Download Lesson 17: Patterns of Inheritance (3

Lesson 17: Patterns of Inheritance (3.2.2)
GENES AND INHERITANCE
Genes, which are specific portions of DNA, determine hereditary characteristics. Genes
carry traits that can be passed from one generation to the next. Alleles are different forms
of a gene. Two alleles make up one gene. For every trait, each parent passes on one
allele to the offspring. Each offspring has at least two alleles for every trait. The
expression of physical characteristics depends on the genes that both parents contribute
for that particular characteristic. Genotype is the term for the combination of alleles
(type of genes) inherited from the parents.
Genes can be either dominant or recessive. The dominant gene is the trait that will most
likely express itself. If both alleles are dominant, or one is dominant and one is recessive,
the trait expressed will be the dominant one. In order for expression of the recessive
gene to occur, both alleles must be the recessive ones. For example, a mother might pass
on a gene for having dimples, and the father might pass on a gene for not having dimples.
Having dimples is dominant over not having dimples, so the offspring will have dimples
even though it inherits one allele of each trait. For the offspring not to have dimples,
both the mother and father must pass along the allele for not having dimples. The
phenotype is the physical expression of the traits. The phenotype does not necessarily
reveal the combination of alleles and can be influenced by the environment surrounding
an organism. For example, the genotype determining a person’s height is a range, such as
5’4” to 5’6”. The nutrition a person receives growing up will influence if that person
only reaches 5’4” or if they grow as tall as 5’6” (or somewhere in between). Poor
nutrition will result in a height around 5’4”, while good nutrition can result in this
individual reaching his maximum height potential of 5’6”.
When studying the expression of the traits, geneticist use letters as symbols for the
different traits. Capital letters are used for dominant alleles and lowercase letters for
recessive alleles. For dimples, the symbol could be D. For no dimples, the symbol could
be d. The genotype of the offspring having one gene for dimples and one gene for no
dimples is Dd. The phenotype for this example is having dimples.
If an individual inherits two of the same alleles, either dominant or recessive, for a
particular characteristic, the individual is homozygous. If the offspring inherits one
dominant allele and one recessive allele, such as in the example in the above paragraph,
the individual is heterozygous.
Geneticists use the Punnett square to express the possible combinations for a certain
trait an offspring may inherit from the parents. The Punnett square shows possible
genotypes and phenotypes of one offspring. Figure 17.2 below shows an example of a
monohybrid cross, which involves one gene (trait).
The Punnett square
The Punnett square is a tool geneticists use to determine the possible genotype of one
offspring. The possible alleles donated by one parent are written across the top and the
possible alleles donated by the other parent are written along the left side. It does not
matter which parent is put on the top and which is put on the left side. In the example,
the cross between two heterozygous parents is examined.
D = allele for dimples
d = allele for no dimples
Each time this male and female produce an offspring, there is a ¾ (or 75%) chance the
offspring will have dimples and a ¼ (or 25%) chance the offspring will have no dimples.
In a Punnett Square for a monohybrid cross, each box is 25%. Because there are four
boxes, the total is 100%.
Figure 17.2 Punnett Square for Dimples/No dimples
The phenotype depends not only on which genes are present, but also on the
environment. Environmental differences have an effect on the expression of traits in an
organism. For example, a plant seed may have the genetic ability to have green tissues,
to flower and to bear fruit, but it must be in the correct environmental conditions. If the
required amount of light, water and nutrients are not present, those genes may not be
expressed.
Temperature also affects the expression of genes. Primrose plants will bloom red flowers
at room temperature and white at higher temperatures. Himalayan rabbits and Siamese
cats have dark extremities like ears, nose and feet, at low temperatures. Warmer areas of
the animals’ bodies are lighter colored.
MENDEL’S CONTRIBUTION TO GENETICS
Around 1850, Gregor Mendel (1822-1884) began his work at an Austrian monastery.
Many biologists call Mendel “the father of genetics” for his studies on plant inheritance.
Mendel and his assistants grew, bred, counted and observed over 28,000 pea plants.
