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FUNDAMENTALS
OF GENETICS
CHAPTER 9
GENETICS
 Genetics
is a field of Biology that is
devoted to understanding HOW
characteristics are passed on from
parents to offspring.
 Gregor
Johann Mendel – the father
of genetics
GREGOR MENDAL
1843 – Gregor Mendal joined the monastery at
the age of 21. His task was to tend to the
garden, which allowed him to observe and think
about the growth of many generations of plants.
 1851 – He entered the U. of Vienna to study math
and science, where he studied statistics
 Statistics helped him in the field of Heredity – the
transmission (passing on) of characteristics from
parents to offspring.

GARDEN PEAS
 Mendel
is remembered most for his work
with Pisum sativum, Garden Peas.
 Seven Characteristics of garden peas
were observed.
 For each characteristic, two contrasting
traits were observed
 A trait is a genetically determined variant
of a characteristic.
GARDEN PEAS
 The
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Seven Characteristics:
Plant Height (traits: Long & Short)
Flower Position along stem (traits: axial &
terminal)
Pod Color (traits: green & yellow)
Pod appearance (traits: inflated and constricted)
Seed Texture (traits: round and wrinkled)
Seed Color (traits: yellow & green)
Flower Color (traits: purple & white)
GARDEN PEAS
 Mendel
collected the seeds of his pea plants
and recorded each plant’s traits and seeds
 He planted the seeds
 FLOWER COLOR: He observed that purple
flowering plants came from most of the
seeds that he had collected from purple
plants
 But there where some white flowering plants
that came from purple plant seeds.
GARDEN PEAS
 PLANT
HEIGHT:
 Mendel observed that while tall plants
grew from most of the seeds that were
obtained from tall plants
 But Mendel also observed some short
plants grow from seeds that were also
obtained from tall plants.
 Mendel wanted to find an explanation for
such variation.
MENDEL’S METHODS

Mendel observed how traits were passed from one
generation to the next by controlling how the
plants were pollenated.
 Pollination – when the pollen grains from the male
reproductive part of the flower (anthers) is
transferred to the female reproduction part of the
flower (stigma).
 Self pollination occurs when pollen is transferred
from anthers to stigma of the same plant
 Cross-pollination occurs between two different
plants
MENDEL’S METHODS
 To
Control his Results:
 Mendel
removed the anthers of a plant and
cross pollinated the plants by manually
transferring pollen from the flower of a
second plant to the stigma of the antherless
plant.
 This way he prevented self pollination and
controlled the specific traits of the parents.
MENDEL’S EXPERIMENTS
Mendel’s Experiments: he first studied each
characteristic and it’s contrasting traits
individually.
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He began growing plants that were truebreeding, or pure for each trait. True-breeding
plants always produce offspring with that trait
when they self pollinate.
Mendel produced true-breeding plants, by self
pollination, for each of the characteristics and
for each trait. So in the end he had 14 true
breeding plants.
MENDEL’S EXPERIMENTS
He began to cross-pollinate pairs of plants
w/ opposite traits of a characteristic. He
called the first true-breeding parents the P
generation. He cross-pollinated a plant
true-breeding for yellow pods with another
plant true-breeding for green pods
MENDEL’S EXPERIMENTS
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Mendel recorded the number of each type of
offspring that resulted from the cross-pollination
of the P generation. He called the offspring of
the P generation the F1 generation – F for Filial
which has its roots in latin for son or daughter.
He then allowed the flowers of the F1 generation
to self- pollinate and then collected the seeds.
He called the offspring of the F1 generation the
F2 generation.
Mendel performed hundreds of crossings and
documented the results of each generation.
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Mendel’s Results &
Conclusions
The resulting F1 generation from the cross of the green
pod plant and the yellow pod plant resulted in only green
pod plants
Mendel saw that no yellow pod plants developed even
though one parent had been true-breeding for yellow
pods.
Mendel then allowed the F1 generation to self pollinate:
Three – Fourths (75%) of the F2 generation produced
green pods and One-Fourth (25%) produced yellow pods
He concluded and hypothesized that something inside
the pea plants controlled the characteristics and traits
that were observed. He reasoned that a pair of control
factors must control each trait.
Mendel’s Results &
Conclusions

