LESSON 5 – Vision: Understanding light perception

 Authors: Joshua Heafield (FBMH), Charlotte Blackburn (FBMH), Sanjai Patel (FBMH), Andreas Prokop (FBMH)

Get this lesson & adjunct materials:

  •  L5-KS5-Vision-LessonPackage.zip [LINK]
  •  L5-Spanish-Vision.zip [LINK]
  •  L5-Turkish-GÖRME_DERSI.zip [LINK]

Curriculum relevance: 

  • Sensory receptors, how photoreceptors are used to detect visual stimuli
  • The anatomy of the eye and the structure of the retina
  • The inheritance pattern of colour blindness

Summary: This lesson is a synoptic resource suitable for Biology A Level (KS5), ideal for end-of-term revision lessons. The lesson starts by recalling fundamental knowledge about our senses, with emphasis on visual information obtained from our environment. It then focuses on light and light perception, starting with the physical nature of visible light as a small fraction of the wide spectrum of electromagnetic waves, which are introduced via a brief interactive PowerPoint animation. The question is posed as to why we see only this narrow fraction of the spectrum, providing our evolutionary origins in the oceans as a likely explanation because visible light is little absorbed by water and reaches fairly deep down. The lesson then explains the principle of seeing an object by reflection (also introducing to the subtractive colour model), and introduces to eye anatomy by comparing a lens eye to a camera. In a comparative approach, lens eyes are compared to compound eyes of the fruit fly Drosophila (as typically found in arthropods, such as insects, crustaceans, arachnoids). For both eye types, the idea of perception in the eye and conduction to the brain for information processing is explained. The next topic is phototransduction (i.e. the transformation of light into nerve impulses). It starts with the stereo-isomerisation of retinal embedded in opsins and the subsequent triggering of a signalling pathway which eventually elicits the nerve impulse sent to the brain. This process is explored using an animation which the pupils interpret step-by-step. A micro experiment uses Drosophila to explore the idea of positive phototaxis (movement towards light) as a measure to explore what colours of visible light an animal can sense. Then sevenless mutantf flies and Ishara plate tests are used to introduce to the idea of colour blindness. The underlying concepts of cone cells with three different colour opsins are introduced together with the additive colour model and the idea of mutations that affect opsin genes. Finally, red-green blindness is used as an example of X-chromosomal inheritance, also reminding of the uses of Punnett squares.

Support information for this lesson:

  1. What are our senses? -> GO
  2.  What is light? -> GO
  3. What is in a wavelength? -> GO
  4. Why do we see only a tiny spectrum? -> GO
  5. How do we use light to detect objects? -> GO
  6. The eyes of flies -> GO
  7. How can light induce a nerve impulse? -> GO
  8. Where does visual processing occur in the human brain? -> GO
  9. What light information do we process? -> GO
  10. What is the colour we see? -> GO
  11. How can we measure colour vision? -> GO
  12. How do we detect colour? -> GO
  13. What is colour blindness? -> GO
  14. The inheritance of colour blindness -> GO
Please, help us further improve this resource by sending any comments, corrections, suggestions to Andreas.Prokop@manchester.ac.uk.

1. What are our senses?

The most commonly known senses are smell, taste, touch, hearing and sight. Smell and taste require specialised cells in our nose (olfactory receptor neurons in nasal epithelium) and mouth (taste receptors in taste buds) which detect certain chemicals that define the aromas and flavours of our environment, food and drinks. Touch requires mechano-receptive neurons in our skin and toungue which detect mechanical forces such as poking or shearing of the skin. Hearing also requires mechano-sensitive cells in our inner ear which sense the vibration of the interstitial fluid induced by sound waves (essentially pressure waves of the air). Vision requires photo-receptor cells in the retina of our eyes which detect light waves. More information about senses and the sensory nervous system is given in our Neuro Resource. Here we will focus on vision.

2. What is light?

Light is a form of energy, known as electromagnetic (EM) radiation. This electromagnetic energy is carried by particles called photons which have no mass. Photons are released when electrons lose energy, and the more energy they lose, the shorter the wavelength of the emitted EM radiation will be (details in Fig. 2).

