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Topic 6 Inheritance, Variation and Evolution By Bethan Poole (6.1…
Topic 6 Inheritance, Variation and Evolution
By Bethan Poole
6.1 Reproduction
:baby_chick:
6.1.2 Meiosis
When a cell divides by meiosis...
Copies of genetic info made
cell divides twice to form 4 gametes
all gametes genetically different to each other
New cell divides by mitosis
The number of cells increases
As embroyo develops cells differentiate
Importance
Halves the number of chromosomes
So fertilisation can restore the full number of chromosomes
6.1.3 Comparison
Asexual
Pros
Only one parent needed
More time and energy efficient
Faster than sexual reproduction
Identical offspring can be produced to make the best use of good conditions
Sexual
Pros
Produces variation
If the environment changes it gives a survival advantage- natural selection
Allows us to selectively breed plants and animals- increase food production
Some organisms can reproduce both ways
Many plants can produce seeds sexually and asexually
Many fungi can make spores by sexual or asexual to give variation
Malaria parasites produce sexually in the mosquito and asexually in humans
6.1.1 Reproduction
Sexual
Fusion of egg and sperm (gametes) in animals
Fusion of pollen and egg cells in flowering plants
Leads to variation due to mixing of genetic information
Asexual
All genetic info from one parent
No fusion of gametes
Ensure successful characteristics are passed on (clone of original)
Allows an organism to quickly habitat a new habitat
6.1.4 DNA and genome
Everyone except identical twins have unique DNA
Pros of the human genome project
search for genes linked to certain disease
understanding and treatment of inherited disorders
used to trace human migration patters from the past
Genetic material in the nucleus of a cell made of chemical- DNA- contained in structures called chromosomes
Gene= small section of DNA on a chromosome
Each gene codes for a particular sequence of amino acid to make a specific protein
Genome of an organism- the entire genetic material of that organism
6.1.5 DNA Structure
DNA- polymer made from four nucleotides
Each nucleotide is made of
a sugar
a phosphate
one of four bases: A, C, G, T
Nucleotides join to form long strands
Each molecule has alternating sugar and phosphate strands- twisted to form a double helix
Each sugar is attached to one of the four bases
Attraction between bases- C with G and T with A
Protein synthesis
Order of bases controls the amino acid made- sequence of three bases is the code
Proteins synthesised on ribosomes using template taken from the DNA and carried out of the nucleus
Carrier molecules bring specific amino acids to add to the growing protein chain in the correct order
When chain is finished it folds to form a unique shape
Unique shape allows them to do their job as enzymes, hormones or structural proteins i.e. Collagen
Mutations
Change in the DNA structure
If any bases are change then it may change the order of amino acids in the protein coded for by the gene
Occur all the time
Most don't alter protein or only alter slightly
Some cause it to change shape
Enzyme- Substrate may no longer fit active site
Structural Protein-May lose its strength
Not all parts of DNA code for proteins
Non-coding parts of DNA an switch genes On/Off so they can/can't make specific proteins
Mutations here may change how genes are expressed
6.1.6 Genetic Inheritance
Terms
Dominant
- Only one allele needed for the trait to be expressed
Recessive
- Both alleles needed for the trait to be expressed
Chromosome
- long molecules found in the nucleus of all cells
Homozygous
- Two of the same allele e.g. BB
Gamete
- a specialised sex cell formed by meiosis
Hetrozygous
- One of each allele e.g. Bb
Allele
- Specific version of the gene, an individual always has two (1 from mother, 1 from father)
Genotype
- Code of genes e.g. BB bb Bb
Gene
- section of chromosome that codes for a trait
Phenotype
- the physical expression (the characteristic shown)
Most characteristics are controlled by several genes working together
If only one genes involved-
monohybrid inheritance
Punnet Squares
e.g. Eye colour
One homozygous , one hetrozygous
Brown eye- dominant (Bb)
Blue eye- recessive (bb)
50% chance of blue eyes
Used to predict the outcome of a monohybrid cross
Can be expressed as a ratio, percentage or probability
They use capital letters for dominant alleles and lowercase letters for recessive alleles
6.1.7 Inherited diseases
Polydactyly
Having extra fingers or toes
Caused by dominant allele
Genotype resulting in it : BB Bb
Cystic Fibrosis
Disorder of cell membranes
Caused by recessive allele
Can be a carrier as its recessive (Bb)
Genotype resulting in it bb
Embryo Screening
cells removed from developing embryo to test for genetic disorders
Economic, social and ethical issues of embryo screening
6.1.8 Sex Determination
1 pair of the 23 pairs of chromosomes carries the genes determining sex- sex chromosomes
Females 2 sex chromosomes are identical (XX)
Males inherit an X and a Y chromosome (much shorter)
Offspring inherit- X from mother and X or Y from father
6.2 Variation and Evolution
:monkey:
6.2.1 Variation
Characteristics are inherited
Info carried in gametes
Changes occur
Environment
Genes
Both
Mutation
Cause all variations
Some minor= no effect
Some lead to a new phenotype- suited to environment
6.2.2 Evolution
A change in the inherited characteristics of a population over time through a process of natural selection- can result in new species being formed
All species have evolved from simple life forms made 3 billion years ago
if two populations of a species become so different in phenotype that they can no longer interbreed they have formed two new species
6.2.3 Selective breeding
Choosing parents with the desired characteristics from a mixed population
Continues over many generations
Artificial selection
Pros
:check:
Disease resistant crops
Animals producing more meat can be made
Domestic dogs- gentle nature
Large/ unusual crops made
Cons
:no_entry:
Can lead to inbreeding
Can inherit defects or be prone to disease
