Heredity
 The passing of traits from parents to their offspring
Causes children to resemble their parents.
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Genetics - The study of heredity
Gregor Mendel - the father of modern genetics
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Austrian monk - trained in mathematics and natural
sciences.
Work conducted over period of 8 years with
common garden peas -1856 to 1865.
Kept careful records.
Applied mathematical studies to his work.
Worked with different kinds of plants.
Selected peas
Selected peas because they:

Grew rapidly.
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Produced many seed (offspring).
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Flower structure made it easy to control Pollination
Transfer of pollen from stamen to pistil of a flower.
Pistil - female reproductive structure - egg at base.
Stamen - male reproductive structure - produces pollen
contains sperm.
Self –pollination - process where pollen from stamen
falls on pistil of the same flower.
Cross-pollination - process where pollen from stamen
of 1 flower falls on pistil of another flower
on another plant.
Identified 7 different characteristics in pea plants
Each had two contrasting forms.
 Seed Shape - Round Vs. Wrinkled
 Seed Color - Yellow Vs. Green
 Flower Color - Purple Vs. White
 Pod Shape - Inflated Vs. Constricted
 Pod Color - Green Vs. Yellow
 Flower Position - Axial Vs. Terminal
 Plant Height - Tall Vs. Short
His experiments were different from earlier workers
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Studied only 1 trait at a time, rather than everything about
the offspring at once.
Studied results of many matings and pooled the results earlier workers had looked at only a few offspring from a
single mating
Counted 7324 peas for seed shape (F2 generation)
Counted 8023 peas for seed color (F2 generation)
Used the large number of offspring to discover definite
ratios of characteristics among the offspring.
5474 Round Seed: 1850 Wrinkled Seed 2.96:1
6022 Yellow Seed: 2001 Green Seed 3.01:1
Mendel's Experiments and Observations
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When the plants were allowed to self-pollinate, the trait always stayed the same
- called them "Truebreeding " or "Pure "plants;
Tall plants x Tall plants produced Tall plants.
Removed stamens from the pure plants that produced wrinkled seeds. Dusted
pistil with pollen from plants that produced only round seeds
Called these the Parental or P1 generation
All of the offspring of this cross resulted in plants that had round seed
Called this the First Filial or F1 generation; Also called Hybrids
Hybrid - offspring from a cross between parents differing in 1 or more traits.
Found that 1 trait of the parents always disappeared in the F1 generation.
The F1 generation plants were allowed to self-pollinate
Called the next generation the Second Filial or F2 generation.
Found that some F2 plants had round seed; some had wrinkled seeds.
Similar results were obtained with the other traits
always 75% of 1 trait; 25% of other trait - a 3:1 ratio.
Mendel’s Cross
P1
Round Seed x Wrinkled Seed
F1
All Hybrid Round Seed
Hybrid Round Seed x Hybrid Round Seed
F2
¾ Round Seed; ¼ Wrinkled Seed
Mendel's Conclusions
 Did not know anything about cell reproduction
Work based on hypothesis that Factors or units carried
the traits he was studying - called Genes today.
 Observed that offspring of true breeding plants with
contrasting traits showed the trait of only 1 parent plant
Called trait Dominant – disappearing trait called
Recessive
 Observation lead to his Law of Dominance - one form of a
hereditary trait, the dominant trait, Dominates or prevents
the expression of the recessive trait.
 Mendel hypothesized that factors exist in pairs since the
plants which had 1 trait could produce seeds with the
opposite trait.
Mendel’s Conclusions
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Mendel hypothesized that paired factors separate or segregate
during gamete formation - lead to Law of Segregation - During
gamete formation the pairs of genes responsible for each trait
separate so that each gamete contains only 1 gene for each trait.
During fertilization the zygote gets 1 gene for the trait from mom
and 1 from dad.
The different forms of a gene for a trait are known as Alleles
Combination of alleles or genetic makeup is the organism’s
Genotype
The appearance of the organism regardless of its genetic makeup
is its - physical appearance - Round, yellow, etc.
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Dominance is expressed by a capital letter - usually the first
letter of the dominant trait; Recessive trait is expressed by a
small letter (same as the dominant trait)
For round Vs. wrinkled seed - R - dominant; r - recessive
Hybrid would be Rr
During gamete formation the pairs of genes responsible for
each trait separate so that each gamete contains only 1 gene
for each trait.
Rr
/
R
\
r
Mendel’s Cross
P1
F1
F2
Round Seed x Wrinkled Seed
RR x
rr
All Hybrid Round Seed (Rr)
Hybrid Round Seed x Hybrid Round Seed
Rr
x
Rr
¾ Round Seed; ¼ Wrinkled Seed
RR, Rr
rr
Genetic Terminology
 Dominant - trait which stays visible
 Recessive - trait which disappeared
 Alleles - alternate forms of a gene for a trait
 Genotype - genetic makeup of a trait
 Phenotype - physical appearance of a trait
 Homozygous - both alleles are the same
 Heterozygous - two alleles are different
 Homozygous Dominant - pure dominant
 Homozygous Recessive - pure recessive
 Heterozygous Dominant - Hybrid with 1 dominant
-
Mendel’s Laws
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LAW OF DOMINANCE - one form of a hereditary
trait, the dominant trait, dominates or prevents the
expression of the recessive trait.
LAW OF SEGREGATION - During gamete formation
the pairs of genes responsible for each trait
separate so that each gamete contains only 1 gene
for each trait.
LAW OF INDEPENDENT ASSORTMENT - Alleles
segregate independently of each other during
gamete formation.
The likelihood of an event occurring as expressed as a
ratio or a percentage.

