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CHAPTER 9
MENDEL & MEIOSIS
10.1 MENDEL’S LAWS OF
HEREDITY
I. WHY MENDEL SUCCEEDED
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Gregor Mendol – father of genetics
1st studies of heredity – the passing of
characteristics to offspring
Genetics – study of heredity
The characteristics passed on called traits
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
Called the “Father
of Genetics"
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Particulate Inheritance
Mendel stated that
physical traits are
inherited as
“particles”
Mendel did not know
that the “particles”
were actually
Chromosomes & DNA
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Genetic Terminology
 Trait - any characteristic that
can be passed from parent to
offspring
 Heredity - passing of traits
from parent to offspring
 Genetics - study of heredity
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Designer “Genes”
 Alleles - two forms of a gene
(dominant & recessive)
 Dominant - stronger of two genes
expressed in the hybrid;
represented by a capital letter (R)
 Recessive - gene that shows up
less often in a cross; represented
by a lowercase letter (r)
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1. MENDEL CHOSE HIS
SUBJECT CAREFULLY
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Used garden peas to study
Have male & female gametes (sex cells)
Male & female same flower
Know what pollination & fertilization mean
He could control the fertilization process
Not many traits to keep track of
Reproduction in Flowering Plants
Pollen contains sperm
Produced by the
stamen
Ovary contains eggs
Found inside the
flower
Pollen carries sperm to the
eggs for fertilization
Self-fertilization can
occur in the same flower
Cross-fertilization can
occur
between
flowers
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How Mendel Began
Mendel
produced
pure
strains by
allowing the
plants to
selfpollinate
for several
generations
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2. MENDEL WAS A CAREFUL
RESEARCHER
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USED CAREFULLY CONTROLLED
EXPERIMENTS
STUDIED ONE TRAIT AT A TIME
KEPT DETAILED DATA
Mendel’s Experimental
Methods
Mendel hand-pollinated
flowers using a
paintbrush
He could snip the
stamens to prevent
self-pollination
Covered each flower
with a cloth bag
He traced traits through
the several generations
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II. MENDEL’S MONOHYBRID
CROSSES
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MENDEL STUDIED 7 TRAITS CAREFULLY
Mendel crossed plants w/ diff. traits to see
what traits the offspring would have
These offspring are called hybrids –
offspring of parents w/ different traits
A monohybrid cross is one that looks at
only one trait (let’s look at plant height –
tall or short)
A. THE 1ST GENERATION
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Mendel crossed two plants – 1 tall & 1
short (they came from tall & short
populations)
These plants are called the parental
generation (P generation)
The offspring were all called the 1st filial
generation (F1 generation)
All the offspring were tall (the short plants
were totally excluded)
B. THE 2ND GENERATION
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Next, Mendel crossed two plants from the
F1 generation
The offspring from this cross are called
the 2nd filial generation (F2 GENERATION)
Mendel found that ¾ of the offspring
were tall & ¼ were short (the short plants
reappeared!!!!!!)
TO GO ANY FURTHER, WE
MUST UNDERSTAND ALLELES,
DOMINANCE, & SEGREGATION
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Genes – a section of DNA that codes for
one protein
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These genes are what control & produce traits
The genes Mendel studied came in two
forms (tall/short - round/wrinkled
yellow/green…….etc.)
Alternate forms of a gene are called alleles
Alleles are represented by a one or two
letter symbol (e.g. T for tall, t for short)
ALLELES CONT’D
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THESE 2 ALLELS ARE NOW KNOWN TO BE
FOUND ON COPIES OF CHROMOSOMES –
ONE FROM EACH PARENT
THE RULE OF DOMINANCE
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A dominant trait is the trait that will always be
expressed if at least one dominant allele is
present
The dominant allele is always represented by
a capital letter
A recessive trait will only be expressed if both
alleles are recessive
Recessive traits are represented by a lower
case letter
DOMINANCE CONT’D
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LET’S USE TALL & SHORT PEA PLANTS
FOR AN EXAMPLE
WHICH OF THESE WILL SHOW THE
DOMINANT & RECESSIVE TRAIT?
