (r), and yellow seeds (Y) are dominant to green seeds (y). With these two genes there are 4 possible phenotypes:

Genotypes Vs Phenotype

RRYY, RRYy, RrYY, RrYy-round yellow

RRyy, Rryy-round green

rrYY, rrYy-wrinkled yellow

rryy-wrinkled green

  • In a Mendel’s dihybrid cross,all 4 possible phenotypes are produced, but always in the ratio 9:3:3:1.
  • Mendel was able to explain this ratio if the factors (genes) that control the two characteristics are inherited independently ; in other words one gene does not affect the other .
  • This is summarised in Mendel’s second law (or the law of independent assortment), what states that alleles of different genes are inherited independently.


  • Sometimes two genes at different loci (i.e. separate genes) can combine to affect one single characteristic.
  • An example of this is coat colour in Siamese cats.
  • One gene controls the colour of the pigment, and black hair (B) is dominant to brown hair (b).
  • The other gene controls the dilution of the pigment in the hairs, with dense pigment (D) being dominant to dilute pigment (d).
  • This gives 4 possible phenotypes:

Genotypes vs. Phenotype vs. F2 ratio

BBDD, BBDd, BbDD, BbDd-“seal” (black dense)-9

BBdd, Bbdd-“blue” (black dilute)-3

bbDD, bbDd-“chocolate” (brown dense)-3

bbdd-“lilac” (brown dilute)-1

  • A more complex example of a polygenic character is skin colour in humans.
  • There are 5 main categories of skin colour (phenotypes) controlled by two genes at different loci.
  • Some other examples of polygenic characteristics are: eye colour, hair colour, and height.
  • The important point about a polygenic character is that it can have a number of different phenotypes, and almost any phenotypic ratio.


  • In epistasis, two genes control a single character, but one of the genes can mask the effect of the other gene.
  • A gene that can mask the effect of another gene is called an epistatic gene.
  • This is a little bit like dominant and recessive alleles, but epistasis applies to two genes at different loci.
  • Epistasis reduces the number of different phenotypes for the character, so instead of having 4 phenotypes for 2 genes, there will be 3 or 2.

Dependent genes

  • In mice one gene controls the production of coat pigment, and black pigment (B) is dominant to no pigment (b).
  • Another gene controls the dilution of the pigment in the hairs, with dense pigment (D) being dominant to dilute pigment (d).
  • This is very much like the Siamese cat example above, but with one important difference: the pigment gene (B) is epistatic over the dilution gene (D) because the recessive allele of the pigment gene is a mutation that produces no pigment at all, so there is nothing for the dilution gene to affect.
  • This gives 3 possible phenotypes:

Genotypes vs.Phenotype vs F2 ratio

BBDD, BBDd, BbDD, BbDd-Black (black dense)-9

BBdd, Bbdd-Brown (black dilute)-3

bbDD, bbDd, bbdd-White (no pigment)-4

Enzymes in a pathway

  • In a certain variety of sweet pea there are two flower colours (white and purple), but the F2 ratio is 9:7.
  • This is explained if the production of the purple pigment is controlled by two enzymes in a pathway, coded by genes at different loci.
  • Gene P is epistatic over gene Q because the recessive allele of gene P is a mutation that produces inactive enzyme, so there is no compound B for enzyme Q to react with.
  • This gives just two possible phenotypes:

Genotypes vs Phenotype vs F2 ratio

PPQQ, PPQq, PpQQ, PpQq-Purple-9

PPqq, Ppqq, ppQQ, ppQq, ppqq-White-7

Duplicate Genes

  • This occurs when genes at two different loci make enzyme that can catalyse the same reaction (this can happen by gene duplication).
  • In this case the coloured pigment is always made unless both genes are present as homozygous recessive (ppqq), so the F2 ratio is 15:1.

Genotypes vs Phenotype vs F2 ratio

PPQQ, PPQq, PpQQ, PpQq, PPqq, Ppqq, ppQQ, ppQq-Purple-15


Genetic Variation in Sexual Reproduction

  • whole point of meiosis and sex is to introduce genetic variation, which allows species to adapt to their environment and so to evolve.
  • There are three sources of genetic variation in sexual reproduction:
  • Independent assortment in meiosis
  • Crossing over in meiosis
  • Random fertilisation
  • Random mating

Independent Assortment

  • This happens at metaphase I in meiosis, when the bivalentsline up on the equator.
  • Each bivalent is made up of two homologous chromosomeswhich originally came from two different parents.
  • Since they can line up in any orientation on the equator, the maternal and paternal versions of the different chromosomes can be mixed up in the final gametes.
  • In this simple example with 2 homologous chromosomes (n=2) there are 4 possible different gametes (22).
  • In humans with n=23 there are over 8 million possible different gametes (223).
  • Although this is an impressively large number, there is a limit to the mixing in that genes on the same chromosome must always stay together.
  • This limitation is solved by crossing over.

