3.33 undersand that the incidence of mutations can be increased by exposure to ionising radiation (for exampe gamma raysX-rays and ultraviolet rays) and some chemical mutagens (for example chmicals in tobacco)

Ionising radiation can induce mutations. So can some chemicals in tobacco as
this is why you can get cancer from smoking (as some cancers are a mutation of a cell)

3.32 understand that resistance to antibiotics can increase in bacterial populations, and appreciate how such an increase can lean to infections being difficult to control

Bacteria can develop mutations too, this changes the characteristics and some bacteria become more resistant to particular antibiotics. This means they have an increased chance of survival, so it lives longer and reproduces producing lots of bacterium that are resistant to that particular antibiotic. This is just natural selection.

This poses a problem for people who become infected with the mutated bacteria as the bacteria will not be killed off by the antibiotic supplied, that or a new antibiotic will need to be found which takes a long time.

3.31 understand that many mutations are harmful but some are neutral and a few are beneficial

Most mutations are harmful. For example...

- If a mutation occurs in reproductive cells, the offspring might develop abnormally or die
- If a mutation occurs in body cells, the mutated cell may start to uncontrollable divide and become a cancerous tumour.

Some mutations are neither harmful nor beneficial. For example...

- If a mutation occurs in an unimportant part if the DNA it will not affect the person/animal/plant.

Some mutations can be beneficial also. For example, it may increase the chances of survival for that particular animal, this is natural selection.

Example source: CGP

3.30 describe the process of evolution by means of natural selection

- Living things show variation
- The recourses living things need are limited. Individuals must compete for these resources to survive - only some will survive
- some of the varieties of a particular species will have a better chance of survival . Those varieties will have an increased chance of breeding and passing on their genes
- This means that a greater proportion of individuals in the next generation will have better alleles, and therefore better characteristics, that will help them to survive
- Over many generations, the species becomes better and better adapted to survive. The best features are naturally 'selected' and the species becomes more and more adapted to its environment.

Base notes: CGP

3.29 understand that a mutation is a rare, randon change in genetic material that can be inherited

A mutation is a rare, random change in an organism's DNA (basically, a mutation in a gene). This can be inherited. Mutations are often harmful as they change the sequence of the DNA bases. This could stop the production of a certain protein, or produce the wrong protein.

3.28 understand that variation within a species can be genetic, environmental or a combination of both

Genetic variation is just variation that you inherit, environmental variation is variation that is affected by your environment. That was a bit confusing, here are some examples to clear your mind...

Genetic variation
- Eye colour
- Natural hair colour
- Blood type
- Inherited disorders (e.g. haemophilia, cystic fibrosis)

environmental variation
- Non-inherited disorders (such as PTSD)
- Bad health (such as smoking) can lead to diseases such as cancer and heart disease

NOTE: Some characteristics are affected by genetic and environmental, such as growth. For example, a baby can be very small if the mother does not eat enough during pregnancy (environmental), having a poor diet can stunt your growth (environmental), but you could also just have a short family )
(genetic). Intelligence is also a mixture of both, whilst your maximum IQ is determined by your inheritance, factors such as your upbringing and school life affect how 'clever' you are. Sporting ability is determined by genes, but you also need to train, again it is a mixture of the two.

3.27 know that in human cells te diploid number of chromosomes is 46 and the haploid number is 23

Quite a simple one... in human cells, the diploid number of chromosomes is 46 and the haploid number is 23.

NOTE: All human cells are diploid (have 46 chromosomes) except gametes which are haploid (and have only 23 chromosomes).

3.25 understand that division of a cell by meiosis produces four cells, each with half the numberof chromosomes, and that this results in the formation of genetially different haploid cells

Gametes (sex cells) are produced by meiosis. Meiosis is when a cell reproduces by splitting itseld to form four haploid cells whose chromosomes are not identical. This is the process...


1- The cell duplicates its DNA (chromosomes) so there is enough DNA for each new cell

2- The chromosomes line up in pairs in the centre of the cell

3- The pairs cross over, some of the DNA from each chromosome is swapped. This means each new cell will have a mixture of the two original chromosomes.

4- The chromosomes are pulled apart and they form 2 separate cells

5- These cells are pulled apart, the arms of the chromosomes are pulled apart too (so there is one arm in each cell).

This results in four haploid gametes (haploid because they have half the number, only 23, of chromosomes rather than 46 as each cell has one arm of a chromosome). this means all four gametes are genetically different.

image source: vce.bioninja

3.24 understand that mitosis occurs in growth, repair, cloning and asexual reproduction

Mitosis is wen a cell reproduces itself by splitting to form two cells with identical sets of chromosomes. It is used for asexual reproduction, growth and repair of damaged cells, and cloning.

3.23 understand that division of a diploid cell by mitosis produces two cells which contain identical sets of chromosomes.

Okay so mitosis is when a cell reproduces by splitting itself in two to form two cells with identical sets of chromosomes (this is the definition we need for exams). This means that when a diploid cell (with 46 chromosomes) divides by mitosis, two diploid cells (both with identical 46 chromosomes) result. This is how it works...

NOTE: we do not need to learn the names of each stage for the exam, this is just the best diagram I could find :)

image source: publications.nigms.nih.gov
 

3.22 describe the determination of the sex of offspring at fertillisation, using a genetic diagram

This is a very good diagram which basically has everything on...



