Download Laboratory 4 Patterns of Inheritance (human)

Laboratory 4 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. Dominant and recessive and incomplete dominance
b. Homozygous and heterozygous
c. Locus and gene and allele
d. Polygenic
e. Chromosome
f. Phenotype and genotype
g. Wild type and mutant
2. Compute the genotypic and phenotypic ratios from the class data generated for specified
traits
3. Consider both the pattern of inheritance and the possible evolutionary significance of
certain human traits.
Evaluation (4%)
Quiz at the beginning of lab on the background reading and terms
Group presentation
Timeline
0:10 – 0:20 Introduction, hand in assignments
0:20 – 0:40 Quiz
0:45 – 1:15 Part A
1:15 – 2:00 Prepare group presentation
2:00 – 2:45 Group presentations (15 minutes each—MAX)
2:45 – 3:00 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.
If you consider humans, each child receives one-half of its genetic information from each parent
or phrased more accurately each parent contributes one allele for each genetic locus (the
specific location of a gene on a chromosome). Because both the father’s sperm and mother’s
egg went through meiosis shuffling the genes around by independent assortment of the
homologous pairs and crossing over of genetic information between non-sister chromatids, even
children of the same parents will receive a different set of genes from each parent. Identical
twins are an exception because they are the result of the fertilization of a single egg and sperm.
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How are traits inherited? Each individual gets one allele for each characteristic from each
parent; the two alleles must be either the same or different. When both alleles are a trait are the
identical the individual is said to be homozygous for that trait; if the alleles are different the
individual is said to be heterozygous. A gene for a trait may have one, two or many alleles in
the population.
What is the interaction between the two alleles for the same trait? If one allele masks the
expression of another allele, the expressed allele is called dominant and the hidden allele is
called recessive. Recessive alleles are only expressed when the dominant allele is absent ( =
homozygous recessive or haploid), while the dominant allele is expressed whether it is
homozygous or heterozygous. The genotype is the combination of allele(s) of a particular gene.
The phenotype is the observable trait—commonly includes the physical, physiological, and
behavioural traits.
In some cases, two different alleles are both expressed. If the resulting phenotype is
intermediate between the phenotypes of the homozygous alleles, the condition is called
incomplete dominance. If the phenotype of heterozygotes is the expression of both alleles, the
condition is called co-dominance.
Sex determination varies from species to species. The pattern of inheritance in sex-linked
traits (traits with genes on the sex chromosomes) is different in males and females. Is a trait
autosomal or sex linked? If a trait appears about equally in males and females, it is likely
autosomal. However, some the genes for traits are not on the sex chromosomes, but their
expression is sex-limited, meaning their expression is limited to one sex or another.
Other traits are not determined by a single gene, but by many genes with various alleles for
each gene; these traits are called polygenic. Some genes influence genes at another locus;
this phenomenon is called epistasis.
Materials
Hand lens
Phenotype charts
Colour charts
Series of dilutions of PTC with Q-tips
Preview
In this lab we will look at some human traits to study Mendelian genetic patterns of inheritance.
Some genes affect our shape, while others affect our physiology (including what we can taste).
Many of the trait descriptions noted as (1) are from a previous course at UTM BIO203S
Introductory Genetics. The other reference (2) is from V. A. McKusick, 1988, Mendelian
Inheritance in Man, 8th ed.
In this lab
1. Part A: You will work in pairs.
2. For each trait you and your partner will first determine your phenotypes and possible
genotypes completing TABLE 1
3. Compile class data to compute the genotypic and phenotypic ratios for specified traits.
4. Examine different modes of inheritance: simple dominance and recessiveness,
incomplete dominance, multiple alleles, sex-linked, sex-limited, and sex -influenced
traits.
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5. Part B: each bench will be assigned one topic to verbally summarize to the class—use
the extra readings and class data in your summary. In your summary describe the
pattern of inheritance and the plausible evolutionary significance. The three topics are
PTC tasters/non-tasters; index-ring finger ratio; colour detection.