TT
Tt
DOMINANT TRAIT
tt
RECESSIVE TRAIT
THE LAW OF SEGREGATION
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MENDEL ASKED HIMSELF……..”HOW DID
THE RECESSIVE SHORT PLANTS
REAPPEAR IN THE F2 GENERATION?”
HE CONCLUDED THAT EACH TALL PLANT
FROM THE F1 GENERATION CARRIED
TWO ALLELES, 1 DOMINANT TALL ALLELE
& ONE RECESSIVE SHORT ALLELE
SO ALL WERE Tt
SEGREGATION CONT’D
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HE ALSO CONCLUDED THAT ONLY ONE
ALLELE FROM EACH PARENT WENT TO
EACH OFFSPRING
HIS CORRECT HYPOTHESIS WAS THAT
SOMEHOW DURING FERTILIZATION, THE
ALLELES SEPARATED (SEGREGATED) &
COMBINED WITH ANOTHER ALLELE
FROM THE OTHER PARENT
The law of segregation states that during
gamete formation, the alleles separate to
different gametes
Applying the Law of Segregation
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F1 GENERATION
TT
FATHER
MOTHER
Tt
T t
Tt
tt
F2 GENERATION
- the law of dominance explained the
heredity of the offspring of the f1
generation
- the law of segregation explained the
heredity of the f2 generation
PHENOTYPES & GENOTYPES
PG. 264
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PHENOTYPE – THE WAY AN ORGANISM
LOOKS AND BEHAVES – ITS PHYSICAL
CHARACTERISTICS (i.e. – TALL, GREEN,
BROWN HAIR, BLUE EYES, ETC.)
GENOTYPE – THE GENE COMBONATION
(ALLELIC COMBINATION) OF AN
ORGANISM – (i.e. – TT, Tt, tt, ETC.)
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HOMOZYGOUS – 2 ALLELES ARE THE SAME
HETEROZYGOUS – 2 ALLELES DIFFERENT
ANSWER ON YOUR SHEET
TRAITS = BLUE SKIN & YELLOW SKIN
BB – IS THIS HOMOZYGOUS OR
HETEROZYGOUS?
HOMOZYGOUS
IS BLUE SKIN OR YELLOW SKIN
DOMINANT?
BLUE
MENDEL’S DIHYBRID CROSSES
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MONOHYBRID – MENDEL LOOKED AT
ONE TRAIT
IN HIS DIHYBRID CROSSES – HE LOOKED
AT 2 TRAITS
WANTED TO SEE IF TRAITS ARE
INHERITED TOGETHER OR
INDEPENDENTLY
DIHYBRID CROSS
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TOOK TWO TRUE BREEDING PLANTS FOR
2 DIFFERENT TRAITS (ROUND/WRINKLED
SEEDS ------- YELLOW/GREEN SEEDS)
1ST GENERATION
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WHAT WOULD HAPPEN IF HE CROSSED JUST
TRUE BREEDING ROUND W/ TRUE BREEDING
WRINKLED (ROUND IS DOMINANT)
ALL THE OFFSPRING ARE
ROUND
DIHYBRID CROSS – 1ST
GENERATION CONT’D
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SO WHAT DO YOU THINK HAPPENED
WHEN HE CROSSED TRUE BREEDING
ROUND/YELLOW SEEDS WITH TRUE
BREEDING WRINKLED/GREEN SEEDS
ALL THE F1 WERE ROUND
AND YELLOW
DIHYBRID CROSS – 2ND
GENERATION
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TOOK THE F1 PLANTS AND BRED THEM
TOGETHER (PHENOTYPE WAS
ROUND/YELLOW X ROUND/YELLOW)
2ND GENERATION
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FOUND ROUND/YELLOW
FOUND ROUND/GREEN
FOUND WRINKLED/YELLOW
FOUND WRINKLED/GREEN
( 9 : 3 : 3 : 1 RATIO)
-9
-3
-3
-1
EXPLANATION OF 2ND
GENERATION
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MENDEL CAME UP W/ 2ND LAW – THE
