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B6 - Inheritance, Variation & Response (Evolution (Survival of the…
B6 - Inheritance, Variation & Response
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Mutations
What is it?
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Most mutations have little/no effect on the protein and some will change it slightly so that its function/appearance is unaffected
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If there is a mutation in the non-coding DNA, it can alter how genes are expressed
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Types
Deletions
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This changes that the way that the base sequence is 'read' and has knock-on effects further down the sequence
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Insertions
This changes the way the groups of three bases are 'read', which can change the amino acids that they code for
Can change more than one amino acid as they have a knock-on effect on the bases further in the sequence
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DNA
Genes
DNA determines what proteins the cell produces (eg. haemoglobin, keratin)
This, in turn, determines what type of cell it is (eg. a red blood cell, a skin cell)
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The Structure of DNA
Each nucleotide consists of one sugar molecule, one phosphate molecule, and one 'base'
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Protein Synthesis
The DNA, however, is found in the nucleus and can't move out of it as it is too big
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To make proteins, ribosomes use the code in the DNA
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When the protein chain is complete, it folds to form a unique shape
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Reproduction
Sexual
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In humans, each gamete has 23 chromosomes - half the number of chromosomes in a normal cell
Instead of having two of each chromosome, a gamete just has one of each
Where genetic information from two organisms (a father and mother) is combined to produce offspring which are genetically different to either parent
The egg (from the mother) and the sperm cell (from the father) fuse together (fertilisation) to form a cell with the full number of chromosomes
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Because there are two parents, the offspring contains a mixture of their parents' genes
|This is why the offspring inherits features from both parents - it's received a mixture of chromosomes from both
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Flowering plants can reproduce in this way. they have egg cells, but their version of sperm is pollen
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Asexual
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There is no fusion of gametes, no mixing of chromosomes and no genetic variation
Bacteria, some plants and some animals reproduce asexually
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Examples
Fungus
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Sexually-produced spores introduce variation and are often produced in response to an unfavourable change in the environment, increasing the chance that the population will survive the change
Plants
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eg. Strawberry plants
They produce 'runners' which are stems that grow horizontally on the surface of the soil away from the plant
At various points along the runner, a new strawberry plant forms that is identical to the original plant
Loads of species of plant produce seeds sexually, but can reproduce asexually
Malaria
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When a mosquito carrying the parasite bites a human, the parasite can be transferred to the human
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Inherited Disorders
Cystic Fibrosis
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For a child to have the disorder, both parents must be carriers or have the disease
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Polydactyly
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The parent that carries the allele will also have the disease as it is dominant - there are no carriers
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Embryo Screening
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SEE Implications
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Against
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There may come a point where everyone wants to screen their embryos so they can pick the most 'desirable' (eg. a blue-eyed, blond-haired, intelligent boy)
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Genetic Diagrams
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Genes
Heterozygous
If the two alleles are different, only one can determine what characteristic is present
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For an organism to display a recessive characteristic, both allels must be recessive (eg. cc)
For an organism to display a dominant characteristic, it can either be CC or Cc as the dominant allele overrules the recessive one
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Variation
Genetic variation
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This is because an organism's characteristics are determined by the genes inherited from their parents
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These genes are passed on in gametes, from which the offspring develop
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Environmental variation
eg. a plant grown in plenty of sunlight would be luscious and green, but the same plant grown in darkness would grow tall and spindly and have yellow leaves
The environment, inc. the conditions that organisms live and grow in, cause differences between members of the same species
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Mutations
They are changes to the sequence of bases in DNA which can lead to changes in the protein a gene codes for
Although very rare, mutations can result in a new phenotype being seen in a species
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If the environment changes, and the new phenotype makes an individual more suited to the new environment, it can become more common through the species - natural selection
Mendel
Conclusions
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2) Hereditary units are passed on to offspring unchanged from both parents, one unit from each parent
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Scientific Advances
His work was a starting point as the observations of many different scientists have contributed to our understanding of genes
In the early 1900s, scientists realised that they were striking similarities in the way that chromosomes and Mendel's "units" acted
Based on this, it was proposed that the "units" were found on the chromosomes
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In 1953, the structure of DNA was determined
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In the late 1800s, scientists becamse familiar with chromosomes
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Pea Plants
Pea Plant Experiment
Second cross
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Offspring: three tall pea plants, one dwarf pea plant
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Mendel found out that the height characteristic was determined by separately inherited "hereditary units" passed on from each parent
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The ratio of tall and dwarf plants in the offspring showed that the units for tall plants, T, was dominant over the unit for dwarf plants, t.
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Evolution
Theory: all of today's species have evolved from simple life forms that started to develop over 3 billion years ago
Survival of the Fittest
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He concluded that the organisms with the most suitable characteristics for the environment would be more successful competitors and would be more likely to survive - survival of the fittest
Darwin knew that within a species there is variation in their characteristics (phenotypic variation)
The organisms that survive are more likely to reproduce and pass on the genes for the characteristics that made them successful to their offspring
He used observations from a round-the-world trip as well as experiments, discussions and new knowledge of fossils and geology to suggest the theory
The organisms less well adapted would less likely survive and reproduce, so they are less likely to pass on their genes to then next generation
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Over time, beneficial characteristics become more common in the population and the species changes - it evolves
New Discoveries
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Darwin's theory wasn't perfect because the relevant scientific knowledge wasn't available at the time
He couldn't give a good explanation for why new characteristics appeared or exactly show individual organisms passed on beneficial adaptations to their offspring
Extinction
Reasons for extinction
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A catastrophic event happens that kills them all (eg. a volcanic eruption/collision with an asteroid)
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Controversy
Darwin couldn't explain why these new, useful characteristics appeared or how they were passed on from individual organisms to their offspring
Back then he didn't know anything about genes/mutations - they were discovered about 50 years after his theory was published
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Lamarck
eg. if a rabbit used its legs to run a lot (to escape predators), then its legs would get longer. The offspring of that rabbit would then be born with longer legs
eg. he though that if a characteristic was used a lot by an organism it would become more developed during it's lifetime and the offspring would inherit the acquired characteristic
Jean-Baptiste Lamarck (1744-1829) argued that changes that an organism acquired during its lifetime will be passed on to its offspring
At the time, there were different scientific hypotheses about evolution
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Meiosis
Gamete Fusion
As the embryo develops, these cells then start to differentiate into the different types of specialised cell that make up an whole organism
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After two gametes have fused during fertilisation, the resulting new cell divides by mitosis to make a copy of itself
Process
In the second division, the chromosomes line up again in the center of the cell
The pairs are then pulled apart so each new cell only has one copy of each chromosome (some of the father's and some of the mother's go into each new cell)
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In the first division, the chromosome pairs line up in the centre of the cell
You end up with 4 gametes, each with only a single set of chromosomes in it
After replication, the chromosomes arrange themselves into pairs
Each of the gametes is genetically different from the others because the chromosomes all get shuffled up during meiosis and each gamete only gets half of them, at random
Before the cell starts to divide, it duplicates its genetic information (forming two armed chromosomes - one arms of each chromosome is an exact copy of the other)
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In humans, it only happens in the reproductive organs (the ovaries in females, the testes in males)
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Gender
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When making sperm, the X and Y chromosomes are drawn apart in the first division in meiosis
A similar thing happens when making eggs but the original cell has two X-chromosomes, so all the eggs have one X-chromosome
There's a 50% chance each sperm cell gets an X-chromosome, and a 50% chance it gets a Y-chromosome
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