PART A: Determine the phenotypes and possible genotype(s) for the following
traits. Add this data to Table 1 & on the class chalk board (Work in pairs; use the
phenotype charts provided).
1. Tongue rolling (1) R or r
In 1940, A. H. Sturtevant reported two classes within the human population, tongue
rollers and nonrollers. The roller phenotype (R) is dominant; individuals unable to roll
their tongue are homozygous recessive (r) for the trait.
2. Widow’s peak (1 and 2) W or w
McKusick (1983) and others postulate that the inheritance of a widow’s peak, a distinct
downward point of the frontal hairline, occurs as a dominant mutant in the human gene
pool. Homozygous recessive individuals possess a straight hairline (p). Notice that the
straight hairline is the wild type though recessive.
3. Earlobe attachment (1 and 2) EL or el
The inheritance of a free (unattached) earlobe is a dominant allele; direct attachment of
the earlobe to the head is a recessive trait
4. Facial dimples (1 and 2) D or d
Natural indentations near the corners of the mouth are dominant to no dimples.
5. Cleft chin (2) C or c
Cleft chin is dominant; no cleft chin is recessive [cc]
6. Ear pits (2) Ep or ep
Pits present is dominant; no pits is recessive [ee]
7. Thumb crossing (2) Lt or lt
In a relaxed interlocking of fingers, left thumb over right indicate the dominant allele is
present; right thumb over left is homozygous recessive.
8. [hand] Hitchhiker’s thumb or straight thumb St or st
B.l .Glass and J.C. Kistler classified distal hyperextensibility of the thumb as a recessive
trait in 1953. The expression of this allele is quite variable; for class purposes, if you can
not bend your thumb backward about 45º, you probably have a least one dominant
allele.
9. [foot] Length of the ‘big toe’ BT or bt
A. R. Koplan (1964) reported that the relative lengths of the hallux (big toe) and the 2nd
toe were Mendelian traits (controlled by dominant and recessive alleles). A long hallux is
recessive: the dominant allele results in a short big toe relative to the 2nd toe. If both toes
are equal in length, or if the big toe is longer than the second toe, you probably carry the
recessive alleles.
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10. [hand] Middigital hair (1) Md or md
Genetic control of midphalangeal hair was first documented by C. H. Danforth in 1921.
Examine the segment between the second and terminal joints of your fingers and look
for hair or hair follicles on these segments (midddigitals). Even the slightest amount of
hair qualifies as the dominant condition. Notice that the same fingers on both hands
have hair (or follicles if the hair is worn off). The presence of hair on specified fingers is
determined by a series of multiple alleles as follows:
Table 2 Class frequency of alleles for middigital hair
Allele Phenotype of middigital hair
#students
in class
H1
Hair on all four fingers (not on the
thumb)
H2
Hair on three fingers—not the index
finger
H3
Hair on two fingers—not the index or
little finger
H4
Hair on one finger---is it the ring or
middle finger?
H5
Hair on no fingers
After determining both you and your partner’s phenotype, write down your possible
genotypes in Table 1 at the end of the lab. When the rest of the class have added their
data to the table on the chalk board write down the # students for each allele in Table 2
above.
Re-write the phenotypes represented in your lab class (about 24 students) in decreasing
frequency: ____ _____ _____ _____ _____
Is the phenotype determined by the recessive alleles the least frequent? _____ Why
might this have happened?
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PART B
All members of the class will participate in the collection of the data for items 1113. One group will be assigned to summarize the class data and to present the
data and background information to the class
11. Index and ring fingers (extra information in Appendix A)
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 sexinfluenced 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 sexlimited 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.
Is the mark for your index finger below or above the line?
Mark second
Finger here
Place Fourth
Finger here
_________
_________
Table 3 Class results comparing the Index/ring finger length in males and females
Gender
# with shorter index
# with longer index
fingers
fingers
females
males
total
12. PTC tasting T or t (Appendix B)
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. You will be asked to test a series of numbered solutions starting with water. Enter
the number of the solution you can first taste something bitter/sour.
