Download Mendel`s Law

Genetics and Mendel’s Rules
People have been attempting to explain inheritance patterns
for years.
Failed propositions:
 “Homunculus” – Little man in sperm
 Pangenesis
o Gametes contain particles that
migrated from the somatic cells
 Acquired traits are passed to future generations
 Blending
o An offspring’s traits are an average of its
parents’ traits
o Can’t explain why traits disappear in one
generation and reappear in later ones.
Gregor Mendel – 1860’s
Used pea plants to determine the fundamental principles of
genetics.
Why were pea plants good specimens for Mendel to use?
1. Short life cycles
2. Reproduce sexually
3. Produce many offspring in a short amount of time
4. Easy to manipulate (control crosses, easy to care for)
Male reproductive
structures
5. Many contrasting characteristics (tall vs. short, purple
vs. white flowers, green seeds vs. yellow, etc)
6. True-Breeding strains – when crossed with self or like
plants, offspring look like parents.
http://www2.edc.org/weblabs/mendel/mendel.html
Questions
1. What conclusion can we draw from the flower color
in the F1 generation?
Purple is dominant
Questions:
1. If the F1 plants are crossed with each other or self,
what color flowers do you expect to see in the
offspring?
How can we explain the observed results?
2. How many alleles does each plant in the P generation
have for flower color?
2
3. Assign alleles to each parent in the P generation
F = purple allele
f = white allele
Purple (FF)
White (ff)
4. What allele combination do the F1 plants have?
Ff
5. What types of alleles could be in the gametes
produced by these plants?
F
or
f
6. What possible allele combinations could be found in
the F2 generation?
FF
Ff
ff
Mendel’s observations lead to some important conclusions:
He determined that parents pass on discrete “factors” to
their offspring that control traits.
What do we call those factors?
Genes
1. There are alternate forms of a gene that account for
the variation in inherited characteristics.
a. What do we call different forms of a gene?
Alleles
2. For each trait, an organism inherits two alleles, one
from each parent. (Genotype)
a. Homozygous – if the two alleles are the same
b. Heterozygous – if the two alleles are different
 Genotype – an organism’s allele
combination.
Ex. GG
Gg
gg
3. If an organism inherits two different alleles
(heterozygous), one allele will determine how the trait
is expressed. (Phenotype)
a. Dominant allele – the expressed allele from a
heterozygous genotype
 Only need one to be expressed
 Not “normal” or more common in nature
b. Recessive allele – the allele not expressed in a
heterozygous genotype
 Must have two to be expressed
 Genotype – an organism’s allele
combination.
Ex. GG - Homozygous Dominant
Gg - Heterozygous
gg - Homozygous Recessive
 Phenotype – an organism’s expressed trait
Ex. Tall or short
4. Law of Segregation – Sex cells (gametes/sperm and
eggs) carry only one allele for a specific trait because
homologous chromosome separate during meiosis.
Test Cross
How do you determine the genotype for an organism that
expresses the dominant phenotype?
What would be the best cross to determine the genotype?
Cross with a recessive individual
What data would you need to support your claim?
Pedigrees – Family trees
Genetic traits can be tracked through families using
pedigrees.
 Allows scientists to determine how traits are inherited
without controlling human matings.
Carriers – People in a pedigree that have one copy of a
recessive trait, but don’t express the symptoms of the
disorder.
 Heterozygous
Consanguineous
marriage
M
Examples:
1. The pedigree below is for a genetic disease or
abnormality. We do not yet know if it is dominant or
recessive. Determine if the trait is autosomal dominant
or recessive. Try the following designations:
A = the trait (a genetic disease or abnormality, dominant)
a = normal (recessive)
a) Assign a genotype to each individual. If more than one
genotype is possible, write both.
Is this a dominant or recessive autosomal trait? Explain
your answer.
Recessive – two recessive parents can’t produce dominant
offspring.
b) Write the genotypes next to the symbol for each person
in the pedigree below assuming that it is for a dominant
trait. If more than one is possible, list both.
c) Is it possible that this pedigree is for an autosomal
dominant trait?
YES
d) What can you conclude from these two examples about
the parents of a person that has a dominant characteristic?
(Circle the correct answer below.)
--If a person has a dominant trait, the parents will not have
the trait.
--If a person has a dominant trait, the parents might have
the trait or they might not have it.
--If a person has a dominant trait, at least one of the parents
will have the trait.
--If a person has a dominant trait, both of the parents will
have the trait.
#3
2. We will determine if the pedigree below can be for a trait
that is autosomal dominant. Use "A" and "a" as you did for
the pedigrees above.
a) Write the genotype of each individual next to the
symbol. If more than one is possible, list both.
b) Is it possible that this pedigree is for an autosomal
dominant trait?
YES
c) In conclusion, can two individuals that have an
autosomal dominant trait have unaffected children? (Circle
the correct answer below.)
--If two individuals have a dominant trait, none of their
offspring will have the trait.
--If two individuals have a dominant trait, their offspring
might or might not have the trait.
--If two individuals have a dominant trait, their offspring
will have the trait.
#2
3. We will determine if the pedigree below can be for a trait
that is autosomal recessive. Use the following designations:
A = normal
a = the trait (a genetic disease or abnormality)
a) Assuming that the trait is recessive, write the genotype
of each individual next to the symbol. If more than one is
possible, list both.
b) Is it possible that the pedigree above is for an autosomal
recessive trait?
NO
c) Assuming that the pedigree below is for a recessive trait,
write the genotype next to the symbol for each person. If
more than one is possible, list both.
d) Is it possible that this pedigree is for an autosomal
recessive trait?
