Humans have two complementary control systems that they can use to respond to their environment: the nervous system and the endocrine (hormonal) system.
Exocrine glands secrete chemicals to the outside, or to body cavities, usually through ducts (tubes).
Endocrine glands do not have ducts but secrete chemicals directly into the tissue fluid whence they diffuse into the blood stream. Once a hormone has diffused into the blood stream it is carried all round the body to all organs.
The steroid hormones are lipid-soluble so can easily pass through membranes by lipid diffusion. The two systems work closely together: endocrine glands are usually controlled by the nervous system, and a response to a stimulus often involves both systems.
In this reaction an amino group is removed from an amino acid to form ammonia and an organic acid. The kidneys remove urea and other toxic wastes from the blood, forming a dilute solution called urine in the process. The proximal convoluted tubule is the longest (14mm) and widest (60µm) part of the nephron. All glucose, all amino acids and 85% of mineral ions are reabsorbed by active transport from the filtrate to the tissue fluid. Small proteins are reabsorbed by pinocytosis, digested, and the amino acids diffuse into the blood. The job of the loop of Henle is to make the tissue fluid in the medulla hypertonic compared to the filtrate in the nephron. As the collecting duct passes through the hypertonic salt bath in the medulla, water leaves the filtrate by osmosis, so concentrating the urine and conserving water. The collecting ducts all join together in the pelvis of the kidney to form the ureter, which leads to the bladder. All homeostatic mechanisms use negative feedback to maintain a constant value (called the set point). So in a system controlled by negative feedback the level is never maintained perfectly, but constantly oscillates about the set point. In humans temperature homeostasis is controlled by the thermoregulatory centre in the hypothalamus.
The thermoregulatory centre is part of the autonomic nervous system, so the various responses are all involuntary. Muscles contract, raising skin hairs and trapping an insulating layer of still, warm air next to the skin. Muscles relax, lowering the skin hairs and allowing air to circulate over the skin, encouraging convection and evaporation.
Muscles contract and relax repeatedly, generating heat by friction and from metabolic reactions.
Glands secrete adrenaline and thyroxine respectively, which increase the metabolic rate in different tissues, especially the liver, so generating heat. Glucose is the transport carbohydrate in animals, and its concentration in the blood affects every cell in the body.
After a meal, glucose is absorbed from the gut into the hepatic portal vein, increasing the blood glucose concentration. The water potential of the blood must be regulated to prevent loss or gain of water from cells.
Mendel investigated simple well-defined characteristics (or traits), such as flower colour or seed shape, and he varied one trait at a time.
Mendel use an organism whose sexual reproduction he could easily control by carefully pollinating stigmas with pollen using a brush. Mendel repeated his crosses hundreds of times and applied statistical tests to his results.
A characteristic can disappear for a generation, but then reappear the following generation, looking exactly the same. The outward appearance (the phenotype) is not necessarily the same as the inherited factors (the genotype) For example the P1 red plants are not the same as the F1 red plants. A simple breeding experiment involving just a single characteristic, like Mendel’s experiment, is called a monohybrid cross. This is summarised in Mendel’s First Law, which states that individuals carry two discrete hereditary factors (alleles) controlling each characteristic.
If the offspring are a mixture of phenotypes in a 1:1 ratio, then the parent must be heterozygous.
Mendel never knew this, but we can explain in detail the relation between an individual’s genes and its appearance.
This table shows why the allele that codes for a functional protein is usually dominant over an allele that codes for a non-function protein.
The X and Y chromosomes don’t just determine sex, but also contain many other genes that have nothing to do with sex determination.
Males always inherit their X chromosome from their mothers, and always pass on their X chromosome to their daughters. Other examples of sex linkage include haemophilia, premature balding and muscular dystrophy. In most situations (and all of Mendel’s experiments) one allele is completely dominant over the other, so there are just two phenotypes.
In this case the enzyme is probably less active, so a smaller amount of enzyme will make significantly less product, and this leads to the third phenotype.
Many human genes also have lethal alleles, because many genes are so essential for life that a mutation in these genes is fatal.