Is the class response to PTC all or nothing or can some people tasted at the different
concentrations?
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What is the proportion of tasters and non-tasters in the class?____
What might be the evolutionary significance (if any) for the phenomenon of being a taster (of
a bitter substance) or a non-taster?
13. Red-green colour vision (Appendix C)
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.
Table 1 Personal inventory and class percentages of phenotypes and possible genotypes
Class size = ______
Trait
Your
Your possible # in class with % of class
phenotype
Genotype(s)
trait
with trait
Tongue roller
RWidow’s peak
WEarlobe free
El-
Dimples
DCleft chin
CEar pits
EpThumb
crossing Lt
Straight
thumb
StBig toe
Bt
Middigital hair
H1,2,3,4 or 5
Index/ring
finger index
PTC taster
T and #
Red-green
vision
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Discussion Questions (for general discussion at the end of lab)
Based on the class data, are the dominant phenotypes the most common? Are dominant
phenotypes usually the most common in a population? Explain your answer.
What affects how often a phenotype occurs in a population?
How could you determine the genotype of a person showing a dominant phenotype?
Appendix A Ring-index 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.
Dr. Peter Hurd initially thought the idea was "a pile of hooey", but he changed his mind when he
saw the data.
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.
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"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
Male scientists are good at research because they have the hormone levels of women and long
index fingers, a new study says. A survey of academics at the University of Bath has found that
male scientists typically have a level of the hormone oestrogen as high as their testosterone
level.
[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.
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.
A further study also suggests that prenatal hormone exposure, and hence index finger length,
can also influence actual achievement levels. In a survey of male and female students on a
JAVA programming course at the University, the researchers found a link between finger length
ratio and test score. The smaller the difference between index and ring finger - the higher the
test score at the end of the year.”
From http://www.laputanlogic.com/articles/2004/11/004-0001-2474.html
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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. With no more data than that gleaned from a passing glance at
your hands, a stranger might infer whether you are likely to have homosexual
inclinations, are highly fertile, may eventually suffer from a heart attack or breast cancer,
have musical aptitude or sporting prowess, and a surprisingly long list of other
characteristics.
Why do the fingers reveal such a wide spectrum of information (albeit very
probabilistic)? 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. He states: “In general, it seems that 2D:4D is the most
reliable of the predictors of hypermasculinization…” (p. 139). One of the consequences
of hypermasculinization, as reflected by the digit ratio, may be higher levels of fertility in
men and lower levels of fertility in women. He also suggests that hypermasculinization
increases the likelihood of homosexuality or bisexuality, in both males and females.
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Manning devotes separate chapters to explore the relationship between digit ratio and a
variety of characteristics, including assertiveness and attractiveness (chapter 3),
reproductive success (chapter 4), hand preference, verbal fluency, autism, and
depression (chapter 5), health and disease (chapter 6), homosexuality (chapter 7),
musical aptitude (chapter 8) and sports aptitude (chapter 9). A brief summary Manning’s
findings (some of which he notes are quite preliminary) is presented below.
Table 2 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
Greater proclivity toward
homosexuality/bisexuality
Higher musical and sports
aptitude
Lower SES (?)
Females
Greater proclivity toward
homosexuality/bisexuality
More aggressive and assertive
More fertile
Higher lifetime reproductive
success
Higher risk of breast cancer
There is, however, substantial overlap between the sexes with respect to digit ratio. It is
not uncommon for a man or woman to have a digit ratio that is typical of the opposite
sex. Across various populations, the offset of the distributions of male and female digit
ratio is a little less than ½ of a standard deviation – e.g., the digit ratio, collapsed across
various populations, has an effect size of about 0.3 – 0.4 (Manning, personal
communication, 10/4/02). This is a small to moderate effect size. The sexual
dimorphism of height has an effect size of about 1.4. A web-based program is available
at my website (at this address: http://bellarmine.lmu.edu/faculty/mmills_fp/software.htm)
that will graph the distributions and calculate the effect size for a trait. Using this
program, we can see that there is far more overlap between the sexes in digit ratio than
there is in the overlap between the sexes in height, as noted in the following diagrams.
