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  • Created on: 14-12-14 10:21


Genetics- the study of the mechanism by which offspring inherits characteristics from its parents.

Genotype- the genetic makeup of an organism. It describes all the allelles a organism contains.

Allele- different version of the same gene.

Phenotype- the observable characteristics of an organism.


  • They can seperate and combine
  • They can mutate
  • They code for the production of specific polypeptides.
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Loci combinations

 One locus on a chromosome has the potential for three different allele combination.

  • Heterozygous-different alleles for the same gene, a dominant and recessive. Each allele is carried on a different chromosome within a homologous pair.
  • homozygous dominant- having two dominant alleles.
  • homozygous recessive- having two recessive alleles.

Dominant- will be expressed over recessive alleles

Recessive- will only be expressed when the individual is homozygous.

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Test crossing

Test cross/ backcross

Used to determine whether a dominant characteristic is defined by one or two alleles.

e.g. a pure black bull wiuld expected to be BB, but his appearance could be that of a Bb.

A backcross would be bredding the bull with a known recessive (e.g. a white cow). If all the offspring are black, the bull must be BB.

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Monohybrid inheritance

Involves the inheritance of a single characteristic such as plant height. It involves the inheristnace of two alleles concerning a single gene.

Mendels first law: Characteristics are defined by factors which come in pairs, but only one can be presnt in a gamete.

When using a punnet square in the exam, you must display the process yu go through.

  1. make sure you understand if the aprents are homozygous r/d or heterozygous/
  2. decide the letter to represent each allele, and display this clearly e.g. P=dom p=rec
  3. Represent the parenst with these letters, and clearly label them.
  4. State the gametes produced by each parent (this is the x and y axis of the square) label it gametes!!
  5. Use punnet bsquare to show resluts of mating.
  6. State the phenotype of each genotype and indiciate the ratio.
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Dihybrid inheritance

Involves the inheritance of two seperate genes and characteristics.

Mendel knew that round seeds were dominant to wrinkled, and that a yellow colour was dominnat to green.

He then crossed these two contrasting characters and found:

  • Dominant plants (yellow and round seeds) + recessive plants produced round, yellow seeds in the first generation.
  • When plants from these seeds were self pollinated they produced 4 different types of seed.

Each of these totals can be divided by the double recessive total (smallest one) to give a whole number ratio between phenotypes. This is a dyhbrid ratio.

Mendels second law; either one of contrasted characters may combine with another pair. 

This is saying that alleles of genes are spearate and independent and can be combined freely.

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A breeding experiment will not come out with the exact predicted outcomes. When you throw a coin you would expect a 50/50 outciome, but you often wontr get thta.

The chance you will get a head or a tail is 100%.

1/2 +1/2 = 1

The probability you will get heads on two coins is 25%

1/2 x 1/2 = 1/4

The probablility of any of the four alleles appreaing in the 2nd generation is:

round 3/4, yellow 3/4, wrinkled 1/4, green 1/4

multiply these toigether to get the probability of any combination.

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The expected ratio of the offspring in a dihybrid is always 9:3:3:1.

We use chi square to see if the data is close enough to this ratio to be considered expected, or if its unexpected ad needs to be explained.

'The result is significant at the 5% level and something other than chance is affecting the result.'



The value of (O-E)2 should always be positive.

x2 is the sum of all (O-E)2/E added together.

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Null Hypothesis- accept??

Null hypothesis:

There will be no significant difference between the expected resulst and the observed results.


  • Our calculated value of X2 is smaller/larger than the critical value.
  • The difference between Observed and expected in insignifiant/significant.
  • We can accept/reject our null hypothesis.
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Co-dominance + sex determination

When the two alleles are both dominant, this is caleld co dominance.

Often, an intermediate is shown e.g. AB blood group, pink snapdragon plants (producing equal amlounts of an enzyme for both red and white pigment)

The parents would have to both be homozygous dominant.

RR+ WW = R+W = RW

Sex Determination:

  • In humnas the male has dissimilar pair or sex chromosomes X and Y, and females have simialr XX.
  • All females eggs contain X chromosome.
  • Sperm contain either X or Y.
  • Gametes combine to make a girl or a boy. XX or XY.
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Sex Linkage

Some alleles on the X chromosome and are described as sex linked.

The Y chromosome carries very few alleles, so any recessive genes on teh X will be expressed. These characteristics cannot be given to the sons because they need to recieve the Y chromosome. All daughters will get the gene, but as they have another X chromosome it might not be dispplayed. They have 50% of carrying it on to their son.

Haemohilia is a recessive gene on the x chromosome.

  • It is potentially lethal as it causes uncontrolled bleeding.
  • It is now possible to extract the clotting factor from donated blood, but the rick of ingeritnace remians.
  • The conditions occursd in males, who must aklways inherit it from their mother.
  • This is because women usually have the dominant clotting fctor gene on one of their X chromosomes.
  • Women would need to inherit two chromosomes with the recessiove gene to be haemophilicas, so it is very rare.
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This is when two different genes are found on the same chromosome. They are inherited together and pass into the gametes togethe,r and then offspring.

