7 Colour Blindness, PTC tasting

Dr. SAA Latheef and Dr. P. Venkatramana

Contents:

1. Introduction

2. Colour blindness

2.1. Genetics of colour blindness

2.2. Types of colour blindness

2.3. Causes of colour blindness

2.4. Incidence of colour blindness

2.5. Importance of colour blindness screening

2.6. Screening of colour blindness

2.6.1. Ishihara test

2.7. Calculation of allele frequency of colour blindness

Phenylthiocarbamide

3.1. Genetics of phenylthiocarbamide

3.2. Association of PTC taste sensitivity with other diseases

3.3. Screening of PTC taste sensitivity

3.3.1. Testing of PTC taste sensitivity

3.4. Calculation of allele frequency of PTC taste sensitivity

Summary

Learning Outcomes:

After reading this module you will:

appreciate that why anthropologists study human variation;
be familiar with definition of colour blindness, its types, causes, incidence, importance of colour blindness screening, tests used for screening and calculation of allele frequency; and
know the discovery of PTC differential taste sensitivity, genetics of PTC, association with other diseases, test used to screen PTC taste sensitivity and procedure used for calculation of allele frequencies.

Introduction

Physical anthropologists are interested in studying human population. In this process, they reconstruct the history using material evidence and biomolecules recovered from fossils. Another approach in understanding human evolution is investigating diversity in human populations (Ruvolo, 1997). Earlier anthropologists studied human diversity or variation using visible physical traits like skin, hair and eye colour, hair form, shape of face and nose (Jurmain et al, 2009). Advances in serology and molecular biology have made it possible to study blood groups and of late, biomolecules like DNA, RNA and proteins.

Human variation arises due to the operation of evolutionary forces like mutation, migration (gene flow), genetic drift and natural selection (Jurmain et al., 2009). It was observed that the percentage of variation within and between populations is 85% and 15% respectively (NIH, 2007). Study of human variation is not only important from anthropological perspective but also helpful in understanding the contribution of genetics to the human disease and long-term survival of human species (Serre and Pääbo, 2004; NIH, 2007; Meier, 2010). Human population genetics, a subfield of physical anthropology, concerned with study the allele frequency of genetic markers like blood groups and also factors responsible for allele frequencies over a period of time (Luxmi and Kapoor, 2011). Colour Blindness and PTC are the best examples of population genetics.

Colour blindness

Light rays reflected from the object pass through cornea, pupil and lens and produce upside image on the retina (Medical Encyclopedia). Retina converts light into visual signals and transmit to the brain, with the help of two photoreceptors namely rods and cones, where image is interpreted as upright (Mustafi et al., 2009). Retina has 100 million rod photo receptors that help in night vision and 6 million cone photoreceptors which are responsible for daylight, colour and spatial vision (Cideciyan et al., 2013). Normal vision is called trichromatic due to the presence of pigments of three type of cone photoreceptors (S,M,L) that are sensitive in the blue (short wave length (420nm) or S), green (middle wavelength (530nm) or M) and red (long wavelength(560nm) or L) of visible light (Deeb, 2006). Among the three cone types, L and M cones are rich in fovea (central retina) while S cone constitute 2%-7% of total cone population. Cone pigments are called opsins, they are composed of 340-370 amino acid residues. They folded into seven membrane structure and embedded in the outer segment membrane (Imamoto and Shichida, 2014).

2.1. Genetics of colour blindness

Gene for S cone opsin (OPN1SW) is located on chromosome 7q31.3-q32, whereas genes for L opsin (OPN1LW) and M opsin (OPN1MW) are located Xq28 in a head to tail tandem array. Expressions of these genes are governed by specific promoters and single upstream locus control region (Cideciyan et al., 2013). Six exons in M and L opsin genes with a total length of 12,036 bp and 14000 bp and 5 exons in S opsin gene with a total length of 1047 bp were observed (Sharpe et al., 2001; Neitz and Neitz, 2011; Tan et al., 2015). The ability to appreciate colour distinguishes humans from non-primates (Shah et al., 2013). Colour blindness or colour vision deficiency is defined as the inability or decreased ability to perceive colour differences under normal lighting conditions (Fareed et al., 2015).