YES
e) If a trait is autosomal recessive, what can you conclude
about the children if both parents are affected? (Circle the
correct answer below.)
--If both parents are affected, none of the children will be
affected.
--If both parents are affected, the children might or might
not be affected.
--If both parents are affected, all of the children will be
affected.
#3
4. We will determine if the pedigree below can be for a trait
that is autosomal recessive. Use "A" and "a" as you did for
the previous example.
a) Write the genotype of each individual next to the
symbol. If more than one is possible, list both.
b) Is it possible that this pedigree is for an autosomal
recessive trait?
YES
c) If a trait is autosomal recessive, what can you conclude
about the children of two parents that are not affected?
(Circle the correct answer below.)
--If two parents have a dominant trait, the children will not
have the trait.
--If two parents have a dominant trait, the children might or
might not have the trait.
--If two parents have a dominant trait, the children will
have the trait.
#2
5. We will determine if the pedigree below can be for a trait
that is autosomal recessive.
a) Write the genotype of each individual next to the
symbol. If more than one is possible, list both.
b) Is it possible that this pedigree is for an autosomal
recessive trait?
YES
c) In this pedigree, two generations have been skipped.
What can you conclude about recessive traits skipping
generations (is it possible or not)? (Circle the correct
answer below.)
--Recessive traits cannot skip generations.
--Recessive traits can skip generations.
#2
Pedigree A
Determine if the trait is dominant, recessive or if the
pedigree is not possible. Assign a genotype to each person.
If more than one genotype is possible, list both.
Recessive Trait
Pedigree B
Determine if the trait is dominant, recessive or if the
pedigree is not possible. Assign a genotype to each person.
If more than one genotype is possible, list both.
Dominant Trait
Dihybrid Cross
A cross involving 2 characteristics/traits, each controlled by
their own gene on different homologous pairs.
How many total chromosomes?
4
How many genes (types of letters)?
2
How many total letters in a genotype?
4
Mendel crossed two true-breeding plants:
Plant 1: Yellow and Round seed
Plant 2: Green and Wrinkled seed
Parental Genotype:
Gametes:
Offspring:
Phenotype:
YYRR
X
yyrr
YR
yr
YyRr
Yellow and Round
Works with other traits as well:
Develop a key for the above Dihybrid Cross:
R = Green
r
=
Yellow
Y
=
Constricted
y
=
Inflated
Law of Independent Assortment: Each pair of alleles
assorts independently of the other pair during meiosis
(gamete formation).
The sides of a Punnett Square only needs to be as big as the
number of different gametes produced by each parent.
Rules of Probability
Rule of Multiplication
 When using two coins (one egg and one sperm) the
outcome for each coin is an independent event.
o The probability of both coins landing heads up is
the product of the separate probabilities.

½
X
½ =¼
 When crossing two heterozygous Rr X Rr
individuals, the probability of a homozygous
genotype RR is ¼. Same for rr.
 What about the heterozygous genotype?
¼ +¼ =½
Rule of Addition – if there are 2+ outcomes, the
probability is the sum of the separate probabilities.
 Trihybrid Cross
o Maximum number of different gametes?
 23 = 8
o Maximum size for Punnett Square?
 8 X 8 (64 squares)
o Easier to do three separate monohybrid crosses
and multiply probabilities
 Given AaBbCc X AaBbCc
Chances of aabbcc?
¼
X
¼
X
¼
= 1/64
 Given AaBbCc X AaBbCc
Chances of AabbCC?
2/4 X
¼
X
¼
= 2/64 (or 1/32)
Most human genetic disorders are recessive
Most genetic disorders are not evenly distributed across
ethnic groups.
 Prolonged geographic and/or “class” isolation leads to
inbreeding.
o Increase the chance that both parents will have a
harmful recessive allele.
Ex. Cystic Fibrosis 1/2,500 Caucasians
Albinism
Sickle-cell disease* 1/400 African-Americans
Tay-Sachs disease 1/3,500 Jewish people from C. Eu.
Disorders can also be caused by dominant alleles.
Dominant alleles that cause lethal diseases are much less
common than lethal recessives.
 Why?
Heterozygotes are affected
Lethal dominant that don’t kill until later in life can be
passed to future generations.
Ex. Huntington’s disease 1/25,000
Achondroplasia (Dwarfism) 1/25,000
Alzheimer’s Disease (some cases)
Genetic Legacy and Technology
With increases in our understanding of genetics and
technology, people are now able to learn more about their
children’s genetic legacy before conception, during
pregnancy and after birth.
Before Conception: Parents are screened for recessive
alleles to determine if they are carriers.
During Pregnancy: Often requires the collection of fetal
cells.
 Chorionic Villus Sampling (CVS) – collects sample
of chorionic villus tissue from the placenta.
(After 8-10 wks)
 Amniocentesis – collection of fetal cells from the
amniotic fluid using a needle (After 14-16 wks)
 Blood tests on the mother (After 15-20 wks)
o Check protein levels
 Down Syndrome
 Ultrasound Imaging – uses sound waves to produce
images.
 Fetoscopy – use of viewing scope and fiber optics
Post-Birth: samples taken in the hospital after birth to
screen for common genetic disorders.
 PKU – can’t break down an amino acid - adjust diet
Greater technology also leads to great ethical questions.
For people with a family history but no symptoms of a
genetic disorder, a predictive test can help determine a
person’s risk for developing the disorder in the future.
People can seek early medical screening or care.
 Colon cancer
 Breast cancer
Information needs to be used/handled responsibly
 Counseling
 Insurance
 Job security