Genetics: The Science of
Heredity
Lesson 1
A Priest-Scientist
Gregor Mendel
Who was Gregor Mendel?
• Gregor Mendel was an Austrian
monk.
• He lived between 1822 to 1884.
• He was a teacher & a botonist
• He did experiments on hundreds
of pea plants.
• Why Pea Plants?
– Simple genetic make up
– Traits are easily observed
– Can cross-pollinate or self-pollinate
Heredity
• Father of Genetics
– Mendel’s work with pea plants
has become the basis of
genetics, the study of heredity.
• Heredity is the passing of
traits from parents to
offspring.
Mendel’s Pea Experiments
• He crossed plants with two
different traits, for example purple
flowers with white flowers.
• He started his experiments with
purebred plants known as True
Breeders.
• Purebred plants ALWAYS
produce offspring with the same
trait as the parent. For example, if
the parent is tall, all offspring will
be tall. If the parent is short, all
offspring will be short.
Some Pea Traits that Mendel
Studied
F1 Generation
• Mendel called the parent plants the P generation.
• He called the offspring from the parents the F1
generation.
• F is from the Latin word, filial, which means son.
• When Mendel crossed pure pea plants with purple
flowers with pure pea plants with white flowers, all the
F1 generation had purple flowers.
P Generation
(Parent Plants)
F1 Generation,
Offspring of the
Parent Plants
F2 Generation
• When he crossed the F1 generation peas with
one another, only some of the offspring had
purple flowers. These formed the F2 generation.
• Mendel found that in the F2 generation, ¾ of the
plants had purple flowers and ¼ of them had
white flowers (3:1 ratio).
F1 Generation,
offspring of
Parent Plants
F2 Generation,
offspring of F1
Generation
Lesson 1 Review
1. Who was Gregory Mendel?
2. Why did he choose peas for his
experiments?
3. What is heredity?
4. What is genetics?
5. What is a purebred plants?
6. What is the P generation?
7. What is the F1 generation?
8. What is the F2 generation?
Lesson 2
Understanding Mendel’s
Experiments
Dominant and Recessive Traits
• It seemed to Mendel,
that for each
characteristic in peas,
one trait was stronger
than the other.
• He called the “stronger”
one, the dominant trait.
• He called the “hidden”
one, the recessive trait.
• This has become known
as “Mendel’s Principle of
Dominance”
Genes and Alleles
• The traits of peas (and
yours) are controlled by
factors that scientists call
genes.
• You inherit your genes
from your parents.
• The different forms of a
gene are called alleles.
• You inherit a combination
of two alleles from your
parents.
Dominant and Recessive Traits in
Peas
• For each of the 7
traits that Mendel
studied in peas, there
is a dominant allele
and a recessive
allele.
• If a plant inherits both
a dominant allele and
a recessive allele, the
dominant allele
masks the recessive
allele.
Understanding Mendel’s
Experiments
Part I
2 alleles
for white
2 alleles
for
purple
pp
PP
1 allele for purple. 1 allele for
white
Pp
Understanding Mendel’s
Experiment
1 allele for purple
Part II
1 allele for white
Pp
Pp
2 alleles for purple
PP
1 allele for purple
1 allele for white
Pp
2 alleles for white
pp
Principle of Segregation
• Based on the outcome of his experiment,
Mendel developed his “Principle of
Segregation” that states that when forming
sex cells, the paired alleles separate so
that each egg or sperm only carries one
form of the allele.
• The two forms of the allele come together
again during fertilization.
Lesson 2 Review
16. What did Mendel find to be the same with all 7
traits of the pea plant that he studied?
17. What are genes?
18. What are dominant alleles?
19. What are recessive alleles?
20.What happens if a pea plant inherits two
dominant allele of the same gene?
21. What happens if a pea plant inherits a
dominant allele and a recessive allele of the
same gene?
22. What happens if the pea plant inherits two
recessive alleles of the same gene?
Lesson 3
Probability and Genetics
Probability
• Probability is the likelihood
that a particular event will
occur.
• The laws of probability
determine what is likely to
occur, not what does occur.
• Mendel was the first
scientist that applied the
principles of probability to
genetics.
Punnett Square
• Punnett square is a table that
shows all the possible
combinations of alleles that
can result when two
organisms cross.
• Using Punnett square,
geneticists can predict the
probability of occurrence of a
particular trait.
• The allele that each parent
will pass to its offspring is
based on chance, just like
tossing a coin.
Genotypes and Phenotypes
Phenotype
• Genotype: Indicates
the alleles that the
organism has
inherited regarding a
particular trait.
• Phenotype: The
actual visible trait of
the organism.
Genotype
Homozygous and Heterozygous
• Homozygous: An
organism with two
identical alleles for a
trait (a purebred
organism).
• Heterozygous: An
organism that has two
different alleles for a
trait (a hybrid
organism).
Review
16. What it probability? How is it related to
genetics?
17. What is the Punnett Square? How is it
helpful to geneticist?