An individual has two copies of each gene, so can only have two alleles of any gene, but there can be more than two alleles of a gene in a population. The cross below shows how all four blood groups can arise from a cross between a group A and a group B parent. So far we have looked at the inheritance of a single gene controlling a single characteristic. Mendel also studied the inheritance of two different characteristics at a time in pea plants, so we’ll look at one of his dihybrid crosses. The gametes have one allele of each gene, and that allele can end up with either allele of the other gene. In epistasis, two genes control a single character, but one of the genes can mask the effect of the other gene. Gene P is epistatic over gene Q because the recessive allele of gene P is a mutation that produces inactive enzyme, so there is no compound B for enzyme Q to react with. As mentioned in module 2, the whole point of meiosis and sex is to introduce genetic variation, which allows species to adapt to their environment and so to evolve. In this simple example with 2 homologous chromosomes (n=2) there are 4 possible different gametes (22).
The points at which the chromosomes actually cross over are called chiasmata (singular chiasma), and they involve large, multi-enzyme complexes that cut and join the DNA.
Crossing over means that maternal and paternal alleles can be mixed, even though they are on the same chromosome. Variation in a population can be studied by measuring the characteristic (height, eye colour, seed shape, or whatever) in a large number of different individuals and then plotting a frequency histogram.
Discontinuous characteristics are rare in humans and other animals, but are more common in plants.
Most people believed in Creationism, which considered that all life was created just as it is now.
Naturalists began systematic classification systems (especially Linnaeus 1707-1778) and noticed that groups of living things had similar characteristics and appeared to be related. European naturalists travelled more widely and discovered more fossils, which clearly showed that living things had changed over time, so were not always the same. Lamark (1809) proposed a theory that living things changed by inheriting acquired characteristics.
Charles Darwin (1859) published "On the origin of species by means of natural selection, or the preservation of favoured races in the struggle for life", which has been recognised as one of the most important books ever written. Far more offspring are generally produced than survive to maturity - they suffer from predation, disease and competition. Darwin's concluded that individuals that were better adapted to their environment compete better than the others, survive longer and reproduce more, so passing on more of their successful characteristics to the next generation. Darwin wasn't the first to suggest evolution of species, but he was the first to suggest a plausible mechanism for the evolution - natural selection, and to provide a wealth of evidence for it. Darwin used the analogy of selective breeding (or artificial selection) to explain natural selection.
In contrast, other glands are exocrine glands because they secrete chemicals through ducts.
Some endocrine glands are so complex and so specific that certain sections of the gland produce different hormones.
Most hormones are either steroids or amines, peptides, proteins, or glycoproteins synthesized from amino acids. All body functions are influenced by the endocrine glands; there are no processes that are NOT affected by hormone signaling.
According to the Biology of Kundalini, there are significant hormonal implications of a Kundalini Awakening. The hyperactivity of the HPA axis and sympathetic nervous system is perhaps one of the reasons why excessive kundalini activity can make one feel out there, that and the excessive production of opiates of course. The adrenal-cortex and the adrenal-medulla make up the adrenal gland, which sits on top of the kidneys.
They bind to a receptor on the membrane, which then activates a transporter, so substances can enter or leave the cell.
They bind to a receptor on the membrane, which then activates an enzyme in the membrane, which catalyses the production of a chemical in the cytoplasm, which affects various aspects of the cell. They diffuse to the nucleus, where the bind to a receptor, which activates protein synthesis. The effect of a hormone is determined not by the hormone itself, but by the receptor in the target cell. The body cannot store protein in the way it can store carbohydrate and fat, so it cannot keep excess amino acids.
The two kidneys have a very extensive blood supply and the whole blood supply passes through the kidneys every 5 minutes, ensuring that waste materials do not build up.
Urea is a small, uncharged molecule, so it can pass through membranes by lipid diffusion and there isn’t much the kidney can do about it.