6.2.4 Genetic engineering
AKA Genetic modification (GM)
Changing the DNA of a living thing to change characteristics
Method
1) Enzymes used to isolate specific gene
2) Gene inserted into the vector
3)Vector used to insert the gene into required cells
If put into cells of animals or plants at egg or embryo cells- all cells in the organism will get the new gene
GM Plants
Disease, pest, frost, herbicide or drought resistant
Longer shelf life
Grow in harsh conditions
Pros
Could result in cheaper foods
Could give bigger yields
Less chemical sprays needed
Cons
GM seeds are expensive
Could cause allergies
Reduce biodiversity
Could accidentally transfer genes to other plants
6.2.5 Cloning
Produced naturally (asexual reproduction)
Plants :sunflower:
Clones can be produced from
Cuttings
Tissue culture
Animals :sheep:
Clones produced by splitting apart cells from an embryo before they become specialised , then transplanting embryos into surrogate
Method
1) Remove nucleus from unfertilised cell
2) Insert the nucleus from an adult body cell of the organism you want to clone into the empty egg cell
3) Stimulate the egg to divide using an electric shock
4) Allow the embryo to develop into a ball of cells
5) Insert embryo into womb of surrogate mother
e.g. Dolly the sheep
Clones are genetically identical individuals
6.3 Genetics and Evolution
:jeans:
6.3.1 Theory of evolution
Evolution is the gradual change in inherited characteristics of a population over time
Can lead to formation of new species
Individuals
Charles Darwin
Theory of Natural Selection- change in genetics causes evolution
Noticed 4 things
1) Organisms often produce a large number of offspring
2) Populations usually stay about the same size
3) Organisms are all slightly different
4) Characteristics can be inherited
Made these conclusions...
There's a struggle for existence
More organisms are born than can survive
Those who survive and breed are those suited to environment
Characteristics passed on to offspring
Over time characteristics of populations change
John- Baptiste Lamark
Based on idea that changes that occur in an organisms lifetime can be inherited
Environmental changes cause evolution
6.3.2 Speciation
Charles Darwin
Darwin publishes book
On the Origin of Species
1859
Lots of controversy over his ideas because...
Theory challenges idea that God creator eveything
Not enough evidence to convince scientists
Mechanism of inheritance and variation not known for another 50 years
Alfred Russel Wallace
Independently proposed theory of evolution by natural selection
Worked worldwide gathering evidence
Evidence led to current understanding
Best known for work on warning colouration of animals and is theory of speciation
Sterile
- can't breed
Fertile
- can breed
Species
- a group of living things that can breed with each other and produce offspring who are able to breed
6.3.3 The understanding of genetics
Gregor Mendel
(Father of genetics)
Around at the same time as Darwin
Carried out breeding experiments on plants- smooth and wrinkly peas
Made the conclusions that
Characteristic are determined by units that are inherited and do not blend together
Characteristics can be dominant or recessive
Recessive characteristics may be inherited but not shown
Certain characteristics can be inherited predictably
The importance wasn't recognised in his lifetime because...
He was a monk working in a monastery not a scientist in a university
He didn't publish his work in a well known book or journal
20th century scientists worked out what the structure of DNA looked like and he mechanism by which genes work- observed that chromosomes and Mendel's units worked in the same way
6.3.4 Evidence for evolution and 6.3.5 Fossils
Fossils
Found in various ways
from the hard parts of animals that don't decay easily
from parts of an organism that have not decayed
when parts of the organisms are replaced by other materials when they decay
preserved traces of the organism i.e. footprints
Scientists use fossils to look at how organisms have evolved
Problems
Gaps in the fossil record
Caused by many early organisms being soft bodied - no bones decay quickly
Traces destroyed by geological activity
Antibiotic resistance in bacteria
Explained by natural selection
Bacteria evolve rapidly- reproduce at a fast rate
When they reproduce mutations occur
Mutated bacteria might not be killed as they're resistant to anti-biotics
They survive and reproduce so resistant strains develop
Still lots of debates over the theory of evolution and the origins of life
6.3.6 Extinction
Occurs when there are no remaining individuals of a species alive
Caused by
Extreme poaching
Natural Disasters
Changes to environment
New predators
New diseases
New, more successful competitors
How to survive
Migrate
Adapt
Change food sources
Separate communities
Find new territories
6.3.7 Resistant bacteria
Can evolve rapidly (reproduce at a fast rate- binary fission)
Mutations cause new strains- some resistant to antibiotics- can't be killed
These survive and reproduce- population rises
No effective treatment/ no immunity- it will spread
MRSA
Resistant to antibiotics
How to reduce rate of development
Doctors shouldn't unnecessarily prescribe antibiotics- i.e for non-serious/ viral infections
Patients should complete course of antibiotics - ensure ALL bacteria are killed
Agricultural use of antibiotics should be restricted
Development of new antibiotics is costly and slow- unlikely to keep up with the emergence of new strains
6.4 Classification of living organisms
:cow:
Carl Linnaeus
Classified living things into
Kingdom → phylum→ Class→ Order→ Family→ Genus→ Species
Organisms are named by the
binomial system
They have two parts to their Latin name
1st part-
Genus
2nd part-
Species
New models of classification were proposed as...
Microscopes improved- more learnt about cells
Biochemical processes became better understood
Carl Woese
Developed the
three domain system
Organisms are divided into
Archaea (primitive bacteria- live in extreme environments)
Bacteria (true bacteria)
Eukaryota (including protists, fungi, plants and animals)