Figure 2. Photons can be generated by electrons. A) Two atoms with the nucleus at their centre (blue), and electrons (little black dots) orbiting around the nucleus (dashed black lines). When electrons move to a higher orbit (red arrows) they gain energy (red dots). B) When electrons fall back to a lower orbit (black arrows), they release energy in form of photons. The further an electron falls, the higher the energy that the photon carries and the shorter the wavelength of EM radiation. Accordingly, the electron in the left atom falls further and releases higher energy photons.

3. What is in a wavelength?

Did you know that the wavelength of EM radiation can be longer than a football field but also much smaller than the size of an atom (Fig. 3, bottom)? Depending on where you are on this length scale, electromagnetic waves have very different properties and can be used for different purposes. Did you know that we use different wavelengths of electromagnetic waves to heat our food in the microwave, to listen to the radio, to perform an X-ray and kill cancer cells in radiotherapy? What we call ‘visible light’ is just a tiny fraction of this wide electromagnetic spectrum (Fig. 3, top).

Figure 3. The electromagnetic wave spectrum. There is a massive length difference in the electromagnetic wave spectrum (bottom), ranging from football field dimensions (left) to sub-atomic scales (right). These different wavelengths have different names (text boxes above the wave spectrum) with different uses (symbols at the top: radio, microwave, remote control, visible light, UV sterilisation, X-ray, cancer therapy).

4. Why do we see only a tiny spectrum of the electromagnetic wave scale?

The reason scientists believe that we detect this small and specific range of electromagnetic radiation is due to our evolutionary past. Life started under water in the oceans, and visible light is absorbed poorly by water (Fig. 4). Imagine diving in a pool on a hot day: you cannot feel the heat of the sun (infrared radiation is absorbed), you don’t get a sun burn (UV radiation is absorbed), but you can still see! Visible light reaches up to 1000 m deep [LINK], and organisms detecting this light can therefore see their environment even in deep waters. When life emerged on land, the preferred wavelength for vision obviously has not changed – likely due to the fact that the retinal/opsin system which had developed for vision detection (Section 7) is limited in its capacity to adapt to further wavelengths and would have to be replaced by a completely new system of light detection.
Figure 4. The absorption spectrum of water (left) shows how visible light is poorly absorbed, as compared to ultraviolet (UV) and infrared light (IR), allowing it to travel deeper than other types of electromagnetic radiation.
Some thoughts about UV and IR
(ultra-violent & infra-dead – according to the Hitchhiker’s Guide)
Interestingly, short-lived animals seem more likely to use ultraviolet (UV) than long-lived animals; the human lens even filters UV out, so that it does not reach the retina. The reason for this may lie in the fact that UV light is damaging to cells and DNA and causes cancer (see our L4-Enzymes resource). It might therefore be that the benefits of seeing UV have to be balanced against the risks of cancer – and this risk is lower in short-lived animals because they die before there was sufficient time for cancerous mutations to accumulate and cause harm. Also note that some snakes pick up infrared (IR) in a vision-like manner (a lensless “eye” called pit organ) which generates an image of very low resolution. This is used for example by snakes in dark caves to detect the body heat of bats which they hunt (see video).

5. How do we use light to detect objects?

Figure 5. Light and object detection and processing in a digital camera (left) compared to a human eye (right).Numbers in squares are explained in the text.

A digital camera (Fig 5A) captures an object and stores it as a digital file on a memory card so that we can view it on a computer or print it. The following steps are involved:

  • Rays of light reflect from an object (a pencil) and enter the camera through the lens (1) which focuses it onto the camera chip (2).
  • On the chip (2), light sensitive cells convert the light energy into an electrical signal.
  • The electrical signal is sent along cable connections (3) to a processing unit.

