Motor Function of the Nervous System

Muscle tissue contracts in response to appropriate nervous impulses.
- impulse travels along a motor neuron to the nerve-muscle synapse (neuromuscular junction)
- at the synapse, neurotransmitters (acetylcholine) are released into the synaptic cleft
- neurotransmitters bind to receptors on the muscles, initiating a depolarization process in the muscle, causing actin filaments to pull in between the myosin filaments, resulting in contraction of the muscle
- once the action potential has passed, the muscle will relax

http://www.hoops.co.il/?p=14856

http://www.zoology.ubc.ca/~gardner/chemical_synapses%20-%20postsynaptic.htm

In skeletal muscle:
- one impulse results in one contraction - this is called muscle twitch (izomrángás)

http://howmed.net/physiology/skeletal-muscle/

- since the action potential has passed by the time the muscle contracts, a new action potential can innervate the muscle before the period of relaxation begins.

In the figure below, in (a) we can see a series of twitches, where the impulses are spaced out in time

Staircase effect
- if impulses arrive one after the other in more rapid succession, we can observe what is referred to as the staircase effect, where each twitch is a bit stronger than the one before.  See (b).  Basically, the second twitch came so quickly that the first one never fully relaxed, so there is leftover calcium in the sarcoplasm.  Since there is some force leftover from the first twitch, they build on each other.  This is called summation of force

Tetanus
- In (c), the twitches are created by such a rapid succession of impulses (at least toward the end), that the individual twitches summate and blend into a smooth sustained contraction, called tetanus.  This is also called frequency summation

http://faculty.stcc.edu/AandP/AP/AP1pages/Units5to9/unit9/summatio.htm

- sustained contractions are normal in living organisms, individual twitches are rare.

Muscle tone/Tonus
- skeletal muscles are constantly in a state of passive, partial contraction, which is called muscle tone
- it helps maintain posture and it decreases during REM sleep
- muscles are always ready to respond to sudden changes (eg. in balancing)
- muscle tone is primarily controlled at the level of spinal reflexes, but some aspects also have higher control.
eg. knee-jerk reflex

http://kids.britannica.com/comptons/art-156424/Knee-jerk-reflex-reaction-and-motor-neuron-connection-to-spinal

- hitting the hammer to the knee causes a slight stretch in the quadriceps (thigh muscle on front of leg)
- the stretch receptor in the quadriceps sends an impulse along the sensory neuron to the spinal cord
- the axon branches to two synapses: one to an inhibiting interneuron which synapses with a motor neuron that leads to the biceps muscle and prevents it from contracting; the other branch of the axon synapses directly with a motor neuron which sends and impulse to the quadriceps causing it to contract.
- lower leg kicks forward

- reflexes play a role in more complex motions, but control over most motion occurs at higher levels of the central nervous system
- the organization and coordination of our motions occurs in the cerebrum, in the posterior region of the frontal lobe
-motor tracts descend through one of 2 systems:  pyrmidal and extrapyramidal motor systems

Pyramidal motor system:
- the pyramidal tracts descend from the cerebrum through the brainstem to the grey matter of the spinal cord
- in the brainstem some fibres connect to motor neurons of brain nerves, but the majority cross-over in the pyramids of the medulla oblongata and continue into the spinal cord where they synapse with motor neurons
- a few nerves don't cross-over in the medulla oblongata, but continue straight down into the spinal cord , where they cross-over right before synapsing with motor neurons
- in the end, the motor innervation of the right side of the body is controlled by the left side of the brain and vice versa.
- the pyramidal motor system controls all intentional movement and learned fine motor coordination (like writing)

http://teachmeanatomy.info/neuro/pathways/descending-tracts-motor/

Extrapyramidal motor system:
- includes parts of the cerebrum, the mesencephalon and the cerebellum, as well as the brainstem
- this system controls automatic learned movements, large motor movements and motions that reflect emotions, as well as taking part in maintaining muscle tone
- the extrapyramidal system is very ancient and 3 of the 4 tracts found in humans are shared by salamanders

-well-controlled movement requires the coordinated function of both the pyramidal and the extrapyramidal systems

- the contraction of smooth muscle is also controlled by stimulation from vegetative motor neurons.
- contraction of the heart muscle does NOT require external stimulation.  The heart contains small muscle cells that are capable of spontaneous stimulation.  These cells are more permeable to sodium than the others and are found grouped at the sino-atrial node and at the atrio-ventricular node.

