Download Biology - Chapter 7

Chapter 7:
Mendel and Heredity
Objectives for the Chapter:
1. Identify the investigator whose studie
genetics.
2. List characteristics that make the gar
study.
3. Summarize the 3 major steps of Greg
4. Relate the ratios that Mendel observe
5. Describe the 4 major hypothesis Men
6. Define the terms homozygous, hetero
7. Compare Mendel's 2 laws of heredity.
8. Predict the results of monohybrid gen
squares.
9. Apply a test cross to determine the ge
dominant phenotype.
10. Predict the results of monohybrid ge
11. Analyze a simple pedigree.
12. Identify the favors that influence pat
13. Describe how mutations can cause g
14. List 2 genetic disorders and describ
15. Evaluate the benefits of genetic coun
es formed the basis of modern
rden pea a good subject for genetic
gor Mendel's garden pea experiment.
ed In his crosses to his data.
ndel developed.
ozygous, genotype, and phenotype.
.
netic crosses by using Punnett
enotype of an organism with a
enetic crosses by using probabilities.
tterns of heredity.
genetic disorders.
be their causes.
nseling.
Vocabulary
Heredity
Genetics
Monohybrid cross
True-breeding
P generation
F1 generation F2 generation
Allele
Dominant Recessive
Homozygous
Heterozygous
Genotype
Phenotype
Law of Segregation
Law of Independent Assortment
Punnett square
Test cross
Probability
Pedigree
Sex-linked gene
Polygenic inheritance
Incomplete dominance
Multiple alleles Codominance
Assignment:
Using the picture on the
front cover explain your
thoughts on how those
Thoroughbreds to be the
fastest possible. Use
your prior knowledge of
genetics to guide your
explanation. 15 points
Mendel's Studies of Characters
The scientific study if heredity (the passing of
traits from parent to offspring) started more than
a century ago with the work of an Austrian monk
named Gregor Mendel. He carried out several
experiments in which he bred different varieties
of pea plants. Although he was not the first
person to do this, he was given the credit
because he documented his findings and
created rules that accurately predicted the
outcomes. 200 years prior to Mendel, T.A.
Knight performed the some of the same
experiments with the exception of Mendel
counted the number of offspring and analyzed
the data.
So...why peas?
1. Several characters exits in peas in 2 clearly
defined variations. See chart below.
2. The male and female reproductive parts are
contained in the same flower. You can control
mating by allowing self-pollination or removing
the stamen and allowing it to pollinate with an
other pea plant.
3. The pea plant is small, grows easily, matures
quickly, and produces many offspring. Thus,
result can be obtained quickly.
Traits Expressed as Simple Ratios
Mendel began his experiment with monohybrid
crosses which is a cross that involves 1 pair of
contrasting traits. His experiments were carried
out in 3 steps.
Each of Mendel's F1 plants showed only one
form of the trait. The contrasting form of the trait
had disappeared. But when the F1 generation
was allowed to self-pollinate, the missing trait
reappeared in some of the F2 generations. Mendel used ratios (a comparison of 2 numbers
that can be written in a fraction) to express his
findings. For each of the 7 tested traits, he found
the same 3:1 ratio expressed in all plants of the
F2 generation.
Assignment:
Answer the following questions
correctly and email your responses
to [email protected]. The
subject of the email should be
Chapter 7, Assignment 1.
1. Describe the contribution of
Mendel to the foundation of modern
genetics.
2. Describe why garden pea plants
are good subjects for genetic
experiments.
3. Summarize the design of
Mendel's pea-plant studies.
4. State the ratio obtained in each
F2 generation for each of the 7 traits
studied.
10 points
Mendel's Theory
The 4 hypotheses (below) were developed as a
direct result of Mendel's experiments. They
now make up the Mendelian Theory of Heredity,
which is the foundation of genetics.
1. For each inherited character, an individual
has 2 copies of the gene-one from each parent.
2. There are alternative versions of genes.
Today they are called alleles.
3. When 2 different alleles occur together, one
may be completely expressed (dominant), while
the other may have no observed effect on the
organism's appearance (recessive).
4. When gametes are formed, the alleles for
each gene in an individual separate
independently of one another. Thus, gametes
carry only one allele for each inherited
character. When gametes unite during
fertilization, each gamete contributes its 1 allele.