Flipping a coin – ½ heads; ½ tails

Cards Chance of drawing an ace –4/52 or 1/13
Chance of drawing a spade - 13/52 or 1/4
Chance of drawing the Ace of Spades
1/13 x 1/4 = 1/52
Product Rule
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To find the probability of 2 events occurring you multiply
the individual probabilities.
Chance of a head – ½ ; Chance of another head – ½
Chance of 2 heads in a row – ½ x ½ = ¼
Chance of 4 heads in a row – ½ x ½ x ½ x ½ = 1/16
Each gamete has ½ chance of getting a particular allele
Homozygous Dominant - RR: Alleles - R or R = 1/1
Homozygous Recessive - rr: Alleles - r or r = 1/1
Heterozygous Dominant - Rr: Alleles - R or r; ½ R; ½ r
PUNNETT SQUARE
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Special chart used to show possible combinations
resulting from a cross of 2 organisms.
Put female gametes along top; male gametes along
left side
Squares show possible genotypes of offspring -used
to determine phenotype and ratio of offspring.
Used to predict; it doesn’t mean it will happen
MONOHYBRID CROSS - involves only one set of contrasting factors for a
trait
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Cross a homozygous yellow with a homozygous green
Yellow – dominant (Y); green - recessive (y)
P1 YY x yy
y
F1 all Yy heterozygous yellow
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y
Cross two of the F1 generation
Yy x Yy
F2 – ¼ YY; ½ Yy; ¼ yy
¾ Yellow; ¼ green
Y
y
Y
YY
Yy
y
Yy
yy
Y
Y
Yy
Yy
Yy
Yy
DIHYBRID CROSS - involves two sets of contrasting
traits at one time; genes are on separate
chromosomes
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Each gamete contains 1 allele for each trait
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LAW OF INDEPENDENT ASSORTMENT - Alleles
segregate independently of each other during
gamete formation.
Het Round, Het Yellow
RrYy
x
Het Round, Het Yellow
RrYy
Gametes:
RY
Ry
rY
ry
INCOMPLETE DOMINANCE/NONDOMINANCE
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Phenotype between dominant and recessive trait
Heterozygous condition
Example - Four-O-Clocks
RR - red flowers
rr - white flowers
Rr - Pink flowers
Cross of 2 pink flower plants - Rr x Rr
Results – ¼ red (RR); 2/4 (½) Pink (Rr); ¼ white
Determination of Sex
 By the Sex Chromosomes - x or y
Other chromosomes are called Autosomes
 Male - xy; Female - xx
 Get ½ males and ½ females on Punnett square
x
x
x xx xx
y xy xy
SEX LINKED CHARACTERISTICS
 Recessive trait linked with a certain sex - usually males
 Carried on the x-chromosome;
Male has only 1 x, the trait is visible
Females with 2 x's - not visible if 1 of the x has a
dominant gene for the trait.
 Female only shows trait in homozygous recessive.
 Woman is called a "Carrier" in heterozygous condition.
 Examples:
Red-Green color blindness
Hemophilia
Male
Female
MULTIPLE ALLELES
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More than 2 alleles exist for a particular trait
In humans - blood types is an example of multiple
alleles
3 different alleles - A, B, O
Alleles A and B are codominant
O is recessive
Blood Type
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Due to presence of antigens on the red blood cells produces antibodies in blood
Type A - Antigen A on cells
Plasma contains anti B
Type B - Antigen B on cells
Plasma contains anti A
Type O - No Antigens on cells
Plasma has anti A and anti B
Type AB - Antigens A and B on cells
Plasma lacks anti A and anti B
Phenotypes and genotype combinations
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Type A - AA or AO
Type B - BB or BO
Type AB - AB
Type O - OO
Blood
Group
Donor
to
Receives
from
O
O, A, B, AB
O
A
A, AB
O, A
B
B, AB
O, B
AB
AB
O, A, B, AB
Universal Donor
Universal Recipient
GENE LINKAGE
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Concerned with the presence of 2 different genes
on the same chromosome
Does not follow usual dihybrid results - follows
monohybrid.
Variation can occur due to crossing over - pieces of
chromatids exchange places during synapsis of
tetrads in meiosis.
LETHAL GENES
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Genes which can cause death or harm in the
homozygous condition.
Examples:
Sickle-cell anemia
PKU
Tay Sachs - Jews of middle eastern European
origin.
Diabetes mellitus
NONDISJUNCTION
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Failure of chromosomes to segregate properly during gamete
formation.
Can involve sex chromosomes as well as the autosomes - zygote
gets an improper number of chromosomes
Examples:
Down's Syndrome - three #21 chromosomes - autosomal
Turner's Syndrome - has only 1 x, no y chromosome - female
Klinefelter's Syndrome - xxy - male
Jacob’s Syndrome- xyy - male –thought to show criminal
behavior at one time; not any higher.
Normal Male
Male – Down’s
Klinefelter’s Syndrome
Turner’s Syndrome
Jacob’s Syndrome
MUTATIONS
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Change in the genetic code or genes of an organism.
Can occur naturally or by exposure to agents that
produce mutations.
Breaks during crossing over which do not reattach.
Increased chance of breakage by exposures to
mutagens
Mutagens - agents that cause mutations.
Radiation - x-rays, UV, gamma
Chemicals