Variation

Variation

Random Fertilisation

• When two gametes (sex cells) combine to form a new diploid cell (zygote) at fertilisation, this results in genetic variation of the offspring

• This is seen across all species which reproduce sexually as every living thing is genetically different, except for identical twins formed from the same zygote (so produced from the same sperm and the same egg) 

This is because the gametes are all genetically different from each other and also because fusion of a sperm and egg at fertilisation is completely random

Diploid & Haploid Cells

• In humans, a normal body cell (produced as a result of mitosis) contains 23 different chromosomes with two copies of each one

• This means there are a total of 46 chromosomes in each cell

• This is called the diploid number

Diploid & Haploid Cells

• In the gametes (sex cells produced as a result of meiosis) the chromosome number is halved

• There is only one copy of each chromosome in each cell, leading to a total chromosome number of 23

• This is called the haploid number

Genes vs Environment

• Variation is simply the many differences within a species It can be caused in two main ways

• It can be genetic – controlled entirely by genes Or it can be environmental – caused entirely by the environment in which the organism lives

• Most often though, it is a mixture of both genes and environment, as very few things are entirely controlled by one or the other

• This is often popularly called ‘nature vs nurture’

Genetic Variation

• Examples of genetic variation in humans include blood group, eye colour, gender, whether ear lobes are free or fixed (see diagram), ability to roll tongue etc

• Whether earlobes are attached or free is an example of genetic variation

Environmental Variation

• Characteristics of all species can be affected by environmental factors such as climate, diet, accidents, culture and lifestyle In this instance ‘environmental’ simply means ‘outside of the organism’ and so can include factors like climate, diet, culture, lifestyle and accidents during lifetime

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Genetic and Environmental Causes

• Often features vary because of a combination of genetic and environmental causes

• tall parents will pass genes to their children for height their children have the genetic potential to also be tall however if their diet is poor then they will not grow very well therefore their environment also has an impact on their height

• Another way of looking at this is that although genes decide what characteristics we inherit, the surrounding environment will affect how these inherited characteristics develop

Open Ended

Why are identical wins not classified as random fertilisation?

Multiple Select

How are normal body cells produced?

• Mieosis

• Mitosis

Multiple Select

How are gametes produced?

• meiosis

• mitosis

Multiple Choice

If there are only one copy of each chromosome in any gamete, what is the total number of chromosomes in those cells?

• 23

• 46

• 26

• 43

Open Ended

Give 3 examples of genetic variation

Open Ended

Give 3 examples of environmental variation

What Are Mutations?

• Mutations are random changes that occur in a gene or a chromosome

• As a mutation is contained in the DNA, it can be passed on from one generation to the next, ie. mutations can be inherited

How Do Mutations Occur?

• Mutations are random changes that occur in a gene or a chromosome This random change happens spontaneously and continuously in our cells

• This can cause an alteration to the base pair sequence in the genetic code If the base pair sequence is different it could code for a different amino acid when translated in the ribosome

How Do Mutations Occur?

• This could lead to a different protein being made that will not carry out its usual function or will work in a different way, causing the body or organs to behave differently

• As a mutation is contained in the DNA, it can be passed on from one generation to the next

Effects of Mutations

• Most mutations have no effect on the phenotype The protein that a mutated gene produces may work just as well as the protein from the non – mutated gene These types of mutation will not affect the chances of survival of species Mutations rarely lead to the development of new phenotypes and if they do, most have a small effect on the organism

• Occasionally, the new phenotype gives the individual a survival advantage over other members of the species, it can lead to rapid change

Effects of Mutations

• For example: A bird develops a mutation leading to a change in feather colours This makes it more attractive to birds of the opposite sex Which causes the bird to breed more frequently and have more chances of passing on the mutated phenotype to the next generation

• Mutations can also lead to harmful changes that can have dramatic effects on the body – for example, cystic fibrosis in humans

Increasing Mutation Rates

• Mutations happen spontaneously and continuously but their frequency can be increased by exposure to the following:

• Gamma rays, x – rays and ultraviolet rays – all types of ionising radiation which can damage bonds and cause changes in base sequences

• Certain types of chemicals – for example chemicals such as tar in tobacco

• Increased rates of mutation can cause cells to become cancerous, which is why the above are linked to increased incidence of different types of cancer

Open Ended

What does phenotype mean?

Open Ended

What does genotype mean?

Open Ended

Give an example how a mutation can be beneficial to an organism

Open Ended

Give two examples of how the rate of mutation can be increased