REVIEW 5
Heredity
Modern society uses scientific knowledge to predict events and to control nature. Sometimes
these uses of scientific knowledge aren't too controversial. For example, scientists know
that germs cause disease and that regular handwashing reduces the spread of disease. So,
scientific knowledge makes a prediction: The dirtier your hands are, the greater the chance
that you (or someone else) will get sick. Scientific knowledge also suggests a way to
control the spread of disease: If you wash your hands regularly, then you and those around
you will not get sick as often.
Sometimes, however, the use of scientific knowledge to predict events and control nature is
more controversial. Consider Gregor Mendel and two of his discoveries in the midnineteenth century: (1) Organisms have "factors"—we call them genes— that control their
physical traits, and (2) organisms pass on their genes to their offspring in predictable ways.
This knowledge isn't too controversial if all you want to do is predict the ratio of purple to
white blooms in a batch of flowers. But scientists now use knowledge based on Mendel's
work to predict events and control nature in ways that could radically change society.
Scientists can predict the likelihood that children not yet conceived may inherit certain
diseases. Scientists are also using the knowledge of genetics to alter the biology of plants,
animals, and humans. None of these practices would be possible without the study of
heredity begun by Gregor Mendel.
Heredity
Heredity refers to the passing of traits from parents to offspring. For example, bacteria, rabbits,
and humans all pass on traits to their offspring. Genes control the traits that appear in those
offspring. When organisms reproduce, they give genes to their offspring, and those genes
cause traits to appear. So, the genes your parents gave you are responsible for your eye
color, your natural hair color, the shape of your ears, and so on. Each gene is a small piece
of a long, complicated molecule called DNA. In the reproductive process, DNA arranges
itself into structures called chromosomes.
Words to Know
allele
chromosome
codominance
DNA
dominant allele
gene
genotype
heredity
heterozygous
homozygous
pedigree
phenotype
Punnett square
recessive allele
trait
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Write the term that matches each definition.
A section of a DNA molecule that controls a trait:
The passing of traits from parents to offspring:
A complicated molecule that contains genes:
A characteristic in an organism:
The structure into which DNA arranges itself:
Phenotype and genotype
Scientists can study traits by examining how organisms look on the outside. The outside
expression of a gene is called the phenotype. For example, suppose Chris has a widow's
peak on his forehead. (A widow's peak is a point formed by a person's hairline.) A scientist
will say that Chris is expressing the widow's peak phenotype. Scientists are also able to
study traits by examining the genes that code for them. Humans have two copies of each
gene, called alleles, one from the biological mother and one from the biological father.
When scientists determine which two alleles a person has, they are determining the
person's genotype.
Just because we can see that Chris has the widow's peak phenotype does not mean we know his
genotype. A dominant allele causes a widow's peak in humans. Dominant alleles hide the
expression of recessive alleles. Scientists use an uppercase letter (W) to indicate a
dominant allele and a lowercase letter (w) to indicate a recessive allele. The widow's peak
gene has three possible genotypes: WW, Ww, or ww. Chris has a widow's peak. Therefore,
we know that Chris has at least one dominant allele. His genotype is either Ww or Ww. If a
person has two identical alleles, that person is homozygous for a trait. If a person has two
different alleles, that person is heterozygous for a trait. Suppose that Chris's genotype is
WW; he is homozygous dominant. Now suppose that his genotype is Ww; he is
heterozygous.
Lupe doesn't have a widow's peak. Write the letters for her genotype.
What is Lupe's genotype: homozygous dominant or homozygous recessive?
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Punnett squares
If you know the genotypes of people who are about to have a baby, you can use a Punnett
square to predict the probability that their children will inherit their alleles. A Punnett
square shows the possible combinations of alleles that children can inherit when two
people produce offspring. Let's look at a Punnett square that shows the mating between
two people that are heterozygous for a widow's peak.
---see diagram
The genotypes written inside the Punnett square are all of the genotypes possible in the
offspring of these two people. The offspring have a 75% chance of having the widow's
peak phenotype and a 25% chance of not having the widow's peak phenotype.
Pedigrees
Punnett squares are useful for finding the probability that a trait will or will not express itself
within offspring. A pedigree goes further by tracking the transmission of a trait from
generation to generation. A pedigree has a few simple rules:
 Circles stand for females; squares stand for males.
 Horizontal lines between individuals represent mating; vertical lines represent parentoffspring relationships.
 Generations are identified by Roman numerals (I, II, III . . .). Individuals are identified by
Arabic numerals (1, 2, 3 . . .).
 A shaded figure indicates an individual in which the trait is expressed.
 An unshaded figure indicates an individual in which the trait is not expressed.