Sensory function of the nervous system

- information from the internal and external environments  is collected by receptors and brought to the central nervous system through spinal and cerebral nerves.
- stimulation of receptors in skin and muscle is transferred to spinal sensory nerves (found in the dorsal ganglion), which carry the stimlation through the sensory tracts of the spinal cord
- some take part in spinal reflexes, these pass into the ventral horn and synapse with motor neurons there.
- most continue on the ascending tracts and cross-over in the spinal cord or the medulla oblongata and enter cerebral centers on the opposite sides.

- stimulation from the touch and heat receptors of the skin enters the dorsal horn, then cross-over in the spine before continuing along the lateral ascending tracts to the thalamus.
- sensory information from the deeper layers of the skin and the muscles enters the dorsal ascending tracts to the medulla oblongata, there the neurons synapse, then cross-over and travel along the opposite side to the thalamus
- information is processed and grouped in the thalamus, then sent on to the appropriate parts of the cerebrum.

http://faculty.southwest.tn.edu/rburkett/A&P1_nervous_syst_organization.htm

Taste receptors are found in tastebuds on the tongue.  The stimulation is carried by cranial nerves to the medulla oblongata and from there to the thalamus and the cerebrum.  All flavours are derived from sweet, salty, sour and bitter.  Typically, it is stated that sweet receptors are at the tip of the tongue, bitter is at the back and the sides contain salty and sour.  I have recently read that this is not entirely true, but....  that is what is in the textbook :)

http://chargedmagazine.org/2012/09/taste-buds/

Smell receptors or olfactory receptors are found on the back surface of nasal passage

http://www.allthingsherbal.biz/2011/10/aromas-and-the-sense-of-smell/

- stimulation of the cerebral olfactory nerves goes directly to the cerebrum without passing through the thalamus

Sight receptors are the eyes! They contain many light receptors, but the eye is more complex that simply being a mass of light receptors.

The eyeball is surrounded by 3 layers:

- the outer layer is protective, the sclera (ínhártya) is is the white layer, which becomes the clear cornea (szarúhártya) at the front of the eye.  Light passes through the cornea to the inside of the eye.  The cornea is constantly moistened by the blinking of the eyelids.

- the middle layer contains many blood capillaries, providing the eye with necessary oxygen and nutrients, it also absorbs excess light to prevent damage.  It is called the choroid (érhártya).  At the front it forms the ciliary body, which to which the muscles controlling the lens attach.  The iris (szivárványhártya) is a continuation of the ciliary body and the hole in the middle, through which light passes into the eye is called the pupil.  The colour of the iris depends on the number of pigments it contains.  If someone's eyes are black or brown, then the iris contains many pigments, while people with green or blue eyes contain fewer pigments.  

- the inner layer is where the receptors are found.  It is called the retina (ideghártya) and it contains two kinds of light receptors, rods and cones.

http://libot-libot.blogspot.hu/2011/05/cataract-is-clouding-of-lens-of-eye.html

http://lpatersonbiotask3.wikispaces.com/Describe+the+anatomy+and+function+of+the+human+eye...

- light passes through several different media before it reaches the receptors

- First the light passes through the cornea

- Then it passes through space between the cornea and the lens, which contains the aqueous humour (csarknovíz), which is watery in consistency

- The size of the pupil decreases when there is lots of light, due to the constriction of the iris, and increases in low light, due to relaxation of the iris.  