Mendel's Findings in Modern Terms
Geneticists have developed specific terms and
ways of representing and individual's genetic
makeup. For example, letters are used to
represent alleles. Dominant alleles are indicated
by writing a capital letter and recessive alleles
are indicated by writing a lower case letter.
R
r
Dominan
Recessiv
t
e
If 2 alleles of a particular gene present in an
individual are the same, the individual is said to
be homozygous. If the alleles present are
different, the individual is heterozygous.
RR rr
Rr
Homozygous dominant
Homozygous recessive
Heterozygou
s
The set of alleles than an individual has for a
character is called its genotype. The physical
appearance of a character is called its
phenotype.
More Ratios:
Genotypic Ratio=____RR:____Rr:____rr (example 1RR:
2Rr:1rr)
Phenotypic Ratio=____:____ (example 3:1)
Laws of Heredity
The Law of Segregation
During meiosis, homologous chromosomes are
separated. This law states the two alleles for a
character separate when gametes are formed.
The Law of Independent Assortment
Mendel found that for the characters he studied,
the inheritance of one character did to influence
the inheritance of another character. This law
states that the alleles of different genes separate
independently of one another during gamete
formation.
Both plants have purple
flowers, but one is tall
and the other is short.
Assignment:
Answer the following question correctly
and email responses to
[email protected]. The subject
of the email should be Chapter 7,
Assignment 2.
1. Differentiate between alleles and
genes.
2. Apply the terms homozygous,
heterozygous, dominant, or recessive to
describe plants with the genotypes RR
and Rr. 3. Identify the phenotypes of rabbits
with the genotypes Bb and bb, where B
= black coat and b = brown coat.
4. Determine whether the rabbits in
question 3 are heterozygous,
homozygous dominant, or homozygous
recessive.
10 points
Studying Heredity
A Punnett Square is a diagram that predicts the
outcome of a genetic cross by considering all
possible combinations. There are 2 types of
Punnett squares we will utilize in this class:
monohybrid cross and dihybrid cross.
Monohybrid cross is a cross that considers on
pair of contrasting traits and a dihybrid cross
considers two pairs of contrasting traits at the
same time.
Monohybrid Cross
Dihybrid Cross
When completing a dihybrid cross, be careful to
keep the alleles in the order originally listed
regardless of its dominant/recessive status.
This example lists A/a first therefore A/a will
always be the first allele listed regardless if it is
A or a.
Determining Unknown Genotypes
Animal breeders, horticulturists, and others
involved in breeding organisms often need to
know whether an organism with a dominant
phenotype is heterozygous or homozygous for a
character. By determining this, breeders can
guarantee the outcome of a certain type of
organism each time. In order to figure it out,
they must perform a test cross. This is a
process where an organism with a dominant
phenotype, but genotype is unknown, is
crossed with a homozygous recessive
individual.
Inheritance of Traits
Imagine you want to learn about an inherited trait
present in your family. How would you find out
the chances of passing the trait to your child?
Geneticists often prepare a pedigree, a visual
family history that shows how a trait is inherited
over several generations. Sometimes individuals
are carriers of a recessive traits and those traits
can show up in their children if the opposite
parent also donates a recessive trait. For
example, albinism. Parents can be heterozygous
for this character and not be effected but if they
both pass the recessive alleles to their child,
their child will be an albino.
Females are depicted as circles and males are
depicted as squares. If the shape is colored in
then that individual is effected with whichever
trait is being traced.
If a gene is autosomal, it will appear in both
sexes equally. If it is sex-linked it will appear in
one sex and not the other. Sex-linked genes are
only located on the X or Y chromosome.
If a gene is autosomal dominant, every individual
with the condition will have a parent with the
condition. If the condition is autosomal
recessive, an individual with the condition can
have one, two, or neither parent with the
condition. Assignment: Answer the following questions and
email responses to
[email protected]. The subject
of the email should be Chapter 7,
Assignment 3.
1. Predict the expected phenotypic and
genotypic ratios among offspring of 2
individuals who are heterozygous for
freckles (Ff) by using a Punnett square.
2. Summarize how a test cross can
reveal the genotype of a pea plant with
round seeds.