Pea plants are very useful when conducting genetic studies because the pea plant has a
very simple genetic makeup. It has only seven chromosomes, its traits can be easily
observed, and it can cross-pollinate (have two different parents) or self-pollinate (have
only one parent).
Table 17.1 lists some of the pea plant traits, along with their attributes. To begin his
experiments, Mendel used plants that were true breeders for one trait. True breeders are
homozygous dominant and when self-pollinated will produce offspring identical to itself.
PRINCIPLE OF DOMINANCE
Through his experiments, Mendel discovered a basic
principle of genetics, the principle of dominance. Mendel’s
principle of dominance states that some forms of a gene or
trait are dominant over other traits, which are called
recessive. A dominant trait will mask or hide the presence
of a recessive trait. When Mendel crossed a true breeding
tall pea plant (TT) with a true breeding short pea plant (tt),
he saw that all the offspring plants were tall. The tallness
trait masks the recessive shortness trait. The crossing of the
true breeders is the parental generation, or the P
generation. The offspring produced are the first filial
generation or F1 generation. The offspring of the F1
generation are called the second filial or F2 generation.
PRINCIPLE OF SEGREGATION
Crossing plants from the F1 generation creates the F2 generation. Mendel soon
discovered that a predictable ratio of phenotypes appeared. For every one plant that
expressed the recessive trait, there were three plants that expressed the dominant trait.
Mendel realized that this ratio could only occur if the alleles separate sometime during
gamete formation.
As a result, Mendel developed his principle of segregation. The principle states that
when forming sex cells during meiosis, the paired alleles separate so that each egg or
sperm only carries one form of the allele. The two forms of the allele come together
again during fertilization.
PRINCIPLE OF INDEPENDENT ASSORTMENT
When Mendel began to study dihybrid crosses,
which involve two traits, he noticed another
interesting irregularity. Mendel crossed plants that
were homozygous for two traits, seed color and seed
texture. Round seed texture and green color are both
dominant traits. Mendel assigned the dominant
homozygous P generation the genotype of (RRGG).
Wrinkled seed texture and yellow color are both
recessive traits. The recessive homozygous P
generation seeds were assigned the genotype (rrgg).
When (RRGG) was crossed with (rrgg) the resulting
F1 generation was entirely heterozygous (RrGg). The
F1 generation was then allowed to self-pollinate,
resulting in an F1 dihybrid cross of (RrGg) with
(RrGg). The result was an F2 generation with a
distinct distribution of traits, as depicted in Figure 8.4.
Counting up the genotypes of the F2 generation should
give you the result that 9/16 of them will have the
round, green phenotype and 1/16 will have the wrinkled, yellow phenotype.
The consistent observation of this trend led to the development of the principle of
independent assortment. This principle states that each pair of alleles separates
independently during meiosis in the formation of the egg or sperm. For example, the
allele for green seed color may be accompanied by the allele for round texture in some
gametes and by wrinkled texture in others. The alleles for seed color segregated
independently of those for seed texture. Because there are different allele combinations
for each parent, this leads to an increase in genetic diversity. This same concept is why a
parent can have a child that does not look like that parent.
Activity
PEDIGREE CHARTS
A pedigree is a graphical chart used to identify the lineage of individuals. Pedigrees are
useful when the genotype of individuals is unknown.
Pedigrees are often used when breeding
domesticated animals like dogs or race horses.
However, pedigrees can also be used to examine the
heredity of any type of living organism, including
humans. Many times, pedigrees help show the
inheritance of genetic disorders of members within
families. Males are represented with a square and
females are represented with a circle. Horizontal
lines found between males and females represent a
mating. Males and females located at the end of a
vertical line represent offspring from the above
mating. Sometimes the generations found in a
pedigree are numbered using roman numerals. For
example, the pedigree in Figure 17.5 shows three
generations in a family.
Members of a family affected with a genetic disease are shown as shaded or colored
shapes. Members of a family unaffected with a genetic disease are found on the pedigree
as un-shaded. Sometimes individuals who are carriers for the disease are shown half
shaded. Examine the sample pedigree in Figure 17.6. A key may be provided to help
you better understand the pedigree.