Recessive and Dominant Traits : Whenever
Mendel crossed traits – one of the P traits failed to
appear in the F1 plants. An every case, the trait
reappeared in the F2 generation at a ratio of 3:1.
This pattern lead Mendel to hypothesize that one
factor in the pair may prevent the other from
having an effect.
• Dominant Trait masks or dominates the
appearing characteristic
• Recessive Traits is only expressed when paired
with another plant or animal displaying that
recessive trait
Results and Conclusions
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The Law of Segregation:
Mendel concluded that each reproductive
cell or gamete, receives one factor from
each parent. When the gametes combine
during fertilization, the offspring have two
factors for each characteristic. The Law of
Segregation states that a pair of factors is
segregated, or separated, during the
formation of gametes.
Results and Conclusions
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The Law of Independent Assortment:
Mendel also crossed plants that differed in
two characteristics, such as flower color
and seed color. The results from these
crosses showed that the traits produced by
dominate factors do not necessarily appear
together. A white flowering plant can
produce green pods. Mendel hypothesized
that the factors for individual characteristics
are not connected.
RESULTS &
CONCLUSIONS
 REMEMBER
that in meiosis, the random
separation of homologous chromosomes
is called independent assortment.
 The Law of Independent Assortment
states that characteristics separate
independently of one another during the
formation of gametes.
SUPPORT FOR
CONCLUSIONS…
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Support for Mendel’s Conclusions: Most of
what Mendel found is supported by what
biologists now know about molecular genetics.
Molecular genetics is the study of the
structure and function of chromosomes and
genes.
A gene is the segment of DNA on a
chromosome that controls a certain hereditary
trait.
Remember that chromosomes occur in pairs so
genes also occur in pairs. An allele is the name
given to one of the alternate forms of a gene
that governs a characteristic.
Section 2: Genetic Crosses
1.
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5.
Differentiate between the genotype and
phenotype of an organism
Explain how probability is used to predict the
results of genetic crosses
Use a Punnett square to predict results of
monohybrid and dihybrid genetic crosses
Explain how a testcross is used to show the
genotype of an individual whose phenotype
expresses the dominant trait
Differentiate a monohybrid cross from a
dihybrid cross
Genotype and Phenotype
Genotype and Phenotype
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Genotype is the organism’s genetic make
up. It is the alleles that the organism inherits
from the parents. When you think of
Genotype – Think GENES
Example: The genotype for a white-flowering
plant would be pp. The genotype of a purple
flowering plant would be Pp or PP.
Genotype & Phenotype
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Phenotype is the organism’s
appearance. When you hear
phenotype – think PHYSICAL.
Example: The phenotype of PP or
Pp would be purple-flowers. The
phenotype of pp would be white
flowers.
GENOTYPE & PHENOTYPE
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Genotype doesn’t necessarily mean that
phenotype and visa versa. A plant with the
genotype to be tall can be short because of
environmental factors.
When both alleles of a pair are alike, the
organism is said to be homozygous. This
means that they can be homozygous
dominant (PP) or homozygous recessive (pp).
When the alleles are different, they are said to
be heterozygous. An example of
heterozygous is Pp.
Probability

Probability is the likelihood that a specific event
will occur. (the chances that “something” will
happen). Probability may be expressed as a
decimal number, a percentage, or a fraction.

Probability is determined by the following
equation:
Probability = the # of times an event is expected to happen
The # of times an event could happen
Predicting Results..
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A monohybrid cross is a cross in which only one
characteristic is tracked.
Monohybrids are the offspring of the monohybrid
cross.
A Punnett square is an aid in the prediction of the
probable distribution of inherited traits in the
offspring.
Homozygous x Homozygous
We are going to cross a Pea Plant that is
homozygous for Purple flowers (the alleles are PP)
and a pea plant that is homozygous for White
flowers (the alleles are pp). The alleles that are
carried by each parent’s gamete are represented
by the letters on the outside of the boxes:
Predicting Results
 The
combinations within the four boxes
represent the possible genotypes that can
result from the cross of the homozygous pea
plants. The outcome was Pp in every case,
so the probability of the offspring having Pp
is 100%. The probability of the flower being
purple is 100% as well
PREDICTING RESULTS..
 We
are going to cross a Vegasaurous
Rex that is homozygous dominant (allele
is CC) for crazy curly hair and another
Vegasaurous Rex (allele is Cc) that is
heterozygous for crazy curly hair.
Predicting Results…
 The
two possible genotypes from this
cross are CC and Cc. The probability of
an offspring having the genotype CC is
2/4 or 50%.
 You could expect about 2/4 or 50% of the
offspring to have Cc. The probable
phenotype of this cross is crazy curly
hair. Thus, there is a 100% probability
that the offspring will have crazy curly
hair.
Predicting Results…
 If
it were the other way around: cc x Cc ,
then the probability of an offspring having
the genotype cc is 2/4 or 50%. You can
expect the other half, 50% or 2/4 to have
Cc. So the phenotype results would change
– the genotype Cc would exhibit crazy curly
hair and the cc would not.
Predicting results…