- Light then passes through the lens (just behind the iris).  The lens is flexible and can be flattened or rounded to focus the light on the fovea (sárgafolt) at the back of the retina.  This is the point in the eye of truly focused vision

-  When the eye is relaxed then focus is on distant objects and the muscles holding the lens are constricted, so the lens is flattened, the opposite occurs when focusing on close objects, the muscles relaxes and the lens becomes rounded. With age, the flexiblilty of the lens decreases, so most elderly people have good distant vision, but difficulty focusing close objects.  Focal problems can and often do occur earlier in life and can be corrected with proper optical lenses.  If someone is far-sighted the focal point is behind the retina, while if someone is near-sighted the focla point is in front of the retina.

- the chamber behind the lens is filled with the vitreous humour, which is a jelly-like substance that helps maintain the eye's shape. 

- when light reaches the retina, it activates the receptors, the rods (pálcikák) and cones (csapok), which got their names from their unique shapes.

- Rods are long and cylindrical.  They work well in low light and produce black and white images.

- Cones are smaller and conical.  They function well in bright light and allow us to see in colour.

- When photons are "caught" by the receptors, their energy is transformed into an electrical impulse, which is transfered down the receptor cell's membrane to the synapse.  The stimulation is then carried by sensory neurons in the optic nerve to the thalamus.  From here, via synapses, the stimulation is sent to the visual areas in the occipital lobes.

http://www.biochemj.org/csb/010/csb010_fig071.htm

Rods and cones are organized as shown below.  Where the optic nerve enters the back of the eye, there is a point on the retina where there are no rods or cones.  This is the blind spot.

http://tiffanybiology.blogspot.hu/2011/05/rods-and-cones.html

http://abcarcade.com/blindspot-test.html

Sound and balance receptors are found in the ears.

http://www.biographixmedia.com/human/ear-anatomy.html

Sound
- The outer ear's shape is designed to bring sounds into the auditory canal to the ear drum (also called tympanic membrane).

- The ear drum is the beginning of the middle ear.  In the space behind the ear drum, the smallest bones in the body called ossicles, the hammer, anvil and stirrup, can be found.  The hammer moves when a sound vibration causes the ear drum to vibrate.  The movement of the hammer moves the anvil, which in turn, moves the stirrup.  The bottom of the stirrup hits the oval window (not labelled in upper diagram) of the inner ear.

- The inner ear begins with the oval window and beyond it, the cochlea, which is filled with fluid called perilymph.  When the stirrup hits the oval window, this begins a wave in the cochlear fluid.  The round window, at the other end, gives the fluid somewhere to go.
- The basilar membrane is a rigid surface that extends the length of the cochlea.  When the stirrup moves in and out, it pushes and pulls on the basilar membrane that is right below the oval window.  This force starts a wave along the surface of the membrane.  The basilar membrane has a peculiar structure.  It is made of about 25 000 reed-like fibers that extend the width of the cochlea.  Near the oval window the fibers are short and stiff.  Toward the other end the fibers are longer and more flexible.  Thus the fibers have different resonant frequencies.  A specific wave frequency will resonate perfectly with the fibers at a certain point, causing them to vibrate rapidly.  When the wave reaches the fibers with the same resonant frequency, the wave's energy is suddenly released.  Because of the increasing length and decreasing rigidity of the fibers, higher-frequency waves vibrate close to the oval window and lower frequency waves vibrate at the other end of the membrane.

http://bio1152.nicerweb.com/Locked/media/ch49/SAVE/cochlea.html

-  The organ of Corti is a structure containing thousands of tiny hair cells.  It lies on the surface of the basilar membrane.  When a wave finally reaches the resonant point, the membrane suddenly releases a burst of energy in that area.  This energy is strong enough to move the organ of Corti hair cells at that point.  When the hair cells move, they send electical impulses through the cochlear nerve to the thalamus and then to the cerebral cortex, where the brain interprets them.  Louder sounds release more energy at the resonant point, thus move greater numbers of hair cells in that area.

