Notes

Inheritance, Variation and Survival

DNA and Inheritance
CHROMOSOMES, DNA and GENES:

Most cells in our body have a NUCLEUS. The nucleus contains CHROMOSOMES.
Chromosomes are LONG, COILED UP LENGTHS of a molecule called DNA.
DNA is a long LIST of CHEMICAL INSTRUCTIONS on how to build an organism.
A GENE is a SHORT SECTION of a chromosome (and therefore, a short section of DNA).
Genes CONTROL many of our CHARACTERISTICS. e.g. hair colour, eye colour, hairiness, etc. Different genes control DIFFERENT characteristics.
Genes WORK IN PAIRS - one will usually be DOMINANT over the other.
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We INHERIT CHARACTERISTICS From Our PARENTS:

Human body cells have 23 PAIRS (46) of (CHROMOSOMES.
SPERM and EGG cells carry only 23 CHROMOSOMES.
During reproduction, when an egg is FERTILISED, the NUCLEUS of the egg FUSES with the NUCLEUS of the sperm.
This means that the fertilised egg contains 23 MATCHED PAIRS of chromosomes. It has ONE COPY OF EACH GENE from the MOTHER and ONE from the FATHER.
Since genes control characteristics, the fertilised egg develops into an embryo with a MIXTURE of the PARENTS' CHARACTERISTICS. This is how you 'INHERIT' your parents' characteristics.
The process by which genes are PASSED DOWN from parents to their offspring is called HEREDITY.
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The STRUCTURE OF DNA Was Only Worked Out RECENTLY:

Scientists STRUGGLED for DECADES to work out the STRUCTURE of DNA.
CRICK and WATSON were the FIRST scientists to build a MODEL of DNA - they did it in 1953.
They used DATA from other scientists, WILKINS and FRANKLIN, to help them that DNA is a DOUBLE HELIX - a SPIRAL made of TWO CHAINS wound together.
By putting all the information together, Crick and Watson were able to BUILD a MODEL showing what DNA looks like.
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Variation
Different SPECIES Have Different GENES:

All living things in the world are DIFFERENT - we say that they show VARIATION.
A human, a cow, a dandelion, and a tree all look different because they're DIFFERENT SPECIES. These DIFFERENCES BETWEEN SPECIES occur because their GENES are very DIFFERENT.
But, you also see variation WITHIN A SPECIES, i.e. plants or animals that have BASICALLY the SAME genes will also show differences between them, e.g. skin colour, height, flower size, etc. Any DIFFERENCE is known as a CHARACTERISTIC FEATURE.
CHARACTERISTIC FEATURES can be INHERITED or they can be ENVIRONMENTAL (caused by surroundings).
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CONTINUOUS and DISCONTINUOUS Variation

Variation WITHIN A SPECIES can either be classed as CONTINUOUS or DISCONTINUOUS.


CONTINUOUS VARIATION - the feature can vary over a RANGE OF VALUES:

Examples of this are things like HEIGHT, WEIGHTM SKIN COLOUR, INTELLIGENCE, LEAF AREA, etc. Where the feature can have ANY VALUE AT ALL - withing a certain RANGE. If you did a survey of kids' heights, you could plot the results on a chart (the categories are NOT distinct - there are NO GAPS between the bars).
The smooth DISTRIBUTION CURVE drawn on afterwards shows much better the CONTINUOUS way that values for height actually vary.


DISCONTINUOUS VARIATION - the feature can only take CERTAIN VALUES:

An example of this is someone's BLOOD GROUP, where there are just FOUR DISTINCT OPTIONS, NOT a whole CONTINUOUS RANGE.
Another example is the COLOUR OF A COURGETTE. A courgette is either yellow, light green or dark green - there's NO RANGE of values.
If you did a survey of the four blood groups, you could plot the results on a chart (the categories ARE distinct - there ARE GAPS between the bars).