Transcript Chapter 14 * Mendel and the gene idea

CHAPTER 14 – MENDEL AND THE GENE
IDEA
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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.
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
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PEA PLANTS
Mendel was actually lucky with his
choice of pea plants because
almost all of the characters show
pure dominance.
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
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GENERATIONS
Mendel started off his
experiments with plants that
were true-breeding
(homozygous)
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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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
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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PUNNENT SQUARES AND VOCABULARY
Genotype = the genes that an organism
has; Ex. AA, Aa, or aa
Phenotype = what the organism looks
like; Ex. purple, white
Homozygous  same alleles; AA or aa;
can be homozygous dominant or
homozygous recessive; also called truebreeding
Heterozygous  has different alleles; one
dominant and one recessive; Aa
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.
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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.
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MONOHYBRID VS.
DIHYBRID
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Monohybrid  ONE trait; ex.
Flower color
 Aa x AA
Dihybrid  TWO traits; ex.
Seed color AND seed shape
 YyRr x yyrr
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
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LAW OF INDEPENDENT ASSORTMENT
The Law of
Independent
Assortment
says that each
pair of alleles
segregates into
gametes
independently.
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.
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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
½x½=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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RULE OF ADDITION
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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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GENETICS PROBLEMS – SAMPLE PROBLEM!
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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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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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Parents  PpyyRr x PpYyrr
1. Chance of having all 3 dominant phenotypes
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
2. Chance of having at least 2 heterozygous
genotypes
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
3. Chance of having at least 2 dominant
phenotypes
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PpYyrr –
PpYyRr –
PPYyrr –
PPYyRr –
PpyyRr PPyyRr ppYyRr -
½ x ½ x ½ = 1/8 = 2/16
½ x ½ x ½ = 1/8 = 2/16
¼ x ½ x ½ = 1/16
¼ x ½ x ½ = 1/16
½ x ½ x ½ = 1/8 = 2/16
¼ x ½ x ½ = 1/16
¼ x ½ x ½ = 1/16
------------------10/16 or 5/8
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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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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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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 far more prevalent than
the dominant allele.
DOMINANT ALLELES
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MULTIPLE ALLELES
Most genes have more than 2 allelic forms
(more than just dominant and recessive).
The best example is the ABO blood groups.
Both the IA and IB
alleles are
dominant to the i
allele.
The IA and IB
alleles are
codominant to
each other.
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
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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.
BLOOD GROUPS
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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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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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POLYGENIC
INHERITANCE
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.
Quantitative characters  traits that
vary along a continuum; ex. Skin color,
eye color, height
AABbcc = AaBbCc….both have 3 dominant
alleles; it is an additive effect.
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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, suntanning 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.
NORM OF REACTION
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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 future.
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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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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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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
Heterozygotes are carriers and do
- Tay Sachs Disease
NOT have the disorder, but have a
- Sickle Cell Disease
50% chance of passing the allele
- Phenylketonuria (PKU)
onto their offspring.
We are also going to look at disorders that follow autosomal dominant inheritance:
- Achondroplasia (dwarfism)
- Huntington’s Disease
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!
NOTE: Consanguineous matings (matings between close relatives) can increase the risk of
producing offspring with a genetic disorder.
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- 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
CYSTIC FIBROSIS
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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
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and average life expectancy is 3-5 years of age.
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
- Being a carrier is beneficial because they are immune to malaria (common in Africa)
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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
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special diet and are usually OK
-Autosomal dominant
-Homozygous recessive = normal height
-Heterozygous = dwarf
- Homozygous dominant = lethal
ACHONDROPLASIA
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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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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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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.
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See the example on the next slide
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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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FETAL TESTING
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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.
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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 done between 8-10 weeks.41
ULTRASOUND
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.
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FETOSCOPY
Fetoscopy is a process when a thin
viewing scope is inserted into the
uterus to view the fetus.
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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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