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Mendel's Laws of Inheritance
1. 1
Gregor Johann Mendel
Between 1856 and
1863, Mendel
cultivated and tested
some 28,000 pea
plants
He found that the
plants' offspring
retained traits of the
parents
2. Gregor Mendel
Called the “Father of
Genetics
Gregor Mendel
(1860’s) discovered
the fundamental
principles of
genetics by
breeding garden
peas.
4. Pea Garden (Pisum sativum)
Easy to grow and can be grown in a small areaan be grown in a small area
Produce lots of offspringProduce lots of offspring
Produce pure plants when allowed to self-Produce pure plants when allowed to self-
pollinate several generations (true breedingpollinate several generations (true breeding
varieties)varieties)
Clearly defined characteristics or traits
Easy to be crossed between parents
6. Mendel cross-
pollinated pea
plants
• Mendel probably chose
to work with peas
because they are
available in many
varieties.
• The use of peas also
gave Mendel strict
control over which
plants mated.
• Fortunately, the pea
traits are distinct and
were clearly contrasting.
7. Mendel’s experimental design
Statistical analyses:
Worked with large numbers of plants
counted all offspring
made predictions and tested them
Excellent experimentalist
controlled growth conditions
focused on traits that were easy to score
chose to track only those characters that varied in an
“either-or” manner
10. Typical breeding experiment
P generation (parental
generation)
F1 generation (first filial
generation, the word filial
from the Latin word for
"son") are the hybrid
offspring.
Allowing these F1 hybrids
to self-pollinate produces:
F2 generation (second
filial generation).
12. 12
Law of Dominance
In a cross of parents that are pure for
contrasting traits, only one form of the trait
will appear in the next generation.
All the offspring will be heterozygous and
express only the dominant trait.
RR x rr yields all Rr (round seeds)
13. The Principle of Dominance
PaternalMaternal
eye color locus
B = brown eyes
eye color locus
b = blue eyes
14. Dominant and Recessive alleles
Dominant alleles – upper-case (B)
a. homozygous dominant (BB – Brown eyes)
Recessive alleles – lower case (b)
a. homozygous recessive (bb – blue eyes)
b. heterozygous dominant (Bb – Brown eyes)
15. Phenotype vs. Genotype
Outward appearance
Physical characteristics
Examples:
1.Brown eyes
2.blue eyes
Arrangement of genes
that produces the
phenotype
Example:
1. TT, Tt
2. tt
17. 17
Law of Segregation
During theDuring the formation of gametesformation of gametes (eggs(eggs
or sperm), theor sperm), the two allelestwo alleles responsibleresponsible
for a traitfor a trait separateseparate from each other.from each other.
Alleles for a trait are thenAlleles for a trait are then
"recombined" at fertilization"recombined" at fertilization,,
producing the genotype for the traits ofproducing the genotype for the traits of
the offspringthe offspring.
19. Punnett Squares
Diagram used to predict genetic
crosses
Tool for calculating genetic
probabilities
A tool to predict the probability of
certain traits in offspring that shows
the different ways alleles can
combine.
Diagram showing the probabilities of
the possible outcomes of a genetic
cross
20. How to use Punnett
Squares
Choose a letter to represent the alleles in the cross.
Write the genotypes of the parents.
Determine the possible gametes (reproductive cells)
that the parent can produce.
Enter the possible gamete at the top and side of the
Punnett square.
Complete the Punnett square by writing the alleles
from the gametes in the appropriate boxes.
Determine the phenotypes of the offspring.
21. Punnet Square Process
1. Determine alleles of
each parent, these are
given as TT, and tt
respectively.
2. Take each possible
allele of each parent,
separate them, and
place each allele
either along the top,
or along the side of
the punnett square.
22. Punnett Square Process
Lastly, write the letter for
each allele across each
column or down each
row. The resultant mix is
the genotype for the
offspring. In this case,
each offspring has a Tt
(heterozygous tall)
genotype, and simply a
"Tall" phenotype.
23. Punnett Square Process
Lets take this a step further
and cross these F1
offspring (Tt) to see what
genotypes and phenotypes
we get.
Since each parent can
contribute a T and a t to the
offspring, the punnett
square should look like
this….
24. Punnett Square Process
Here we have some more
interesting results: First
we now have 3 genotypes
(TT, Tt, & tt) in a 1:2:1
genotypic ratio. We
now have 2 different
phenotypes (Tall &
short) in a 3:1
Phenotypic ratio. This
is the common outcome
from such crosses.
Monohybrid cross
(cross with only 1 trait)
27. Dihybrid cross
Take the offspring and cross them since they are
donating alleles for 2 traits, each parent in the f1
generation can give 4 possible combination of alleles.
TW, Tw, tW, or tw.