Our visual system (Fig. 5B) uses analogous structures and processes to generate an image impression:

  • The lens (1) focuses light onto the retina (2).
  • Photoreceptor cells in the retina (2) convert the light energy into nerve impulses.
  • The nerve impulses are sent through the optic nerve (3) towards the brain (black arrow). Neurones send information via electrical impulses known as action potentials (see more details here)

Note that in Fig. 5 the yellowish light from the lamp reflects back in a different colour from the tip and the end of the pen. To understand this, we need to briefly discuss the two colour systems which were developed for the human colour perception:

  • The additive colour system (Fig. 6A; and see Section 10) refers to light that enters our eyes: if you shine red light on a white wall it will reflect and enter our eyes giving the impression of red colour. If you project a green light over the red, the mixed reflected light will appear yellow, and it will turn white if you add also blue light (if all are at the right amount and wavelength as shown in Fig. 6A; see also this link).
  • The subtractive colour system (Fig. 6B) works the other way around and refers to light-absorbing pigments: if you shine white light (e.g. sunlight containing all wavelengths) at an object having yellow colour pigment, it absorbs all wavelengths except yellow, which is therefore reflected, or passes through in the case of coloured glass. If this reflected/non-absorbed yellow enters our eyes, the object/glass appears therefore yellow. If you add a magenta pigment (which absorbs all wavelength except magenta), the mixed yellow and magenta pigments jointly absorb a wider spectrum of light (i.e. subtract it from the white light) so that the reflected/non-absorbed light appears red. If all wavelengths are absorbed by the pigments of an object, no light is reflected/passes through and the object/glass appears black.

Taken together, the colour of an object depends on its pigments which absorb/subtract certain wavelengths from the visible light (following the rules of the subtractive colour system), thus turning white light into coloured light which is defined by the reflected/non-absorbed wavelengths that enters our eyes (and follows therefore the rules of the additive colour system).

Figure 6. The (A) additive and (B) subtractive colour models. See text for details.

6. The eyes of flies

Not all eyes are the same – insects such as the fruit fly Drosophila have eyes known as compound eyes (see Fig 7). They are made of tiny individual units called ommatidia. Each ommatidium has its own lens and 8 photoreceptive cells, pointing in one direction and seeing one fraction of the environment. All ommatidia together jointly provide information of the entire surrounding, a bit like pixels on a camera chip or pieces of a mosaic. When the composed information from all ommatidia comes together at the optic stalk/nerve (no. 3 in Fig. 8), the information content is fundamentally the same as it is at the level of the optic nerve of the human eye.

Figure 7. A) A diagrammatic view of a compound eye, like those found in Drosophila. Note how compared to the human eye in B) the steps in detecting light are the same, it is simply the anatomy of the eye and the structure of cells that differ (note also that 3, the nerve connection, is referred to as optic stalk in flies). C) A magnified view of an ommatidium, the single units that make up compound eyes; each ommatidium carries out all three major steps of the vision process: focussing the light, activation of photoreceptors and the sending of action potentials.

Figure 8. Left: A common misconception about fly vision is that flies see many copies of the same object through each lens of each ommatidium. Right: Instead, each ommatidium points at a different point in space, and a composite image is put together at the level of the optic nerve.

7. How can light induce an electrical message?

The process of transforming light energy into a nerve impulse is referred to as photo-transduction. The first step is that light induces a shape change of an organic molecule, the photopigment retinal. Retinal is a product of vitamin A (= retinol; see Fig. 9A). When retinal absorbs light, it performs an internal rotation that changes its arrangement in space (referred to as stereo-isomerisation; see Fig. 9B). Retinal is embedded in specific proteins called opsins, found in the cell membrane of the outer segment of photoreceptor cells in the retina (Fig. 9C). Note that there are 5 major opsins in the human visual system (Tab. 1), and all of them are classified as G-protein-coupled-receptor proteins.