2.2. Types of colour blindness:

Achromatopsia: Persons with achromatopsia are characterized by reduced visual acuity (20/200 in complete achromatopsia and 20/80 in incomplete achromatopsia), pendular nystagmus, photophobia, small central scotoma, and eccentric fixation, reduced or complete loss of colour discrimination. In complete achromatopsia, impaired colour discrimination and lack of function in all three cone types is observed. Incomplete achromatopsia afflicted persons may have partial functioning of one or more cone types and may have less severe symptoms than individuals with complete achromatopsia.

Monochromacy:

(a) Typical monochromacy: It is caused by mutations in the gene encoding the cone specific alpha and beta subunits of the cat ion channel. Typical characteristic features of persons with this type of colourblind are nystagmus (involuntary shaking of eyes), differentiation of colour by brightness, photophobia, insensitive to red light and low visual acuity (6/36 – 6/60).

(b) Blue cone monochromacy: This type of colour blindness is characterized by presence of S (blue) cone and absence of L (red) or M(green) cones, differentiation of colour by brightness, rudimentary colour vision in mesoscopic vision from rod and cone activation, photophobia, nystagmus, insensitivity to red light and low visual acuity(6/12-6/24) in the affected persons.

Dichromacy:

(a) Protanopia (Red): In the affected persons with this type of colour blindness have no L(red) cone pigment, confuse red, yellow and green, white and green, and blue and purple and reduced sensitivity to red light.
(b) Deuteranopia (Green): This type of colour blindness is characterized by absence of M(green) cone pigment, confuse red, yellow and green and white and green.
(c)Tritanopia (Blue): In persons with tritanopia, S(blue) cone pigment is absent, confuse blue with blue green and green and white with yellow.

Anamalous Trichomacy:

(a) Protanomaly: Characteristic feature of this anomaly is shifting of L (red) cone pigment absorption spectrum to shorter wavelength(blue(S) cone pigment. Afflicted persons confuse red, yellow and green, reduced sensitivity to red light, abnormal colour matching (add excess red in the colour match i.e red + green = yellow)
(b) Deuteranomaly: In this type of anomaly, shifting of M(green) conge pigment absorption spectrum to long wavelength of light(red(L) cone pigment) is observed. Further, confuse white with green and confuse red, yellow and green, match abnormal colour (add excess red in the colour match i.e red + green = yellow)
(c) Tritanomaly: in this anomaly, partial loss of S(blue) cone is observed and also failure in discriminating blues, blue-greens and greens(Kohl et al., 2004; Cole, 2007 ).

2.3. Causes of colour blindness

Aetiology of colour blindness is either genetical or acquired. The acquired (disturbance occurring on visual pathway from photoreceptors to the cortex) causes of colour blindness include damage to the eyes, nerves, brain, diseases (diabetes, glaucoma, hypertension, macular degeneration, optic atrophy, optic neuritis and sickle cell anemia), exposures to drugs (digoxin, barbiturates, anti-tuberculosis drugs, ethambutol, chloroquine and sildenafil) (Franzco et al., 2008; Xie et al., 2014; Shah et al., 2013; Marey et al., 2015).

