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Mendel & Heredity
Genes are segments on the chromosomes that are
responsible for inherited traits such as eye color, hair color,
skin color, height, etc.
Now we’re going to learn
how these traits are inherited by the study of
genetics.
I. Genetics: The branch of biology that studies heredity.
• Heredity: The passing of characteristics from parents to
offspring.
• Mendel, an Austrian monk, carried out the 1st important
studies of heredity.
• The “father” of genetics.
• He is credited with stating the
– principle that traits among off– spring result from combinations
– of dominant and recessive genes.
• Traits :Characteristics that
– are inherited.
Monohybrid Cross: A cross involving 1 pair of
contrasting traits.
• Mendel’s 1st experiments are called monohybrid
crosses because the 2 parent plants differed by a
single trait—height.
• Example: crossing a tall pea plant with a short pea
plant.
Mendel carried out his experiments in three steps:
• 1. Mendel allowed each variety of garden pea to selfpollinate for several generations. This method ensured that
each variety was true-breeding (purebred) for a particular
trait; that is all the offspring would show only one form of a
particular trait.
• Self-pollination-occurs when pollen is transferred from the
anthers of a flower to the stigma of either the same flower or
another flower of the same plant.
• For example: All the tall pea plants had to produce many
generations of nothing but tall plants. All the short pea plants
had to produce many generations of nothing but short plants.
• These true-breeding plants served as parental generation in
Mendel’s experiments.
• Parental generation = P1 generation (or P generation)
2. Mendel then cross-pollinated two P1 generation
plants that had contrasting forms of a trait, such as a
tall and a short pea plant. Mendel called the
offspring of the P1 generation the 1st filial generation,
or F1 generation. He then examined each F1 plant
and recorded the number of F1 plants expressing
each trait.
Offspring of the P1
generation = 1st Filial
or F1 generation
Cross-pollinationinvolves flower of
two separate plants.
 3. Finally, Mendel allowed the F1 generation to
self-pollinate. He called the offspring the F1
generation plants and the 2nd filial generation, of F2
generation. Again, each F2 plant was characterized
and counted.
 P1 = 1st generationTall x short
**The “x” means crossed. Mendel transferred
pollen from one plant to another plant.
 F1=2nd generation= Tall Tall Tall Tall
**All of the f generation is tall. It’s as if the shorter
parent had never existed.
 F2=3rd generation=Tall Tall Tall short
** ¾ (75%) of the F2 generation is Tall & ¼ (25%) of
the F2 generation is short. A ratio of 3:1.
*It was as if the short (recessive) trait had
reappeared from nowhere!
*A dominant trait appears in every generation of the
offspring. A recessive trait does not.
 III. Mendel’s Theories of Heredity—The
foundation of genetics.
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•
•
•
1. Mendel is known as the father/founder of genetics.
2. For each inherited trait, an individual has two copies of
the gene—one from each parent.
3. Alleles are different gene forms (different versions).
An individual receives one allele from each parent.
Each allele can be passed on when the individual
reproduces.
4. When two different alleles occur together, one of them
may be completely expressed, while the other may have
no observable effect on the organism’s appearance.
 Mendel’s 3rd Theory Cont.
•
•
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•
Mendel described the expressed form of the trait as
dominant. And the trait not expressed when the
dominant form of the trait was present was described as
recessive
Dominant: The expressed form of the trait. (The trait is
visible).
Observable trait of an organism that masks a recessive
form of the train.
Dominant genes (alleles) are represented by capital
letters and the same letter in small case represents
recessive genes. The dominant allele is always written
first.
Ex. If tallness is dominant over short pea plants. Tt would
be used if the pea plant is tall.
Blonde Hair
VS.
Brown Hair
 Mendel’s 3rd Theory Cont.
•
•
•
•
•
Recessive: The trait that is not expressed when the
dominant form of the trait is present.
The hidden trait of an organism that is masked by a
dominant trait.
For every pair of contrasting forms of a trait that Mendel
studied, the allele for one form of the trait was always
dominant and the allele for the other form of the trait was
always recessive.
Example: Each of Mendel’s pea plants had 2 alleles that
determined its height.
A plant could have: 2 alleles for tallness, TT
2 alleles for shortness, tt
1 for tall and 1 for short, Tt
 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 trait.
 When gametes unite during fertilization, each
gamete contributes one allele. Each parent can
contribute only one of the alleles because of the
way gametes are produced during the process of
meiosis (haploid).
+
Sperm (1n)
=
Egg (1n)
Zygote (2n)