3. Calculate the probability that an
individual heterozygous for a cleft chin
(Tt) and an individual homozygous
recessive for a cleft chin (tt) will
produce offspring that are homozygous
recessive (tt).
10 points
Complex Patterns of Heredity
When genes are influenced by several genes, the
character is said to be polygenic inheritance.
Examples in humans can be height, weight, skin
pigmentation, and intelligence.
Intermediate Characters
Sometimes offspring can display a phenotype
that is intermediate to their parents. This is a
condition known as incomplete dominance.
When this occurs, there is no dominant allele,
therefore a blending effect occurs.
Genes with 3 or more alleles are said to
have multiple alleles. For example human
blood. An individual can have Type A,
Type B, or Type O.
2 Forms of the Same Trait
For some characters, 2 dominant alleles are
expressed at the same time. This phenomenon
is called codominance. Codominance is
different from incomplete dominance because
both traits are displayed.
Environmental Influence
An individual's phenotype often depends on its
environment. In plants, Hydrangea flowers range
from blue to pink. Depending on the pH of the
soil certain colors are produced.
The color of an arctic fox is affected by
temperature.
Summe
r
Winter
Fur color of a Siamese cat (originated in Syberia)
is also influenced by temperature. The dark
areas attract more sunlight keeping those areas
of the body warmer and preventing frost bite.
Human height is also influenced by the
environment. While developing, if a child does
not receive proper nutrition, they are more likely
to be short. If we have prolonged sun exposure,
our skin darkens.
Genetic Disorders
Changes in genetic material is called a mutation.
Many mutations are carried by recessive alleles by
heterozygous individuals. Meaning, they do not
display the condition, but their child may if both
parents donate the recessive allele. The harmful
effects produced by inherited mutations are called
genetic disorders. Sickle Cell Anemia
A recessive genetic disorder caused by a mutated
allele that produces a defective form of the protein
hemoglobin. Hemoglobin is found on blood cells
and allows oxygen to attach to be transported all
over the body. the name Sickle Cell refers to the
shape of the cell. An individual with this disorder
does not have normal, round cells, but instead
sickle shaped cells. This cause a decreased amount
of oxygen to be transported, thus making the
individual lethargic and increases the risk of blood
clots. Also, individuals with this disorder, are
immuned to Malaria. The sickle shape of the cell
doesn't allow for enough oxygen to keep the
parasitic protozoan alive. African heritages have a
higher tendency to contract sickle cell anemia.
Cystic Fibrosis
This genetic disorder is the most common fatal,
hereditary, recessive disorder among Caucasians.
1 in every 25 Caucasians have at least 1 copy of the
defective gene. This genetic disorder causes the
airways of the lungs to become clogged with thick
mucus, and the ducts to the liver and pancreas
become blocked. There is no cure.
Hemophilia
Another recessive disorder is hemophilia, a
condition that I,pairs the blood's ability to clot. It is
a sex-linked trait and occurs mostly in males.
Huntington's Disease
Huntington's disease is a dominant autosomal
disorder. Unfortunately, this disorder is normally
diagnosed in the later stages due to the mild side
effects in the beginning. The first symptoms
include forgetfulness and irritability. In time this
disorder will cause loss of muscle control, spasms,
mental illness, and death.
Treating Genetic Disorders
Most genetic disorders cannot be cured,
although progress is being made. A person with
a family history of a genetic disorder may wish
to undergo genetic counseling before becoming
a parent. This is a process that informs people
about genetic problems that could affect them
or their offspring.
In some cases, a genetic disorder can be treated
if the diagnoses is early enough. Through gene
therapy, geneticists can take some cells of the
diseased organ or affect area and replace the
bad genes with good ones from donor cells or
by "piggy-backing" the new genes to viruses
(Cystic Fibrosis). The "fixed" cells will then
multiply naturally, eventually replacing all
defective cells.
Assignment: Answer the following question
correctly and email responses to
[email protected]. The
subject of the email should be
Chapter 7, Assignment 4.
1. Differentiate between
incomplete dominance and
codominance.
2. Identify 2 examples of
characters that are influenced by
the environment. 3. Summarize how a genetic
disorder can result from a
mutation.
4. Discuss 2 genetic disorders.
10 points
End of Chapter 7