Download Lesson 12: Single Trait Inheritance student notes

Single Gene Inheritance
A single gene influences whether or not a person will go bald. Male-pattern
baldness has a very interesting pattern of inheritance. Unlike the traits you will
consider in this lesson, male pattern baldness trait depends on the “hormonal
environment” the genes find themselves in. The exact same combination of alleles
(Bb) in a woman’s body will not result in baldness, while it will result in baldness
in a man’s body. A 4000-year old body was recently found and analyzed for a
variety of genetic signatures. This body belonged to a hunter who would have been
bald, if the gene behaved then as it does now. 1
This lesson introduces you to (or reminds you of) the fundamentals necessary to understand genetic
inheritance. Because the genes that influence a person’s response to drugs are often complex and
interact with each other to produce that drug response trait, it is best to start with a single gene and
the rules for how parents will transmit that gene to their children. If you are unfamiliar with the
events (not the phases) of meiosis, you may want to start with a primer – several are recommended in
the supplemental readings. Meiosis, the making of sex cells that are poised to unite and form a child,
is where this story starts. Some understanding of probability will also be helpful. Punnett squares
help make thinking about probability very visual and manageable. But human families are not
always amenable to Punnett squares, and we need therefore to consider how Pedigree analysis can
also help solve genetics problems in human families.
Objectives
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To relate your understanding of genes and DNA to chromosomes
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To relate your understanding of alleles to Mendelian concepts of segregation and
dominance
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to understand how meiosis leads to the production of haploid gametes
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to relate the outcome of meiosis to the establishment of Punnett squares
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to extend your understanding of genetics to include incomplete dominance
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to learn how to set up and interpret pedigrees
Before you begin!
o
1
Your ideas
http://www.npr.org/templates/story/story.php?storyId=123567603
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Single Gene Inheritance
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What is a gene and how does that relate to a chromosome?
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In what fundamental way is meiosis different from mitosis (what is achieved)?
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In a Punnett square, what information goes on the outside?
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On the inside?
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What is 1/2 x 1/2 ?
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In humans, hair texture shows incomplete dominance. What kind(s) of kids can a curly haired man
have with a straight haired woman (three phenotypes – curly, wave, and straight)?
o
Previously learned material
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What is the molecular basis of the word “allele”?
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What are some genes important for us to study with regard to mood-altering drugs?
Lesson: Single Gene Inheritance
Guiding Questions
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How does DNA relate to genes and to chromosomes?
How are chromosomes divided during meiosis?
How are chromosomes reunited during fertilization?
What rules enable us to make predictions for specific matings?
What is dominance? What are some alternatives to dominance?
How are Punnett squares helpful for solving genetics problems?
How are Pedigrees helpful for solving genetics problems?
Key Terms
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Diploid
Haploid
Heterozygous
Homozygous
Dominant
Recessive
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Chromosome
Trisomy
Sex Chromosome
Autosome
Incomplete dominance
Activity One: Web Site Review
Please review the Web sites below.
To consider heredity visit http://learn.genetics.utah.edu/content/begin/tour/ and select the tutorial
called “What is Heredity”. While there, also visit “What are Traits” which is very long. Repeatedly
press “next” until the thumb icon (hitchhikers thumb) is highlighted.
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Give two examples of both a physical and a behavioral trait. One example should be NOT
found in the tutorial.
How do parents pass traits on to their children?
Of the parental chromosomes, what proportion of them are passed to children?
How does one define a dominant allele?
What are two ways that traits can be more complicated than ones like hitchhiker’s thumb?
To consider Punnett Squares http://anthro.palomar.edu/mendel/mendel_2.htm .
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What do the single-letter symbols mean – “G” or “Y”?
How do you fill in the boxes of a Punnett Square?
How can the Punnett Square be used to predict probabilities of genetic outcomes?
How is cystic fibrosis inherited?
What is an example of a dominantly inherited trait?
History Side Bar
Activity Two: Chromosomes: How Do They Relate to Genes and DNA?
Each of our 46 chromosomes is composed of one VERY long
DNA
double helix. That DNA is associated with proteins that
complex with, stabilize, and regulate that DNA. Our focus
will be
on the DNA. Our chromosomes are between 50 million and
250
million nucleotides.
Human chromosomes are “named” based on their size.
Chromosome 1 is largest (250 million nucleotides) and
chromosome 21 is smallest (chromosome 22 was mistakenly
named
to be the smallest). When a human is conceived with too many or too few of most of these chromosomes the
fetus is almost always inviable. Three chromosomes 21 are viable but lead to many unrelated symptoms. A
“syndrome” is a term used to describe unrelated symptoms caused by a single cause. Trisomy 21 is also called
Down’s Syndrome.