Water would follow by osmosis, but it can’t, because the ascending limb is impermeable to water. The filtrate, now called urine, is produced continually by each kidney and drips into the bladder for storage. The importance of this cannot be over-stressed, and a great deal of the hormone system and autonomic nervous system is dedicated to homeostasis.
Negative feedback means that whenever a change occurs in a system, the change automatically causes a corrective mechanism to start, which reverses the original change and brings the system back to normal. It receives input from two sets of thermoreceptors: receptors in the hypothalamus itself monitor the temperature of the blood as it passes through the brain (the core temperature), and receptors in the skin monitor the external temperature. Less heat is carried from the core to the surface of the body, maintaining core temperature. Chemicals called pyrogens released by white blood cells raise the set point of the thermoregulatory centre causing the whole body temperature to increase by 2-3 °C. Some mammals release hormones that reduce their set point to around 5°C while they hibernate. The pancreas has glucose receptor cells, which monitor the concentration of glucose in the blood, and it also has endocrine cells (called the islets of Langerhans), which secrete the hormones glucagon, and insulin. Here we are concerned with classical genetics, which is the study of inheritance of characteristics at the whole organism level. He was a keen gardener and scientist, and studied at Vienna university, where he learnt statistics. Previous investigators had tried to study many complex traits, such as human height or intelligence. Mendel was able to explain this by supposing that each individual has two versions of each inherited factor, one received from each parent.
The two alleles segregate (or separate) during meiosis, so each gamete carries only one of the two alleles. If an individual shows the recessive trait (white flowers in the above example) then they must be homozygous recessive as it’s the only genotype that will give that phenotype. A gene was originally defined as an inherited factor that controls a characteristic, but we now know that a gene is also a length of DNA that codes for a protein. This mutated gene codes for non-functional enzyme, so the red pigment can’t be made, and the flower remains white. For example the gene for seed shape in peas (round or wrinkled) actually codes for an enzyme that synthesises starch! In a heterozygous cell, some functional protein will be made, and this is usually enough to have the desired effect. Since these are non-homologous they are called heterosomes, while the other 22 pairs are called autosomes. Note that female gametes (eggs) always contain a single X chromosome, while the male gametes (sperm) can contain a single X or a single Y chromosome. The Y chromosome is very small and seems to contain very few genes, but the X chromosome is large and contains thousands of genes for important products such as rhodopsin, blood clotting proteins and muscle proteins.
Red eyes (R) are dominant to white eyes (r) and when a red-eyed female is crossed with a white-eyed male, the offspring all have red eyes, as expected for a dominant characteristic (left cross below). But in some cases there are three phenotypes, because neither allele is dominant over the other, so the heterozygous genotype has its own phenotype.
The gene for haemoglobin Hb has two codominant alleles: HbA (the normal gene) and HbS (the mutated gene). All haemoglobin is abnormal, and molecules stick together to form chains, distorting the red blood cells into sickle shapes. If two Manx cats are crossed the litter has ratio of 2 Manx kittens to 1 normal (long-tailed) kitten. If the lethal allele is expressed early in embryo development then the fertilised egg may not develop enough to start a pregnancy, or the embryo may miscarry.
This simplification allows us to understand the basic rules of heredity, but inheritance is normally much more complicated than that.
Mendel was able to explain this ratio if the factors (genes) that control the two characteristics are inherited independently; in other words one gene does not affect the other. This gives 4 different gametes for the second generation, and 16 possible genotype outcomes. Just like we saw with the monohybrid cross, these four genotypes can be distinguished by crossing with a double recessive phenotype. The only difference is that here, we are looking at a single characteristic, but with a more complicated phenotype ratio than that found in a monohybrid cross. There are 5 main categories of skin colour (phenotypes) controlled by two genes at different loci.
The important point about a polygenic character is that it can have a number of different phenotypes, and almost any phenotypic ratio.