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

Phenotype: The way an organism looks. It is the
physical appearance of a trait.
Example: The phenotype of a tall plant is tall
regardless of the genes it contains.
Genotype:
The gene combination on organism
contains (set of alleles). Genotype describes the
genetic make up of a trait. The genotype of a tall
plant that has 2 alleles for tallness is TT.
**You can’t always know an organism’s genotype
simply by looking at its phenotype.
 Homozygous:
Two alleles for a trait are the
same. (The presence of two identical alleles for a
trait.)
 Example—The true breeding tall plant that has two
alleles for tallness (TT) would be homozygous for
the trait height.
 Because tallness is dominant, a TT individual is
homozygous dominant for that trait.
 A short plant would have 2 alleles for shortness (tt);
it would always be homozygous recessive for the
trait height.
 Heterozygous: (hybrid) Two alleles for a
trait are different. (The presence of two
different alleles for a trait.)
 Example: The tall plant that has one allele
for tallness and one allele for shortness (Tt)
is heterozygous for trait height.
Mendel’s hypotheses brilliantly predicted the results of
his crosses and also accounted for the ratios he
observed. Because of their importance, Mendel’s ideas
are often referred to as the laws of heredity
Law of Dominance:
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•
The dominant gene hides the recessive gene; the recessive gene only
shows when the dominant gene is NOT present.
Law of Segregation:

•
•
The first law of heredity describes the behavior of chromosomes during
meiosis when homologous chromosomes, and then chromatids are
separated.
States that the two alleles for a trait segregate (separate) when gametes
are formed.
 Punnett Squares: It is a diagram that
predicts the possible offspring of
crosses.
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We solve genetic problems and predict possible
offspring by using the Punnett Squares.
The simplest is four boxes inside a square.
The possible gamete that one parent can produce is
written along the top.
The possible gamete that the other parent can produce
is written along the left side.
Each box inside the square is filled with two letters
obtained by combining the allele along the top of the
box with the allele along the side of the box.
The letters in the boxes are the possible genotypes of
the offspring.
A.
Monohybrid Cross: A
cross involving one
pair of contrasting
traits.
t
T
t
Tt
Tt
tt
tt
t
Example #1: Cross a
heterozygous tall plant
Possible offspring:
with a homozygous short Genotype: 2 Tt; 2 tt
plant. T=dominant for tall Phenotype: 2 Tall; 2 short
and t=recessive for short. Genotype ratio
Tt
X
tt
_2:2______________
Phenotype ratio
_2:2_____________
_50__ % tall
_50__ % short
T
Example #2: Cross a
heterozygous tall plant
with a heterozygous tall
plant.
Tt
X
Tt
T
t
TT
Tt
Tt
tt
t
Possible offspring:
Genotype: 1TT, 2Tt; 1tt
Phenotype: 3 Tall; 1 short
Genotype ratio
_1:2:1______________
Phenotype ratio
_3:1_____________
_75__ % tall
_25__ % short
t
Example #3: A homozygous
tall plant is crossed with a
homozygous short plant.
T
t
Tt
Tt
Tt
Tt
T
TT
X
tt
Possible offspring:
Genotype: 4Tt
Phenotype: 4 Tall
Genotype ratio
_4:0______________
Phenotype ratio
_4:0_____________
_100__ % tall
_0__ % short
T
Example #4: A homozygous
tall plant bred to a
heterozygous tall plant.
T
t
TT
Tt
TT
Tt
T
TT
X
Tt
Possible offspring:
Genotype: 2TT;2Tt
Phenotype: 4 Tall
Genotype ratio
_2:2______________
Phenotype ratio
_4:0_____________
_100__ % tall
_0__ % short
s
s
Example #5: A dog that is
Ss
Ss
heterozygous for curly
S
hair is crossed with a dog
that is homozygous
ss
ss
recessive for straight
hair.
s
ss= straight hair
Possible offspring:
SS=wavy hair
Genotype: 2Ss;2ss
Ss=curly hair
Phenotype: 2 curly; 2 straight
Ss
X
ss
Genotype ratio
_1:1______________
Phenotype ratio
_1:1_____________
_50__ % tall
_50__ % short
Probabilities Can Also Predict
The Expected Results of
Crosses:
Probability—is the likelyhood
that a specific event will occur.
Consider the possibility that
a coin tossed into air will land
on heads (1 possible outcome.)
The total number of all possible outcome is two—
heads or tails. Thus, the probability that a coin will
land on heads is ½ or 50%.
Probability = number of 1 kind of possible outcome
total number of all possible outcomes
Example : Assume that in
humans there is a 50/50
chance that a child will
be a boy. If a certain
mother and father have
four sons, what are the
chances that their fifth
child will be a daughter.
X
X
XX
XX
XY
XY
Y
number of 1 kind of possible outcome OR 1
total number of all possible outcomes
2
XY
X
XX
X
Traits Influenced by the Environment:
Environmental Influences:
• Making inheritance more difficult to understand
are interactions between genes and the
environment.
• The expression of some traits is affected by
internal environments that are governed by age
or sex.
• Expression of other traits is affected by external
factors in the environment such as temperature,
chemicals, or light.
• Example: Environmental influences fur color in
the artic fox—in the winter it has white fur; it has
dark fur in the summer.
Summer
Winter
Human Genetic Disorders Caused by Mutations:
• Changes in genetic material are called mutations.
• The harmful effects produced by inherited mutations are
called genetic disorders.
• Many mutations are carried by recessive alleles in
heterozygous individuals. This means two phenotypically
normal people who are heterozygous carriers of a
recessive mutation can produce children who are
homozygous for the recessive allele.
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A genetic trait that appears in every
generation of offspring is called dominant.
Scientists can determine several pieces of
genetic information from a pedigree.
Autosomal trait—if a trait is autosomal, it
will appear in both sexes equally.
Autosomal does not involve the sex
chromosomes.
Each chromosome carries genes for certain
traits.
Recall that in humans, the diploid number
of chromosomes is 46 or 23 pairs.
There are 22 pairs of matching homologous
chromosomes called autosomes.
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Autosome is a chromosome other than X or Y
sex chromosomes.
The 2 chromosomes in a homologous pair of
autosomes look exactly alike.
In a pair of homologous chromosomes, 1
comes from the mother, and one comes from
the father.
In other words, out of 46 chromosomes, 23
comes from mom and 23 from dad.
Homologous chromosomes have genes for the
same traits arranged in the same order.
However, because there are different possible
alleles (versions) for the same gene, the two
chromosomes in a homologous pair are not
identical to each other.
The 23rd pair (sex chromosomes) of chromosomes
differs in males and females. These two
chromosomes determine the sex of an individual and
are called sex chromosomes.