http://bio1152.nicerweb.com/Locked/media/ch50/cochlea.html

http://nl.bu.edu/research/projects/moneta/moneta-v2-0/auditory-system/

 Balance

- The 3 semi-circular canals, along with the utricle and vestibule form the organ that senses our position

http://en.wikipedia.org/wiki/File:Balance_Disorder_Illustration_A.png

- The semi-circular canals are filled with a fluid called endolymph and lined with tiny cilia

- The canals are positioned at right angles to each other and movement in any direction will cause the fluid in the corresponding canal to move.  Fluid movement in the horizontal canal corresponds to rotation of the head in a horisontal axis, while the posterior and superior canals correspond to vertical movments of  the head.  Movement of the fluid causes movement of the cilia, which sends electrical impulses to the brain.

- Otoliths are small crystaline structures found in the utricle and vestibule.  They rest in a jelly-like matrix over receptors cells.  Their movement mechanically stimulates the receptor cells, giving information about gravity and linear motion    

Békésy György (1899-1972), biophysicist, worked in Budapest, Stockholm and the US

-Studied hearing and won a Nobel prize in 1961 for his discovery of the physical mechanism of how stimuation occurs in the cochlea.  His research clarified how sound waves travel in the cochlea and how the vibration frequency is associated with stimulation at a specific point.

Bárány Róbert (1876-1936), medical doctor, worked in Austria and Sweden

- Studied the physiology and disease of balance.  He developed experimental techniques to study balance and received a Nobel prize for his work in 1914.

Nervous Control of Human Behaviour

Almost all information about our environment that is collected by receptors is taken to the cerebral cortex.
The cortex's massive neural network processes, stores and, if necessary, recalls the information.  This is where sensory information becomes emotions, where responses are organized and impulses are sent out through motor neurons.

The Limbic System
The limbic system is a complex set of brain structures that lie on both sides of the thalamus, right under the cerebrum.  It is not a separate system, but a collection of structures including olfactory bulbs, hippocampus, amygdala and hypothalamus.

http://webspace.ship.edu/cgboer/limbicsystem.html

It supports a variety of functions including emotion, behaviour, long-term memory, motivation and olfaction (smelling).  It can be divided into 2 functional areas, the inner and outer rings.  The outer ring is closely associated with the hypothalamus, thus in this way the limbic system is in direct connection with the hormonal system.  The other main function of the outer ring is the control of emotional behaviour, including such emotional reactions like pain, joy, fear  and anger.  These emotions are associated with specific areas in the outer ring.    The inner ring is primarily associated with the storage of important memories.  The memories are not actually stored in the limbic system, but it controls where memories will be stored in the cerebrum.  Memories can be short-term memories, which only last for a few minutes before being replaced by other information, or they can be long-term memories which are stored for many years, if not whole lifetimes.  The long-term storage of memories requires the creation of synapses. The more often a synapses is activated the greater the amount of neurotransmitter, thus creating a more stable synapse and a more lasting memory.

Memory

Memory and memory storage is an area of constant research.  To read a good, clear overview of memory creation, storage and retrieval click here.

Features of human memory:
1. Memorization is easier if the information as meaning to the individual.
2. Well-organized information is easier to retrieve.
3.  Storage and retrieval of information is easier if them memory can be associated with other key stimuli.
4.  Storage is improved if there is emotional weight to the memory.
5. Interference between memories makes storage and retrieval more difficult (eg. learning large amounts of new material can make it more difficult to retrieve old information)

Speech
Speech assists humans in their social interactions.  At birth humans are capable of creating nearly 200 different sounds (this is a biological ability).  In any given social environment (nation, culture), only 25-45 of these sounds are used.   Humans also use writing and gestures (metacommunication) to communicate.  In greater than 85% of people the speech association areas are found in the left lobe of the cerebrum.  The auditory association area, found in the temporal lobe, also takes part in the creation and understanding of speech.  Most of the time, it is also in the left brain that these sounds are converted into thoughts.  

