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Biology - Variation and Inheritence Cheat Sheet by

variation and inheritance natural selection plate tectonics


• Variation
Variation is the differ­ences between indivi­duals of the same species.
Those variations that can be inherited are determined by genes. They are genetic variat­ions. Variations may be brought about by genes, but can also be caused by the enviro­nment, or a combin­ation of both genes and the enviro­nme­nt.T­here are variations that are not heritable but determined by factors in the enviro­nment. The tan is an acquired charac­ter­istic. You cannot inherit a suntan. Black skin, on the other hand is an inherited charac­ter­istic.

Discon­tinuous Variation

* Discon­tinuous variation In discon­tinuous variation, the variations take the form of distinct, altern­ative charac­ter­istics with no interm­edi­ates. Some people can roll their tongue into a tube. Others are unable to do it. They are known as non-tongue rollers. Again, there are no interm­edi­ates. Discon­tinuous variation cannot usually be altered by the enviro­nment. You cannot change your eye colour by altering your diet. A genetic dwarf cannot grow taller by eating more food. You cannot learn how to roll your tongue. Discon­tinuous variation is under the control of genes.

Continuous Variation

* Continuous variation An example of continuous variation is height. There are no distinct categories of height; people are not either tall or short. There are all possible interm­ediates between very short and very tall. Continuous variation is influenced by a combin­ation of both genetic and enviro­nmental factors.

Causes of Variation

Enviro­nmental variation :
Inherited Variation :
Variation in the charac­ter­istics caused by an organism's surrou­ndings
variation in charac­ter­istics caused by and organism's parent
eg : scar by an accident
eg: natural eye and hair colour



Is the substance found in the nucleus of every cell in an organi­sm.It carries inform­ation about how the organism develops and functions moreover DNA causes inherited variation. DNA is also known as the genetic material.



Structure containing a molecule of DNA, which carries genetic inform­ation in genes
nucleus ----> chromosome ----> DNA -----> Gene
One extremely long DNA molecule is folded up together with some proteins to form a structure called a chromo­some.


Certain sections of a DNA molecule in a chromosome contain instru­ctions for a charac­ter­istic. These sections are called genes.G­enes control and determine the charac­ter­istics

Sexual Reprod­uction

For sexual Reprod­uction to occur an organism need to produce gametes ( male gametes - sperm ) ( female gametes - egg). These are specia­lised cells that only contain half the normal number of cells and their nuclei only contain half the normal number of chromo­somes.

human gametes -----> 23 chromo­somes
total chromo­somes -----> 46 chromo­somes


A male gamete and a female gamete need to fuse so that their nuclei become on which will cause 23 + 23 = 46 chromo­somes. This process is known as fertil­ization and forms a fertilized egg cell.

Fertil­ization in humans

The tip of the sperm cell contains acrosome which is a jelly like substance that allows it to break through the jelly layer and cell membrane of the egg cell.Once the sperm cell has broken through the egg cell the nuclei of the sperm cell and the egg cell fuse.nThe sperm cell and the egg cell nucleus each have half the number of chromosome needed to make a new human . When they fuse the new cell contains a new combin­ation of DNA , half of the mother and half of the father.

Small ball of cells that develops from a fertilised gg cell. It becomes attached to the uterus lining and develops into the fetus


Mitosis is nuclear division giving rise to geneti­cally identical cells.
meosis is reduction in which chromosome number is halved from diploid to haploid resulting in cells which are geneti­cally different
it occurs in somatic cells
it occurs in germ cells
nucleus divides only one
nucleus divides twice
equational division
reduction division
two daughter cells are formed
four daughter cells are formed
daughter cell from somatic organs
daughter cells form gamete
number of chromosome is same in parent cell and daughter cells
number of chromo­somes is daughter cell reduce to hald
no crossing over in chromo­somes
crossing over occurs in chromosome
daughter cells are geneti­cally identical
daughter cells ar egenit­ically different
note :
A haploid nucleus is a nucleus containing a single set of unpaired chromo­somes present , for example : egg and sperm cells
A diploid nucleus containing two sets of chromo­somes preset , for example : body cells

The Earth's structure and plate movement

Tectonic Plates
The Earth has four main layers - the inner core, the outer core, the mantle and the crust.