Crossing Over

This happens at prophase I in meiosis, when the bivalents first form.

  • While the two homologous chromosomes are joined in a bivalent, bits of one chromosome are swapped (crossed over) with the corresponding bits of the other chromosome.
  • The points at which the chromosomes actually cross over are called chiasmata (singular chiasma), and they involve large, multi-enzyme complexes that cut and join the DNA.
  • There is always at least one chiasma in a bivalent, but there are usually many, and it is the chiasmata that actually hold the bivalent together.
  • There are always equal amounts crossed over, so the chromosomes stay the same length.
  • Ultimately, crossing over means that maternal and paternal alleles can be mixed, even though they are on the same chromosome
    i.e. chiasmata result in different allele combinations.

Random Fertilisation

  • This takes place when two gametes fuse to form a zygote.
  • Each gamete has a unique combination of genes, and any of the numerous male gametes can fertilise any of the numerous female gametes.
  • So every zygote is unique.

Gene Mutation also contributes to Variation

  • Mutations are changes in genes, which are passed on to daughter cells.
  • bases of nucloetides can change when DNA is being replicated.
  • Normally replication is extremely accurate but very occasionally mistakes do occur (such as a T-C base pair).

The actual effect of a single mutation depends on many factors:

1. A substitution on the third base of a codon may have no effect because the third base is less important (e.g. all codons beginning with CC code for proline).

2. If a single amino acid is changed to a similar one (e.g. both small and uncharged), then the protein structure and function may be unchanged, but if an amino acid is changed to a very different one (e.g. an acidic R group to a basic R group), then the structure and function of the protein will be very different.

3. If the changed amino acid is at the active site of the enzyme then it is more likely to affect enzyme function than if it is part of the supporting structure.

4. Additions and Deletions are Frame shift mutations and are far more serious than substitutions because more of the protein is altered.

5. If a frame-shift mutation is near the end of a gene it will have less effect.

6. If the mutation is in a gene that is not expressed in this cell (e.g. the insulin gene in a red blood cell) then it won't matter .

7. If the mutation is in a non-coding section of DNA then it probably won't matter .

8. Some proteins are simply more important than others.

  • For instance non-functioning receptor proteins in the tongue may lead to a lack of taste but is not life threatening, whereas non-functioning haemoglobin is fatal.

9. Some cells are more important than others.

  • Mutations in somatic cells (i.e. non-reproductive body cells) will only affect cells that derive from that cell, so will probably have a small local effect like a birthmark (although they can cause widespread effects like diabetes or cancer).

10. Mutations in germ cells (i.e. reproductive cells) will affect every single cell of the resulting organism as well as its offspring.

These mutations are one source of genetic variation.

As a result of a mutation there are three possible phenotypic effects

  • Most mutations have no phenotypic effect. These are called silent mutations shhh~ , and we all have a few of these.
  • Of the mutations that have a phenotypic effect, most will have a negative effect .
  • Most of the proteins in cells are enzymes, and most changes in enzymes will stop them working
  • When an enzyme stops working, a metabolic block can occur, when a reaction in cell doesn't happen, so the cell's function is changed.
  • An example of this is the genetic disease phenylketonuria (PKU), caused by a mutation in the gene for the enzyme phenylalanine hydroxylase.
  • This causes a metabolic block in the pathway involving the amino acid phenylalanine, which builds up, causing mental retardation.

Mutation Rates and Mutagens

Mutations are normally very rare, which is why members of a species all look alike and can interbreed. However the rate of mutations is increased by chemicals or by radiation. These are called mutagenic agents or mutagens, and include:

  • High energy ionising radiation such as x-rays, ultraviolet rays, a, b, or g rays from radioactive sources. These ionise the bases so that they don't form the correct base pairs.
  • Intercalating chemicals such as mustard gas (used in World War 1), which bind to DNA separating the two strands.
  • Chemicals that react with the DNA bases such as benzene, nitrous acid, and tar in cigarette smoke.
  • Viruses. Some viruses can change the base sequence in DNA causing genetic disease and cancer.


  • Suitable for: Edexcel Biology A Level modules 5 or 6, but also other exam boards.
  • Written by: SnowLady
  • From this thread