Achromatopsia (total colour blindness) is caused by mutations in the genes expressed by three cone opsins, guanine nucleotide binding protein gene(GNAT2), phosphodiesterase 6C gene and cyclic nucleotide gated channel beta 3. Six mutations in the OPN1SW gene have been shown to cause blue-yellow colour blindness or tritan involving amino acid substitutions at different positions of the sequence ( arginine in place of glycine at position 79; proline in place of serine at position 209; serine in place of proline at position 264; arginine in place of glutamine at position 283, leucine in place of proline at position 56 etc.,). Mutation in OPN1LW causes loss of L cone function resulting in protanopia. When a partially functional hybrid pigment gene replaces normal OPN1LW it results in protanomaly. A polymorphism (Ser180Ala) of OPN1LW gene determines the severity of colour vision in persons with red-green colour blindness. Mutation in OPN1MW that causes red-green colour blindness replaces the amino acid cysteine with arginine at position 203. Mutations in OPN1MW when causes loss of M cone it is called deutaranopia and when a partially functional hybrid pigment gene replaces the normal OPN1MW it result in deutaranomaly. Genetic changes that prevent the opsin pigments (formed from both the OPN1MW and OPN1LW genes) prevent from functioning normally, whereas deletion of LCR (locus control region), result in blue cone monochromacy. Achromatopsia is a autosomal recessive inherited colour vision defect. Red-green colour vision defect and blue cone monochromacy follow X linked recessive pattern, whereas, blue-yellow colour vision defect are inherited in autosomal dominant pattern and incomplete penetrance also observed (Neitz and Neitz, 2011; Genetic home reference, National library of Medicine)

2.4 Incidence of colour blindness

Among all colour blindness types, frequency of red-green colour blindness is high. It was observed in higher proportion in males than in females. Highest frequency of this colour defect was reported from Northern Europe (1 in 12 males and 1 in 200 females). Blue cone monochromacy frequency was also found to be higher in males than in females. The incidence of blue cone monochromacy was reported as 1 in 100,000 in world populations. Blue-yellow colour blindness was observed equally in males and females. The observed incidence of blue-yellow was 1 in 10,000 populations (Genetic home reference, National library of Medicine).

2.5. Importance of colour blindness screening

Apart from using as a tool to study population variation, colour blindness is of socio-economic importance. While driving vehicles, recognition of colour of traffic lights and colour of articles of daily use in shopping is essential. Various occupations demand colour recognition and these occupations those without colour vision defects can be productive and successful. For jobs such as deck and navigational watch keeping in maritime industries; pilots in aviation industry; train drivers and workers in railways; drivers in road transport; aircraft pilots in air force; officers and deck crew in navy; traffic police, fire fighters in fire and rescue services; technicians and line men in telecommunications; and electrician, individuals with colour vision defects are not preferred. In some jobs such technicians in diagnostics, pathologists, dentists, optometrists, horticulturist, fruiterer, architect interior decorator, graphic artists and fashion designers can be at disadvantage because of colour vision defects than without it (Cole, 2007).

2.6. Screening of colour blindness

Colour blindness is not curable. Colour vision deficiency can be managed to some extent by using adaptive strategies; informed choice can be made in careers and disappointments can be avoided. This is only possible if screening is done at an early stage of life. Colour vision defect can be diagnosed by using 18 different pseudoisochromatic plate tests, 8 sorting/arrangement tests, 5 anomaloscopes and 5 lantern tests. No single test provides all the information on the colour vision defects. The ideal test should be easily available, low cost, administered quickly, interpreted easily and reliable (Cole, 2007). Four tests such as Ishihara test, Richmond HRR test, Medmont c100 test and Fransworth D15 are used for detection of colour vision defects. Among all tests, ishihara test is widely used, it requires little guidance for administration and interpretation. It was extensively used by anthropologists for detection of colour vision defects in various populations (Reddy, 1983; Bhasin and Chahal, 1996: Reddy, 2015). Therefore, ishihara test is described here.