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The following pedigree shows how the trait for a widow's peak was passed down among three
generations of two families. Remember, the trait for a widow's peak (W) is dominant over
not having a widow's peak (w).
---see diagram
What are the genotypes of individuals 12, 13, and I4?
What are the genotypes of individuals II1, 112, and II3?
What is the genotype of individual 1113?
Multiple alleles
So far we have looked at traits controlled by a single gene pair (two alleles). Many traits are
controlled by more than just a single gene pair. For example, scientists have located three
gene pairs (six alleles) that control eye color, and they suspect that there may be more. By
tracking two of the known gene pairs, however, scientists can usually predict whether a
child will have brown, blue, or. green eyes. One gene pair has alleles for brown (B) and
blue (b) eyes. A second gene pair has alleles for green (G) and blue (b) eyes. The alleles in
the two gene pairs are dominant or recessive to each other in the following ways:
 Brown (B) is dominant over both blue (b) and green (G).
 Green (G) is dominant over blue (b) and recessive to brown (B).
 Blue (b) is recessive to both brown (B) and green (G).
In this model of heredity, each person has two gene pairs that control eye color.
For example, a person with the genotype BbGG has brown eyes, because the brown allele in the
first gene pair is dominant over the blue and green alleles. A person with the genotype
bbGb has green eyes, because the green allele in the second gene pair is dominant over the
blue alleles.
What is the genotype of a person with blue eyes?
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As you might expect, a Punnett square for eye color is more complicated than a Punnett square
for a trait controlled by just one gene pair. For example, suppose that one parent has the
genotype BbGG, and the other parent has the genotype bbGb. Each parent contributes two
alleles (one from each gene pair) so that each offspring ends up having a total of four
alleles for eye color. The following Punnett square represents this pairing. One possible
outcome of the pairing has already been filled in for you.
---see chart
Fill out the rest of the Punnett square shown above.
How many times does each of the following genotypes appear in the Punnett square above?
Express, as a percentage, the probability that an offspring of this pairing will have the
following phenotypes:
Again, this model of the heredity of eye color does not explain all outcomes. For example,
sometimes two blue-eyed parents can have a brown-eyed offspring. Eyes can also be gray,
or hazel, or many different shades of brown. The third gene pair, not accounted for in this
model, clearly influences eye color. Still, scientists often use a two gene-pair model to
predict the eye color of children.
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Codominance
Alleles are not always in a dominant-recessive relationship. Sometimes alleles are codominant,
which means that both alleles express themselves equally in the phenotype of an organism.
In cattle, for example, the allele for red hair (R) is codominant with the allele for white hair
(W). Cattle that are homozygous for the red allele (RR) have red hairs. Cattle that are
homozygous for the white allele (WW) have white hairs. Cattle that are heterozygous for
hair color (RW) have both red hairs and white hairs. Codominance also occurs in human
alleles and traits, a fact that you will explore in the upcoming activity.
Keys to Keep
 Gregor Mendel's studies of heredity increased the ability of scientists to predict and control
the traits of organisms.
 Heredity is the study of how traits are passed on through generations of organisms. I
Genotype refers to which alleles are in an organism's DNA.
 Phenotype refers to an organism's physical appearance.
 You can use Punnett squares to determine the probability of an offspring having a trait.
 You can use pedigrees to follow a trait through generations of offspring.
 Alleles can be dominant and recessive to each other or codominant to each other.
 James Watson and Francis Crick discovered the structure of the DNA molecule. (See page
74.)
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Explore It Yourself
Human blood comes in four types: A, B, AB, and 0. Blood type is determined by three alleles
with differing relations to each other. The allele for type A blood (IA) and the allele for
type B blood (IB) are codominant with each other. The allele for type O blood (i) is
recessive to both the IA and IB alleles. In the following activity, you will use Punnett
squares to explore the inheritance of the alleles for blood type.
Step 1: Suppose that a man who is homozygous for type A blood and a woman with type O
blood marry and have children. Use the following Punnett square to find the possible blood
type genotypes of their offspring.
---see chart
Step 2: Now suppose that a child from Step 1 grows up to marry and have children with a
person who is heterozygous for type B blood. Use the following blank Punnett square to
find the possible genotypes of their children. Be sure to label which parent is which, as
above.
---see chart
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What Does It Mean?
1. Express as a percentage the probability that a child from Step 1 will have one of the
following blood types.
2. Express as a percentage the probability that a child from Step 2 will have one of the
following blood types.
3. Suppose that the couple in Step 2 have four children. Is it possible for all four children to
have type O blood? Explain your answer.
4. Suppose that a married couple, both with type B blood, are thinking of having children. The
couple does not know what genotypes they have for blood type. Can they be certain that
their children will all have type B blood? Explain your answer.
