lecture

1. CHAPTER 14 – MENDEL AND THE GENE
IDEA
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2. GREGOR MENDEL
Gregor Mendel is the father of
genetics. He came up with the Law
of Segregation and the Law of
Independent Assortment. In 1857 he
began breeding garden peas to study
inheritance. He was also a monk.
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Blending Hypothesis  proposes
that the genetic material contributed
by each parent mixes; similar to how
blue and yellow paint mix to make
green
Particulate Hypothesis  proposes
that parents pass on discrete heritable
traits (genes) which retain their
SEPARATE identities in offspring; this
was Mendel’s idea

3. PEA PLANTS
Mendel used pea plants for
several reasons:
-They have distinct
characters (TRAITS) that
are easily observable
-They have male and
female sex organs
- He could control the
mating
-They produced many
offspring and have a short
generation time
-They were easy to
manage
Mendel was actually lucky with his
choice of pea plants because
almost all of the characters show
pure dominance. 3

4. GENERATIONS
P1 = Parents
F1 = Offspring of P1 x P1
F2 = Offspring of F1 x F1
F3 = Offspring of F2 x F2….etc
The “F” in F1, F2, etc. stands
for the word “filial” which comes
from the Latin word “filius”
which means son.
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Mendel started off his
experiments with plants that
were true-breeding
(homozygous)

5. LAW OF SEGREGATION
The Law of Segregation
encompasses 4 general ideas:
- Alternate versions of genes
(alleles) account for variations in
inherited characteristics
- For each character, the offspring
inherits 2 alleles (mom, dad)
- If the 2 alleles are different, the
dominant one is expressed
- The 2 alleles separate during
meiosis.
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Dominant  Trait that is seen in the
phenotype; represented with an
uppercase letter
Recessive  trait that is hidden in the
phenotype; represented with a
lowercase letter

6. PUNNENT SQUARES AND VOCABULARY
A punnent square is a tool that helps you
predict the results of a genetic cross
where the genotypes of the parents are
known. They provide you with the
probability ratios.
Genotype = the genes that an organism
has; Ex. AA, Aa, or aa
Phenotype = what the organism looks
like; Ex. purple, white
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Homozygous  same alleles; AA or
aa; can be homozygous dominant or
homozygous recessive; also called
true-breeding
Heterozygous  has different alleles;
one dominant and one recessive; Aa

7. TESTCROSS
A testcross is needed if you are
trying to find out the genotype of
a certain organism. You can
cross the organism in question
with a homozygous recessive
organism. The offspring will tell
you the genotype of the original
parents.
Purple plant….Aa or AA? Cross
with a white (aa) to see what the
results are.
IF the results are all purple, you
know the original plant was AA.
IF half of the plants are purple
and the other half are white, you
know that the original plant was
Aa. 7

8. MONOHYBRID VS.
DIHYBRID
 Monohybrid  ONE trait;
ex. Flower color
 Aa x AA
 Dihybrid  TWO traits; ex.
Seed color AND seed
shape
 YyRr x yyrr
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When Mendel did a dihybrid
cross of a homozygous
dominant with a homozygous
recessive, all the F1 plants
were heterozygous. When he
crossed two F1 plants to get
an F2 generation, he
observed a 9:3:3:1 ratio

9. LAW OF INDEPENDENT ASSORTMENT
The Law of
Independent
Assortment
says that each
pair of alleles
segregates into
gametes
independently.
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This law applies only to genes located on different, non-homologous
chromosomes. Genes that are located on the SAME chromosome tend
to be inherited together and are called Linked Genes.