2.6.1. Ishihara test: The author of this test is Shinobu Ishihara, who served as a surgeon in Japanese army and later was appointed as a professor of Ophthalmology in the University of Tokyo. International congress of Ophthalmology held in Hollond in the year 1929, has recommended for testing army persons. The test is available in 38 plate edition, abridged 24 plate edition and a 14 plate edition (Figure 1). Each plate has figures printed in colour dots of varying sizes against a background of dots printed in different colours. Colours are chosen to ensure that colourblind person either not read or misread. In 38 plate edition, 1-25 plates have numbers and meant for literates (Figure 2), whereas 26-38 (Figure 3) has winding lines to be used by illiterates. Plates 1-21 of 38 plate editions are used to distinguish normal from person with colour blindness. Remaining 22-38 plates are used to differentiate protanope and protanomols from deuteranope and deuteranomols (Venkatramana, 2012). This test can be used to test children as early as five years. It was recommended to use plates 1, 6, 7,10, 14 and 24 as they contain only the numbers 1, 2, 3, 4 and 5 or use of test meant for unlettered persons(Plate 10) as it contain symbols only(Cole,2007). This test detect protan and deutan with high sensitivity and specificity. Disadvantages of this test are that it cannot differentiate protan and deutan in 30%-40% of cases, fail to report severity of colour vision defect and unable to diagnose titan colour vision defects (Cole, 2007).

Procedure:

For Lettered subjects:

The ishihara plates are held at 75 cm distance from the subjects. The plates are lit by daylight fluorescent lamp (colour temperature 6500 K, colour rendering index > 90). Subject is requested to read the numbers in plates 1-25 within 3 seconds. The numerals reported by the subject is compared to the standard table of Ishihara (Table 1) for detecting colour vision defect. If subject is able to read the numerals in 17 or more plates, he/she is considered as having normal colour vision. If subject is able to read numerals in equivalent or less than 13 plates, he/she is considered as having colour vision defect. Those who read numerals as 5,2, 45 and 73 in plates 18,19,20 and 21 and read numerals more easily than those on plates 14, 10,13 and 17 were recorded as abnormal(Fareed et al., 2015) The plates 22,23,24 and 25 are used to differentiate protan and deutan types of colour vision. Cole (2007) has observed that five or more errors on numeral plates indicates red-green colour defect. Further, it was reported that failure to see the red numeral indicates protan, whereas failure to see the red-purple is considered as deutan. Colour blindness can be tested by using available oneline resources (http://www.color-blindness.com/ishihara_cvd_test/ishihara_cvd_test.html).

For unlettered persons:

The unlettered person is requested to trace the winding lines between two X’s on the plate with soft brush within ten seconds. Detection of normal and colour vision defect can done based on the tracing of wind line by the following criteria described by Bhasin and Chahal,1996 and Venkatramana, 2012 (Table 2).

2.7. Calculation of allele frequency of colour blindness:

Shal et al (2013) has found the prevalence of 48 colour blind in a total 534 male and 12 colour blind in a total of 590 women. There were 12 colour blind women in total of 590 women. The allele frequency can be calculated in the following manner.

Men

colour blind persons designated in small c= 48

normal vision persons designated as capital C =486

Allele frequency of colour blindness

c = number of colour blind men/total men = 48/534= 0.089

c=0.089 and

Allele frequency of normal vision (C ) = C=1-c= 1-0.089 = 0.911

Women

Colour blind women(c) = 12

Normal vision women(C) = 578

Allele frequency of colour blindness (c ) = number of colour blind women/total women

= 12/590= 0.02=square root of 0.02 =0.14

Allele frequency of normal vision (C ) = 1-c= 1-0.14=0.86

Combine allele frequency is calculated by using the following formula

Colourblindness allele frequency(c )= 1/3 X c (men) + 2/3 X c(women)(Fareed et al., 2015)

= 1/3 X 0.089 + 2/3 X 0.14 = 0.33 X 0.089 + 0.66 X 0.14= 0.029 + 0.092 =0.121 Note: 1/3=0.33 and 2/3=0.66

Combined allele frequency for normal vision (C )= 1/3 x C (men) + 2/3 x C(women)