Chromosome pairs 1 through 22 are termed “autosomes”. These are chromosomes that are the same
regardless of sex. Our 23rd pair is the sex chromosomes. These sex chromosomes are quite different in size and
composition. The X chromosome is long and contains many necessary genes for BOTH sexes. The Y
chromosome is much smaller and has very small percentage of coding sequences. Humans can also tolerate
gain or loss of a sex chromosome. A single X chromosome leads to a female with Turner’s syndrome. XXY
leads to Kleinfelter’s syndrome male. XYY is very uncommon, and leads to a male with relatively few
differences from an XY male. It used to be believed that there was a disproportionately large proportion of the
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Single Gene Inheritance
incarcerated population that was XYY (suggesting that an extra Y chromosome leads to criminal behaviors).
This has been disproven since.
While chromosomal gain and loss is relatively inviable, it does happen reasonably frequently. Mistakes in
meiosis lead to these anomalies (called non-disjunction). The reason it seems uncommon is that fetuses which
gain or loss of large chromosomes will generally result in miscarriage. Small deletion of portions of
chromosomes is far more common and viable.
TEST OF CONTENT
Nucleotides, DNA, genes, chromosomes, and codons are all terms used to describe our genetic
information. Take each pair of terms and describe their relationship.
Nucleotides and genes
Genes and Chromosomes
DNA and codons
Chromosomes and DNA
Activity Three: How are Chromosomes Transmitted to Children?
Remind yourself of what meiosis accomplishes by visiting this interactive site
http://www.johnkyrk.com/meiosis.html . The cell here started with ONE PAIR of REPLICATED
chromosomes. A chromosome looks like a rod until it receives a signal that the cell needs to divide. Then the
chromosome duplicates to resemble an X. Thus XX refers to a PAIR of REPLICATED chromosomes. A cell
that starts with this pair, if it is in the ovaries or testes (gonads) will receive a signal to replicate and then
divide twice. Follow how those divisions divvy up the chromosomal material.
We say that the starting XX cell is DIPLOID. It has TWO copies of each unique chromosome type. The
resulting cell is HAPLOID, it only has one copy of each chromosome type. You might have learned about
phases (metaphase, etc.) but here you only need to know how chromosomes are divided up to make sex cells,
sperm and eggs (also called gametes).
Of course, our cells do not have just one pair of chromosomes, in humans, we have 23 pairs. Next visit the
interactive site at http://www.biologyinmotion.com/cell_division/index.html which has a cell with TWO
PAIRS of chromosomes. You’ll have to drag the chromosomes into the dividing cells and will be rewarded
when you’ve done that correctly.
Complete your study of meiosis at http://www.biology.arizona.edu/cell_bio/tutorials/meiosis/main.html
TEST OF CONTENT
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Single Gene Inheritance
Test your understanding of meiosis by completing question 1, 4, and 6 at
http://www.biology.arizona.edu/cell_bio/tutorials/meiosis/problems.html
WHICH parent, male or female, has had a faulty meiosis if an XYY child is conceived?
HARD – which division of meiosis was at fault (in previous XYY case)?
Activity Four: What did Mendel Infer About Gene Transmission BEFORE Even Seeing
Chromosomes?
As you view the tutorial linked here to get started thinking about the work of Gregor Mendel, keep in mind
that this work was presented in the 1865 and chromosomes were not identified until 1882.
http://anthro.palomar.edu/mendel/mendel_1.htm
Activity Five: Dominant and Recessive Alleles
Presentation to post here.
Follow this link to a presentation on dominant vs. recessive alleles and the molecular mechanism that
distinguishes them. Link presentation here.
Activity Six: Chromosome Movements Predict Gamete Genotypes
Few lessons connect the idea of meiosis in with the idea of Punnett Squares. In activity one, you learned that
on the OUTSIDE of the Punnett square, one will distribute the alleles that a parent can contribute via meiosis.
If a person has the genotype Aa, it means that on one chromosome, perhaps the one inherited from the
person’s father, there is an allele “A” that is dominant to the allele “a” on the other (maternal) chromosome.
Those alleles are located on paired chromosomes, say chromosome 3.
DNA sequence for “A” allele
DNA sequence for “a” allele
Paternal 3
Maternal 3
These chromosome 3s will undergo meiosis when this person makes gametes. Apply what you have already
learned about meiosis to this problem. Take these chromosomes through two rounds of meiosis, now labeling
your chromosomes with “A” or “a”. Check your diagram when you are finished.
Chromosomes replicate. An “X” shaped structure,
each half of each replicated chromosome has an
allele of the A gene.
Replicated chromosomes pair –
meiosis I.
AA aa
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Single Gene Inheritance
Paired chromosomes separate. Each cell has two
A gene alleles. The genotype is given below the
chromosomes
aa
AA
A
a
a
A
A
A
Each replicated chromosome breaks in to two
separate chromosomes
a
a
Four haploid gametes, two with
paternal “A” and two with
maternal “a” alleles
So when you put “A” and “a” on the side (or top) of a Punnett square, it represents the GENOTYPE of the
GAMETE that is contributing to the trait we are following. The “inside” boxes on a Punnett square represent
the possible combinations of gametes and the relative probabilities of these combinations.
