Download Laboratory 3 Patterns of Inheritance

Laboratory 3 Patterns of Inheritance (human)
Before the lab
Read in Freeman Chapter 13 (Mendel and the Gene) pp 273, 281-282, 282, Table 13.2
Print and read this lab material.
Objectives
1. Understand the meaning and connections between the following terms:
a. Phenotype and genotype
b. Locus, gene and allele
c. Chromosome and chromatid
d. Homologous chromosomes (homologous pairs)
e. Sister chromosomes and non-sister chromosomes
f. Independent assortment
g. Homozygous and heterozygous
h. Dominant and recessive
i. Incomplete dominance and co-dominance
j. Sex linked, sex limited, sex influenced
2. Consider both the pattern of inheritance and the possible evolutionary significance of
certain human traits.
Evaluation 6%
1% Quiz at the beginning of lab on the background reading and terms
5% Assignment due at the beginning of Lab 4 (see assignment description posted separately on
the Lab folder on the course web page).
Timeline
2:10 – 2:20 Introduction
2:20 – 2:40 (20 minutes) Quiz
2:45 – 4:15 (90 minutes) Collect data for parts 1,2,3
4:15 – 4:45 (30 minutes) Record bacteria data from Lab 2—write due in Lab 4
4:45– 5:00 (15 minutes) Clean up and wrap up
Introduction
Though Mendel and others asked the question of why offspring resemble their parents, the
reverse question is also important—why do even closely related individuals vary considerably in
appearance and behaviour? These differences exist because all individuals inherit unique
combinations of genes from their parents. Unique combinations of genes are a direct result of
meiosis acting on the shuffling of different alleles. In this lab we will look at some human traits to
study Mendelian genetic patterns of inheritance.
Materials
Hand lens or illuminated magnifier
Phenotype charts
Colour charts
Series of dilutions of PTC with Q-tips
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Part 1
Red-green colour vision (Additional information in Appendix A)
Red-green ‘colour blindness’ is inherited as an X-linked recessive trait. During lab you will be
screened for this trait.
Please RECORD your phenotype on the CLASS master chart on the board and in Table 1 in
your lab notes.
(Assignment lab 3: you will hand in a table summarizing the class results for re-green
colour vision and write a short paragraph describing the evolutionary
advantages/disadvantages to having impaired colour vision)
Part 2 Index and ring finger ratio
(additional information in Appendix B)
Some work has been done measuring the relative length of the index and ring fingers and
correlating this ratio to the amount of testosterone present in the uterus during development—
the ring fingers of boys and men are typically longer than their index fingers while in girls and
women these fingers are usually the same or the index finger is slightly longer. More
background information is in Appendix B
The shorter index finger to ring finger may also be a sex-influenced trait such that the trait is
dominant in the male but recessive in the female. Another example of a likely sex-influenced
trait is pattern baldness.
Sex-influenced traits are different from sex-linked traits. Sex-linkage refers to the gene loci are
on a chromosome associated with sex-determination. Sex-influence or sex-limited traits
(expressed only in one sex such as egg production in chickens and milk production in cows)
represent gene actions associated with the unique phenotypes and the internal environment
related to maleness and femaleness.
Place your right hand on the sheet with your fourth (ring) finger just barely touching the line
below. Be sure your fingers are vertical (up and down) on the page. Make a mark across the
uppermost tip of the second (index) finger.
Place second
Finger here
Place Fourth
Finger here
_________
_________
Is the mark for your index finger below or above the line?
From http://www.laputanlogic.com/articles/2004/11/004-0001-2474.html
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Table 1 Class results comparing the Index/ring finger length in males and females
Gender
# with shorter index
# with longer index
fingers
fingers
females
males
total
(Assignment lab 3: you will hand in a graph of the class results comparing the index/ring
finger length in males and females and write a short paragraph describing the
evolutionary advantages/disadvantages to having impaired colour vision)
Part 3 PTC tasting T or t (Extra information in Appendix C)
The inability to taste phylthiocarbamide (P.T.C.) or phenylthiourea is conditioned by a recessive
allele (t). The majority of the North American Caucasian population are ‘tasters’ (T),
experiencing a striking bitter or sour sensation if this substance is put on the tongue. Children of
two non-tasters have non-taster children. However, expression of the allele is variable.