= 0.33 X 0.911 + 0.66 X 0.86 = 0.300 + 0.56=0.86

Phenylthiocarbamide

After blood group, the widely studied genetic marker in human populations is sensitivity to Phenylthiocarbamide (PTC). In predawn age of molecular biology, PTC was used for settling paternity disputes and also considered as honorary blood group (Guo and Reed, 2001). PTC was synthesized by Arthur Fox initially for making dye in 1931(Newcomb et al., 2012). Dimorphism in the taste sensitivity to PTC in human was discovered accidentally. Fox was pouring the chemical in the bottle to store it, accidentally co-worker came into exposure to the particles of chemical, who complained bitter taste of the chemical while Fox found the chemical tasteless (Wooding, 2006). This observation triggered many studies on the trait of taste sensitivity to PTC in different human populations.

Human sense of taste is of five types namely bitter, sweet, sour, salt and umami (taste in response to glutamate) (Fareed et al., 2012). Taste sensitivity to PTC, is a phenotype which has genetic, epidemiological, evolutionary interest and also correlated with food preference (tasting naturally occurring bitter substances and avoiding), nutritional, survival and susceptibility to diseases (Wooding et al., 2004). L-5-vinyl-2-thio-oxazolidone, a natural analog to PTC, was discovered in the cabbage and rapeseed and later identified as a causative agent for goiter (Wooding, 2006). Vegetables (Cabbage, cauliflower etc.) belonging to the Brassicaceau family contain higher levels of glucosinolates which when breakdown gives rise to isothiocynates. These isothicynates were found to interfere with uptake of iodine by the thyroid gland (Tepper, 2008). Tastes avoid vegetables because of bitter taste which deprive them of trace elements. It was observed that vegetables like broccoli, Brussels sprouts and cauliflower taste bitter to tasters and because of this reason they avoid consuming it which also deprive them of isothiocynates which are abundant in these vegetables and these were proven to have anti-inflammatory and anticancer properties. Because of bitter taste of the medicine, they are avoided by children that may land them into the health problems (Floriano et al., 2006). Selective disadvantage among homozygous tasters in third and fourth decade of life due to hyperthyroidism was proposed (Koertvelyessy et al., 1982).

Taste serves as both warning signal (presence of toxins) and attractant. Taster were shown to be ectomorphic, whereas, nontasters as endomorphic (Tepper, 2008). Nontaster were found to have low whereas, taster had high density of fungi form papillae on the anterior surface of tongue (Tepper, 2008). Up to puberty, no difference in taste sensitivity was found in both sexes, after puberty, comparatively more males were found to nontaster whereas, more female were taster (Tepper, 2008). Lower threshold sensitivity to PTC was observed among women than in men (Hussain et al., 2014).In one study it was observed that 15% of subjects who were non-tasters to PTC at sea level tested tasters at high altitude. It was hypothesized that high altitude hypoxia changes the hormonal profile and alter the sensitivity to the taste of PTC resulting in some of the individuals shifting to lower PTC sensitivity (Singh et al., 2000). Identical twins were found to be similar in the perception of bitter compounds (Newcomb et al., 2012). Sensitivity to PTC decreases with age. Smoking before testing, mental status, ingestion of chemicals, radiation, metabolic disturbances, menstrual cycle, ear infection and head injury were reported to cause variability in the sensitivity to PTC (Whissell-Buechy, 1990; Guo and Reed, 2001).

Another chemical used to test the taste sensitivity was 6-n-propylthiouracil due to lack of sulphur odour of PTC and its usage as a medication in Graves’s disease (Guo and Reed, 2001). The bitter sensitivity of PTC and 6-n-propylthiouracil was found to be due to the presence of thiocynate moiety (N-C=S). PTC was found to bind more strongly and reported lower threshold sensitivity than 6-n-propylthiouracil (Tepper, 2008). With respect to tasting PTC, humans are classified as tasters and non-tasters. Generally, individuals with two recessive alleles (tt) are termed as nontasters and people with one dominant allele (Tt) or two dominant alleles (TT) are considered as tasters for PTC. Incomplete dominance, two locus and polygenic models are reported with regard to the inheritances of tasting sensitivity to PTC (Malini et al.,2007).