The second division is a bit like mitosis, but the first division is different in many important respects. Each bivalent is made up of two homologous chromosomes, which originally came from two different parents (they’re often called maternal and paternal chromosomes). While the two homologous chromosomes are joined in a bivalent, bits of one chromosome are swapped (crossed over) with the corresponding bits of the other chromosome. There is always at least one chiasma in a bivalent, but there are usually many, and it is the chiasmata that actually hold the bivalent together. Each gamete has a unique combination of genes, and any of the numerous male gametes can fertilise any of the numerous female gametes. This graph has the values of the characteristic on the X axis (grouped into bins if necessary) and the number of individuals showing that characteristic on the Y axis.
The frequency histogram is a smooth curve (usually the bell-shaped normal distribution curve).
Some examples are height, hair colour, heart rate, muscle efficiency, intelligence, growth rate, rate of photosynthesis, etc. A very similar theory was also proposed by Alfred Wallace, and Darwin and Wallace agreed to publish at the same time.
Darwin used the memorable phrases survival of the fittest, struggle for existence and natural selection.
In a population of horse-like animals there would be random genetic variation in neck length. In selective breeding, desirable characteristics are chosen by humans, and only those individuals with the best characteristics are used for breeding.
There is therefore selective pressure for species to change in response to the environmental change. These light coloured moths are well camouflaged from bird predators against the pale bark of birch trees, while rare mutant dark moths are easily picked off. Antibiotics kill bacteria, but occasionally a chance mutant appears that is resistant to that antibiotic. The most common type of heart disease is the narrowing or blockage of the coronary arteries, blood vessels that supply blood to the heart.
Chemicals secreted by the glands of this system have effects on numerous organs of multiple systems and control or regulate various mechanisms of every other body system. Blood then transports the hormones to all parts of the body, but the hormone has an affect only on target cells.
For example, the anterior lobe of the pituitary produces six different hormones that regulate vital body functions.
For example, the hypothalamus secretes hormones that stimulate other endocrine glands to release hormones. These target organs have specific receptor molecules in their cells to which the hormone binds. The renal artery carries blood to the kidney, while the renal vein carries blood, now with far lower concentrations of urea and mineral ions, away from the kidney.
The glomerulus is enclosed by the renal capsule (or Bowman’s capsule)- the first part of the nephron. In this part of the nephron over 80% of the filtrate is reabsorbed into the tissue fluid and then to the blood.
Since this is a passive process, urea diffuses down its concentration gradient until the concentrations of urea in the filtrate and blood are equal.
The loop of Henle does this by pumping sodium and chloride ions out of the filtrate into the tissue fluid. So the tissue fluid becomes more salty (hypertonic) and the filtrate becomes less salty (hypotonic). These aquaporin channels can be controlled by the hormone ADH, so allowing the amount of water in the urine to be controlled.
The bladder is an expandable bag, and when it is full, stretch receptors in the elastic walls send impulses to the medulla, which causes the sphincter muscles to relax, causing urination. Animals that maintain a fairly constant body temperature are called homeotherms, while those that have a variable body temperature are called poikilotherms. Note that some of the responses to low temperature actually generate heat (thermogenesis), while others just conserve heat.
This helps to kill bacteria (+ white blood cells work best at this temperature) and explains why you shiver even though you are hot.
In insulin-dependent diabetes (also known as type 1 or early-onset diabetes) there is a severe insulin deficiency due to autoimmune killing of b cells. It contains osmosreceptor cells, which can detect changes in the water potential of the blood passing through the brain.
It is also known as transmission genetics or Mendelian genetics, since it was pioneered by Gregor Mendel.
One allele comes from each parent, and the two alleles are found on the same position (or locus) on the homologous chromosomes.
It is the proteins that actually control phenotype in their many roles as enzymes, pumps, transporters, motors, hormones, or structural elements.
Almost any mutation in a gene will result in an inactive gene product (usually an enzyme), since there are far more ways of making an inactive protein than a working one. The functional enzyme makes lots of starch and the seeds are full and rounded, while the non-functional enzyme makes less starch so the seeds wrinkle up. In particular, enzyme reactions are not usually limited by the amount of enzyme, so a smaller amount will have little effect. In humans the sex chromosomes are homologous in females (XX) and non-homologous in males (XY), though in other species it is the other way round.