Father’s gametes – a and a
Mother’s gametes – A and a
1/4 of
kids
1/4 of
kids
1/4 of
kids
1/4 of
kids
Fill out this Punnett square and check yourself against the “key”.
Mother’s gametes – A and a
Father’s gametes – a and a
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Single Gene Inheritance
TEST OF CONTENT
Visit the interactive Punnett square activity found at:
http://www2.edc.org/weblabs/Punnett/punnettsquares.html.
Activity Seven: Incompletely Dominant Genes
If you persevered through the entire tutorial found at http://learn.genetics.utah.edu/content/begin/tour/ you’ve
already met the idea of incomplete dominance. Go back there if you find you need a little reinforcement after
this activity is complete. Incomplete dominance happens for traits whose alleles do not show complete
“Mendelian” dominance. One of Mendel’s dominant traits was purple flower color.
When purple-flowered plant was crossed to white-flowered plant:
x
But sometimes, two contrasting traits are crossed and rather than getting one trait to dominate the other, an
intermediate trait is seen:
x
In each case, a heterozygote is produced, but in one case, the heterozygote resembles one of the homozygotes
(purple) and in the other case, the heterozygote has a phenotype that is intermediate to the two extremes.
In the “Mendelian” dominance case, having one allele make the functional gene product is sufficient, and
dosage is not important. In incomplete dominance, allele dosage is very important. Making functional
product from two alleles is noticeably different from making functional product from one allele. “RR”
homozygotes make a LOT of red pigment and are therefore darker red than “Rr” heterozygotes who make a
moderate amount of red pigment and are therefore paler in color.
Remember how important the heterozygote was in determining which allele is dominant previously. The
heterozygote is even more key in determining if a trait shows complete (“Mendelian”) or incomplete
dominance.
Because incompletely dominant traits do not show dominance, it is problematic to call one allele dominant and
the other recessive. Instead, we say that one allele is contributing (makes a contribution of color or size or any
other feature) while the other allele is non-contributing (is neutral, making no contribution to the trait).
TEST OF CONTENT
A gene called “Insulin-like growth factor” (I) controls size in dogs. A Great Dane is homozygous
for the I allele, whereas a toy poodle is homozygous for the “I” allele. A mating between the two gives a
middle sized dog. Assume there is only this one gene that influences size and determine what sizes (and in
what proportion) would be seen if the Great Dane mated with the middle-sized GreatOodle. The middle
sized dog makes two types of gametes, “I” and “I”, while the large dog makes only “I” gametes.
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Activity Eight: Pedigree Analysis
While Punnett squares are useful for solving problems with organisms that have large numbers of offspring,
human pairs often have small enough families that proportions are not particularly handy to solve the
inheritance pattern of a trait. Often a geneticist will examine MANY families to determine how a trait is
inherited. Each family’s information is summarized in this kind of a study by making a “family tree” or a
pedigree. Pedigrees are diagrams that show familial relationships in a standard (and therefore easy to
interpret by others) way and represents the phenotype of each individual in the family.
Conventions used to make and solve pedigrees:
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Circles represent women and girls
Squares represent men and boys
Filled symbols represent people with “the trait” (a disease or a phenotype of interest).
Empty symbols represent people who do NOT have “the trait”
If the trait shows incomplete dominance, the symbols will be hatched or have a dot in the middle
Matings are connected by a line between a male and a female
Offspring from a mating arise from a stem below that line, and are given in order of birth from a “sib
line”
 Twins descend from the sib line as a pair, either directly from the sib line (fraternal twins) or from a
stem off the sib line that then forks (identical twins)
Visit the interactive pedigree-making site at http://learn.genetics.utah.edu/content/addiction/pi/
Activity Nine: Practice Problems
Genetics is an area of biology that some students find challenging because it is more problem solving than
memorization. To give yourself practice with this type of approach to studying, do as many practice problems
as you can. In the sites below, you will find an interactive quiz for genetics problems.
http://anthro.palomar.edu/mendel/quizzes/mendqui2.htmhttp://anthro.palomar.edu/mendel/mendel_3.htm
Activity Ten: Reading
Required Reading
Introductory biology textbook
Chapter(s) or section(s) on Meiosis and Punnett Squares
Chapter(s) section(s) on Mendel – single gene traits, dominant, recessive, incomplete dominance
Internet
 http://learn.genetics.utah.edu/content/begin/tour/ (introduction to heredity)
 http://anthro.palomar.edu/mendel/mendel_2.htm (introduction to Punnett squares)
 http://www.biology.arizona.edu/cell_bio/tutorials/meiosis/main.html (introduction to meiosis)
Supplemental Reading
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Single Gene Inheritance
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For anyone requiring a little back-to-basics on genes, heredity, chromosomes.
http://www.youtube.com/watch?v=eOvMNOMRRm8&feature=related
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For a reminder of what meiosis can do that mitosis cannot, visit
http://www.pbs.org/wgbh/nova/baby/divi_flash.html
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