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Figure 1 – The Sex Difference in Height in inches (American data)
Figure 2 – The Sex Difference in Digit Ratio, Liverpool, UK Data (data from Manning, 2002)
One of the especially interesting theoretical discussions in Manning’s book is his speculation
(starting on page 54) regarding why there is any overlap between the sexes in sexually
dimorphic traits in the first place, including digit ratio. If being taller than women is adaptive for
men in general, why aren’t all men taller than all women? And why don’t all men have a lower
digit ratio than all women? Why the overlap? Manning interprets this as an evolutionary stable
equilibrium point in a conflict between sexually antagonistic genes. Genes that tend to
masculinize the fetus will increase when there is an advantage to having male offspring (e.g.,
4-Patterns of Inheritance - 12 of 21
when the operational sex ratio favors men, or there is a polygynous mating system). The
benefits of more masculinized male fetuses (increased adult sperm count, higher libido,
inclination toward sexual promiscuity, etc.) will compensate for the reduced fertility of female
offspring due to their relatively higher than normal fetal exposure to male hormones. When the
mating system pendulum swings to the other side, and a monogamous mating system is in
place, or operational sex ratio favors females, genes that tend to feminize the fetus will be
favored. The result is an overlap in male/female distributions for many sexually dimorphic traits
– a somewhat middle ground as parents hedge their bets regarding the relative reproductive
potential of male or female offspring.
Somewhat surprisingly, the effect size for digit ratio between the sexes varies substantially as a
function of geography and race. For example, among the Zulu the effect size was found to be as
low as about .2, while the effect size for a German sample was .56. Surprisingly, the females in
some cultures may have a lower digit ratio than males of other cultures, although men have a
lower digit ratio than women within populations in all cultures for which there is data. It is unclear
why the effect size of the digit ratio of the sexes varies between different populations. This is a
curious fact, one for which Manning provides little in the way of definitive conclusions -- and the
reader may be left to wonder whether some of Manning's interpretations are threatened by this
between population variability in effect sizes. However, the fact that the average height of men
of some populations is lower than women of other populations doesn't negate the sex difference
in height, nor does the fact that the gender effect size of height varies in different populations.
Height, like digit ratio, is still sexually dimorphic. But the causes of between population variation
in sexually dimorphic traits, such as digit ratio, is certainly puzzling, and it is a fertile area for
future research.
One might be tempted to speculate that racial variation in digit ratio may correspond to Ruston’s
(1997) theory that populations closer to the equator are more r-selected, and thus are relatively
more masculinized in utero and have higher adult testosterone levels. By this reckoning, one
might expect to find low digit ratios near the equator, and progressively higher digit ratios in
populations farther away from the equator. Indeed, Caucasians tend to have higher digit ratios
than do Blacks. Data for Asian populations has not yet been published, however, some
preliminary data suggests that Asians tend to have low digit ratios (Manning, personal
communication, 10/5/02). Manning suggests that populations in middle latitudes may have
higher digit ratios compared to populations nearer to the equator or nearer to the poles. This is
inconsistent with Rushton’s theory, and Manning, so far, has not speculated about a possible
theoretical explanation for this curvilinear pattern. The curious relationship between digit ratio
and geographic latitude may require much additional research to disentangle what may be
some complex determinants of these population variations.
It is clear why men and women have sexually dimorphic reproductive organs. But why did they
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. Today, this sex difference
may be seen in male superiority in throwing darts. And, it would be interesting to know if men
with lower digit ratios were better dart throwers and men with higher digit ratios.
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
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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.