Heterozygous x Heterozygous
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In rabbits the allele for brown coat color
(B) is dominant over white coat color (b).
Now, we are going to cross to rabbits
that are heterozygous Bb. They are both
brown rabbits.
Predicting results…
 As
you can see, ¼ or 25% of the offspring
predicted will have the genotype BB
 Another 25% or ¼ of the offspring
predicted will have the genotype bb and
the rest, 50% or ½ will have the genotype
Bb.
 The phenotype results in 75% or ¾ being
brown rabbits while the rest, 25% or ¼ will
have white fur.
Predicting Ratios…
 Ratio:
 Genotype
Ratio: the ratio of the genotype
that appear in the offspring
 Example: 1 BB: 2Bb: 1bb
 Phenotype Ratio: the ratio of the phenotype
that appear in the offspring
 Example: 3 brown: 1 white
TESTCROSS
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Is performed when the genotype of an individual is
unknown.
In a test cross the individual with the unknown
genotype is crossed with a homozygous recessive
individual.
Example: Brown Bunnies. Is it BB or Bb? Cross it
with Homozygous Recessive Bunny
If all the offspring appear Brown then the bunny is
Homozygous Dominant (BB)
If half the bunnies are white, then the bunny is
Heterozygous Dominant (Bb)
INCOMPLETE DOMINANCE
 When
the dominant allele completely masks
recessive allele is called complete dominance.
 But there are some traits, where the recessive
allele comes through a little bit. (Incomplete
dominance)
 For Example: When crossing a certain type of
Flower: Red Flowers (RR) and White Flowers
(rr) The F1 generation are all (Rr) but the
phenotype is Pink.
 In humans, skin color, eye color and hair
shades are an example of incomplete
dominance
Codominance
 Occurs
when BOTH alleles for a gene are
expressed in a heterozygous offspring
 In codominance – NIETHER allele is
dominant or recessive, and they don’t blend
as in incomplete dominance.
 Example: is in Blood Types
Predicting Results of
Dihybrid Crosses
A
dihybrid cross is a cross in which two
characteristics are tracked.
 Dihybrids are the offspring of these
crosses
 Naturally, with the addition of a trait, more
combinations are possible.
Homozygous x Homozygous
 By
using a Punnett square, we are going
to predict the possible offspring of a cross
between two Pea plants with are
Homozygous for

Round, Yellow Seeds (RRYY)
&
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Wrinkled, Green Seeds (rryy)
Predicting Results…
Homozygous x Homozygous
 Assort
the alleles
 RY, RY, RY, RY
 ry, ry, ry, ry
 Each box gets filled by the letters above it
and to the left
 In this case each box has RrYy, so ach
plant would have round yellow seeds.
Predicting Results
Heterozygous x Heterozygous
 So,
now lets take two pea plants from the
F1 generation that is heterozygous yellow
round (RrYy)
 This cross would result in NINE different
genotypes
 This cross would result in FOUR different
phenotypes
Heterozygous x Heterozygous
 9/16
would have the phenotype of round,
yellow seeds (genotypes: RRYY, RRYy,
RrYY, RrYy)
 3/16 would have the phenotype of round,
green seeds (genotypes: RRyy & Rryy)
 3/16 would have the phenotype of wrinkled,
yellow seeds (genotypes: rrYY, rrYy)
 1/16 would have the phenotype of wrinkled,
green seeds (genotype: rryy)
Pedigree Charts

Chart which shows how a trait and the genes
that control it are inherited within a family
 Identifies the presence or absence of
particular trait in members of each generation
Pedigree Charts
 Males
=
 Females =
 Generations = Roman Numerals
 Individuals = Numbered sequentially
 Trait Expressed = Filled in
 Non Carriers = Empty
 Carrier (not ill) = Half filled
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Individual who carries a recessive allele that
is not expressed