Crosses and ratios for genes on the same chromosome do not follow mendelian patterns so rations will not be obtained.

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Linkage and recombination

Recombination takes place when alleles are exchanged between homologous pairs as a result of crossing over. The further apart two genes are on a chromosome, the more chance there is of crossing over taking place.

Most linked genes remain together, and crossing iver only occurs in 5%-10% of meiotic divisions.

Recombinants separate linked genes.

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A change in the amount, arrangemnt or structure in the DNA of an organism.

What does it effect?

  • It may affect a single gene or a whole chromosome.
  • Most mutations occur in somatic body cells.
  • Oh those mutations which occur during the formation of gametes can be inherited.

How often?

  • Mutations are spontanous events which could provide a source of material for natural selection and evolution.
  • Mutation rates are very small, tehrefore mutation has less impact on evolution than other sources of variation.
  • In general, organisms with short life cycles and more frequent meiosis show a greater level of mutation.
  • Mutations can be increased by ionising radiation and mutagenic chemicals.
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Gene mutations

These are changes in the base pair within the gene and produce a different allele of a gene. The rates of mutaion vary from one gene to another within an organism. They can take form of duplication, insertion, inversion or subsitution. Modifies the polypeptide.

They rarely show up in the phenotype as they are generally recessive. 

mutation > phenotype?

  1. Specific sequence of nucleotides.
  2. A chnage in one or more of these nucleotides  could lead to a different amino acid sequence.
  3. The enzyme or proetin will be aunable to do its function perfectly. e.g. the enzyme which assists the chemical melanin to develop is broken, this results in someone being an albino.
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Gene mutation; sickle cell anaemia

Sickle cell anaemia

One mutation results in a different amini acid (glutamic acid instead of valine in the 6th position of the Beta globulin chain) in haemoglobin. This reuslts in it beocming a sickle cell shape, and less able to carry oxygen, resulting in anaemia and possible death.

Glutamic acid = CTT or CTC. Valine = CAT or CAC. Only one mutationneeded to change these two.

The mutant allele is co-dominant, so a homozygous sufferer will have all haemoglobin S. A heterozygous carrier will have 30-40% sickle cells and the rest are normal. This is reffered to as sickle-cell trait.

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Chromosome mutations :structure

These are muyataions that cause changes in the structure or whole number of chromosomes in a cell. They are most likely to occur in meiosis  when the prioces of lining up on the equator at metaphase 1 can go wrong. Chromosomes can not be shared equally among daughter cells.

Changes in structure

During prophase i or meiosis, homologous chromosomes pair up and exchnage material at chiasmata. Errors arise when chromosomes rejoin with the corresponding piece of chromosome on its homologous partner. The chromosome can end up with a different gene sequnce, and so pairing up in meosisi is impossible.

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Chromosone mutations: Numbers

Changes in numbers

Non-dijunction is a faulty cell division where one of the cells recieves two copies of a chromosome while the other gets none. In Downs syndrome this affects number 21, where the cell which got two copies survives. This is known as trisomy 21 and results in downs syndrome.

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Chromosome mutations: sets of chromosomes

Changes in sets of chromosomes

Occasionally a mutation effects a whole set of chromosomes, increasing the number of complete sets- this is called polyploidy.Often results as a defect of meiosis when a cell recieves two sets of chromosomes, and is then fertilised by a normal haploid gametes, resulting in a triploid zygote. 

Tetraploidy is when there a 4 sets of chromosomes e.g. when genetic material is doubled during mitosis but the cell doesnt separate.

Polyploidy is often found in flowering plants and is associated with beneficial characteristics such as disease resistance. Tomoatoes and wheat. They are usuallky infertile as they cannot form homologous pairs.

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Mutations happen naturally but the rate can be increased by exposure to mutagens. e.g

  • x-rays, gamma rays, and UV.
  • chemicals, e.g. polycyclic hydrocarbons.


Any agent that cause cancer. They affect the DNA in cells resulting in a mutation which affects cell division. Cells accumulate and become a tumour. They can spread around the body and get in tissues- these are called malignant tumours.

Tobacco smoke contains harmful chemicals, including tar, carbon monoxide and nicotine. Tar collects in the lungs as tobacco cool  and contains carcinogens. 25% of cancer deaths are due to tar exposure.


Division is usually halted when enough cells have been made for their purpose. The interaction of proto-oncogenes and tumour suppressors helps this process. Proto-oncogenes  stimulate cell dividon; suppressors slow it down. Mutations in either can result in tumours.

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Importance of mutations

Increase variation in a population. Most mutations are harmful, but beneifcial ones(rare) can lead to a selective advantage.

  • Somatic mutations can lead to cancer. e.g. UV.
  • If it is a gamete, it will effect the offspring not the parent.
  • Mutations cause sudden and distinct differences, and are the basis of discontinuous variation.
  • Most muations are recessive to normal allele, and therefore must await many generations in the gene pool before chance brings recessive alleles together.
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