Activity
Make a pedigree of three or four generations of individuals in your family. Select a
genetic trait like eye color, earlobe attachment, curly hair or having dimples and trace the
inheritance of this trait through your family. Similar to some activities from:
Resource from NC Biology SCOS support document: Alien Encounters Pages 189-215
http://www.dpi.state.nc.us/curriculum/science/units/high/
Lesson 17 Section Review: Part 1
Genetics
A. Define the following
Gene
Allele
Genotype
Dominant gene
Recessive gene
Self-pollinate
phenotype
homozygous
heterozygous
Punnett square
monohybrid cross
cross-pollinate
Gregor Mendel
true breeder
principle of dominance
principle of segregation
dihybrid cross
principle of independent assortment
B. Choose the best answer.
1. What is the combination of inherited alleles found?
A. heterozygote
C. genotype
B. phenotype
D. zygote
2. What is the expression of traits called?
A. phenotype
B genotype
C. mutation
D. allele
3. If an individual inherits one dominant and one recessive allele, what is the genotype?
A. homozygous
C. heterozygous
B. recessive
D. phenotype
4. If an individual inherits two of the same alleles, either both dominant and both
recessive for a particular characteristics, what is the individual’s genotype?
A. heterozygous
C. homozygous
B. phenotype
D. mutated
5. Use a Punnett square to predict the cross of a homozygous green parent with a
homozygous yellow parent if yellow is dominant over green. What is the possible
phenotype of the
offspring?
A. all yellow
C. neither yellow nor green
B. all green
D. some yellow and some green
C Complete the following exercises.
1. The gene for cystic fibrosis is a recessive trait. This disorder causes the body cells to
secrete large amounts of mucus that can damage the lungs, liver and pancreas. If one out
of 20 people is a carrier of this disorder, why is only one out of 1,600 babies born with
cystic fibrosis?
2. What is the relationship between phenotype and genotype?
3. Compare homozygous alleles to heterozygous alleles.
4. What specifically determines hereditary characteristics in an individual?
5. Is it possible for two individual who do not have Huntington disorder to have a child
that has Huntington’s disorder
MODES OF INHERITANCE
Recall that humans have 46 chromosomes. These chromosomes can be divided into two
groups, autosomes and sex chromosomes. Humans have 44 autosomes (chromosome
pairs 1-22) which contain the DNA those codes for almost every trait in the body. Sex
chromosomes are the chromosomes responsible for determining the sex of an organism.
These chromosomes carry the genes responsible for sex determination as well as other
traits. They are the 23rd pair of chromosomes and are sometimes called X or Y
chromosomes. Males have the genotype XY and females have the genotype XX. In
females, one X comes from their mother and one X comes from their father. In males,
the X chromosome comes from their mother and the Y chromosome comes from their
father. We will look at several genetic disorders found on both the autosomes and the sex
chromosomes.
SIMPLE OR COMPLETE DOMINANCE
There are a number of genetic
disorders that can be found on the
autosomes. Sickle cell anemia,
cystic fibrosis, and Huntington’s
disease are just three of them.
Sickle cell anemia is a recessive
genetic disorder found in people
with African descent. Sickle cell
affects the red blood cells, which
carry oxygen throughout the body.
A normal red blood cell is shaped
like a doughnut. Individuals with
sickle cell have red blood cells shaped like a crescent moon. These cells do not carry
oxygen. However, people who are heterozygous do not express any symptoms of sickle
cell, but cannot get malaria.
Look at the pedigree in Figure 17.7. It can be determined that person I-1 has the
genotype aa because he is completely shaded, and sickle cell is a recessive disorder. To
determine the genotype of person I-2, you must look at the children. Person I-2 is not
shaded, so she does not have sickle cell, and thus has at least one dominant allele, A. She
has 5 children (II-1, II-3, II-5, II-6, and II-8). Three of these children are shaded and
therefore have the genotype aa. The only allele dad (I-1) had to donate was an a.
Therefore, the second a had to have come from mom (I-2). She is therefore a carrier, and
you could shade her circle half way.