However, when the opposite cross was done (a white-eye male with a red-eyed female) all the male offspring had white eyes (right cross below).

As explained on p31, the genes for green-sensitive and red-sensitive rhodopsin are on the X chromosome, and mutations in either of these lead to colour blindness.
The red blood cells are slightly distorted, but can carry oxygen, so this condition is viable. The embryo doesn’t develop and is absorbed by the mother, so there is no evidence for its existence. If the lethal allele is expressed later in life, then we call it a genetic disease, such as muscular dystrophy or cystic fibrosis. The red blood cell antigen is coded for by the gene I (for isohaemaglutinogen), which has three alleles IA, IB and IO. Round seeds (R) are dominant to wrinkled seeds (r), and yellow seeds (Y) are dominant to green seeds (y). This is summarised in Mendel’s second law (or the law of independent assortment), what states that alleles of different genes are inherited independently. This is a little bit like dominant and recessive alleles, but epistasis applies to two genes at different loci.
In mice one gene controls the production of coat pigment, and black pigment (B) is dominant to no pigment (b).
In a certain variety of sweet pea there are two flower colours (white and purple), but the F2 ratio is 9:7.
This occurs when genes at two different loci make enzyme that can catalyse the same reaction (this can happen by gene duplication). Since they can line up in any orientation on the equator, the maternal and paternal versions of the different chromosomes can be mixed up in the final gametes. Although this is an impressively large number, there is a limit to the mixing in that genes on the same chromosome must always stay together. The chiasmata can be seen under the microscope and they can give the bivalents some strange shapes at prophase I. These histograms show that there are two major types of variation: discontinuous and continuous. In an environment where there were trees and bushes, the longer-necked animals were better adapted and so competed well compared to their shorter-necked relatives.
During the industrial revolution in the 19th century, birch woods near industrial centres became black with pollution. In an environment where the antibiotic is often present, this mutant has an enormous selective advantage since all the normal (wild type) bacteria are killed leaving the mutant cell free to reproduce and colonise the whole environment without any competition. The nervous system is one of motor control; the endocrine system controls through chemicals. Target cells are specific to the hormone; a hormone specific to a target cell can affect only that target tissue because of the presence of receptors for the hormone on the target cells. The posterior lobe produces different hormones entirely—and the pituitary gland is about the size of a pea! For example, some glands respond directly to changes in the body’s chemical environment. For example, the adrenal medulla secretes a hormone in response to sympathetic nerve impulses. These receptors are protein molecules, and they form specific hormone-receptor complexes, very much like enzyme-substrate complexes. This process of converting ammonia into urea shown above is not a single reaction, but is a summary of another cyclic pathway, called the ornithine cycle. The blood pressure in the capillaries of the glomerulus forces plasma out of the blood by ultrafiltration.
So in each pass through the kidneys half the urea is removed from the blood and half remains in the blood. The first part of the loop (the descending limb) is impermeable to ions, but some water leaves by osmosis. Since the filtrate is most concentrated at the base of the loop, the tissue fluid is also more concentrated at the base of the medulla, where it is three times more concentrated than seawater. More ADH opens the channels, so more water is conserved in the body, and more concentrated urine is produced. This is an involuntary reflex response that we can learn to control to a certain extent when we are young. Here we shall look at three more examples of homeostasis in detail: temperature, blood glucose and blood water. Negative feedback applies to electronic circuits and central heating systems as well as to biological systems. The homeotherms maintain their body temperatures at around 37°C, so are sometimes called warm-blooded animals, but in fact piokilothermic animals can also have very warm blood during the day by basking in the sun. Similarly some of the responses to heat actively cool the body down, while others just reduce heat production or transfer heat to the surface. In non insulin-dependent diabetes (also known as type 2 or late-onset diabetes) insulin is produced, but the insulin receptors in the target cells don’t work, so insulin has no effect. In response, the hypothalamus controls the sensation of thirst, and it also secretes the hormone ADH (antidiuretic hormone).