From this we can understand that there is a 50% chance of an offspring being male (XY) and a 50% chance of the offspring being female (XX).

image source: biologymad.com

3.21 understand that the sex of a person is controntrolled by one pair of chrmosomes, XX in a female and XY in a male

Firstly, it is important to understand that there are 23 pairs of chromosomes in each human cell, resulting is 46 individual chromosomes. (NOTE: there are only 23 chromosomes in sex cells, this is known as haploid). The 23rd pair is the one that determines whether you are male or female. Every male has 'XY' as his 23rd pair and every female has 'XX' as their 23rd pair. The female can only give X alleles, whilst the male can give X or Y alleles. This results in a combination of 2 XX genotypes and 2 XY genotypes, meaning there is a 50% chance of being male and a 50% chance of being female. This diagram may help for understanding...


image source: boiledpizza.blogspot

3.20 predict the probabilities of outcomes from monohybrid crosses

The each parent gives one allele to the offspring. There are two possible alleles it could give (for example, 'Bb' there is 'B' and 'b'). Therefore, if there are two parents, and each parent can give a possibility of one of two alleles, this means there are 4 possibilities overall for the offspring. I explained that so oddly, here is a diagram which will probably help more...



 

If you are asked to work out the probability of a child inheriting a genotype/phenotype, just count how many times it comes up and divide it by 4 (then x by 100 to get a %).

For example, if you are given the diagram above and asked to work out the probability the offspring will inherit 'Bb'. First you count how many 'Bb' are present, the answer is 2.Now divide 2 by 4 (this is because there are 4 possibilities) = 0.5. Now x0.5 by 100 to make a %. 0.5 x 100 = 50. So there is a 50% chance the offspring will inherit 'Bb'.

Image credit: scienceaid

3.19 understand hwo to interpret family pedigrees

A pedigree diagram enables someone to easily show a specific gene within a family. Here's how they work...

- A square represents a male
- A circle represents a female
- Coloured in represents the person has that allele/characteristic
- Blank implies that the characteristic/allele is not present
- You may see half a square/circle coloured in. This implies the allele is present but recessive so the characteristic is not present.

NOTE: This is not always the case but most often is, in an exam situation there will be a key so you do not need to learn this.


For example, in this particular family, the mother carries a particular gene, which the sons have inherited. However, the father and daughters do not carry this gene...

3.17 understand the meaning of the terms: dominant, recessive, homozygous, heterozygous, phenotype, genotype and codominance

Dominant - If an allele is dominant, the characteristic controlled by a dominant allele develops if the allele is present on one or both chromosomes in a pair. A dominant allele is always shown with capital letters. For example, 'BB'

Recessive - if the allele is recessive the characteristic controlled by a recessive allele develops only if the allele is present on both chromosomes in a pair. If there is only one recessive allele present, the characteristics of the dominant allele will show. A recessive allele is portrayed with lowercase letters. For example, 'ee'.

NOTE:  If you have two alleles, the version of the characteristic that appears will be dominant. For example, if you inherit one allele for brown eyes (BB) which is dominant and one allele for blue eyes (ee) which is recessive, the dominant characteristic will show, so you will have brown eyes. The only way you could have blue eyes is if you inherited two alleles for blue eyes. as this allele is recessive.

Homozygous - If you are homozygous, you have inherited two of the same alleles (e.g. BB or ee). All this means is that you have inherited, for example, two alleles for blue eyes or two alleles for brown eyes.

Heterozygous - If you are heterozygous, you have inherited two different alleles for that particular gene (e.g. Bb). All this means is you have most likely inherited one recessive and one dominant, in which case the characteristics of the dominant allele will show.

Phenotype - The phenotype is the characteristic that the alleles produce. For example, brown eyes, blonde hair, brown hair, blue eyes and so on.

Genotype - The genotype is the allele configuration you have inherited. For example, 'Bb', 'BB', 'ee' and so non.

Codominance - If you happen to inherit two dominant alleles, this is called codominance. This is where neither allele is recessive so you show characteristics from both alleles, as one is not dominant over the other. For example, blood group A is dominant and so is blood group B, so if you inherit an allele for A and an allele for B, you will end up with blood group AB.

3.16 understand that genes exist in alternative forms called alleles which give rise to differences in inherited characteristics

Different forms of a gene are known as alleles. For example, for the gene that codes for eye colour there are alleles that code for brown eye colour and alleles that code for blue eye colour.

3.15 describe a DNA molecule as two strands coiled to form a double helix, the strands being linked by a series of paired bases; adenine (A) with thymine (T), and cytosine (C) with guanine (G)

a molecule of DNA is made up of two strands that are coiled together and held in place by chemicals known as bases. the four bases are as follows... adenine, thymine, cytosine and guanine. The bases are paired together. Adenine ALWAYS pairs with thymine and cytosine ALWAYS pairs with guanine. This ensures that you will always have an equal amount of adenine to thymine and cytosine to guanine. This diagram may help...


NOTE: A good way to remember which base pairs with which is with the rhyme 'Apples on a Tree, Cars in a Garage' (Apple = Adenine, Tree = Thymine. Cars = Cytosine, Garage = Guanine)

3.14 understand that a gene is a section of a molecule of DNA and that a gene codes for a specific protein

A gene is just a section of DNA, and each separate gene codes for a particular protein.

3.13 understand hat the nucleus of a cell contains chromosomes on which genes are located

Okay so basically every cell contains a nucleus, every nucleus contains chromosomes (humans are diploid and have 46 chromosomes, except for sex cells which are haploid and have 23). Each chromosome is made up of a long coil of DNA. DNA can be sectioned, a small section of DNA is known as a gene.