What was the probability that person III-8 would have cystic fibrosis? To
solve, figure out the genotypes of her parents and work out the Punnett
Square. Her mom (II-7) is a carrier, Aa, and her father has the disorder,
aa. When the Punnett Square is worked out, as in Figure 17.8 there was a
50% chance of getting sickle cell anemia.
Cystic fibrosis is another recessive genetic disorder. It is found in people with European
descent. Cystic fibrosis causes a buildup of thick mucus in the lungs, and digestive
organs. Think about the last time you had an upper respiratory infection. What were
your symptoms? Your lungs were tight, you tired easily, and you probably coughed up
mucus. People with cystic fibrosis have these same symptoms on a daily basis and to a
much higher degree. They are prone to upper respiratory infections and will most likely
need a lung transplant because of the damage caused to the lungs by the buildup of
mucus. Examine the predigree below in figure 17.9.
Look at person 1. Her mother has cystic fibrosis, but her father does not. Because person
1 is not shaded, she also does not have cystic fibrosis. This means she has at least one
dominant allele, F. Because her mom has the recessive disorder and therefore a genotype
of ff, the only allele mom could give was an f.
Therefore, person 1 is a carrier and we could shade
in her circle half way.
Huntington’s disease is a dominant genetic disorder
that causes deterioration of the basil ganglia, a group
of nerves at the base of the brain
where the spinal cord meets.
Symptoms generally appear in
people between the ages of 3545, and are gradual.
Uncontrollable and jerky muscle
movements, memory loss,
dementia, and restlessness are
just a few symptoms. Because Huntington’s is a progressive
disorder, it always leads to death. Because Huntington’s disease is a
dominant disorder, the genotype of all the non-shaded circle and
squares would be hh. Every shaded circle and square must have a
genotype with at least one dominant allele, HH or Hh.
Activity
Make a pedigree of three or four generations of individuals I your family. Select
a genetic trait like dimples, earlobe attachment, or hitch hackers’ thumb and trace
the inheritance of this trait through your family.
Similar to activities from :
NC Biology SCOS support document: Alien Encounters Pages 189-215
http://www.dpi.state.nc.us/curriculum/science/units/high/
INCOMPLETE DOMINANCE
Incomplete dominance is the situation when one trait is not completely dominant over
the other. Think of it as blending of the two traits¸ like paint for example. Red and white
paint, when mixed, create pink paint. All of the offspring in this F1 generation will show
a phenotype that is a blending of both the parents. If the F1 generation is self-pollinated,
the ratio of the offspring will appear in a predictable pattern. One offspring will look like
one parent, one offspring will look like the other parent, and two offspring will have a
phenotype that is an intermediate of both parents.
For example, a cross between a red and a white four o’clock
flower demonstrates this point. One flower in the parental
generation is red with genotype R1R1. The other flower is
white with genotype R2R2. The offspring of this cross appear
pink and have a genotype of R1R2. Sometimes R1R1 is
written as R’R’, R2R2 is written as RR, and R1R2 can be
written as R’R. See Figure 8.11 for the genotypes and the
phenotypes of the P, F1, and F2 generations.
CO-DOMINANCE
In co-dominant traits, there are no recessive alleles. Both
alleles are dominant, each dominant allele coding for a
different trait. One example occurs in cows in which the trait for red hair is co-dominant
with the trait for white hair. If a red haired cow is crossed with a white haired cow, the F1
generation is roan. The cow appears to look pinkish-brown from far away. However, if
you look closely at the coat of this animal, you will notice that both solid red and solid
white hairs found on the coat give the animal its unique color. This animal expresses
both the red and the white trait. This cross is shown below:
RR x WW
Though they sound similar, there are two main differences between the situations of
co-dominance and incomplete dominance. When one allele is incompletely dominant
over another, the blended result occurs because neither allele is fully expressed. That is
why the F1 generation four o’clock flower is a totally different color (pink). In contrast,
when two alleles are co-dominant, both alleles are completely expressed. The result is a
combination of the two, rather than a blending. The roan horse’s hair may look pink for
afar, but it is actually a combination of distinct red hair and white hair.