Inability to taste PTC was suggested due to the failure to activate G protein (Floriao et al., 2006). Smokers, coffee or tea drinkers tend to be nontasters due to the presence of bitter compounds which compete for the PTC (Floriano et al., 2006). R.A. Fisher proposed the hypothesis that balanced natural selection maintains the taster and non-taster alleles in human populations (Fisher et al., 1939) and further, evidence of biomodal distribution of taster and non-taster alleles was found in the chimpanzee and orangutan (Fisher et al., 1939). Wooding et al., (2004) had observed that PTC alleles conferring non-taster alleles evolved separately in humans and apes. In Brazilian Indians, no nontasters were observed (Guo and Reed, 2001). Frequency of non-tasters were found to 25-57% in Caucasians and from 24.5% – 71.5% in Indian populations (Fareed et al., 2012; Aimba et al., 2010; Hussain et al., 2014), whereas, the frequency of tasters ranged from 10%-98% (Pal et al., 2004) in world populations. In India, high frequency of tasters were observed in island populations, north and south India, whereas, low frequency of tasters was found in central India and tribal populations (Hussain et al., 2014). In terms of proportions, the frequency of taster population in India was reported to be 42%- 66% (Hussain et al., 2014).

3.1. Genetics of phenylthiocarbamide

Bitter sense is experienced due to the presence of bitter receptors located on the surface of the taste cells of tongue (Hussain et al., 2013). Binding of bitter substances to these receptors initiates neural signaling via activation of G proteins (Floriano et al., 2006). Receptor for bitter taste response is encoded by the gene TSA2R38 belong to the family of bitter taste receptor genes consists of 25 members. These members have 25%-89% of identical amino acid sequence (Hussain et al., 2013). The major locus of TSA2R38 gene is on the chromosome 7q35-q36 and secondary locus is identified on chromosomes 16p (Drayna,2005) . It has a single coding exon of 1002 base pair length that encodes 7-transmembrane domain G protein coupled receptor 333 amino acids long. TAS2R38 receptor do not recognize compound that lack thiourea moiety (Tepper, 2008). Taster and non-taster alleles of TAS2R38 gene differ at three locations, encoding amino acid differences at position 49 within the first intracellular domain, position 262 within the six transmembrane domain and position 296 within the seventh transmembrane domain. These differences can observed at two haplotypes(set of single nucleotide polymorphism closely linked on chromosome are inherited together). The taster or PAV haplotype encode a proline, an alanine and a valine, whereas, the non-taster or AVI haplotype encode an alanine, a valine and an isoleucine, at these three positions (Drayna, 2005). Frequency of AVI and PAV were found to be 47% and 47% in European populations and 30% and 70% in Asian populations (Kim and Drayna, 2005). AVI haplotype was not observed in South West native Americans, whereas, other rare haplotypes such as PVI and AAI haplotypes were found only in sub-saharan African ancestry (Kim and Drayna, 2005).

3.2. Association of PTC taste sensitivity with other diseases

Higher proportion of adenomatous goiter, nodular goiter, congenital athyreotic cretinism, dental cariers, Cystic fibrosis, schizophrenia, idiopathic and symptomatic epilepsy was observed in nontasters to PTC (Koertvelyessy et al., 1982; Hussain et al., 2014; Manlapas et al., 1965; Moberg et al., 2007; Sharma, 2004; Pal et al., 2004). Goitre, diabetes, mucoviscidosis, duodenal and gastric ulcers were shown to be associated with ability to taste PTC (Terry, 1950; Saldanha, 1956; Manlapas et al., 1965; Pal et al., 2004).

