MENDELIAN GENETICS

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MENDELIAN GENETICS
Mrs. Woytowich Regents Biology
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OBJECTIVES Upon completion of this unit students
will be able to 1. Define genetics. 2. State
who arrived at the basic principles of heredity
and how. 3. State the Law of Dominance. 4.
Perform a Punnett Square. 5. Analyze the
offspring of a Punnett Square. 6. Differentiate
the P generation from the F1 and F2
generation. 7. State the Law of Segregation. 8.
Understand that chromosomes carry the hereditary
factors called genes. 9. Understand and
recognize an allele. 10. Explain the difference
between homozygous and heterozygous alleles. 11.
Understand that genotype determines
phenotype. 12. State the Law of Probability.
13. Differentiate and perform both monohybrid
and dihybrid crosses . 14. State the Law of
Independent Assortment. 15. Recognize and
understand what a test cross is and why it must
be done. 16. Understand what incomplete
dominance is and perform a cross properly. 17.
Understand what codominance is and perform a
cross properly. 18. Understand what multiple
alleles are and perform a cross properly.
KEY WORDS
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• I. INTRODUCTION
• What is genetics? THE BRANCH OF BIOLOGY THAT IS
  CONCERNED WITH THE WAYS IN WHICH HEREDITARY
  INFORMATION IS TRANSMITTED FROM PARENTS TO
  OFFSPRING
• GREGOR MENDEL AUSTRIAN MONK WHO ARRIVED AT THE
  BASIC PRINCIPLES OF HEREDITY
• Mendel used PEA PLANTS to investigate inheritance
  of traits.
• Traits that he observed
• Tall vs. short
• Green vs. yellow
• Smooth vs. wrinkled

Observable traits
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II. THE LAW OF DOMINANCE

• Law of Dominance WHEN AN ORGANISM IS HYBRID FOR
  A PAIR OF CONTRASTING TRAITS, IT SHOWS ONLY THE
  DOMINANT TRAIT
• T is dominant and t is recessive

TT homozygous tall (pure tall) tt homozygous
short (pure short) Tt heterozygous tall (hybrid
tall)
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• EXAMPLE USING A PUNNETT SQUARE
• TT homozygous tall
• tt homozygous short
• Tt heterozygous tall (hybrid)
• Lets do a cross with parents TT x tt

T
T
The offspring are 100 tall and HETEROZYGOUS
t t
T
t
T
t
T
t
T
t
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Now, lets do a cross with parents Tt x tt
T
t
t t
The offspring are 50 HOMOZYGOUS short and 50
HETEROZYGOUS tall
t
t
T
t
T
t
t
t
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• Generations P , F1 ,F2
• TT x tt

T T
t t
Tt
Tt
P generation (P for parent) ? Tt, Tt, Tt,
Tt F1 generation (F for filial)
Tt
Tt
T t
T t
Tt
T T
Tt
tt
? TT, Tt, Tt, tt F2 generation (F for filial)
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III. THE LAW OF SEGREGATION

• Law of Segregation FACTORS THAT OCCUR IN PAIRS
  ARE SEPARATED FROM EACH OTHER DURING GAMETE
  FORMATION AND RECOMBINED AT FERTILIZATION

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• THE CONCEPT OF THE GENE
• Mendels results could be explained by assuming
  that CHROMOSOMES CARRY THE HEREDITARY FACTORS
  (GENES)
• The separation of homologous chromosomes during
  MEIOSIS and their recombination during
  FERTILIZATION would account for the separation
  and recombination of GENES

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• V. ALLELES
• Each body cell of an organism has TWO copies of
  the gene for each trait these may be alike (
  HOMOzygous) or different (HETEROzygous)
• EXAMPLES

• ALLELE DIFFERENT COPIES OR FORMS OF A GENE
  CONTROLLING A PARTICULAR TRAIT (T, t, B, b, s, y,
  etc.) TT HOMOZYGOUS TALL tt
  HOMOZYGOUS SHORT Tt HETEROZYGOUS TALL

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• VI. GENOTYPES AND PHENOTYPES
• The GENETIC MAKEUP of an organism is its GENOTYPE
• The PHYSICAL TRAITS that an organism expresses AS
  A RESULT of its genotype is the PHENOTYPE

• Two different individuals may have the same
  phenotype (like brown eyes) but have different
  genotypes

DIFFERENT GENOTYPE
BB brown eyes Bb brown eyes
SAME PHENOTYPE
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• THE LAW OF PROBABLILITY
• To explain the numerical results of Mendels
  experiments, we must apply the law of PROBABLITY
  to the SEPARATION AND RECOMBINATION OF ALLELES
• The Basic Law of Chance (Probability) IF THERE
  ARE SEVERAL POSSIBLE EVENTS THAT MIGHT OCCUR, AND
  NO ONE OF THEM IS MORE LIKLEY TO OCCUR THAN ANY
  OTHER, THEN THEY WILL ALL OCCUR IN EQUAL NUMBERS
  OVER A LARGE NUMBER OF TRIALS