Look at Table 17.2 below. It summarizes the difference between simple Mendelian
dominance, co-dominance, and incomplete dominance using hamster’s fur color as an
example. Keep in mind the fur of hamsters may not express all three types of dominance.
This is just an example to help you understand the difference between these three.
Table 17.2 Types of Inheritance Patterns
Simple Dominance
BB
(black fur)
Co-dominance
BB
(black fur)
Bb
(black fur)
BW
(black and white
fur)
Incomplete dominance BB
BW
(black fur)
(gray fur)
**AB blood type is another example of co-dominance.
bb
(white fur)
WW
(white fur)
WW
(White fur)
SEX-LINKED TRAITS
A trait that is carried on the X chromosome is sex-linked. If a recessive trait, like color
blindness, is located on the X chromosome, it is not very likely that females will have the
phenotype for this condition because they have two X chromosomes. In one type of color
blindness (recessive disorder sex-linked trait), individuals might see red and greens as
shades of gray. For a heterozygous female, the X with the dominant allele will mask the
recessive allele on the second X chromosome. It is more likely that males will have the
phenotype for this condition because they only have one X chromosome. Because males
only have one X chromosome, if this chromosome carriers a recessive allele, there is not
a second X chromosome to potentially mask the recessive allele. A heterozygous female
that has one X chromosome with a dominant allele and one X chromosome with a
recessive allele is called a carrier. A carrier is a person that has the ability to pass a trait
or disorder onto their offspring, but does not express the phenotype or symptoms of that
disorder. In sex linked traits, only females can be carriers because males do not have two
X chromosomes.
Sex
Male
Genotype
XBY
XbY
Phenotype
Not colorblind
Colorblind
XBXB
Not colorblind
B b
X X
Not colorblind (carrier)
XbXb
Colorblind
Figure 17.13 Sex-linked Traits
Female
Examine the Punnett square in Figure 17.14, which shows that cross of a female who is
heterozygous for color blindness with a normal male (not colorblind). This Punnett
square shows how a mother contributes to the color
blindness of her sons.
Another sex-linked trait is hemophilia, a recessive genetic
disorder in which individuals are missing one or more
blood clotting factors, as a result of a defective gene.
These factors help the body to clot blood quickly, in a
normal individual. The severity of hemophilia depends on
which blood clotting factor is missing. Individuals with
hemophilia are at a higher risk of bruising, internal
bleeding, or bleeding to death. Below in Figure 17.15 is a
pedigree of a recessive sex-linked genetic disorder, such as
hemophilia. Notice that mainly males are affected. This is because sex-linked disorders
are found on the X chromosome, and males only have one X chromosome. Because
there is no second X chromosome to potentially mask the X chromosome carrying the
recessive trait, males are, therefore, more susceptible to inheriting sex linked traits.
MULTIPLE ALLELES AND POLYGENIC TRAITS
Certain traits like blood type, hair color, and eye color, are determined by two or more
genes for every trait, one from each parent. Whenever there are different forms of the
same gene, each form is called an allele. Although each individual only has two alleles,
there can be many different combinations of alleles in that same population. For
instance, hamster hair color is controlled by one gene with alleles for black, brown,
agouti (multi-colored), gray, albino and others. Each allele can result in a different
coloration.
Polygenic traits are the result of the interaction of multiple genes. It is commonly
known, for instance, that high blood pressure (hypertension) has a strong hereditary
linkage. The phenotype for hypertension is not, however, controlled by a single gene that
lends itself to elevating or lowering blood pressure. Rather, it is the result of the
interaction between one’s weight (partially controlled by one or more genes), their ability
to process fats in general and cholesterol in particular (several metabolic genes), their
ability to process and move various salts through the bloodstream (transport genes) and
their lifestyle habits, such as smoking and drinking (which may or may not be the result
of the expression of several genes that express themselves as addictive behavior).
Polygenic inheritance leads to a wide range of phenotypes. Skin color, hair color, and
eye color are three other examples of phenotypic traits controlled by more than one gene.
This is why people may have fair skin, very dark skin, or one of many shades in between.