F2 Generation
28. Dihybrid cross
Note that there is a 9:3:3:1
phenotypic ratio. 9/16
showing both dominant
traits, 3/16 & 3/16 showing
one of the recessive traits,
and 1/16 showing both
recessive traits.
Also note that this also
indicates that these alleles are
separating independently of
each other. This is evidence
of Mendel's Law of
independent assortment
29. Mendel’s Principles
The inheritance of biological characteristics are
determined by genes.
For two or more forms of a gene, dominance
and recessive forms may exist.
Most sexually reproductive organisms have two
sets of genes that separate during gamete
formation.
Alleles segregate independently.
30. Law of Independent
Assortment
Alleles forAlleles for differentdifferent traits aretraits are
distributed to sex cells (& offspring)distributed to sex cells (& offspring)
independently of one another.independently of one another.
Different genes on different
chromosomes segregate into
gametes independently of each other
35. Three Conclusions of
Mendel Experiment
1. Principle of Dominance and Recessiveness
One allele in a pair may mask the effect of the other
1. Principle of Segregation
The two alleles for a characteristic separate during
the formation of eggs and sperm
1. Principle of Independent Assortment
The alleles for different characteristics are distributed
to reproductive cells independently.
36. Variations on Mendel’s
Laws
The relationship of genotype to phenotype is rarely
simple
Mendel’s principles are valid for all sexually
reproducing species
But genotype often does not dictate
phenotype in the simple way his laws
describe
There is an exceptional to Mendel Laws
37. Exceptions To Mendel’s
Original Principles
Incomplete
dominance
Codominance
Multiple alleles
Polygenic traits
Epistasis
Pleiotropy
Environmental effects on
gene expression
Linkage
Sex linkage
38. Incomplete dominance
The phenotype of the
heterozygote is intermediate
between those of the two
homozygotes.
Neither allele is dominant and
heterozygous individuals have an
intermediate phenotype
For example, in Japanese “Four
o’clock”, plants with one red
allele and one white allele have
pink flowers:
P Generation
F1 Generation
F2 Generation
Red
CR
CR
Gametes CR
CW
×
White
CW
CW
Pink
CR
CW
Sperm
CR
CR
CR
Cw
CR
CRGametes
1
⁄2 1
⁄2
1
⁄2
1
⁄2
1
⁄2
Eggs
1
⁄2
CR
CR
CR
CW
CW
CW
CR
CW
40. Co-dominance
Phenotype of both
homozygotes are
produced in
heterozygotes
individuals.
Both alleles are
expressed equally.
Examples:
Roan Cattle
White-feathered birds
are both homozygotes
for both B and W
alleles
41. Multiple Alleles
More than three alleles for a gene
Found among all individuals in a population
Diploid individuals only have two of the alleles
Phenotype depends on relationship
between different pairs of alleles
Still follows Mendel’s principles
43. Human ABO Blood Group
Antigens
Glycoproteins on surface of red blood cells
IA
allele produces A antigen (dominant)
IB
allele produces B antigen (dominant)
i allele produces neither A nor B (recessive)
Blood types (phenotypes)
IA
IA
or IA
i = type A blood
IB
IB
or IB
i = type B blood
ii = type O blood
IA
IB
= type AB blood
45. Epistasis
Type of polygenic inheritance where the alleles at one gene locus
can hide or prevent the expression of alleles at a second gene locus.
Allele of one locus inhibits or masks effects of allele at a different
locus
Some expected phenotypes do not appear among offspring
Labrador retrievers one gene locus affects coat color by controlling
how densely the pigment eumelanin is deposited in the fur.
A dominant allele (B) produces a black coat while the recessive allele
(b) produces a brown coat
However, a second gene locus controls whether any eumelanin at all
is deposited in the fur. Dogs that are homozygous recessive at this
locus (ee) will have yellow fur no matter which alleles are at the first
locus:
47. Labrador Retrievers
Melanin pigment gene
B allele: black fur color (dominant)
b allele: brown fur color (recessive)
Pigment deposition gene
E allele: pigment deposition normal (dominant)
e allele: pigment deposition blocked (recessive)
Phenotypes
Black fur: BB EE, BB Ee, Bb EE, Bb Ee
Brown fur: bb EE, bb Ee
Yellow fur: BB ee, Bb ee, bb ee
50. Polygenic Inheritance
Most traits are not controlled by a single gene locus, but
by the combined interaction of many gene loci. These
are called polygenic traits.
Several genes at different loci interact to control the
same character
Produces continuous variation
Phenotypic distribution: Bell-shaped curve
Often modified by environmental effects
53. Pleiotropy
One gene affects more than one character
For example, in Labrador retrievers the gene
locus that controls how dark the pigment in the
hair will be also affects the color of the nose,
lips, and eye rims.
