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INHERITANCE
Chapter 6
Early Idea of Heredity
• “Blending Theory of Inheritance”
• Parents’ traits mixed in offspring
Gregor Mendel
• Called “the father of genetics”
• Pea plant study
• Tall vs. short…
Gregor Mendel’s Bio
• https://www.youtube.com/watch?v=QmSJ
GhPTB5E
Mendel & Genetics
• http://www.biologycorner.com/bio2/genetic
s/notes_mendel.html
Ppt of Mendel and his pea experiments
• (This is where the Mendel rap is.)
Mendel’s Results
• Mated tall plant with short plant
• Produced only tall plants
Mendel Tries Again
• Self-fertilized the offspring
• Tall and Short plants produced
Mendel Figures it Out!
• Parents pass on alleles that
remain separate
• No blending!
• Particulate Theory of
Inheritance 
Mendel’s Peas
• http://ed.ted.com/lessons/how-mendel-spea-plants-helped-us-understandgenetics-hortensia-jimenez-diaz
Genes vs. Alleles
• Gene
• Height
• Allele: different
forms of a
gene
• Tall
• Short
Types of Alleles
• Dominant Allele: capital letter
•
Dominant Trait: phenotype with at least
one dominant allele (TT or Tt)
• Recessive Allele: lowercase
letter
•
Recessive Trait: phenotype with both
recessive alleles (tt)
Genotype
• Genetic makeup
• Represented by letters (alleles)
• 2 alleles = Genotype of one trait
(one allele from each parent)
• Ex: TT, Tt, tt
• Homozygous: genotype if alleles are
the same
• TT, tt
• BB,bb
• Heterozygous: genotype if alleles are
different
• Tt
• Bb
Phenotype
• Appearance
• Determined by genotype
• Ex: tall, short
brown eyes, blue eyes
Practice Questions
1. If having a widow’s peak is dominant,
what will be the phenotype of a person
with the genotype Dd?
2. If not having a cleft chin is dominant,
what is the phenotype of a person with
genotype aa?
3. Curly hair is dominant. What could be the
following person’s genotype?
HH
Hh
Principle of Segregation
• Alleles separate during meiosis when
homologous chromosomes are divided
among gametes
Probability in Genetics
• Probability: predicts chances that
certain events will occur
• Used to predict offspring traits
Inheritance of Alleles
• Monohybrid Cross: cross organisms that
differ in one trait
Punnett Squares
TT x tt
t
t
T
T
Tt
Tt
Tt
Tt
100% Tt = tall
Tt x Tt
T
t
T
TT
Tt
t
Tt
tt
• 25% TT = Tall
• 50% Tt = Tall
• 25% tt = Short
75% Tall, 25% Short
Monohybrid Cross Problem #1
• A yellow flowered plant (Yy) is
crossed with a white flowered plant
(yy). What are the genotypes and
phenotypes of the offspring?
Y
y
y
Yy
yy
y
Yy
yy
50% Yy
50% yellow
50% yy
50% white
Monohybrid Cross and Punnett Squares
https://www.youtube.com/watch?v=i-0rSv6oxSY&safe=active
Dihybrid Cross
• Cross between two traits
• Pea shape and pea color
Dihybrid Cross
RRYY x rryy
• Step 1: Find all gamete
combinations
• Parent 1 gametes: RY, RY, RY, RY
• Parent 2 gametes: ry, ry, ry, ry
• R=round and Y=yellow
Dihybrid Cross
RY
RY
RY
RY
ry
RrYy
RrYy
RrYy
RrYy
ry
RrYy
RrYy
RrYy
RrYy
ry
RrYy
RrYy
RrYy RrYy
ry
RrYy
RrYy
RrYy RrYy
Phenotypes
• 16 Round and yellow
RrYy x RrYy
• Parent 1 (RrYy)
RY, Ry, rY, ry
• Parent 2 (RrYy)
RY, Ry, rY, ry
R=round r= wrinkled
Y=yellow y=green
Dihybrid Cross
RY
Ry
rY
ry
RY
RRYY
RRYy
RrYY
RrYy
Ry
RRYy
RRyy
RrYy
Rryy
rY
RrYY
RrYy
rrYY
rrYy
ry
RrYy
Rryy
rrYy
rryy
Phenotypes
• 9 round and yellow
• 3 round and green
• 3 wrinkled and yellow
• 1 wrinkled and green
Dihybrid Cross Practice
Questions
• What are the gamete combinations for the
genotype RrTT?
RT,RT,rT,rT
• What are the gamete combinations for the
genotype rrTt?
rT,rt,rT,rt
• What are the gamete combinations for the
genotype RrTt?
RT,Rt,rT,rt
Dihybrid Cross
https://www.youtube.com/watch?v=qIGXTJLrLf8
Principle of Independent
Assortment
• The principle of independent
assortment states that genes for
different traits can segregate
independently during the formation of
gametes
• During meiosis, alleles divide
independently of alleles for other
characteristics
Sex Determination
• Sex chromosomes:
X and Y
• Females = XX
• Eggs all have X
• Males = XY
• Half of sperm have
X, half have Y
Polygenic Traits
• Traits determined by more than one
gene
• Height
• Skin Color
• Hair Color
• Eye Color
EYE COLOR
• https://www.youtube.com/watch?v=MjBZa
ed9yzM
Multiple Alleles
•
•
•
•
•
•
•
•
Genes with more than 2 alleles
Ex: Blood type
I= makes enzyme that adds A/B to blood cell
i= does not make enzyme
IAIA or IAi = type A
IBIB or IBi = type B
IAIB = type AB
ii= type O
Practice Problem
• If a man with the genotype ii marries
a woman who is IAi, what are the
genotypes and phenotypes of their
offspring?
Codominance
• Both alleles are
fully expressed
• Ex: Blood Type
• IAIB = AB blood
Incomplete Dominance
• No dominant or recessive alleles
• Both alleles are partially expressed
Incomplete dominance
• Red flowered
snapdragon
crossed with
• White flowered
snapdragon
produces
• Pink flowers
Incomplete Dominance Problem
• Among certain flowers the allele for red
flowers (R) is incompletely dominant to the
allele for white flowers (r). What would be
the genotypes and phenotypes of the
offspring between two pink flowers?
X-Linked Traits
• Trait whose gene is carried only on X
chromosome
• Expressed in males more often
Nondisjunction
• Nondisjunction: Failure of paired
chromosomes to separate (to disjoin) during cell
division, so that both chromosomes go to one
daughter cell and none go to the other.
Nondisjunction causes errors in chromosome
number, such as trisomy 21 (Down syndrome)
and monosomy X (Turner syndrome). It is also a
common cause of early spontaneous abortions.
Turner’s Syndrome
XO
Klinefelter’s Syndrome
Detection of Disorders:
Pedigree
Pedigrees
• Family tree showing genetic traits
• Shows genotypes and phenotypes
of each family member
• Squares = males
• Circles = females
• Solid colored = have trait
• Horizontal lines = marriage
• Vertical lines = offspring
Sample Pedigree #1
• Autosomal
recessive (not
X-linked)
Aa
Aa/
AA
Aa
Aa
aa
Aa
Aa
Aa
aa
Sample Pedigree #2
XcX
XX
XcX
• X-linked
recessive
XY
XX
XcX
XcY
XY
XY
XcX
XcY
• Great review PPTs for you!
http://www.slideshare.net/jayswan/chapter14-11546833
http://slideplayer.com/slide/231794/