Download Mendel`s Law

Mendel’s Rules
People have been attempting to explain inheritance patterns
for years.
Failed propositions:
 Pangenesis
o Gametes contain particles from the somatic
cells
 Acquired traits are passed to future generations
 Blending
o An offspring’s traits are an average of its
parents’ traits
 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.
Law of Independent Assortment: Each pair of alleles
assorts independently of the other pairs 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
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
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
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
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:
F1 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 pairs 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.
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)