Manning's book summarizes a cutting-edge area of research. He provides an outstanding (and
currently the only) review and synthesis of the literature on digit ratio. This is a book that will
serve as a valuable resource for researchers conducting studies in this area. He provides a
wealth of statistical data (right down to the number of subjects and F test values) from a variety
of studies. Many of these studies he has conducted himself. However, to a casual reader, lay or
professional, trudging through statistical information will likely be an obstacle. Manning may
have been able to capture a far wider readership if much of the detailed statistical data from
specific studies had been relegated to footnotes or end notes. Indeed, this is a fascinating topic
– what other easily visible physical trait (other than sex itself) is likely to be associated with such
a variety of behavioral, reproductive, and health characteristics?
In the final chapter, Manning provides an interesting discussion about possibilities for further
research that may help to further disentangle the consequences of early fetal hormonalization.
Given the results from such future research, a brief glance at the hands may be even more
revealing.
Don't wish others to glean too much personal information about you from a brief glance?
Manning might suggest that you may wish to keep your hands in your pockets…
References
Manning, J. T. (2002). Digit ratio: A pointer to fertility, behavior and health. NJ: Rutgers
University Press.
Rushton, J. P. (1997). Race, evolution and behavior, 2nd Ed. New Brunswick, NJ: Transaction
Publishers
Appendix B 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.
"We found evidence at the molecular level that natural selection has maintained the variation in
the gene that allows us to taste or not taste PTC," said geneticist Stephen Wooding, Ph.D.,
corresponding author on the study and a post-doctoral fellow at the Eccles Institute of Human
Genetics at the University of Utah School of Medicine.
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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."
The researchers recently published their findings in the American Journal of Human Genetics.
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.
The PTC gene is only one of 24 bitter taste genes. Wooding and the others would like to
research the remaining genes to make a stronger correlation to smoking and diet.
"I would like to know which genes contribute most to smoking tendencies," Wooding said.
Other authors of the latest study include: Lynn B. Jorde, Ph.D., professor of human genetics at
the U of U School of Medicine; Michael J. Bamshad, M.D., associate professor of pediatrics, U
of U School of Medicine; Un-kyung Kim and Jennifer Larsen, both of the National Institute of
Deafness and Other Communication Disorders, National Institutes of Health.
Contact: Stephen Wooding
4-Patterns of Inheritance - 15 of 21
801-585-7423
University of Utah Health Sciences Center
2. [abridged] THIOUREA TASTING OMIM 171200 Online Mendelian Inheritance in Man
http://www.ncbi.nlm.nih.gov/entrez/dispomim.cgi?id=171200 accessed June, 2005
PHENYLTHIOCARBAMIDE TASTING, INCLUDED
PTC TASTING, INCLUDED
PROPYLTHIOURACIL TASTING, INCLUDED
PROP TASTING, INCLUDED
Gene map locus 7q35-q36
DESCRIPTION
Humans worldwide display a bimodality in sensitivity to the bitter taste of PTC, with
approximately 75% of individuals perceiving it as intensely bitter, whereas the rest perceive it as
tasteless. This difference has been the basis of study of taste perception in humans for over 70
years. Kim and Drayna (2004) provided an historical review of the subject.
Propylthiouracil (PROP) and PTC are members of a class of compounds known as thioureas.
The compounds carry the chemical group N-C=S, which is responsible for their characteristic
bitter taste (Bartoshuk et al., 1994; Drewnowski and Rock, 1995).
CLINICAL FEATURES
Variation in the ability to taste PTC was discovered by Fox (1931). Supplementation of the
standard test using quinine in the intermediate cases was suggested by Kalmus (1958). There
has been a suggestion that PTC tasting in man is related to a component of saliva: Cohen and
Ogdon (1949) claimed that PTC tasters can taste PTC only if it is dissolved in their own saliva. If
the tongue is dried and then presented with PTC dissolved in someone else's saliva, it is
tasteless. Jones and McLachlan (1991) described a technique for fitting mixture distributions to
data on PTC sensitivity.