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People in Science
"People in Science" doesn't normally cover two people at once, but we'll make an
exception for James Watson and Francis Crick. It's hard to speak of one without
speaking of the other.
In 1947, though the two men would not meet for four years, they already shared one
experience: They had both read the book What Is Life? by the scientist Erwin
Schrodinger. Schrodinger wrote that biologists should study heredity—the
transmission of genetic traits from one generation to the next—at the level of
molecules. Such information, he argued, could be used to treat disease and
improve human lives. Schrodinger's book convinced both Watson and Crick to
study molecular biology.
When Watson and Crick met at Cambridge University in England in 1951, they quickly
became friends. They decided to work together to find out what kind of molecule
could carry and transmit traits. They knew that such a molecule had to have three
qualities. It had to be complex enough to carry traits, stable enough to survive in
the cells of organisms, and capable of copying itself so its information could be
passed on to the next generation. Such a molecule would be very complicated,
indeed.
Watson and Crick reviewed evidence suggesting that molecules called nucleic acids
could be the molecules of heredity. To determine whether nucleic acids fit the bill,
they began studying one especially complex molecule: deoxyribose nucleic acid,
or DNA. The key thing was to discover the structure of the DNA molecule. Other
scientists had already shown which atoms made up DNA, but nobody knew how
those atoms fit together. Could DNA have a structure that was complex, stable,
and self-replicating? To figure this out, Watson and Crick built large tinker-toy-like
models that represented possible structures of DNA. Two colleagues, Maurice
Wilkins and Rosalind Franklin, provided data that helped Watson and Crick
improve their models.
In 1953, Watson and Crick arrived at a structure for DNA that was complex, stable,
and self-replicating: the double helix, which looks like a spiral staircase. Watson
and Crick proposed that when the DNA molecule needs to copy itself, it splits right
down the middle. Each side becomes the basis for one new DNA molecule, so the
two sides end up making two new DNA molecules. Barring mistakes, the two new
DNA molecules are identical to each other and to the original DNA molecule. Later
studies confirmed that nucleic acids are indeed the molecules of heredity, and that
Watson and Crick's model of DNA is correct. Watson, Crick, and Wilkins received
the 1961 Nobel Prize for their work on DNA. (Franklin died in 1958, and the Nobel
committee does not award the prize to people who have died.) James Watson and
Francis Crick went on to make more accomplishments as individual scientists, but
they will best be remembered as the team of Watson and Crick, the scientists who
discovered the structure of the DNA molecule.
James D. Watson
(United States 1928—)
Francis Crick
(England 1916-2004)
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AIMS Science Practice
1 The gene for unattached earlobes, E, is dominant over the gene for attached earlobes, e.
Ricky has unattached earlobes, his father has unattached earlobes, and his mother has
attached earlobes. Lucy has attached earlobes, her father has attached earlobes, and her
mother has unattached earlobes.
Suppose Ricky and Lucy marry and have children. Which Punnett square correctly predicts the
probabilities of their children having unattached or attached earlobes?
2 Amanda is not sick, but she is the carrier of a disease allele. What is another way to describe
her condition?
A homozygous dominant for the disease gene
B heterozygous for the disease gene
C recessive for the disease gene
D homozygous recessive for the disease gene
3 Genes code for inherited traits. Which of the following best describes a gene?
A a section of a DNA molecule
B a small protein molecule
C a carbohydrate molecule
D a chromosome
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DIRECTIONS: Base your answers to question 4 on the diagram below, which shows a
model of human inheritance.
4 The allele for the condition is recessive. What is the genotype of individual 12 for this
condition?
A homozygous recessive
B homozygous dominant
C heterozygous
D It cannot be determined.
5 Look at the following partial Punnett square for the widow's peak trait.
Which of the following can be concluded about the genotype of each parent?
A One parent is heterozygous; the other parent is homozygous recessive.
B Both parents are homozygous dominant.
C One parent is homozygous dominant; the other parent is heterozygous.
D Both parents are homozygous recessive.
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DIRECTIONS: Base your answers to questions 6 and 7 on the diagram below, which
shows the result of sexual reproduction in two mice.
6 In each offspring, what percentage of the genetic material comes from the female parent?
A 25%
B 50%
C 75%
D 100%
7 In this species of mouse, white color (W) is dominant over black color (w). Which of the
following states possible genotypes for the parents?
A male = Ww, female = Ww
B male = WW, female = ww
C male = ww, female = ww
D male = WW, female = WW
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8 A human being can have one of four blood types: A, B, AB, and O. The type A allele (IA)
and type B allele (IB) are codominant to each other and dominant over the type O allele (i).
Suppose that two people, both heterozygous for type A blood, marry and have a child. What is
the probability that the child will have type A blood?
A 25%
B 50%
C 75%
D 100%
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