Figure 9. A) The oxidation of 11-cis retinol – or vitamin A – to 11-cis retinal, the photopigment which is used in light detection. B) When retinal absorbs light, it causes a stereo-isomerisation event, converting it to all-trans retinal which is a “straighter” molecule. C) Zooming in on the location of retinal within the human eye: 1– Cornea – involved in focussing light; 2 – aqueous humour – maintains eye pressure; 3 – iris, the coloured section of the eye: by changing size it allows more or less light into the eye; 4 – lens: fine-focussing of light, allowing focussing of images at different distances; 5 – vitreous humour: a clear fluid which forms the main body of the eye; 6 – retina, the light sensitive cell layer; 7 – optic nerve; 8a – outer segment of rod cell containing the retinal and arranged into ‘disks’ to increase its membrane surface area harbouring the retinal; 8b – inner segment of rod cell, similar to the cell body of a nerve cell; 9 – extracellular space (white); 10 – plasma membrane; 11 – intracellular space (orange); 12 – opsin protein (opsin 2 / rhodopsin in the case of rods; see Tab. 1) holding retinal and transmitting the stereo-isomerisation of retinal into a cellular response; 13 – retinal located in the middle of the opsin protein.

Table 1. A list of human opsin genes encoding for proteins which associate with retinal to act in light reception. They are found in different locations/cell types in our eyes, absorb different wavelengths of light and are involved in retrieving different types of visual information (see also Fig. 12).

G-protein-coupled-receptor proteins usually receive signals at the cell surface and translate them into a chemical response inside the cell. For this, they activate a so-called signalling pathway/cascade which is essentially a chain of enzymatic reactions. This is also the case when an opsin/retinal complex detects light and, in this case, the signalling cascade eventually triggers specific ion channel proteins to open – thus eliciting a nerve impulse (action potential).  How this works is explained in Fig. 10 for the phototransduction cascade of the fruit fly Drosophila, which is a bit simpler than that of humans but clearly illustrates the fundamental concept.

Figure 10. Schematic of the phototransduction process in Drosophila. 1) Light waves hit 11-cis retinal (yellow). This causes a conformational change into all-trans retinal which, in turn, induces a conformational change in the rhodopsin protein (brown), which leads to the detachment and activation of the associated G-protein (green). 2) The smaller subunit of the G-protein, known as the γ-subunit, then binds to the phospholipase C (PLC) enzyme (black). 3) The enzyme then breaks down PIP2 (light blue), a phospholipid found in the plasma membrane into IP3 and DAG which, in turn, (4) cause a channel protein (green) to open. The opening lets a current in (yellow arrow), which represents an electrical message that can travel down the axon. In larger insects these axons are much longer than in Drosophila and the electrical signal has to be propagated actively via a nerve impulse (or action potential; see explanations in our L3-Neuron Resource). The phototransduction cascade shown here for Drosophila, deviates from that in higher animals or humans, but the principle is the same (see video1 and video2).

Did you know that ..
.. the phototransduction cascade in flies was actually one of the first complex biochemical pathways that was “fully” understood? Photreceptors and opsins are great for this kind of research, as they are densely packed and contain high concentrations of the proteins involved in the phototransduction process. These proteins can therefore be easily isolated in amounts suitable for experimentation.

8. Where does visual processing occur in the human brain?

 In humans, visual processing already begins in the retina which has layers of different neurons, all connected to each other into a complex network (Fig. 11). The nerve cells that form the optic nerve leading towards the brain, are called ganglion cells (green in Fig. 11).  Spatial and motion information is provided by the sum of photoreceptor cells across the retina (see also Fig. 5) which measure spatial differences in contrast or colour (e.g. patterns on a football) without distinguishing whether the conditions are dim or bright (although the darker it becomes the less we use colour vision). The nerve cell networks of the retina perform image processing which enhances the contrast (differences) within the image we see. The brightness of light is measured by photosensitive ganglion cells, specialised ganglion cells which contain melanopsin (Tab. 1, Fig. 12).
Figure 11. Visual processing in the human brain, with a magnified section of the retina to show the basic circuitry. Note that photoreceptor cells (orange) are located at the far end of the retina, pass on the signal to other layers of retinal cells (pink) and, eventually, to ganglion cells (green). These cellular layers provide a filter that enhances the image contrast. Also note that some ganglion cells are photosensitive themselves (not shown, but see Tab. 1; Fig. 12). Axonal processes of the ganglion cells pass this  information in form of nerve impulses through the optic nerve to the brain: first to the lateral geniculate nucleus (LGN), and from there to the visual cortex (VC) in the occipital lobe.

