Causes of Variation Flashcards
Sexual Reproduction
Sexual reproduction mixes up genetic material in three : crossing over, independent assortment and fertilisation. Producing new genotypes and so variations in phenotype.
SR: Crossing over
Homologous chromosomes pair up as meiosis begins
Genetic material is exchanged between chromatids…
SR: Independent Assortment
Homologous pairs may line up in different ways, producing different combinations of genes in the gametes OR
There are four different gametes from just two pairs of chromosomes. Each human produces many different gametes in this way.
SR: Fertilisation
Any male gamete can combine with any female gamete.
Define Mutation
A mutation is a change in the type or amount of DNA.
How can mutations arise?
because or:
- mistakes in the copying of DNA as cells get ready to divide > pairing with the incorrect base
- damage to the DNA > some environmental factor might alter the bases present in the DNA
- uneven distribution of chromosomes during the division of cells
Chromosome mutations
They occur when cell division fails to work with complete accuracy. For example, when human gametes are formed each gamete should receive 23 chromosomes, occasionally and error occurs. The consequences of chromosome mutations are often serious (i.e. Down Syndrome).
Gene mutations
They occur when part of the DNA on a single chromosome is changed. As a result a defective protein may be produced or no protein at all. This can lead to a considerable in a characteristic (i.e. sickle cell anaemia)
What are mutagens
Mutagens are factors that can increase the rate of mutations.
Important Mutagens
Are:
- radiation = gamma, ultraviolet and X-radiation can all damage the DNA and so cause mutations
- chemicals = tar in tobacco, high concentrations of some preservatives and some plant control hormones can cause mutations
- carcinogen = a mutagen that causes uncontrolled cell division resulting in cancer (e.g. asbestos, tobacco smoke)
Beneficial Mutations
Some mutations give benefits to the organism and aid adaption to the environment. They may cause harm in one environment but be a benefit in another (i.e. Sickle Cell Anaemia)
Types of Mutations
Substitution Deletion Insertion Inversion Nonsense
Substitution
One base is substituted for another (e.g. C instead of U)
Deletion
One base is deleted from the chain.
Insertion
An extra base is inserted into the chain.