1. Count your tastebuds (fungiform papillae)
Accessed August 12, 2006 and reprinted from
http://www.bbc.co.uk/science/humanbody/body/articles/senses/supertaster.shtml
Conduct a simple scientific experiment to find out whether you have a supertaster tongue or not.
Food colouring
Cotton buds
Reinforcement rings for hole-punched paper
Magnifying glass
•
•
•
•
1. Using a cotton Q tip, swab some blue food colouring on to the tip of your tongue
2. Place a reinforcement ring on your tongue
3. Count the pink dots within the reinforcement ring (easier with a magnifying glass)
The pink dots are your fungiform papillae. They don't take up the food colouring. These papillae
are the tiny bumps on your tongue that house your taste buds. The more papillae you have, the
more taste buds you have and the more sensitive to taste you are. On average, non-tasters
have fewer than 15 papillae in that area, while supertasters have over 30.
2. PTC taste threshold
1. You will be asked to test a series of numbered solutions starting with the water and
going from most dilute to most concentrated solution.
2. The taste threshold is the number of the solution you can first taste something
bitter/sour.
3. Note your threshold on the class data posted on the chalk board.
4. Copy the threshold numbers for the students in your lab in the table below.
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PTC solutions (prepared for you)
Stock solution: 0.43 g PTC dissolved in 500 ml tap water (0.86 g/ml = 860 mg/ml); to dissolve
PTC use heat and stir gently on a hotplate. Tap water is used because distilled water tastes
terrible!
Table 2 Tabulating the PTC taste threshold
Tube #
Dilution factor (parts
Calculate the
stock in total parts
PTC concentration
diluted with tab water)
mg/ml
8
1 in 2
860/2=430
7
1 in 4
6
1 in 8
5
1 in 16
4
1 in 32
3
1 in 64
2
1 in 128
1
1 in 256
0
(Control)
# students at each
threshold
0 (tap water)
3. Does taste sensitivity affect eating habits?
How does your PTC sensitivity and number of fungiform papillae relate to your preference for
food and atmosphere while eating relate?
“Find out whether you live in the intense world of a supertaster or whether most foods you eat
taste pretty much the same.” This survey was developed with Professor Virginia Utermohlen
from Cornell University. Reprinted from
http://www.bbc.co.uk/science/humanbody/body/interactives/supertaster/
Add your letter answers to each question to the class data for you to compare to PTC sensitivity
and number of fungiform papillae in you Lab 3 assignment.
SURVEY
You are planning to go to a fine restaurant with friends. Which restaurant would you prefer?
Question 1 of 5
a. A small, quiet, cozy restaurant with simple décor
b. An elegant restaurant with mirrors and an outdoor patio
c. A popular busy restaurant with a large dining room where you can see everything that's
going on
You're in a restaurant with friends, deciding what to eat. Which best describes you?
Question 2 of 5
a. Everything on the menu looks so good it's hard to choose what to have
b. The food described on the menu isn't quite the way you'd like it to be
c. The atmosphere of the restaurant and the people you are with are usually more important
than what's on the menu.
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The waiter takes your order and asks you how you'd like it served. How do you like your meal to
be prepared?
Question 3 of 5
a. With low-fat versions of any dressings or sauces, because, for you, healthiness tends to be
as important as taste
b. With dressings or sauces 'on the side' so that you can find out how the food tastes first and
avoid ruining the dish
c. You go with the chef's recommendations.
15 minutes later, the food has just arrived. Do you..
Question 4 of 5
a. Tuck in straight away without really thinking about it
b. Taste a small mouthful and try to distinguish the different flavours
c. Take in the food's aroma - the food has to smell good for you to really enjoy eating it
The food is really good. Do you…
Question 5 of 5
a. Stop conversing because you want to concentrate on eating your meal
b. Stop conversing because you're busy trying out the different foods on your plate one at a
time
c. Keep talking because you are with friends and are enjoying the conversation
The authors of the survey relate the answers to the questions on the survey to the number of
fungiform papillae & sensitivity to better tasting substances (e.g., PTC) as follows:
Normal taster
Like a large variety of foods but care about how their food is prepared. Have an average
number of medium-sized papillae. Around 50% of people are said to be normal tasters.