It has long been proposed that there is a relationship between athyreotic hypothyroidism
(218700; formerly called athyreotic cretinism) and PTC nontasting (e.g., Shepard, 1961). Both
PTC and PROP are structurally similar to the naturally occurring antithyroid substance l-goitrin;
all members of this class of chemicals have antithyroidal activity and are not tasted by PTC
nontasters (Shepard, 1961). Nearly all individuals with athyreotic hypothyroidism are nontasters.
Tepper (1998) reviewed the literature for the ability to taste PTC and PROP and the implications
for food preference and dietary habits [her article is linked for extra reading-not required, but
interesting]. Tepper (1998) discussed the classic explanation for the persistence of the PROP
polymorphism, i.e., a selective advantage for avoidance of harmful compounds in the
environment that are often bitter tasting (Drewnowski and Rock, 1995). This taste aversion may
have special relevance for the avoidance of certain bitter-tasting vegetables. PROP and PTC
are chemically related to the isothiocyanates and goitrin, bitter-tasting compounds that are
present in cruciferous vegetables such as cabbage, broccoli, brussels sprouts, turnips, and kale.
When eaten in large quantities, these compounds interfere with iodine metabolism, producing
thyroid enlargement and goiterlike symptoms. Tepper (1998) noted that the incidence of thyroid
deficiency disease is relatively rare among PTC tasters. In modern society, however, avoidance
4-Patterns of Inheritance - 16 of 21
of bitter-tasting foods may have health disadvantages, since epidemiologic studies indicate that
diets low in fruits and vegetables and high in fat may be associated with increased risk of heart
disease and cancer.
INHERITANCE
Reddy and Rao (1989) examined the genetics of PTC taste thresholds by studying 100 nuclear
families. They concluded that variability in thresholds is controlled by a major locus with
incomplete dominance, as well as by a multifactorial component. Olson et al. (1989) studied 120
families and concluded that the data fitted best a 2-locus model in which one locus controls PTC
tasting and the other locus controls a more general taste ability.
Appendix C 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)
4-Patterns of Inheritance - 17 of 21
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
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
4-Patterns of Inheritance - 18 of 21
Figure 6 Poor red-green separation
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 called "pseudo-isochromatic"
plates or colour confusion plates. The plates are 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. Other more specific tests, such as the
anomaloscope, can pinpoint the more subtle defects in colour vision and provide a more
accurate classification.
4-Patterns of Inheritance - 19 of 21
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.
The most common, hereditary colour blindnesses are known as red-green colour vision
deficiencies, which are associated with disturbances in either the L-cone photopigment (protan
defects) or M-cone photopigment (deutan defects).
Tritan defects affect the S-cones. They are often referred to as yellow-blue disorders, but the
term blue-green disorder is more accurate because they affect the ability to discriminate colours
in the short- and middle-wave regions of the spectrum. Tritan defects arise from alterations in
the gene encoding the S-cone photopigment and are autosomal dominant (linked to
chromosome 7). Incidences are equivalent in males and females. In the UK, the frequency of
inherited tritan defects has been estimated as being as low as 1:13,000 to 1:65,000. Like many
autosomal dominant disorders, it is complicated by frequent incomplete manifestation.
Although congenital tritan defects are rare, the most frequently acquired colour vision defects,
whether due to ageing or to retinal or neural disorders are the acquired blue-yellow defects.
These are similar, but not identical to tritan defects. Unlike tritan defects, which are assumed to
be stationary, acquired defects are usually progressive and have other related signs, such as
associated visual acuity deficits.
Total colour blindness or monochromacy occurs when a person has only a single functioning
cone class (blue or S-cone monochromacy, green or M-cone monochromacy or red or L-cone
monochromacy) or has no functioning cones (complete achromatopsia or rod monochromacy).
These forms of colour blindness are extremely rare.
4-Patterns of Inheritance - 20 of 21
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
4-Patterns of Inheritance - 21 of 21
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.”