9. What light information do we process?

As explained in Section 8, information from our photoreceptor cells allows us to see spatial differences in colour and contrast which enables us to extract a great deal of information from the light entering our eyes, with shape, location, and movement being the most prominent. This information can be further refined: for example, research suggests we have an area in our brain purely for the recognition of faces! Within the retina, there are different types of photoreceptor cells gathering different kinds of information: rod cells are responsible for night vision, in that they are very sensitive to light but do not distinguish colour (black-and-white vision); cone cells are less sensitive to light but detect colour.

Figure 12. Various types of light information we obtain form the environment. A) Shape and location; B) Movement (right: detected by following an object with our eye balls; left: detected by keeping our eyeballs still but the image of the object moves across our retina); C) Brightness (bulb: bright; candle: dim); D) Colour (image shows a prism splitting white light into the different wave components of the rainbow). E) The major light sensitive cells: see Fig. 11 for their position in the retina and Tab. 1 for the opsins they contain; the abbreviations at the bottom of A-D (Co, Ro, GC; as explained in panel E) indicate what types of photosensitive cells are required to obtain the different types of visual information.

10. What is the colour we see?

Remember, that different wavelengths in the electromagnetic spectrum correspond to different types of EM radiation (Fig. 3). This can be further refined, in that different wavelengths within the visible light spectrum correspond to different colours. Colours are just our own way of labelling different wavelengths we can distinguish (Fig. 13A). White light contains all wavelengths, hence colours, and is therefore not classified as a colour!

Figure 13. A) The colours of the rainbow that humans perceive correspond to the different wavelengths of light. The shortest wavelength of light we can detect is (~400nm) is usually perceived as blue, and the longest wavelength of light (~700nm) usually as red. B) When you superimpose  red and green light, you get yellow – yellow wavelength falls in between red and green, so by mixing the two, the effective ‘average’ is taken and that is the wavelength you detect. C) In order to make orange, the proportions of red and green need to be altered to more red and/or less green (in this case less/ darker green). Note how, by reducing green wavelengths, the white overlap has also disappeared.

11. How can we measure colour vision?

Measuring colour vision and interpreting the results is a tricky task, because colour is subjective. It is impossible to know whether other people experience the world like you do! Certainly you will have had discussions with others, for example as to whether a flower or dress you see is pink or purple? This is why the caption for Fig. 13 stays vague by saying that a certain wavelength is usually “perceived” as a certain colour. Thinking a step further, would a chimpanzee see the colour in the same way as you do, let alone a honey bee? You can’t ask them! However, there are clever experimental means to assess colour vision and what wavelengths can be detected by a human individual, or even an animal. For example, many animals show negative/positive phototaxis, meaning they are repelled by/attracted to light: just think of the swarms of insects around a street lantern on a warm summer evening. They are attracted by light (referred to as positive phototaxis, see Fig. 15). Other animals, such as the flat worm Planaria avoid light (referred to as negative phototaxis; see Fig. 14). Drosophila displays positive phototaxis, and we can use this to perform experiments: we predict that flies will show phototactic behaviour only if they are able to detect a certain wavelength of light, and that they will show no behaviour if the used wavelength cannot be detected.

Figure 14. The negative phototactic behaviour displayed by Planaria in response to light. To detect light, they use simple photosensitive organs referred to as eye spots (the two dark spots seen in the left image).

To carry out phototaxis experiments with Drosophila you can use simple experiments for example with light emitting diodes (LED) of specific wavelength; if the fruit flies can detect this wavelength they are expected to accumulate near the light source (Fig. 15 and video below).

Figure 15. A simple experiment showing positive phototaxis of Drosophila to UV light. Flies are in a container with UV-emitting LEDs on both ends. A) Before LEDs are switched on, flies are evenly distributed. When the right LED is switched on flies move to the right (B), when the left is switched on they move to the left (C).

Experiments like this have shown that Drosophila possesses colour vision, as it shows positive phototaxis towards light sources that have their maximal emission at wavelengths of 500nm (‘green’ light), 400nm (‘blue’ light) and 350nm (UV light). But, unlike humans, flies cannot detect longer wavelengths of light, such as ‘red’ light (see more detail further down).