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• EXAMPLES

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VIII. THE PUNNETT SQUARE Lets do some examples
using the letter b for the allele for eye
color. We will analyze the offspring by genotype
AND phenotype, too!   1.  Cross of homozygous
dominant and homozygous recessive
B
B
Genotype 100 Bb
b b
B
b
B
b
Phenotype 100 brown eyes
B
b
B
b
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  2. Cross of 2 heterozygous brown parents
B
b
Genotype 25 BB 50 Bb 25 bb
B b
B
B
B
b
Phenotype 75 brown eyes 25 blue eyes
B
b
b
b
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• THE LAW OF INDEPENDENT ASSORTMENT
• All of the crosses we have done so far are called
  MONOHYBRID because only ONE pair of contrasting
  traits is being studied
  (for
  example, the cross Tt x tt is monohybrid---they
  are all ts)
• A DIHYBRID cross involves TWO contrasting traits

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• EXAMPLE
• Y is for yellow and R is for round
• Lets do a dihybrid cross for YYRR x yyrr and
  analyze the offspring

YR
YR
YR
YR
yr
YyRr
YyRr
YyRr
YyRr
Genotype 100 YyRr
yr
YyRr
YyRr
YyRr
YyRr
Phenotype 100 Yellow and Round
yr
YyRr
YyRr
YyRr
YyRr
yr
YyRr
YyRr
YyRr
YyRr
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• The Law of Independent Assortment DURING
  MEIOSIS, GENES FOR DIFFERENT TRAITS ARE SEPARATED
  AND DISTRIBUTED TO GAMETES INDEPENDENTLY OF ONE
  ANOTHER (NOT ALWAYS TRUE!!!)

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• THE DIHYBRID CROSS
• Look at the dihybrid cross that we did
  alreadywhat are the phenotype and genotype s?
•  
•  
•  
• Now, do a cross of two dihybrids from the
  dihybrid cross that weve just done. What are
  the genotype and phenotype ratios?

Genotype 100 YyRr
Phenotype 100 Yellow and Round
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PARENTS ARE TWO DIHYBRIDS YyRr x YyRr
YR
Yr
yR
yr
____________ Yellow and Round ____________ Yello
w and wrinkled _______________ green and
Round _______________ green and wrinkled
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YR
YYRR
YYRr
YyRR
YyRr
3
Yr
YYRr
YYrr
YyRr
Yyrr
3
yR
YyRR
YyRr
yyRR
yyRr
1
yr
yyRr
yyrr
YyRr
Yyrr
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• THE TEST CROSS
• The test cross is used to show if AN INDIVIDUAL
  SHOWING A DOMINANT TRAIT IS HOMOZYGOUS OR
  HETEROZYGOUS
• In a test cross, an individual of unknown
  genotype is mated with a homozygous recessive
  individual. Is an unknown plant TT or Tt? To
  find out, cross it with tt and analyze the
  offspring.

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• EXAMPLE
• In peas, the allele for a tall (T) plant
  height is dominant over the allele for short
  plant height (t). A tall pea plant (T?) was
  crossed with a short pea plant (tt). Of the 95
  offspring that resulted, 55 were tall and 40 were
  short. Determine the genotype of the tall
  parent.

Since there were some offspring with the
recessive phenotype (short), the tall parent must
have been heterozygous.
T
T
T
t
t t
t t
T
t
T
t
T
t
t
t
T
t
T
t
T
t
t
t
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• XII. INCOMPLETE DOMINANCE
• In some organisms, BOTH alleles contribute to the
  phenotype of a HETEROZYGOUS individual this is
  called INCOMPLETE DOMINANCE or BLENDING
  INHERITANCE
• EXAMPLES using the Japanese 4 oclock plant

Alleles written in caps since BOTH influence
phenotype
Red RR White - WW Pink - RW
Example 1 A cross between a red plant and a
white plant Example 2 - A cross between a red
plant and a pink plant      
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• XIII. CODOMINANCE
• Codominance is when TWO DOMINANT ALLELES ARE
  EXPRESSED AT THE SAME TIME
• How is this different from incomplete dominance?
  This is different because BOTH ALLELES ARE
  EXPRESSED THERE IS NO BLENDING OF TRAITS
• EXAMPLES Roan cattle and sickle cell anemia in
  humans
• Roan cattle have both red and white in their
  coat
• CR red coat
• CW white coat
• CRCW roan coat
• Notice that all letters are in caps because there
  is no recessive trait
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• Cross a homozygous white coat with a homozygous
  red coat

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• XIV. MULTIPLE ALLELES
• Multiple alleles is when MORE THAN TWO ALLELES
  EXIST IN THE SPECIES BEING STUDIED
• EXAMPLE human blood type (the ABO blood system).
  There are 3 alleles that we will study that
  control for blood type A, B, O
• Type O blood is recessive
• Types A and B are dominant over type O, but
  neither is dominant over the other
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• IA Type A
• IB Type B
• i type O
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• When both A and B are present in the genotype,
  both alleles are expressed.
• IAIB Type AB
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•  

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• Lets make a chart of all blood types and their
  genotypes

IAIA or IAi
IBIB or IBi
IAIB
ii
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EXAMPLE A woman with type A blood whose
father was type O married a man with type AB
blood. What will be the possible genotypes and
phenotypes of their children?