The inner core is 5,500°C - extremely hot. It is a very dense solid made from iron and nickel.

The outer core is 2,000 km thick and is a liquid.

The mantle is semi-m­olten and about 3,000 km thick.

The crust is the rocky outer layer. It is thin compared to the other sections, approx­imately 5 to 70 km thick.

The crust is made up of pieces called plates.
There are two types of crust: oceanic and contin­ental crust.

The oceanic crust is found under the sea and is thinner and more dense than the contin­ental crust.

Plate bounda­ries: where two or more plates meet.

Plate Movement

It is believed that heat from the Earth's core caused convection currents in the mantle and that these currents slowly moved the crust around.

Earthq­uakes and volcanoes are primarily found at plate bounda­ries. The plates are like giant rafts that slowly move around. At the boundaries between plates, molten magma is able to force its way to the surface and escape as lava.

Constr­uctive (diver­gent) plate boundary:

Two plates move away from each other.

• When two oceanic plates move away, magma rises to the surface (conve­ntion current) and solidifies when it comes in contact with cold ocean water.
• The magma turns to lava and forms new basaltic ocean crust.
• They can also form shield or basic volcanoes and have non-ex­plosive eruptions.
• Small Earthq­uakes are triggered.
• If two contin­ental plates move away from each other, a rift valley may form.

An example of a constr­uctive plate boundary is the mid-At­lantic Ridge.

Destru­ctive (conve­rging) plate boundary:

Two plates move towards each other.
• When an oceanic plate and contin­ental plate move towards each other, the denser (oceanic) plate is forced down (subdu­cted) under the lighte­r(c­ont­ine­ntal) plate.
• This happens in the subduction zone and an ocean trench is formed.

• The friction between the plates triggers Earthq­uakes.
• The heat produced due to friction turns the descending plate into magma.
• The magma starts to rise and erupt (due to pressure) through a weakness in the crust as an explosive composite volcano.

An example of a destru­ctive plate boundary is where the Nazca plate is forced under the South American Plate.

• Fold mountains are also formed.
• The magma that erupts at the surface forms a chain of volcanic islands
• If two contin­ental plates move towards each other, the sediments between the two plates are compressed (collision zone) and pushed upwards to form fold mountains.
Eg Himalayas

• Earthq­uakes occur, but no volcanic activity as there’s no subduction of oceanic plate.

Conser­vative plate boundary:

• Two plates slide past each other.
• They move in different speeds.
• The plates get locked together and pressure builds up until it is released as an Earthq­uake.
The magnitude (strength) of an Earthquake is measured using a seismo­meter on the Richter scale.


A fossil is the preserved remains or traces of a dead organism. The process by which a fossil
is formed is called fossil­isa­tion.

It’s very rare for living things to become fossil­ised. Usually after most animals die their bodies
just rot away and nothing is left behind. However, under certain special condit­ions, a fossil can

After an animal dies, the soft parts of its body decompose leaving the hard parts, like the
skeleton, behind. This becomes buried by small particles of rock called sediment.
As more layers of sediment build up on top, the sediment around the skeleton begins to
compact and turn to rock.

The bones then start to be dissolved by water seeping through the rock. Minerals in the water
replace the bone, leaving a rock replica of the original bone called a fossil.

Magnetic Reversal

At the center of the ridge, hot magma pushes up through the crust and hardens into rock. Once the magma hardens, the alignment of magnetic domains in the rock is frozen in place forever. Magnetic domains are regions in the rocks where all the atoms are lined up and pointing toward Earth’s north magnetic pole.

The newly hardened rock is gradually pushed away from the ridge in both directions as more magma erupts and newer rock forms. The alignment of magnetic domains in this new rock is in the opposite direction, showing that a magnetic reversal has occurred.


where do i go to find the template like yours? everytime i click create thers nothing there on the intro stuff...this is frustrating lol

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