But why fly towards light?
Why exactly flies seek the light remains a task to be resolved, but some theories have been put forward. Night-active insects seem to aim for the moon as a reliable navigation cue. Since they are so far away, they can keep flying towards it continuously, thus maintain a straight path. A street light confuses this behaviour. Artificial light might confuse the day-night cycle of day-active insects, and if they wake up at night they seek the light because thy need their visual senses to navigate. See also this Science ABC blog about this topic.

Drosophila is famous for its many genetic mutations, the study of which has told us much about biology (see Why fly?). In one of these mutations, called sevenless, the mutant flies show no movement towards UV light, but are still attracted to green and blue. This shows that genes are involved in colour vision and leads over to the problem of colour blindness, a phenomenon likewise known in humans. To assess human colour vision, one can use the Ishara Plate test (Fig. 16). Can you spot the number in the left field?

Figure 16. The Ishara plate test. In the example on the left, only people with normal red-green vision will spot the number 57 that is outlined on the right. If an individual cannot see the number, they are colour blind for that particular colour range.

12. How do we detect colour?

Figure 17. (A) Red, green and blue cones are distributed throughout the retina. (B) The absorption peaks along the EM spectrum of the red, green and blue cone types and that of isolated retinal (on the right).

As explained before, colour vision is mediated by cone cells throughout our retina (Fig. 17A). There are three different types of cone cells which detect different wavelengths of light. As is shown in Figure 17B, retinal in isolation requires high energy UV light to undergo isomerisation (see Fig. 9). However, when retinal is incorporated into an opsin, its isomerisation is made easier and light of lower energy (i.e. longer wavelength) is sufficient to trigger isomerisation, a bit like how an enzyme lowers the activation energy needed for a chemical reaction to take place. Three different opsins are depicted in Figure 18, which all embed retinal in a distinct way and therefore shift its isomerisation point to different wavelength requirements. Only one opsin type is found in any one cone cell, explaining why there are three different types of cone cells. When deciphering the wavelength of light entering our eyes, we integrate the activation of these cone types. For example, if light with a wavelength of 600nm entered our eyes it would activate both red and green cones: Red + Green = Yellow!

Figure 18. The blue, green and red opsins, top to bottom respectively, all interact with 11-cis retinal in a slightly different way which changes what wavelength of light that will cause it to become all-trans retinal.

Drosophila also uses rhodopsin, as shown in the animation of Fig. 10. However, the opsin genes in flies are slightly different, and cause retinal to absorb different wavelengths of light (see Fig 19A), thus explaining why flies do not move towards red light (long wavelength) but, unlike humans, can detect UV (short wavelength).

Figure 19. – A comparison between the rhodopsins of A) Drosophila and B) humans.

13. What is colour blindness?

There are different forms of inherited colour blindness: monochromacy (total colour blindness), anomalous trichomacy (three opsins exist but display a mutational shift in their wavelength specificty) and dichromacy (absence of one functional opsin or cone type; see Fig. 20). Causes may lie in defects of the opsin genes, but they may also affect the cone cells and the way the cone cells work. For example, the sevenless mutation in Drosophila (Section 11) causes UV-specific colour blindness due to the fact that the specific photoreceptor cells containing UV-sensitive rhodopsin  (the 7th photoreceptor cell in ommatidia) is not formed during development (hence the name sevenless). In the following we focus on human dichromacy.

In human dichromacy, the respective wavelength of light is no longer accurately detected because one of the opsin genes makes non-functional or no protein. For example, in deuteranopia, there are no functional green cones (Fig. 20). If light with a wavelength of 600nm enters the eyes of an individual with deuteranopia, then only the red cones are being activated, meaning that the fine interpretation of longer wavelengths is much more difficult. Affected individuals can still see across the whole range of the visible spectrum because the remaining two opsins are still functional, but colour distinction is far less reliable and colour interpretation of the different wavelengths in the visible light spectrum is shifted (see spectra in Fig. 20 and more detailed explanations here).