The Cerebral Cortex

The cerebral cortex, often referred to as grey matter, is considered to be responsible for "higher" brain function.  The surface of the cortex is folded in large mammals, increasing its surface area.  

http://gerardkeegan.com/glossary/cerebral-cortex

The two halves of the cerebrum are asymmetreical, both in structure and in function.  The two sides complement each other: while one side takes part in speech and deduction, the other deals with physical and temporal connections (representations).  In women, this asymmetry is less apparent.

http://www.as.wvu.edu/~rbrundage/chapter5b/sld014.htm

http://bio1152.nicerweb.com/Locked/media/ch48/cerebral.html

Asymmetry of the cerebrum

Health and the Nervous System

Drugs (compounds which can be addictive) affect the nervous system.  Stiumulants are drugs that increase the speed of communication within nervous system (at synapses or in centres).  They usually increase alertness and physical activity.  They include caffeine, amphetamines, cocaine and crack.  Depressants (narcotics)  slow down the activity of the nervous system.  For medical purposes they can calm nerves, relax muscles and help with insomnia.  They include alcohol, marijuana and opium.  Hallucinogens interfere with the nervous sstem in such a way that it results in distortions of the user's perception of reality.  Profound images, sounds and sensations will be experienced, but they do not actually exist.  Marijuana, in high doses can have this effect.  LSD and magic mushrooms are other examples of hallucinogens.  Drugs affect all parts of the body and can have a variety of effects depending on the user's genetic makeup, current physical and physiological state and the quality/quantity of the drug and its method of use.  What all drugs share as a common problem is that with time and continued use, the body accommodates to them, so the user feels the need to increase dosages to acheive the same effect.

Stress is a common problem in modern society.  Stress affects the sympathetic nervous system, which results in the increased production of adrenalin by the adrenal glands.  Continuous stress can bring on many illnesses as the body's normal functions cannot occur.

Mental States
When an individual continuously feels that he or she is unable to meet the expectations of his environment, it can lead to neurosis.  The unmet expectations can cause constant stress, which can lead to exhaustion, insomnia and loss of concentration.  A balanced lifestyle can help to prevent/solve this problem.
Depression is psychological state when the individual's level of interest drops.  There can be real, external reasons for this, but it can also occur without obvious external factors.
Paranoia is one of the most common psychological diseases, where the patient feels a constant fear.  Schizophrenia is when the patient believes at times that he is someone else.

Physical States
Headaches can be cause by many factors  and can be indicators of many diseases - tension, poor eyesight, encephalitis, low blood sugar, blood pressure change, low brain oxygen and many others.  Migraines are intense headaches, which often affect a whole lobe of the brain and can have other symptoms, such as vision problems and nausea.
Hemorrhagic strokes (gutaütés) occur when a blood vessel in the brain is damaged (due to atherosclerosis and high blood pressure) and then oxygenated blood no longer reaches an area of the brain.  Ischemic strokes (agytrombózis) occurs when a blood clot  or a narrowing blocks a blood vessel in the brain, also leading to a lack of oxygen to that part of the brain.  These are the most common type of strokes.
Epilepsia is the result of chaotic messages being sent through motor areas, resulting in sudden loss of conciousness and seizures
Concussions are the result of physical trauma to the brain.  They can result in unconciousness, headaches, nausea and vomitting.  If a person suffers from many concussions, it can lead to permanent damage.
Encephalitis (agyhártyagyulladás) is caused by viruses or bacteria that infect the membranes surrounding the brain and spinal cord. Typical symptoms include headaches, stiff neck and back, high fever, vomitting and light sensitivity.