LAW OF INDEPENDENT ASSORTMENT
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GENES FOR DIFFERENT TRAITS ARE
INHERITED INDEPENDENTLY FROM EACH
OTHER
THIS IS WHY MENDEL FOUND ALL THE
DIFFERNENT COMBONATIONS OF TRAITS
Summary of Mendel’s laws
LAW
DOMINANCE
SEGREGATION
INDEPENDENT
ASSORTMENT
PARENT
CROSS
OFFSPRING
TT x tt
tall x short
100% Tt
tall
Tt x Tt
tall x tall
75% tall
25% short
RrGg x RrGg
round & green
x
round & green
9/16
pods
3/16
pods
3/16
pods
1/16
pods
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round seeds & green
round seeds & yellow
wrinkled seeds & green
wrinkled seeds & yellow
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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
r
RR = red flower
rr = white flower
R
R
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Codominance Problem
Example:
(IBIB)
homozygous male Type B
x
heterozygous female Type A (IAi)
IA
i
IB
IAIB
IBi
IB
IAIB
IBi
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1/2 = IAIB
1/2 = IBi
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Codominance
Question:
If a boy has a blood type O and
his sister has blood type
AB,
what are the
genotypes and
phenotypes of their parents?
boy - type O (ii) X girl - type AB
(IAIB)
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Codominance
Answer:
IA
IB
i
i
IAIB
ii
Parents:
genotypes = IAi and IBi
phenotypes = A and B
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Sex-linked Traits
Traits (genes) located on the sex
chromosomes
Sex chromosomes are X and Y
XX genotype for females
XY genotype for males
Many sex-linked traits carried on
X chromosome
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Sex-linked Traits
Example: Eye color in fruit flies
Sex Chromosomes
fruit fly
eye color
XX chromosome - female
Xy chromosome - male
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Codominance
Answer:
IA
IB
i
i
IAIB
ii
Parents:
genotypes = IAi and IBi
phenotypes = A and B
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PUNNETT SQUARES
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A QUICK WAY TO FIND THE GENOTYPES
IN UPCOMING GENERATIONS
1ST DRAW A BIG SQUARE AND DIVIDE IT
IN 4’S
PUNNETT SQUARE
CROSS T T X Tt
CONT’D
TT X Tt
T
T
T
T
T
T
T
t
T
t
T
t
DIHYBRID CROSSES
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A LITTLE DIFFERENT
HhGg X HhGg
MUST FIND OUT ALL THE POSSIBLE
ALLELIC COMBONATIONS
USE THE FOIL METHOD LIKE IN MATH
FOIL – FIRST, OUTSIDE, INSIDE, LAST
H hGg X HhGg
1. HG
2. Hg
3. hG
4. hg
BOTH PARENTS
ARE THE SAME
NOW LET’S DO A DIHYBRID
CROSS
HhGg X HhGg
HG
Hg
hG
HG
HHGG
HHGg
HhGG
HhGg
Hg
HHGg
HHgg
HhGg
Hhgg
hG
HhGG
HhGg
hhGG
hhGg
hg
HhGg
Hhgg
hhGg
hhgg
hg
WHAT ARE THE PHENOTYPIC
RATIO’S?
HhGg X HhGg
DD:
9
HhGg
Dr:
3
HhGg
Hhgg
rD:
3
HhGg
hhGG
hhGg
rr:
1
Hhgg
hhGg
hhgg
HG
Hg
hG
HG
HHGG
HHGg
HhGG
Hg
HHGg
HHgg
hG
HhGG
hg
HhGg
hg
PROBABILITY
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WILL REAL LIFE FOLLOW THE RESULTS
FROM A PUNNETT SQUARE?