Figure 20. Dichromacy (two-colour vision) as one mild form of colour blindness. Mutations in the genes encoding for red-, green- or blue-sensitive opsins cause two-colour vision referred to as protanopia, deuteranopia or tritanopia, respectively.  The respective rhodopsins are not formed or disfunctional (left) leading to loss of their contribution to the absorption spectrum (curves on the right) which leads to different ways to interpret the colour spectrum (spectra above curves on the right).

As mentioned before, colour blindness is often a genetic condition, so the children of a colour blind individual are at a statistically predictable risk of having the same condition (see next section). The most prominent form is red-green colour blindness which can be due to lacking the red-sensitive or the green-sensitive opsin.

14. The inheritance of colour blindness

Inheritance is the term used for traits or diseases which can be passed on genetically from parents to their children. For most genes, humans carry two copies, one inherited from the mother the other from the father. Colour blindness usually is the result of a gene mutation causing non-functional opsin protein; the condition does not become apparent if only one copy of the gene is affected (referred to as heterozygous condition) since it is “rescued” by the second gene copy which still makes “healthy” protein. In genetic terms, the mutated gene copy is considered a “recessive” allele because its mutant “phenotype” (i.e. the respective form of colour blindness) is masked by the “dominant” healthy allele. Only if both gene copies are mutated (referred to as homozygous condition) is there a total functional loss of the respective opsin protein, and the affected person shows the respective form of colour blindness.

You may wonder why a heterozygous individual is not partially colour blind. This can be due to two reasons: (1) the total amount of protein produced by two gene copies is so much that one copy produces enough anyway (i.e. production is above required threshold); (2) the healthy gene copy can be subject to compensatory gene regulation, i.e. the machinery of gene expression senses underproduction of protein and stimulates the healthy gene to make more protein. In some cases of genes (but not the opsin genes), such compensation does not take place and a partial (or “intermediate”) phenotype is seen, a condition referred to as haplo-insufficiency.

Tritanopia is very rare with only 1 in 10.000 people being affected. In contrast, protanopia or deuteranopia (i.e. red-green colour blindness) is far more frequent, with male individuals being particularly affected: 1 in 12 male indivicuals are affected but only 1 in 200 female individuals. Men are more likely to inherit a non-functional allele of the red- or green-sensitive opsin genes. This large bias in men being colour blind is due to the fact that the red and green opsin genes are both located on the X chromosome. Female individuals have two X chromosomes (XX), whereas male individuals have one X and one Y chromosome (XY; note that the Y chromosome contains hardly any genes, but determines the male gender). Consequently, male individuals display only two possible conditions: they are either colour blind (Xb/Y; referred to as hemizygous condition) or normal (XB/Y). In contrast, female individuals display three conditions: they can be normal (XB/XB), colour blind  (Xb/Xb; homozygous for the condition) or be heterozygous (XB/Xb).

Figure 21. A series of Punnett squares showing the statistical distribution of children from different combinations of mothers and fathers which are either colour blind (yellow), genetically normal (red) or heterozygous (orange; only mothers). As these examples show, female individuals (upper rows in all Punnett squares) can be colour blind only if the father is colour blind AND the mother is either heterozygous (E) or homozygous (F); the condition of male individuals (lower rows in all Punnett squares) is independent of their fathers (from whom they inherit the Y) but they can be colour blind when their mothers are either hetero- or homozygous (same male distribution in B,C versus E,F).

Fig. 21 shows all possible combinations of pairings between normal, colour blind and heterozygous indivicuals and their effect on their children. If you consider that it is the most common case that only one parents carries a colour blind allele (Fig. 21B,D), the distributions shown in the figure make clear why men are more affected. If a mother carries one mutant allele (i.e. is heterozygous and does not display colour-blindness herself), her sons have a 50% chance of being colour blind (Fig. 21B). Colour-blind fathers pass on the mutant allele only to their daughters who will in most cases be heterozygous and not show the condition (Fig. 21D).

To read more about colour blindness, visit this website. Note that, as beneficial as it is to our lives, there are advantages to not seeing in colour – to understand this, have a go at the Monkey Opsin games.