Genetics - Some definitions before we get into the nitty gritty

Gene: a segment of DNA that codes for one characteristic

Locus:  the physical position on a strand of DNA where a gene is found

Allele: an alternative form of a gene (one member of a pair) that is located at a specific location on a specific  chromosome

Wildtype allele:  the most commonly occurring allele in a population

Phenotype:  the appearance, structural and functional characteristics of an organism

Genotype:  the genes of an organism that produce the phenotypic characteristic

Dominant allele:  an allele that produces the same phenotypic characteristic, whether its paired allele is the same or different.

Recessive allele: an allele that only produces its characteristic phenotype when its paired allele is identical.

Haploid: an organism (or cell) with only one copy of its genetic material (DNA)

Diploid: an organism (or cell) with two copies of its genetic material (DNA)

Homozygote: a diploid cell in which the alleles for a given gene are identical.

Heterozygote: a diploid cell in which the alleles for a given gene are different.

Classical Mendelian Genetics

Johann Gregor Mendel (1822-1884)

Mendel is considered to be the "Father of Genetics".  In the mid-1800's he carried out systematic studies of the inheritance of traits in the common garden pea.  He then published this body of work in 1866, but it was not well received, as most people simply didn't grasp was he was explaining!!  It wasn't until the early 1900's that the value of his work was recognized.

Mendel chose to work with the garden pea because it grew quickly, it required little space to grow, its seeds were easy to handle, it could self-pollinate or be cross-pollinated, and there were many sub-species.

Mendel focused his studies on 7 different  traits:

Mendel self-pollinated the plants until he got purebreeding strains (offspring always had the same characteristics as the parent plant).  Then he cross-pollinated plants with particular traits and observed the outcome over many generations.

Some of his first observations showed that when he cross-pollinated purebreeding parental plants showing different characteristics (for example, purple flowers or white flowers), all of the offspring were identical (in this example all purple).  The parental generation is refered to as P (parental generation) and the offspring is F1 (or first filial generation).

http://bioserv.fiu.edu/~walterm/GenBio2004/new_chap13_inheritance/pics.htm

He observed this same pattern when he studied the other traits as well.

Parent Pea Plants	F1 Pea Plants
tall stem x short stem	all tall stems
yellow seeds x green seeds	all yellow seeds
green pea pods x yellow pea pods	all green pea pods
round seeds x wrinkled seeds	all round seeds
axial flowers x terminal flowers	all axial flowers

This led Mendel to conclude that certain traits were dominant over other traits and he created the Rule of Dominance  to explain this.

Rule of Dominance:  In a cross of parents that are pure for contrasting traits, only one form of the trait will appear in the next generation.  Offspring that are hybrid for a trait will have only the dominant trait in the phenotype.

Mendel then took the F1 generation and let them self-pollinate.

http://bioserv.fiu.edu/~walterm/GenBio2004/new_chap13_inheritance/pics.htm

He consistently observed that in the F2 (second filial generation) there were 3 purple flowers for ever white flower.  He consistently observed this 3:1 ratio with other traits as well.

http://www.biology.iupui.edu/biocourses/N100/2k4ch10genetics.html

Mendel realized that these results could be explained if three things were true. He hypothesized that:

1. Every trait (like flower color, or seed shape, or seed color) is controlled by two "heritable factors". [We know now that these are genes - we each have two copies of every gene].

2. If the two alleles differ, one is dominant (will be observed in the organism's appearance or physiology) and one is recessive (cannot be observed unless the individual has two copies of the recessive allele). Dominant traits mask the appearance of recessive traits.

3. Alleles are randomly donated from parents to offspring - the factors (alleles) separate when the gametes are formed by meiosis, allowing all possible combinations of factors to occur in the gametes.