Supertaster
Perceive all tastes as more intense than other taster types, particularly bitter tastes. Tend to be
fussy about their food and have strong food likes and dislikes. Usually don't like coffee,
grapefruit, cabbage, Brussels sprouts and spinach. Have lots of papillae that are closely packed
together and small. Around 25% of people are said to be supertasters
Non-taster
Perceive all tastes as less intense than other taster types. They are particularly insensitive to
bitter tastes. Are happy with most foods, irrespective of the type of food or its preparation
Have few papillae that are spread out and large. Around 25% of people are said to be nontasters.
Assignment for Lab 3 (due in Lab 4)—see the expanded explanation for this assignment
linked to the LAB folder on the course web page.
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For the assignment will compare the results for the PTC taste threshold, the # fungiform
papillae, and eating habits (survey)
1. Graph and write a short paragraph, comparing the classification of individuals as
non-tasters, tasters, and supertasters in the class according to their PTC taste
threshold and to the # fungiform papillae.
2. Write a short paragraph describing comparison of the proportion of non-tasters,
tasters, and supertasters in the class to the proportions of each normally found in the
population.
3. Write a short paragraph describing the comparison of the class answers to the eating
survey to what you would predict based on individual’s classification as non-tasters,
tasters, or supertasters. Also include a statement whether the class data for the
eating survey supports the conclusions presented by the authors of the survey?
4. In now more than two paragraphs, briefly discuss what might be the evolutionary
significance for the phenomenon of being a taster (of a bitter substance) or a nontaster?
Table 3 Comparison of PCT taste sensitivity to number of taste buds and eating habits
Student
PCT
# papillae
taster category
Letter (a,b,c) for eating
ID
threshold
nt = <15
survey questions
t = 15-30
st = >30
1
2
3
4
5
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
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Appendix A
Colour detection
The series of plates you will be tested with provides a quick and accurate assessment of colour
vision deficiency of congenital origin which is the most common form of colour disturbance.
Colour Blindness
The following information is based on http://www.tiresias.org/guidelines/colour_blindness.htm
developed with the help of Prof Lindsay Sharpe, Professor of Vision Science, University of
Newcastle.
What is it?
Colour blindness is the reduced ability to distinguish between certain colours or wavelengths of
light. To see colours properly, light detecting photoreceptor cells, called cones, are needed in
the retina of the eye. Three different types of cones exist, each containing a different
photopigment: the short-wave (S, sometimes called 'blue'), middle-wave (M, sometimes called
'green')- and long-wave (L, sometimes called 'red') sensitive cones. These have distinct,
spectral sensitivities, which define the probability curve of photon capture as a function of
wavelength. The absorbance spectra of the S-, M- and L-cone photopigments overlap
considerably, but have their wavelengths of maximum absorbance in different parts of the
visible spectrum. If one or more of these types of cells is faulty then colour blindness results.
To help you understand the differences consider the normal visible spectrum of light:
Figure 3 Electomagnetic spectrum of ‘light’ (http://www.arpansa.gov.au/images/nir/spectrum.gif)
The following section is from www.city.ac.uk/ colourgroup/colourblind.html :
Every colour can be defined by 3 properties:
• Hue - type of colour, ie red, green etc
• Saturation - depth of colour from grey
• Luminance - brightness
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1. HUE (type of colour)
People with normal colour vision are able to differentiate colour through 3 colour sensitive light
receptors in the eye. The receptors are sensitive respectively to red, green and blue incident
light. The spectral response of the 3 receptors is shown below:
Figure 4 Normal colour perception
Colour blind persons may have one or more receptors missing or more frequently the receptor
responses are less separated so that colour differences cannot be perceived or can only be
seen with great difficulty. In the most common form - 'red-green' colour blindness this means
that sufferers at best will have difficulty distinguishing colour differences in the red-green part of
the spectrum so that separating reds, greens and yellows is extremely difficult. At worst,
sufferers will only perceive blues, yellows and shades of grey in between.