You can find out which by performing a test cross with a pure-breeding homozygous recessive.
There are techniques for separating X and Y sperm, and this is used for planned sex determination in farm animals using IVF. This surprising result was not expected for a simple dominant characteristic, but it could be explained if the gene for eye colour was located on the X chromosome.
The diagram below shows two crosses involving colour blindness, using the symbols XR for the dominant allele (normal rhodopsin, normal vision) and Xr for the recessive allele (non-functional rhodopsin, colour blind vision).
Since there is no dominance we can no longer use capital and small letters to indicate the alleles, so a more formal system is used. However these red blood cells cannot support the malaria parasite, so this phenotype confers immunity to malaria. This gets more complicated, partly because there are now two genes to consider, but also because the two genes can interact with each other. One gene controls the colour of the pigment, and black hair (B) is dominant to brown hair (b). Epistasis reduces the number of different phenotypes for the character, so instead of having 4 phenotypes for 2 genes, there will be 3 or 2. Another gene controls the dilution of the pigment in the hairs, with dense pigment (D) being dominant to dilute pigment (d).
This is explained if the production of the purple pigment is controlled by two enzymes in a pathway, coded by genes at different loci.
In this case the coloured pigment is always made unless both genes are present as homozygous recessive (ppqq), so the F2 ratio is 15:1. This is because the two sexes (a discontinuous characteristic) each have their own normal distribution of height (a continuous characteristic). This is now known to be wrong, since many experiments (and experience) have shown that acquired characteristics are not inherited, but nevertheless Lamark's theory was the first to admit that species changed, and to try to explain it. All domesticated species of animal and plant have been selectively bred like this, often for thousands of years, so that most of the animals and plants we are most familiar with are not really natural and are nothing like their wild relatives (if any exist).
In this changed environment the black moths had a selective advantage and became the most common colour, while the pale moths were easily predated and became rare. Some farmers routinely feed antibiotics to their animals to prevent infection, but this is a perfect environment for resistant bacteria to thrive.
The nervous system is very fast acting; when you sense a stimulus, it does not take you very long at all to react. When the blood glucose is elevated, the pancreas secretes insulin, and when the blood glucose falls, it secretes glucagons.
Both the capillary walls and the capsule walls are formed from a single layer of flattened cells with gaps between them, so that all molecules with a molecular mass of <70k are squeezed out of the blood to form a filtrate in the renal capsule. The body thus has a range of responses available, depending on the internal and external temperatures. ADH is stored in the pituitary gland, and its target cells are the endothelial cells of the collecting ducts of the kidney nephrons. Note that in these crosses the alleles are written in the form XR (red eyes) and Xr (white eyes) to show that they are on the X chromosome. The gene is represented by a letter, and the different alleles by superscripts to the gene letter.
The other gene controls the dilution of the pigment in the hairs, with dense pigment (D) being dominant to dilute pigment (d). This is very much like the Siamese cat example above, but with one important difference: the pigment gene (B) is epistatic over the dilution gene (D) because the recessive allele of the pigment gene is a mutation that produces no pigment at all, so there is nothing for the dilution gene to affect. So in the next generation there were more long-neck genes than short-neck genes in the population. The analogy between artificial and natural selection is a very good one, but there is one important different - Humans have a goal in mind, nature does not.
The hormone-receptor complex can affect almost any aspect of a cell’s function, including metabolism, transport, protein synthesis, cell division or cell death. Glucagon causes the liver to break down some of its glycogen store to glucose, which diffuses into the blood. These cells are unusual in that water molecules can only cross their membranes via water channels called aquaporins, rather than through the lipid bilayer.
This means that the inheritance of these genes is different for males and females, so they are called sex linked characteristics. If this continued over very many generations, then in time the average neck length would increase. The level of chemicals in the blood has to reach a certain threshold amount for the response to occur. Once the level of chemical in the blood has been achieved, the response occurs and is long lasting. The response will continue until the level of the chemical decreases enough to stop the response. This mechanism is called the negative feedback mechanism because when the level decreases, the feedback stops the effects.

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