NO!!!!!! – A PUNNETT SQUARE ONLY
SHOWS WHAT WILL PROBABLY OCCUR
IT’S A LOT LIKE FLIPPING A COIN – YOU
CAN ESTIMATE YOUR CHANCES OF
GETTING HEADS, BUT REALITY DOESN’T
ALWAYS FOLLOW PROBABILITY
10.2 MEIOSIS
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GENES, CHROMOSOMES, AND NUMBERS
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CHROMOSOMES HAVE 100’S OR 1000’S OF
GENES
GENES FOUND ON CHROMOSOMES
DIPLOID & HAPLOID CELLS
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ALL BODY CELLS
(SOMATIC CELLS)
HAVE
CHROMOSOMES
IN PAIRS
BODY CELLS ARE
CALLED DIPLOID
CELLS (2n)
HUMANS HAVE
THE 2n # OF
CHROMOSOMES
DIPLOID AND HAPLOID CELLS
CONT’D
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HAPLOID CELLS
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ONLY HAVE 1 OF EACH TYPE OF
CHROMOSOME (DIPLOID CELLS HAVE 2 OF
EACH TYPE)
SYMBOL IS (n)
SEX CELLS HAVE THE n # OF
CHROMOSOMES
HOMOLOGOUS CHROMOSOMES
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HOMOLOGOUS CHROMOSOMES ARE THE
PAIRED CHROMOSOMES THAT CONTAIN THE
SAME TYPE OF GENTIC INFORMATION, SAME
BANDING PATTERNS, SAME CENTROMERE
LOCATION, ETC.
THEY MAY HAVE DIFFERENT ALLELES, SO NOT
PERFECTLY IDENTICAL
WHY DO THEY HAVE DIFFERENT ALLELES?
CAME FROM DIFFERENT
PARENTS
WHY MEIOSIS?
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MITOSIS – RESULTS IN GENETICALLY
IDENTICAL OFFSPRING – INCLUDING THE
# CHROMOSOMES
WHAT WOULD HAPPEN IF THE EGG AND
SPERM HAD THE SAME # OF
CHROMOSOMES AS THE BODY CELLS?
EGG = 46 CHROMOSOMES
SPERM = 46 CHROM.
ZYGOTE = 46 + 46 = 92 CHROMOSOMES =
NOT HUMAN
MEIOSIS
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A TYPE OF CELL DIVISION WHICH
PRODUCES GAMETES CONTAING HALF
THE NUMBER OF CHROMOSOMES AS THE
BODY CELLS
2 STAGES – MEIOSIS I & MEIOSIS II
START W/ 1 DIPLOID CELL, END UP W/ 4
HAPLOID CELLS (GAMETES)
4 DAUGHTER CELLS ARE GENETICALLY
DIFFERENT FROM EACH OTHER AND
MOTHER CELL
INTRO TO MEIOSIS CONT’D
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SPERM – MALE GAMETE (n)
EGG – FEMALE GAMETE (n)
FERTILIZATION PRODUCES A ZYGOTE
(2n)
THIS TYPE OF REPRODUCTION IS
CALLED SEXUAL REPRODUCTION
STAGES OF MEIOSIS
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MEIOSIS I
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PROPHASE I, METAPHASE I, ANAPHASE I,
TELOPHASE I (PMAT)
MEIOSIS II
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PROPHASE II, METAPHASE II, ANAPHASE II,
TELOPHASE II (PMAT)
IMPORTANT THINGS TO KNOW
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CROSSING OVER – OCCURS DURING
PROPHASE I
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CREATES GENETIC VARIABILITY (RECOMBINATION
OF GENES)
IN MEIOSIS I, HOMOLOGOUS CHROMOSOMES
SEPARATE (ANAPHASE I)
IN MEIOSIS II, SISTER CHROMATIDS SEPARATE
TETRAD – WHAT THE HOMOLOGOUS
CHROMOSOMES ARE CALLED WHEN THEY PAIR
UP DURING PROPHASE I