Figure 5 Colour perception with the red receptor missing
Figure 6 Poor red-green separation
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2. Saturation –density of colour (see figure below)
Figure 7 Comparison of hue and saturation
3. Brightness
Our perception of brightness depends mainly on the eyes combined response to the red and
green receptors. This means that if a red green colour blind person perceives two colours to
have the same brightness they will not be able to distinguish between them, unless the blue
receptor is stimulated differently, eg 'yellow and bright green' have a similar 'blue' content so will
appear as the same tone of grey. A yellow and a darker green with a high blue content will
appear as a pale and dark grey and will be able to be distinguished as separate parts of the
image.
Detecting colour deficiencies
Colour blindness is normally diagnosed through clinical testing and a number of tests have been
devised. The most common test is the use of special test plates made up of a series of spots of
varying hues and lightnesses so that a central number or letter stands out from the background.
Those with defective vision are unable to distinguish these figures or will see a different figure
due to the different appreciation of the hues. By changing the figure and background colours,
the basic types of defective colour vision can be identified.
There is no cure for colour blindness, however there are techniques that can be used to help
discriminate between colours, for example: hand held filters, tinted spectacles and monocular
contact lenses. However, such devices must be used with caution. For instance, wearing a
coloured filter over one eye reduces luminance, and can actually diminish colour discrimination
and visual acuity, induce visual distortions, alter stereopsis (binocular vision) and impair depth
perception. And, indeed, a review of research on whether tinted lenses or filters improve visual
performance in low vision concluded they actually worsen colour vision. It should be
emphasised that improving test scores on specialized colour vision tests is not the same thing
as curing colour blindness.
Types
Phenotypically, there are 3 main types of inherited colour blindness, resulting from alterations in
the cone photopigments:
(i) One of the three cone pigments is altered in its spectral sensitivity, but normal threedimensional colour vision is not fully impaired.
(ii) One of the cone pigments is missing and colour is reduced to two dimensions.
(iii) Two or all three of the cone pigments are missing and colour and lightness vision is reduced
to one dimension.
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Frequency of occurence
•
Because these defects are inherited as recessive traits, the incidences are much higher
in males whose cells have a single X-chromosome, than in females whose cells have
two.
• Incidences of red-green colour deficiencies vary between human populations of different
racial origin. The highest rates are found in Europeans and the Brahmins of India (c. 8%
of males) and Asians (c. 4%); the lowest in Africans (c. 2.5%) and the aborigines in
Australia (c. 2%), Brazil, North America (c. 2.0%) and the South Pacific Islands (c. 1.0%)
(Source: Opsin genes, cone photopigments, color vision, and color blindness in Color Vision:
from Genes to Perception, Cambridge University Press, New York, 1999)
Problem
Official term
% of males
per 1000 males
Weak in red
"protanomalous"
0.5 %
5
No red
"protoanopia"
0.8 %
8
Weak in green
"deuteranomalous"
3.3 %
33
No green
"deuternopia"
0.6 %
6
Fruit stall http://www.tiresias.org/guidelines/colour_blindness.htm
A fruit stall as seen by colour normal (A), protanopic - a form of red-green blindness
(B), deuteranopic - another form of red-green blindness (C) and tritanopic - a form of
blue-green blindness (D) shoppers.
A. Colour Normal
C. Deuteranopic
B. Protanopic
D. Tritanopic
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What are the consequences of being ‘colour blind’?
Below is a partial list of potential problems—can you think of others? “Life's minor frustrations
(and occasional dangers) for the color blind:” [the following list is from
http://www.toledo-bend.com/colorblind/aboutCB.html]
•
•
•
•
•
•
•
•
Weather forecasts - especially the Weather Channel - where certain colors just can not
be distinguished on their weather maps. Also, maps in general because of the color
coding on the legends.
Bi-color and tri-color LEDs (Light Emitting Diodes): Is that glowing indicator light red,
yellow, or green?