54. Environmental Effects
on Gene Expression
The phenotype of
an organism
depends not only
on which genes it
has (genotype),
but also on the
environment
under which it
develops.
56. Extranuclear inheritance
Some genes are passed from parent to offspring without
being part of nuclear chromatin
Mitochondria (and chloroplasts in plants) are randomly
assorted into gametes and daughter cells
In animals, mitochondrial traits are maternally inherited
Example:
Leaf color in four o'clock plants
Human mitochondrial disorders
58. A gene located on
either sex
chromosome (X in
humans)
Examples:
Color blindness
Hemophilia
Sex-linked traits
59. 6.Autosomal gene is present in both sexes but
expression depends on sex of individual (it’s dominant
in one sex but recessive in the other)
Example:
Baldness in males:
− Man with one copy of gene will be bald
− Female needs two copies of gene to be bald
Milk production in females
− Man with one copy does not lactate
− Female with one copy lactates
Sex-limited traits
60. Probability
The likelihood that a specific event
will occur.
The principles of probability can be
used to predict the outcomes of
genetic crosses.
61. Using probability in
Mendelian genetics
Segregation and random assortment are random
events, and can thus be characterized by
probability
The two rules of probability state that:
a. The probability of an outcome ranges from 0 to 1
b. The probabilities of all possible outcomes for an event
sum to 1
The outcome of a random event is unaffected by
the outcome of previous events
62. Laws of Probability Govern
Mendelian Inheritance
Mendel’s laws of segregation and independent
assortment reflect the rules of probability
The multiplication rule
States that the probability that two or more
independent events will occur together is the
product of their individual probabilities
The rule of addition
States that the probability that any one of two or
more exclusive events will occur is calculated by
adding together their individual probabilities
63. Laws of Probability -
Multiplication Rule
The probability of two or more independent events
occurring together is the product of the probabilities
that each event will occur by itself
Following the self-hybridization of a heterozygous
purple pea plants (Pp), the probability of a homozygous
offspring such as the production of white flowers (pp):
a. Probability that a pollen seed will carry p: ½
b. Probability that an egg will carry p: ½
c. Probability that the offspring will be pp:
1/2 X 1/2 = 1/4
64. Laws of Probability -
Addition Rule
The probability of either of two mutually exclusive events occurring
is the sum of their individual probabilities
Following the self-hybridization of a heterozygous purple pea plant
(Pp), the probability of purple offspring:
a. Probability of maternal P uniting with paternal P: 1/4
b. Probability of maternal p uniting with paternal P: 1/4
c. Probability of maternal P uniting with paternal p: 1/4
d. Probability that the offspring will be purple:
1/4 + 1/4 + 1/4 = 3/4
66. Probability in Mendel’s
Crosses
Purple-flowered × white-flowered (PP × pp)
Probability of Pp zygote = ½ × ½ = ¼
Probability of pP zygote = ½ × ½ = ¼
Total probability of heterozygote = ¼ + ¼ = ½
67. Probability in Mendel’s
Crosses
Heterozygous cross (Pp × Pp)
Genotype probabilities
PP zygote = ½ × ½ = ¼
pp zygote = ½ × ½ = ¼
Pp zygote = ¼ + ¼ = ½
Phenotype probabilities
Purple flowers = PP + Pp = ¼ + ½ = ¾
White flowers = pp = ¼
70. Statistical Testing
Used by biologists to find out if observed results
differ significantly from expected results.
Biologists want more than 95% confidence which
means the probability that the deviation of the
observed from that expected is due to chance
alone (no other forces acting).
In a genetic experiment, it can be used to decide if
observed data fits any of the expected Mendelian
ratios or if data is too “far off” and should be
rejected.
71. Observed Values Expected Values
315 Round, Yellow Seed (9/16)(556) = 312.75 Round, Yellow Seed
108 Round, Green Seed (3/16)(556) = 104.25 Round, Green Seed
101 Wrinkled, Yellow Seed (3/16)(556) = 104.25 Wrinkled, Yellow Seed
32 Wrinkled, Green (1/16)(556) = 34.75 Wrinkled, Green Seed
5556 Total Seeds 556.00 Total Seeds
72. • Xcalc 2
= 0.47 (this is the answer, do not √ it)
• Find the correct critical value on the following table.
• Find the degrees of freedom (n-1) in your data.
• Xtab 2
= 7.82 (Xcalc2
<<<< Xtab2
)
• If calculated chi-square is lower than the critical value,
this shows there is no significant difference between the
expected and observed values and the results are within
the range of acceptable deviation.
• If it is above, the difference is too great and the results
are outside the range of acceptable deviation and should
be rejected!