Sex-linked trait— Traits controlled by genes located
on sex chromosomes.

In humans, the chromosomes that control the
inheritance of sex characteristics are indicated by the
letters X and Y.
•
If you are a female, XX, your 23rd pair of
chromosomes look alike.
•
If you are a male, XY, your 23rd pair of
chromosomes look different
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If a trait is sex linked, it is usually seen only in
males. Most sex-linked traits are recessive.
Because males have only one X chromosome, a male
who carries a recessive allele on the X chromosome
will exhibit the sex-linked condition.
A female who carries a recessive allele on one X
chromosome will not exhibit the condition if there is
dominant allele on her other X chromosome.
She will express the recessive condition only if she
inherits two recessive alleles.
MOST TRAITS ARE NOT CONTROLLED BY
SIMPLE DOMINANT/RECESSIVE ALLELES
Patterns of heredity Can Be Complex:
State Test Question
Humans have 23 pairs of chromosomes.
How many pairs are sex chromosomes
1
2
3
4
Sickle Cell Anemia
An example of an autosomal recessive gene disorder is sickle cell
anemia.
A condition caused by a mutated allele that produces a defective
form of the protein hemoglobin.
Sickle cell anemia is caused by a recessive gene on chromosome 11.
In sickle cell anemia, the defective
form of hemoglobin causes many
red blood cells to bend into a sickle
shape.
 Normal RBCs last ~120 days in blood
–
stream, while sickle RBCs last ~ 10–
20 days. Sickle cells tend to get stuck
–
in small blood vessels and block the
–
flow of blood.
Sickle cell anemia is a recessive
disease..
Sickle Cell Anemia
S

Example: A woman
who is a carrier (Ss)
SS
S
for sickle cell has
children with a man
who is free of the
SS
disease (SS). What are S
the chances of the
couple having a child
with sickle cell
0%
________ chance
anemia?
SS
X
Ss
s
Ss
Ss
Tay-Sachs Disease
TSD is an extremely rare autosomal
recessive genetic disorder. It causes
deterioration of mental & physical abilities;
destruction of the Central Nervous System,
and fatty material builds up in the nerve
cells of the brain.
The child appears normal at first. At 6
months, the child becomes blind, deaf, &
unable to swallow over time. Muscle begin
to atrophy & paralysis sets in; death by age
4.
No treatment or cure; found on chromosome
15.
Tay-Sachs Disease
Cystic Fibrosis
An autosomal recessive gene disorder found to affect
chromosome 7. This disorder causes production of a
protein that makes a person’s body produce
unusually thick, sticky mucus.
This mucus can lead to severe infections, & it
prevents enzymes from the pancreas from helping
the body to break down food & absorb nutrients.
This person usually has a persistent cough, frequent
lung infections, salty tasting skin, infertility, and
weight loss even when he/she or she eats well, &
shortness of breath.
There is no known cure; therefore, cystic fibrosis is
often fatal. However, medical treatment may help
patients live longer.
Huntington’s Disease
Huntington’s disease is a genetic disorder
caused by a dominant allele located on an
autosome.
In time, HD causes loss
of muscle control, uncontrollable physical
spasms, severe mental
illness, and eventually
death.
h

Example: If one
parent is heterozygous
for the trait and the
other has normal
alleles, what are their
chances of having an
offspring with the
disease?
H
Hh
Hh
hh
hh
h
50%
________ chance
Hh
X
hh
h
Phenylketonuria (PKU)
A recessive disorder that results from the absence
of an enzyme that converts one amino acid,
phenylalanine cannot be broken down, it and its
by-products accumulate in the body and result in
severe damage to the central nervous system and
mental retardation.
PKU