Traffic lights, and worst of all, Caution lights: Color blind people always know the
position of the colors on the traffic light - in most states, Red on top, Yellow in the center,
Green (or is that blue?) on the bottom. It isn't good when we go to a city or state where
they put traffic lights horizontal - it takes a couple of days to get used to that one! But
caution lights present an entirely different problem. In this situation there is only one
light; no top or bottom, no right or left, just one light that is either red or yellow - but
which is it?
Getting in the sun with your friend: So, you're out in the boat or on the beach with
your friend and soaking up the rays. But I can't tell until far too late if I'm getting red - or if
she/he is. If I can tell it's red, by that time it's fire engine red and a painful sunburn is
already present.
Color observation by others: "Look at those lovely pink flowers on that shrub". My
reply, looking at a greenish shrub "What flowers?"
Purchasing clothing: I've got some really neat colors of clothes. Not everyone
appreciates them like I do though; they seem to think the colors are strange. I just don't
know why!
Test strips for hard water, pH, swimming pools, etc.: A color blind person is
generally unable to :
o interpret some chemical reactions
o see that litmus paper turns red by acid
o identify a material by the color of its flame such as lead blue or potassium purple
o interpret the chemical testing kits for swimming pool water, test strips for hard
water, soil or water pH tests - all of which rely on subtle color differences and a
band of similar colors to compare against.
Cooking and foods:
o When cooking, red deficient individuals cannot tell whether their piece of meat is
raw or well done. Many can not tell the difference between green and ripe
tomatoes or between ketchup and chocolate syrup.
o Some food can even look definitely disgusting to color deficient individuals. For
example, people with a green deficiency cannot possibly eat spinach which to
them just look like cow pat. They can however distinguish some citrus fruits.
Oranges seem to be of a brighter yellow than that of lemons.
from http://vischeck.com/info/wade.php Dr Alex Wade, Research Fellow at Stanford
University April 2000
“On the positive side, there is some evidence that colour-blind people are much better than
average at certain jobs. They are very good at finding green things hidden against green
backgrounds - for example grass or leaves. They tend to find things by shape and get less
confused by camouflage. Because of this, colour-blind entomologists still catch lots of bugs and
in wartime, armies prize their colour-blind snipers and spotters.”
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Appendix B Index/ring finger ratio
http://www.futurepundit.com/archives/002647.html March 04, 2005
Index and Ring Finger Lengths Partially Predict Violent Tendencies
Higher prenatal testosterone has already been found to be correlated with a higher ratio of ring
finger length to index finger length. Now University of Alberta researchers Peter Hurd and
Allison Bailey have shown that the higher ring finger to index finger ratio is correlated with
physically aggressive behavior in men.
Hurd and his graduate student Allison Bailey have shown that a man's index finger length
relative to ring finger length can predict how inclined that man is to be physically aggressive.
Women do not show a similar effect.
A psychologist at the University of Alberta, Hurd said that it has been known for more than a
century that the length of the index finger relative to the ring finger differs between men and
women. More recently, researchers have found a direct correlation between finger lengths and
the amount of testosterone that a fetus is exposed to in the womb. The shorter the index finger
relative to the ring finger, the higher the amount of prenatal testosterone, and--as Hurd and
Bailey have now shown--the more likely he will be physically aggressive throughout his life.
"More than anything, I think the findings reinforce and underline that a large part of our
personalities and our traits are determined while we're still in the womb," said Hurd.
Hurd and Bailey's research, published this March in Biological Psychology, was determined
from surveys and hand measurements of 300 U of A undergraduates.
In their study, they found there were no correlations between finger lengths and people who are
prone to exhibit verbally aggressive, angry, or hostile behaviors, but there was to physically
aggressive behavior.
Hurd is conducting ongoing research in this area, including a study that involves measuring
hockey players' finger lengths and cross referencing the results with each player's penalty
minutes. He also has a similar study showing that men with more feminine finger ratios are
more prone to depression; a paper on this will be published later this year in Personality and
Individual Differences.
"Finger lengths explain about five per cent of the variation in these personality measures,
so research like this won't allow you to draw conclusions about specific people. For example,
you wouldn't want to screen people for certain jobs based on their finger lengths," Hurd said.