18. What is a genotype?
19. What is a phenotype?
20. What is a homozygous organism?
21. What is a heterozygous organism?
Lesson 4
• Alleles
Genetics
• Alleles
1. Alternative forms of genes.
2. Homozygous alleles are exactly the same.
3. Dominant alleles – capitalized
(TT - tall pea plants)
a. Homozygous dominant
4. Recessive alleles - lowercase
(tt - dwarf pea plants)
a. Homozygous recessive
5. Heterozygous (different) (Tt - tall pea plants)
Phenotype
• Outward appearance
• Physical characteristics
• Examples:
1.
2.
tall pea plant
dwarf pea plant
Genotype
• Arrangement of genes that produces the
phenotype
• Example:
1. tall pea plant
TT = tall (homozygous dominant)
2. dwarf pea plant
tt = dwarf (homozygous recessive)
3. tall pea plant
Tt = tall (heterozygous)
Practice! Practice! Practice!!!
In pea plants the Tall (T) allele
is dominant over the dwarf (t)
allele.
Practice! Practice! Practice!!!
In pea plants the Tall (T) allele
is dominant over the dwarf (t)
allele.
1. What is the genotype of a
homozygous tall plant?
Practice! Practice! Practice!!!
In pea plants the Tall (T) allele
is dominant over the dwarf (t)
allele.
2. What is the genotype of a
homozygous short plant?
Practice! Practice! Practice!!!
In pea plants the Tall (T) allele
is dominant over the dwarf (t)
allele.
3. What is the genotype of a
heterozygous tall plant?
Practice! Practice! Practice!!!
In pea plants the Tall (T) allele
is dominant over the dwarf (t)
allele.
4. A plant has a genotype of Tt.
What is its phenotype?
Practice! Practice! Practice!!!
In pea plants the Tall (T) allele
is dominant over the dwarf (t)
allele.
5. A plant has a genotype of tt,
what is its phenotype?
Practice! Practice! Practice!!!
In pea plants the Tall (T) allele
is dominant over the dwarf (t)
allele.
6. What are the two alleles for the
height of a pea plant?
Answers:
1. TT
2. tt
3. Tt
4. Tall
5. short
6. T (tall) and t (short)
Lesson 5
Mono-Hybrid Crosses
Monohybrid Cross
• A breeding experiment that tracks the inheritance
of a single trait.
• Mendel’s “principle of segregation”
pairs of genes separate during gamete
formation (meiosis) & reform during
ferlization.
Homologous Chromosomes
eye color locus
B = brown eyes
eye color locus
b = blue eyes
This person would
have brown eyes (Bb)
Paternal Maternal
Monohybrid Cross
• Example:
Cross between two heterozygotes
for brown eyes (Bb)
BB = brown eyes
Bb = brown eyes
bb = blue eyes
B
b
B
Bb x Bb
b
female gametes
male
gametes
Monohybrid Cross
B
b
B
BB
Bb
b
Bb
bb
Bb x Bb
1/4 = BB - brown eyed
1/2 = Bb - brown eyed
1/4 = bb - blue eyed
1:2:1 genotype
3:1 phenotype
Practice! Practice! Practice!!!!
In cocker spaniels black (B) is
dominant to red (rust) (b).
1. What would be the phenotypic
ratio of a cross between a true
breeding black crossed with a
true-breeding rust?
Step 2 – List the possible
gametes from each parent
BB
B
bb
B
b
b
Step 3 Draw punnett square and
place the gametes on the sides.
BB
B
bb
B
b
B
B
b
b
b
Step 4 Fill in the punnett
square to find the possible
zygotes
BB
bb
B
B
b
b
b
Bb
Bb
B Bb
Bb
B
b
Step 5 Determine the genotypic
and Phenotypic ratios
BB
B
bb
B
b
B
Bb
B Bb
b
b
b
Bb
Phenotype
100% Black
Bb
Genotype
100% Bb
Practice Problems
Complete a Punnett Square for each of
the following:
T = tall plant
P = purple flowers
1. PP x pp
2. Tt x TT
3. Pp x PP
t = short plant
p = white flowers
4. Pp x pp
5. tt x TT
6. Tt x tt
Lesson 6
Dihybrid Cross
• A breeding experiment that tracks the inheritance
of two traits.
• Mendel’s “principle of independent assortment”
a. during Metaphase I, each pair of alleles
seperates independently
b. formula: 2n (n = # of heterozygotes)
Independent Assortment
• Question: How many gametes will be produced
for the following allele arrangements?
• Remember:
2n (n = # of heterozygotes)
1.
RrYy
2.
AaBbCCDd
3.
MmNnOoPPQQRrssTtQq
Answer:
1. RrYy: 2n = 22 = 4 gametes
RY Ry rY ry
2. AaBbCCDd: 2n = 23 = 8 gametes
ABCD ABCd AbCD AbCd
aBCD aBCd abCD abCD
3. MmNnOoPPQQRrssTtQq: 2n = 26 = 64
gametes
Dihybrid Cross
• Example:
R
r
Y
y
= round
= wrinkled
= yellow
= green
cross between round and yellow
heterozygous pea seeds.