Mendel's Law of Segregation - The two factors (alleles) separate when the gametes are formed, and only one factor (allele) is present in each gamete.

http://facultylounge.whfreeman.com/?q=node/1438

Punnett Squares

To help in solving genetic problems, Punnett squares are often used.  The genotype of each parent is written on the top and left side, respectively.  The two alleles are separated.  Then the table is filled in with the allele from the respective side and top positions.  Those are the offspring produced by the cross.  

http://kids.britannica.com/comptons/art-90108/Mendels-principle-of-segregation-states-that-during-gamete-formation-the

http://en.wikipedia.org/wiki/Punnett_square

Doing a genetic cross (monohybrid = 1 gene):
Geneticists use letters to represent alleles.

• A capital letter = Dominant trait, a lowercase letter = a recessive trait.
• The same letter is used to indicate both alleles.

Examples

Flower color: P= purple, p= white

Seed color: Y= yellow, y = green

Seed shape: W = wrinkled, w = round

E-Z steps for doing genetics problems:

1. Indicate the genotype of the parents using letters

2. Determine what the possible gametes are

3. Determine the genotype and phenotype of the children after reproduction. To consider every type of offspring possible, use a Punnett Square in which all possible types of sperm are lined up vertically and all types of eggs are lined up horizontally:

4. Fill in the squares by "multiplying" the alleles from mom and dad.

Try to figure out the following genetics problems.  Use a Punnett square.  We will discuss them in class to check answers.

Genetics Problem 1: (a) A man with a widow's peak (WW) marries a woman with a continuous hairline (ww). A widow's peak is dominant over a continuous hairline. What kind of hairline will their children have?

(b) Suppose one of their children (Ww) marries someone who is also heterozygous (Ww). What type of hairline will their children have?

Genetics problem 2: A man and a woman are heterozygous for freckles. Freckles (F) are dominant over no freckles (f). What are the chances that their children will have freckles?

Genetics problem 3: A woman is homozygous dominant for short fingers (SS). She marries a man who is heterozygous for short fingers (Ss). Will any of their children have long fingers (ss)? yes / no   Could any of their grandchildren potentially have long fingers? y / n Why or why not?

Genetics problem 4: Jane and John are expecting a baby and know that they are both carriers (ie heterozygous) of cystic fibrosis (Cc). What is the probability that their child will have cystic fibrosis (cc)? What is the probability that their child will be a carrier of cystic fibrosis?

Dihybrid crosses, Mendel's 2nd Law and test crosses

As Mendel did his work, he would sometimes observe 2 traits in a plant that differed from the plant it was crossed with.  This is what is meant by "dihybrid" crossing.  Simply, when the cross is carried out, two traits are observed.
For example, Mendel carried out a cross of a pea plant producing round, yellow peas, with a plant that produced wrinkled green peas.  All of the F1 were round and yellow.  When the F1 were crossed he made the following observation:

http://www.biozoomer.com/2011/03/dihybrid-cross.html

Mendel observed a 9:3:3:1 ratio of the different phenotypes, which led him to the conclusion that the genes for the two traits are inherited independently.

Mendel's 2nd Law:  The Law of Independent Assortment

Mendel noticed that the different traits had no impact on each other, so he concluded that genes for different traits assort independently of one another in the formation of gametes.

It is interesting to note that if you consider 1 trait at a time we get the "usual" 3:1 ratio that we saw in the single hybrid cross.  Just compare the colour trait in the offspring - 12 green and 4 yellow or a 3:1 ratio.  The same is seen with seed texture, showing that the traits are inherited independently of each other.