"But finger length can you tell you a little bit about where personality comes from, and that's
what we are continuing to explore."
2. Academics find that the finger of destiny points their way
[abridged] http://www.bath.ac.uk/pr/releases/fingerlength.htm accessed June, 2005
The study drew on work in the last few years which established that the levels of oestrogen and
testosterone a person has can be seen in the relative length of their index (second) and ring
(fourth) fingers. The ratio of the lengths is set before birth and remains the same throughout life.
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The length of fingers is genetically linked to the sex hormones, and a person with an index
finger shorter than the ring finger will have had more testosterone while in the womb, and a
person with an index finger longer than the ring finger will have had more oestrogen. The
difference in the lengths can be small – as little as two or three per cent – but important.
A survey of the finger lengths of over 100 male and female academics at the University by
senior Psychology lecturer Dr Mark Brosnan has found that those men teaching hard science
like mathematics and physics tend to have index fingers as long as their ring fingers, a marker
for unusually high oestrogen levels for males.
It also found the reverse: those male academics with longer ring fingers than index fingers – the
usual male pattern – tended not to be in science but in social science subjects such as
psychology and education.
3. Book Review Digit Ratio: A Pointer to Fertility, Behavior and Health
by John T. Manning NJ: Rutgers University Press. 2002
Reviewed by Michael Mills, Psychology Department, Loyola Marymount University, Los
Angeles, CA 90045. USA. http://human-nature.com/nibbs/02/manning.html
Take a look at your right-hand. Which of your fingers is longer: your ring finger, or your index
finger? Surprisingly, a passing stranger who noticed a difference in length between these two
fingers (and who had handy a copy of John Manning's book Digit Ratio: A Pointer to Fertility,
Behavior and Health) might infer some very personal characteristics about you.
Manning reviews evidence to suggest that the ratio of the length between the ring and index
finger is somewhat sexually dimorphic, that this ratio is determined during early fetal
development, and that it is influenced by sex hormones, particularly testosterone. If this is true,
the fingers may provide a permanent, and easily visible, historic marker of important hormonal
events that occurred during a critical time of fetal development, the latter part of the first
trimester. This is a critical time of sexual differentiation of both the brain and body.
Specifically, it is the ratio of the length of the index finger (digit 2, or "2D") and the ring finger
(digit 4, or "4D") that is sexually dimorphic. Generally, males have a ring finger that is longer
than their index finger. Females typically have index and ring fingers of about the same length.
The ratio of index finger length to ring finger length is called the “2D:4D digit ratio,” or more
simply, the “digit ratio.” Manning reports that, for males, the index finger is generally about 96
percent of the length of the ring finger, which gives an average digit ratio for males of .96. The
digit ratio would be 1.00 if the ring and index fingers were the same length, and greater than
1.00 if the index finger was longer than the ring finger. Males generally have a digit ratio below
1.00 -- they have what is termed a "low digit ratio." Women generally have a digit ratio of about
1.00 (the index and ring fingers are of about equal length), or a "high digit ratio."
Manning links the proximate causes of digit ratio sexual dimorphism to the effects of sex
hormones during early fetal development. He believes the evidence is persuasive, but not yet
definitive, that higher levels of testosterone during this critical developmental stage facilitates
the growth of the ring finger, while higher levels of estrogen facilitates the growth of the index
finger.
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Table Some Characteristics That May Be Associated with Digit Ratio (from Manning, 2002)
High 2D:4D ratio
Low 2D:4D ratio
Presumably due to relatively
Presumably due to relatively
greater fetal exposure to
greater fetal exposure to
estrogen in the 1st trimester.
testosterone in the 1st
trimester
Higher risk of early heart
More fertile
Males
disease
Higher lifetime reproductive
success
More aggressive and
assertive
Higher musical and sports
aptitude
Females
More aggressive and
assertive
More fertile
Higher lifetime reproductive
success
Higher risk of breast cancer
There is substantial overlap between the sexes for digit ratio. It is not uncommon for a man or
woman to have a digit ratio that is typical of the opposite sex. There is far more overlap between
the sexes in digit ratio than there is in the overlap between the sexes in height.