Each of the genes involved may also have multiple alleles, which vastly expands the
complexity of the interaction, but environment influences which alleles are expressed.
In blood typing, there are three different alleles, which can combine to form four different
blood types. Alleles A (IA) and B (IB) are both dominant, and O (i) is recessive. An
individual who has one of each dominant allele (IAIB) is said to also be co-dominant. The
only way to express Type O blood is to have two recessive alleles (ii).
Figure 17.16 ABO Blood Group
If a heterozygous male for type A blood and a heterozygous female for type B blood have
a child, what is the probability that they will have a child with type A blood? Type B
blood? Type AB blood? Type O blood? The cross (IAi x IBi) is shown above: In this
cross (Figure 17.17) there is a 25% chance of a child having type AB blood, a 25% of
type B blood, a 25% of type A blood, and a 25% chance of type O blood.
TEST CROSS
Have you ever bought packet of seeds from the store based on the picture on the outside
of this package? Did you wonder how the company selling these seeds knew the flowers
were all going to be red, or yellow, or purple, for example? It is not just random luck that
every seed in the packet will develop into a red flower! If red is dominant and white is
recessive, then there are two possible genotypes that will yield a red phenotype. A plant
can be either RR or Rr. To determine which genotype the red flower has, a test cross can
be done. A test cross is used to determine the unknown genotype of an organism. In a
test cross, the organism with an unknown genotype is crossed with an organism that has a
recessive phenotype. By looking at the phenotypes of the offspring, you can determine
the unknown genotype. The possible outcomes of a test cross are shown below in Figure
17.18.
If 100% of the offspring express the dominant phenotype, then the unknown parental
genotype was homozygous dominant, in this case RR. If 50% of the offspring express
the dominant phenotype and 50% express the recessive phenotype, then the unknown
parent genotype was heterozygous, in this case Rr.
DETERMINING GENOTYPE BASED ON PHENOTYPE
What if you do not know the genotype of both
parents and all you know are the phenotypes of their
offspring? How can you determine the genotypes of
both parents based only on the offspring
phenotypes? Assume tall (T) is dominant and short
(t) is recessive in a pea plant. If the cross between
two plants yields 78 tall offspring and 22 short
offspring, what are the genotypes of the P
generation? 78 out of a 100 is very close to 75%
and 22 out of 100 is very close to 25%. This is a 3:1
ratio. Two heterozygous individuals will yield
Lesson 17 Review: Part II
Modes of Inheritance
A. Define the following terms.
autosomes
Huntington’s disease
Sex linked
sickle cell anemia
Incomplete Dominance
carrier
cystic fibrosis
codominant
hemophilia
B. Choose the best answer.
1. A male has the genotype XY which parent is responsible for giving the son the Y
chromosome?
A. mother
C. both the father and the mother
B. father
D. neither the father or the mother
2. A student has dimples (D, dominant); his mother has no dimples (d, recessive) and the
father have dimples. What are her parent’s possible genotypes?
A. DD and dd
B. Dd and Dd
B. dd and dd
C. all are possibilities
3. Which disorders listed below would be carried on the X chromosome
A. sickle cell and hemophilia
C. color blindness and hemophilia
C. Huntington’s and sickle cell
D. cystic fibrosis and Huntington’s
4. A person with AB blood marries a person with type O blood and all their children have
types A and B blood. What does this confirm about type AB blood groups?
A. AB blood groups are dominant
C. AB blood groups are co-dominant
B. AB blood groups are recessive
D. AB blood groups are recessive to type O
5. Pink flowers are crossed with red flowers and all white flowers are produced. What
mode of
Inheritance is exhibited in these flowers?
A. Co-dominance
C. incomplete dominance
B. Multiple allele inheritance
D. multiple dominance
C. Answer the following questions.
1. Cystic fibrosis is a recessive. If one out of 20 people is a carrier of this disorder,
why is only one out of 1,660 babies born with cystic fibrosis?
2. What is the relationship between phenotype and genotype?
3. Compare homozygous alleles to heterozygous alleles.
4. Distinguish between multiple alleles and polygenic traits.