RrYy x RrYy
RY Ry rY ry x RY Ry rY ry
possible gametes produced
Dihybrid Cross
RY
RY
Ry
rY
ry
Ry
rY
ry
Dihybrid Cross
RY
RY RRYY
Ry RRYy
Ry
RRYy
RRyy
rY
RrYY
RrYy
ry
RrYy
Round/Yellow:
9
Round/green:
3
Rryy
wrinkled/Yellow: 3
rY RrYY
RrYy
rrYY
rrYy
wrinkled/green:
ry
Rryy
rrYy
rryy
9:3:3:1 phenotypic ratio
RrYy
1
Lesson 7
Exceptions
Incomplete Dominance
• F1 hybrids have an appearance somewhat in
between the phenotypes of the two parental
varieties.
• Example: snapdragons (flower)
• red (RR) x white (rr)
R
RR = red flower
rr = white flower
r
r
R
Incomplete Dominance
R
R
r
Rr
Rr
produces the
F1 generation
r
Rr
Rr
All Rr = pink
(heterozygous pink)
Codominance
• In codominance, the
alleles are neither
dominant, nor
recessive. Neither
allele is masked by
the other.
Roan Cow
Is both white and red
Lesson 8
Mutations
MUTATIONS
• Mistakes that occur during the duplication
of the chromatin material
• GERM CELL MUTATIONS - occur in
gametes, passed on to offspring without
affecting parent
• SOMATIC MUTATIONS - occur in body
cells, not usually passed on to offspring
Mutations
• Chromosomal Mutations affect the entire
chromosome
• Occur during the Crossing Over period of
Meiosis
– Duplication Mutations – one chromosome
carries two copies of a gene or set of genes
– Deletion Mutations – post cell division, the
new cell will lack the genes that were carried
by the piece that broke off
Chromosomal Mutations, cont.
– Inversion Mutations – the segment that
breaks off reattaches itself but in the
backwards position
– Translocation Mutations – a chromosome
segment attaches itself to a non-homologous
chromosome
Chromosome Mutations, cont.
NONDISJUNCTION-failure of a
chromosome to separate from
its homologue during meiosis
*one gamete will receive an extra copy
of a chromosome while the other
gamete will not have the chromosome
Results in: *DOWN SYNDROME
(TRISOMY 21)-an extra copy of the
21st chromosome
Mutations
• Gene Mutations affect individual genes
on a chromosome.
• Occur by:
– Insertion of a nucleotide
– or
– Deletion of a nucleotide
The Number of Chromosomes
• Humans that have even
one missing or one extra
chromosome usually die
before birth or have
serious defects.
• Down Syndrome happens
when a person is born with
an extra copy of
chromosome number 21.
Chromosomes
• Males are XY and females are XX
• Genes found on sex chromosomes…SEX
LINKED
• Genes on an X chromosome are XLINKED. Genes on a Y chromosome are
Y-LINKED.
• Eye color in fruit flies is X-Linked. Only
male fruit flies have white eyes.
Sex linked traits
Examples of sex linked traits are
1. Blood clotting factor – this factor is located
on the X chromosome and the dominant
allele allows your blood to clot normally.
The recessive form does not allow your
blood to clot.
Two recessive alleles causes the disease
hemophilia.
Boys are more likely to get the disease
because they only have one X.
2. Red-green color vision is another sexlinked trait. The dominant allele allows
you to see reds and greens. The
recessive allele prevents seeing red or
green.
Boys are most often affected because of
having only 1 X chromosome.
Sex Influenced Traits
• Influenced by male or female sex
hormones
• Baldness
Multiple Alleles
• Traits that are determined by 2 or more
alleles & produce different colorations
based on the original traits.
• Include:
– blood type
– Hair color
– Eye color
Polygenic Traits
• The results of the interaction of multiple
genes
• Example: hypertension
– Caused by the interaction of one’s:
•
•
•
•
•
Weight
Ability to process fats
Cholesterol count
Ability to process salts
Lifestyle - i.e. Smoking, drinking, exercise
Mutagens
• Environmental factors that can cause
mutations
• Include:
– Radiation exposure via X-rays or UV light
– Natural or made-made chemicals
– Pollutants
– Extremely high temperatures
– Viruses
Human Karyotype showing
homologous chromosome pairs
This individual has inherited three copies of chromosome 21
and has a condition called Down syndrome.
Nondisjunction, cont.
• KLINEFELTER’S SYNDROME- (XXY)feminine characteristics, mentally
impaired, infertile
• TURNER’S SYNDROME- (XO)- female
appearance, no sexual maturity, infertile
Human Genetic Disorders
• Cystic fibrosis-(CF)-difficulties with
breathing and digestion
• Sickle cell anemia -forms sickle
shaped RBCs because of a defective
protein called hemoglobin, leads to
lack of O2 & circulatory problems
Fig. 11.12, p. 183