10. RULE OF MULTIPLICATION
 This rule is used to determine the probability that two or more
independent events will occur together in some specific combination.
 Probability that two coins tossed at the same time will both lands
heads up is ¼
 Chance of coin A landing heads up = ½
 Chance of coin B landing heads up = ½
 ½ x ½ = ¼
 Probability that a heterozygous pea plant (Pp) will self-fertilize to
produce a white-flowered offspring (pp) is the probability that a
sperm with a white allele will fertilize an ovum with a white allele.
This probability is 1/2 × 1/2 = 1/4.
 Chance of parents having 3 kids that are ALL boys
 Chance of kid A being a boy = ½ (same for kid B, C)
 ½ x ½ x ½ = 1/8
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11. RULE OF ADDITION
 This rule is used to determine the probability when an event can occur in
two or more mutually exclusive ways.
 The probability of getting Pp as offspring with both parents being
heterozygous:
 The probability of obtaining an F2 heterozygote by combining the
dominant allele from the egg and the recessive allele from the sperm
is 1⁄4.
 The probability of combining the recessive allele from the egg and
the dominant allele from the sperm also 1⁄4.
 Using the rule of addition, we can calculate the probability of an F2
heterozygote as 1⁄4 + 1⁄4 = 1⁄2.
 The chance of having 3 kids with 2 boys and 1 girl:
 B, B, G = ½ x ½ x ½ = 1/8
 B, G, B = ½ x ½ x ½ = 1/8
 G, B, B = ½ x ½ x ½ = 1/8
 SO, the chance of having a family with two boys and one girl at 3/8
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12. GENETICS PROBLEMS – SAMPLE PROBLEM!
 Determine the probability of an offspring having recessive
phenotypes for at least two of three traits resulting from a trihybrid
cross between pea plants that are PpYyRr and Ppyyrr.
 The probability of producing a ppyyRr offspring:
 The probability of producing pp = 1/4.
 The probability of producing yy = 1/2.
 The probability of producing Rr = 1/2.
 So, the probability of all three being present (ppyyRr) in one offspring
is 1/4 × 1/2 × 1/2 = 1/16.
 For ppYyrr: 1/4 × 1/2 × 1/2 = 1/16.
 For Ppyyrr: 1/2 × 1/2 × 1/2 = 1/8 or 2/16. (must keep
denominators the same!)
 For PPyyrr: 1/4 × 1/2 × 1/2 = 1/16.
 For ppyyrr: 1/4 × 1/2 × 1/2 = 1/16.
 Therefore, the chance that a given offspring will have at least two
recessive traits is 1/16 + 1/16 + 2/16 + 1/16 + 1/16 = 6/16 or 3/8.
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13. PRACTICE GENETICS PROBLEMS:
 Parents  PpyyRr x PpYyrr
 1. Chance of having all 3 dominant phenotypes
 2. Chance of having at least 2 heterozygous
genotypes
 3. Chance of having at least 2 dominant phenotypes
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14.  PPYyRr – ¼ x ½ x ½ = 1/16
 PpYyRr – ½ x ½ x ½ = 1/8 = 2/16
-------------
3/16
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Parents  PpyyRr x PpYyrr
1. Chance of having all 3 dominant phenotypes

15.  PpYyrr – ½ x ½ x ½ = 1/8 = 2/16
 PpYyRr – ½ x ½ x ½ = 1/8 = 2/16
 PpyyRr – ½ x ½ x ½ = 1/8 = 2/16
 ppYyRr – ¼ x ½ x ½ = 1/16
 PPYyRr – ¼ x ½ x ½ = 1/16
--------------
8/16 or 1/2
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Parents  PpyyRr x PpYyrr
2. Chance of having at least 2 heterozygous
genotypes

16.  PpYyrr – ½ x ½ x ½ = 1/8 = 2/16
 PpYyRr – ½ x ½ x ½ = 1/8 = 2/16
 PPYyrr – ¼ x ½ x ½ = 1/16
 PPYyRr – ¼ x ½ x ½ = 1/16
 PpyyRr - ½ x ½ x ½ = 1/8 = 2/16
 PPyyRr - ¼ x ½ x ½ = 1/16
 ppYyRr - ¼ x ½ x ½ = 1/16
-------------------
10/16 or 5/8
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Parents  PpyyRr x PpYyrr
3. Chance of having at least 2 dominant
phenotypes

17.  In the 20th century, geneticists extended Mendelian principles
both to diverse organisms and to patterns of inheritance more
complex than Mendel described.
 Mendel had the good fortune to choose a system that was
relatively simple genetically.
 Each character that Mendel studied is controlled by a single
gene. (There is one exception: Mendel’s pod shape character is
determined by two genes.)
 Each gene has only two alleles, one of which is completely
dominant to the other.
 The heterozygous F1 offspring of Mendel’s crosses always looked
like one of the parental varieties because one allele was
dominant to the other.
 The relationship between genotype and phenotype is rarely so
simple.
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18. DOMINANCE
Codominant – When both alleles
are dominant; Red + White = a
flower with BOTH red and white;
the heterozygote shows a
phenotype representative of both
alleles.
Incomplete Dominance – when the
dominant allele is not COMPLETELY
dominant; the heterozygote is a mix
between the dominant and recessive
phenotype; EX. red + white = pink
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19. DOMINANT ALLELES
 It is important to recognize that
an allele is called dominant
because it is seen in the
phenotype, not because it
somehow subdues a recessive
allele. Alleles are simply
variations in a gene’s nucleotide
sequence.
 A dominant allele is not
necessarily more common in a
population than the recessive
allele.
 For example, one baby in 400 is
born with polydactyly, a
condition in which individuals
are born with extra fingers or
toes. Polydactyly is due to a
dominant allele. Clearly,
however, the recessive allele is 19

20. MULTIPLE ALLELES
Most genes have more than 2 allelic
forms (more than just dominant and
recessive). The best example is the
ABO blood groups.
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Both the IA and IB
alleles are
dominant to the
i allele.
The IA and IB
alleles are
codominant to
each other.