Hypothesis of the evolutionary significance
Why did men and women evolve a sexually dimorphic digit ratio? Manning notes that it has
been suggested that the male digit ratio pattern may be functional -- a longer ring finger may
help to stabilize the third digit (the middle finger) when throwing objects, thus increasing
throwing accuracy. This implies that the throwing accuracy required for successful hunting
and/or tribal warfare was of sufficient importance to drive the evolution of this sexually dimorphic
trait. While gathering, ancestral women presumably did not need this extra stability for the third
finger.
Another hypothesis for the origin of this sexually dimorphic trait was that it was driven by direct
sexual selection -- female choice. If so, it is surprising that women today are not conscious of
being particularly attracted to men with low digit ratios. However, it is interesting that women
sometimes comment that they were attracted to a man's "masculine looking" hands, albeit
without commenting directly on digit ratio. One wonders if the appearance of "masculine looking
hands" includes an (unconscious?) assessment by females of male digit ratio? If so, this would
lead more credence to the direct sexual selection hypothesis.
Reference
Manning, J. T. (2002). Digit ratio: A pointer to fertility, behavior and health. NJ: Rutgers
University Press.
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Appendix C Additional background on PTC tasters/non-tasters
1. Natural selection at work in genetic variation
28 Jun 2004 Medical News Today accessed June 8, 2005
to taste
A genetic variation seen worldwide in which people either taste or do not taste a bitter, synthetic
compound called PTC has been preserved by natural selection, University of Utah and National
Institutes of Health researchers have reported.
Phenylthiocarbamide (PTC) is not found in nature, but the ability to taste it correlates strongly
with the ability to taste other bitter substances that occur naturally, especially toxins. Eons ago,
the ability to discern bitter tastes developed as an evolutionary mechanism to protect early
humans from eating poisonous plants.
Today, the ability to taste, or not taste, the compound influences what people eat and even
whether they smoke cigarettes.
People who can taste PTC are less likely to eat cruciferous vegetables such as broccoli,
according to Wooding, which could be a problem because these vegetables contain important
nutrients. If the ability to discern bitter tastes discourages PTC tasters from eating broccoli, it
also may have the advantage of dissuading them from inhaling the acrid smoke of cigarettes.
"Among smokers, there seems to be an excess of PTC non-tasters," Wooding said. "So it
seems that PTC tasters are less likely to smoke."
Typically, over hundreds of thousands years, genetic drift takes place, a process in which gene
frequencies and genetic traits change randomly within a population. Under that expectation,
everybody either would be a PTC taster or non-taster by now. But worldwide the ratio has
remained at roughly 75/25 between PTC tasters and non-tasters.
The Utah researchers found that two versions of the PTC allele (genes) are present worldwide,
from America to Africa. After comparing thousands of genes, the researchers found that the
presence of such divergent alleles is highly unusual. But the existence of two PTC alleles can
be explained by evolutionary pressure to avoid the toxins that plants produce to defend
themselves against herbivores.
Everybody carries two copies of the PTC taster gene, meaning any individual could carry two
copies of the PTC taster allele, two of the non-taster allele, or one of each. "We hypothesize that
people carrying one copy of each allele are able to taste a broader range of toxic, bitter
compounds, and have an evolutionary advantage," Wooding said.
Last year, researchers at the National Institutes of Health and the University of Utah discovered
the PTC gene and found that it comes in two major alleles. One allele encodes the receptor to
bind PTC, and the other, which differs by three amino acids from the first, encodes a receptor
that probably binds with different bitter compounds. Those researchers included the U of U
medical school's Mark F. Leppert, Ph.D., professor and co-chair of the Department of Human
Genetics, and Hilary Coon, Ph.D., associate professor of psychiatry.
The ability to taste or not taste PTC was discovered in 1930. An American chemist named
Arthur Fox accidentally let loose a quantity of PTC in a laboratory and noticed that while some
people could taste it, others could not. After that, it was long hypothesized that alleles were
responsible for the ability to taste PTC, according to Wooding.
BIO152H5F 2006