TEST CROSSES

Often when we observe the phenotype of an organism, we don't know from the phenotype whether the individual is homozygous or heterozygous for that trait.  Carrying out a test cross can solve this dilemma.  A test cross is simply when the individual with the unknown phenotype is crossed with a homozygous recessive individual and the ratio of offspring are observed.  If all of the offspring show the dominant phenotype, then the unknown individual must have a homozygous genotype, if half the offspring are recessive, then the unknown individual must be heterozygous.

http://tipstoscore.blogspot.hu/2011/10/test-cross-mendelian-genetics.html

Other Types of Inheritance

Intermediate Inheritance/Incomplete Dominance

One parent, carrying 2 identical alleles, shows one phenotype.  The other parent, carrying two different indentical alleles, shows a different phenotype. The offspring, carrying one copy of each of the alleles, shows a third phenotype - usually intermediate between the two parental phenotypes, but clearly distinguishable from either.

http://bio4esobil2011.wordpress.com/2012/03/04/incomplete-dominance/

http://www.quora.com/If-genes-are-digital-in-replication-meaning-that-when-coupled-white-eyes-and-green-eyes-will-have-either-white-or-green-eyes-how-do-the-children-of-mixed-race-parents-have-skin-colourthat-is-intermediate

Codominance

In this case, both alleles are equally dominant and both traits will show up equally in the offspring.

http://www.biologycorner.com/bio2/genetics/notes_incomplete_dominance.html

The above picture is just an illustration, but this can be seen in real examples, such as the situation with horses and cows, where a white individual (WW) is crossed with a chestnut individual (RR) and the offspring show both.  They are called roan (RW).

http://www.buzzle.com/articles/codominance-explained-with-examples.html

And it is often seen in flowers as well:

Human blood type also shows codominant inheritance:

http://igbiologyy.blogspot.hu/2014/03/codominance.html

Blood type inheritance video

Lethal Genes

The basic idea here is that if the alleles are homozygous for a lethal gene, then they cause death.  The lethal gene may be dominant, in which case the dominant homozygote dies, or it may be recessive, in which case the recessive homozygote dies.

The following is a very typical example of a dominant lethal gene:

http://www.cas.miamioh.edu/~wilsonkg/old/gene2005/syllabus.htm

In the case of a dominant lethal gene, a 2:1 ratio is seen of the heterozygotes to the homozygous recessive.

With a recessive lethal gene, if it is simple dominant recessive inheritance, all the phenotypes will be the same, but if there is intermediate or codominance involved, then once again, a 2:1 ratio will be seen for the heterozygote to homozygote ratio, just in this case the homozygote will be homozygous dominant.

http://lifestunes.blogspot.hu/2010/12/lethal-white-syndrome.html

Polygenic Inheritance

Polygenic inheritance refers to a situation where one phenotype is controlled by 2 or more genes.

Difactorial inheritance is the term used when 2 genes control one phenotype, for example the shape of chicken combs.

http://www.bio.miami.edu/dana/250/250SS13_6.html

If in the parental generation a rose-comb (RRpp) is crossed with a pea-comb (rrPP), then the F1 are all walnut-comb (RrPp).  If the F1 are crossed then we see the following:

http://www.extension.org/pages/65363/poultry-genetics-for-small-and-backyard-flocks:-an-introduction#.VDGWBPl_to0

Or 9 walnut: 3 rose: 3 pea: 1 single.  The ratio of a dihybrid cross, just with one phenotype being dealt with, not 2!

Quantitative Inheritance

This kind of inheritance is observed when many genes are involved in producing a phenotype and the frequency of the phenotypes usually exhibit a normal continuous distribution (aka bell curve).  Examples of this include skin colour, hair colour, eye colour, height and weight in humans.

http://bioserv.fiu.edu/~walterm/FallSpring/inheritance/Fall03_lecturea.htm

Pleiotropy

In this situation, 1 gene is responsible for more than 1 characteristic or trait.  

A good example of this in humans is sickle-cell anemia, where one gene causes the cell to have a sickle-shape, which results in many different symptoms, including anemia, skull deformation and enlarged liver and resistance to malaria.

Phenylketonuria (PKU) is another human example caused by a lack of phenylalanine hydroxylase (an enzyme that converts phenylalanine to valine), which results in mental retardation and reduced pigmentation.