21. BLOOD GROUPS
 Because each individual carries
two alleles, there are six possible
genotypes and four possible blood
types.
 Individuals who are IAIA or IAi
are type A and have type A
carbohydrates on the surface
of their red blood cells.
 Individuals who are IBIB or IBi
are type B and have type B
carbohydrates on the surface
of their red blood cells.
 Individuals who are IAIB are
type AB and have both type A
and type B carbohydrates on
the surface of their red blood
cells.
 Individuals who are ii are type
O and have neither
carbohydrate on the surface of
their red blood cells.
 Matching compatible blood groups
is critical for blood transfusions
because a person produces
antibodies against foreign blood
factors. 21

22. PLEIOTROPY
Pleiotropy is when
one gene affects
more than one
phenotype. In sickle
cell anemia, even
though it is only a
change in one amino
acid, it affects many
things in the body.
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23. EPISTASIS
Epistatic genes are genes that
affect the expression of another
gene at a different locus.
Example 1 – Mice: B (black) is
dominant to b (brown). However,
the gene for color in the fun is
epistatic to it. SO, if the mice have
cc as their genotype, then
regardless of whether they should
be brown or black, they will be
white because they will have no
color deposited into their fur.
Example 2 – Hair: Curly hair (H) is
dominant to straight hair (h). If
someone has a gene for baldness,
it won’t matter if they have straight
or curly, because they won’t have
hair to begin with.
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24. POLYGENIC
INHERITANC
E
Polygenic traits is when several
genes all affect the same
phenotype. It is the opposite idea of
pleiotropy. It has an additive effect
and usually spans a continuum.
AABbcc = AaBbCc….both have 3
dominant alleles; it is an additive effect. 24
Quantitative characters  traits that
vary along a continuum; ex. Skin
color, eye color, height

25. NORM OF REACTION
 Phenotype depends on both
environment and genes.
 Hydrangea plants may be pink or
blue depending on the acidity of
the soil.
 For humans, nutrition influences
height, exercise alters build, sun-
tanning darkens skin, and
experience improves
performance on intelligence
tests.
 Even identical twins, who are
genetically identical, accumulate
phenotypic differences as a
result of their unique
experiences.
 The product of a genotype is
generally not a rigidly defined
phenotype, but a range of
phenotypic possibilities, the
norm of reaction, determined
by the environment.
 Norms of reaction are broadest
for polygenic characters. 25

26. PEDIGREES
Pedigrees are family trees that can follow
genetically inherited traits through several
generations. Based on this information, you
can tell how a trait is inherited (autosomal
dominant, autosomal recessive, sex-linked,
etc). Pedigrees are used to study
heredity…instead of manipulating mating
patterns of humans, doctors analyze the
matings that have already occurred. This
can help understand the past and predict the
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27. MENDELIAN INHERITED TRAITS IN HUMANS
DOMINANT
Some traits in humans follow
Mendelian Inheritance. Some
of the traits that show
dominance and follow this
type of inheritance are:
- Dimples
- Freckles
- Mid-digital hair
- Polydactly
- Tongue rolling
- Widow’s peak
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28. MENDELIAN INHERITED TRAITS IN HUMANS
RECESSIVE
Some of the traits that are
recessive and follow this type of
inheritance are:
- Hitchhickers Thumb
- Attached Earlobes
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29. GENETIC DISORDERS
Genetic Disorders can be caused by several different things. They can be carried on the
autosomal chromosomes or on the sex chromosomes. They can be caused by a
dominant allele, or a recessive allele. Further, they can be the result of an incorrect
number of chromosomes (due to nondisjunction – more on that in Ch. 15). Refer to the
Genetic Disorders Chart for notes on each of the following diseases/disorders:
We are going to look at disorders that follow autosomal recessive inheritance:
- Cystic Fibrosis
- Tay Sachs Disease
- Sickle Cell Disease
- Phenylketonuria (PKU)
We are also going to look at disorders that follow autosomal dominant inheritance:
- Achondroplasia (dwarfism)
- Huntington’s Disease
NOTE: Consanguineous matings (matings between close relatives) can increase the risk
of producing offspring with a genetic disorder. 29
Heterozygotes are carriers and do
NOT have the disorder, but have a
50% chance of passing the allele
onto their offspring.
Lethal dominant alleles are much LESS common than lethal recessives because a lethal
dominant most likely kills the person before they can reproduce (although there ARE
exceptions) but a lethal recesivce can hide in a heterozygote and that person would be
phenotypically normal!