3.3. Screening of PTC taste sensitivity

Subjects are screened for PTC taste sensitivity using threshold and suprathreshold methods. In the former method, the lowest concentration at which subject able to taste PTC is detected, whereas in the later methods employ rating scales to assess the taste intensity at higher concentrations. The limitations of threshold methods are they fail to detect taste intensity for higher concentrations of PTC, cannot distinguish medium tasters from supertasters and nontaster perceive higher concentrations of PTC as intense as perceived by tasters. In suprathreshold methods, multiple samples were used to detect taste intensity over concentrations or single samples for screening as point estimate of taste intensity or ratings of PTC/ 6-n-propylthiouracil were compared to other stimulants like sodium chloride or audible tones. Disadvantages of suprathreshold methods were found to be varying in sensitivity in detecting the taste intensity (Tepper, 2008). The most widely used method for screening PTC taste sensitivity is of threshold method reported by Harris and Kalmus (1949).

3.3.1. Testing of PTC taste sensitivity

Principle: Depending on the genetic makeup subjects taste the PTC either bitter or tasteless.

Reagents:

PTC solution (0.13% w/v) in double distilled water: 1.3g of PTC is dissolved in one litre of double distilled water. Fourteen dilutions are prepared by diluting the stock solution as shown in table 3. The stock solutions can be prepared for 100ml also depending on the requirement and number of subjects to be investigated (Table 3)

Procedure: The subjects who are participating in the taste sensitivity testing are asked to rinse the mouth with double distilled water. The last solution (14) is designated solution 1 and strong solution (stock solution) is labeled as solution 14. Each subject is instructed to consume 4ml of each solution. After each solution testing, subject may be instructed to rinse their mouth with double distilled water. After testing 14 solutions for taste sensitivity, if subject is unable to report no taste he/she may be labeled as non-taster. In case of tasters, the solution at which he/she report bitter taste is record for determination of taste threshold. The allele frequency (taster and non-taster) and heterozygosity is calculated using Hardy-Weinberg method.

3.4. Calculation of allele frequency of PTC taste sensitivity

To explain the calculation of allele frequencies of taster and nontasters, Aimba et al., (2010) study on taste sensitivity to PTC is taken as an example. In their study they recruited 232 males and 220 females and found the frequencies of tasters and non-tasters among males (154 tasters and 78 nontasters, n=232) and females (165 tasters and 55 non tasters, n=220). Allele frequencies of taster (T) and nontaster (t) can be calculate using Hardy-wein berg law =p2+ 2pq + q2 p=frequency of dominant allele in the population

q=frequency of recessive allele in the population

Here frequency of taster (p) (dominant)(TT)= 154 in males and 165 in females

Frequency of nontaster (q)(recessive)(tt)or q2 =78 in males and 55 in females

Nontaster allele frequency (q)

Male

q2=78/232= 0.33, q=√(square root) 0.33=0.574(allele frequency of non-taster)

Female

55/220=0.25, q=√(square root) 0.25=0.50(allele frequency of non-taster)

Taster allele frequency (p)= q-1

Male =1- 0.574(q)=0.426

Female=1-0.50(q)=0.50

Homozygotes and heterozytes i.e TT, Tt and tt can be calculated in the following manner

Males

q2(tt)=0.33, TT=p2=0.426 x 0.426=0.181, Tt=2pq= 2x 0.426x 0.574=0.482

Females

q2(tt)=0.25, TT=p2=0.50 x0.50 =0.25, Tt=2×0.50x 0.25=0.25

The significant difference between frequencies can be compared and significance is calculated using Chi-square test.

Summary:

Human variation arises due to the operation of evolutionary forces like mutation, migration (gene flow), genetic drift and natural selection.

Study of human variation is not only important from anthropological perspective but also helpful in understanding the contribution of genetics to the human disease and long-term survival of human species. Population Genetics is one of the subfields of Physical anthropology, wherein calculation of allele frequencies to understand the genetic differences.

Colour Blindness and PTC tasting are worth studying genetic markers.

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