30. CYSTIC FIBROSIS
- Autosomal Recessive
- Most common lethal genetic disease in the US
- Problem with the Cl- ion transport channels which
leads to a high concentration of Cl- outside the cells
- This higher concentration leads to mucus production
which can build up in the pancreas, LUNGS, and
digestive tract…which leads to infections
- When the white blood cells come to the site of
infection, their remains stay there and add to the
mucus…this is a bad cycle
- Many respiratory problems
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31. TAY-SACHS DISEASE
- Autosomal Recessive (incomplete dominance at molecular level)
-- Brain cells have a defective enzyme that cannot break down lipids; this leads to a
build up on the brain
-- The buildup causes the brain not to function properly and progressively destroys the
central nervous system. This can lead to seizures, blindness, and degeneration of
motor and mental capabilities
-A baby with TSD appears to develop normally for the first few months, then there is a
relentless deterioration of mental and physical abilities. The child gradually becomes
blind, is unable to swallow, and has inefficient pulmonary function. Muscles begin to
atrophy, paralysis sets in, and response to the environment diminishes. There is no cure
or treatment and average life expectancy is 3-5 years of age. 31

32. SICKLE
CELL
DISEASE
-Autosomal recessive, demonstrates pleiotropy; codominant at molecular level
-Caused by a substitution of one amino acid in the hemoglobin protein of RBC’s
-When there is a low level of oxygen, the RBC’s change their shape to a sickle shape
- Symptoms range over a wide spectrum: low # of RBC’s, fatigue, sharp pains, and
infections 32

33. PHENYLKETONURIA (PKU)
-Autosomal
recessive
-Screened for
at birth
-Body cannot properly break
down the amino acid
phenylalanine, which, if
accumulated, can reach toxic
levels and cause mental
deficiencies
- If its confirmed that a baby is
afflicted, they are put on a
special diet and are usually33

34. ACHONDROPLAS
IA
-Autosomal dominant
-Homozygous recessive = normal
height
-Heterozygous = dwarf
- Homozygous dominant = lethal
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35. HUNTINGTON’S DISEASE
-Autosomal Dominant
-This is a deterioration of the
nervous system
-It does not show up until the
person’s late 30’s or early 40’s, so
by this point the gene has probably
already been passed on if they
have already procreated
-This leads to death
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36. MULTIFACTORIAL DISORDERS
 Some disorders are multifactorial, and have
a genetic component plus significant
environmental influence.
 Multifactorial disorders include heart disease,
diabetes, cancer, alcoholism, and certain
mental illnesses, such as schizophrenia and
manic-depressive disorder.
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37. GENETIC COUNSELING
 Genetic counseling is based on Mendelian
genetics and the laws of probability.
 Many hospitals have genetic counselors to
provide information to prospective parents
who are concerned about a family history of
a specific disease.
 See the example on the next slide
37

38.  A hypothetical couple, John and Carol, are planning to have their first
child. Both John and Carol had brothers who died of the same
recessive disease.
 John, Carol, and their parents do not have the disease. Their parents
must have been carriers (Aa × Aa).
 John and Carol each have a 2/3 chance of being carriers and a 1/3
chance of being homozygous dominant.
 The probability that their first child will have the disease is 2/3
(chance that John is a carrier) × 2/3 (chance that Carol is a carrier) ×
1/4 (chance that the offspring of two carriers is homozygous
recessive) = 1/9.
 If their first child is born with the disease, we know that John and
Carol’s genotype must be Aa and they are both carriers.
 In that case, the chance that their next child will also have the
disease is 1/4.
 Mendel’s laws are simply the rules of probability applied to heredity.
 The chance that John and Carol’s first three children will have the
disorder is 1/4 × 1/4 × 1/4 = 1/64.
 Should that outcome happen, the likelihood that a fourth child will
also have the disorder is still 1/4.
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39. FETAL TESTING
39

40. AMNIOCENTESIS
Amniocentesis is a process that is done if a
woman is having a high risk pregnancy. A
needle is inserted into the amniotic sac and
some of the fetal cells are extracted. Those
cells are then cultured in a petri dish until
enough cells form. Then the cells are used
to make a karyotype, which will show
genetic disorders.
40

41. CHORIONIC
VILLI
SAMPLING
(CVS)
This is a fetal testing procedure that suctions out
some of the fetal cells through the cervix.
Because the cells are mature enough and enough
are in the sample, a karyotype can be done
immediately and the results of the test are
returned usually within 24 hours. This test can be
41

42. ULTRASOU
ND
An ultrasound is a non-invasive procedure that
allows doctors to see anatomical features of the
baby. Typically this is used to determine the sex of
the child.
42

43. FETOSCOPY
Fetoscopy is a process when a
thin viewing scope is inserted
into the uterus to view the fetus.
43

44. GENETIC TESTS
 Newer techniques can isolate fetal cells or
DNA from the mothers blood – HARMONY
test
 This test is performed around 10-12 weeks
 Some genetic traits can be detected at birth
by simple tests that are now routinely
performed in the hospitals as soon as the
baby is born.
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