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Mendelian Genetics
Chapter Four
Theories of Inheritance
Homunculus
(Ancient Greeks – 17th ce) sperm caries a miniature human that uses
egg as a growth medium (spermists)
Problem: Doesn’t explain why kids sometimes look like their mom
Blending
Descendents possess traits that are intermediate between
those of parents, become mixed and forever changed in the offspring.
Problem: Over time, a population would become uniform in appearance
Once blended, traits should not reappear in subsequent generations
Pangenesis
Heredity units (pangenes) are formed in all organs, spread
through blood to genitals
Problem: Blood transfusions into experimental animals
did not change characteristics of progeny.
Moravian Sheep Breeders
Association (1837)
Why do valued traits sometimes disappear
and then reappear in some offspring?
Breeders could predict the traits of offspring if
they could answer the basic questions:
What is inherited?
How is it inherited?
What is the role of chance in heredity?
Gregor J. Mendel
(1822-1884)
Monastery of St. Thomas
Brno, Czech Republic
Versuche über Pflanzen-Hybriden
"Experiments in Plant Hybridization"
Society for the Study of the Natural
Sciences Proceedings (1866)
Pisum sativum
Mendel chose a great
“model organism”
• Self fertilizing plants that can be cross-fertilized
• Relatively quick generation time
• Can grow large numbers of plants in limited space
• Used pure-breeding lines (8 generations) to form
hybrid lines: offspring of dissimilar parents
• Can follow discrete traits – no intermediate forms
Mendel’s Experiments
Studied 7 characteristics of
pure-breeding lines:
• Seed color (yellow vs. green)
• Seed shape (round vs. wrinkled)
• Flower color (purple vs. white)
• Pod color (green vs. yellow)
• Pod shape (round vs. pinched)
• Stem length (long vs. short)
• Flower position (along stem vs. at the tip)
“either - or” phenotypes with no intermediates
Mendel’s Experiments
• Pure Breeding Lines:
– Crossing two of same phenotype always
produces one phenotype
• Hybrids:
– Crossing two of same phenotype can lead to
offspring of two phenotypes
– Example – cross two tall plants, offspring are
a combination of tall and short plants
Mendel’s Experiments
Cross fertilization
emasculation
Mendel was careful:
• many controls
• reciprocal crosses
Mendel’s Experiments
Monohybrid Cross
P
parental
x
F1
first filial
dominant – a trait “unchanged” in the hybrids
recessive – a trait that disappears in the hybrids
(but may re-appear in subsequent generations)
Mendel’s Experiments
Monohybrid Cross
P
parental
F1
first filial
F2
5474 smooth, 1850 wrinkled
smooth : wrinkled 2.96 : 1
x
Mendel’s Experiments
Monohybrid Cross
Seed shape
Seed color
Flower color
Pod color
Pod shape
Stem length
Flower position
5474 smooth, 1850 wrinkled
6022 yellow, 2001 green
705 purple, 224 white
428 green, 152 yellow
882 round, 299 pinched
787 long,
277 short
858 stem,
651 tip
3:1
Dominant : Recessive
2.96 : 1
3.01 : 1
3.15 : 1
2.82 : 1
2.95 : 1
2.84 : 1
3.14 : 1
Mendel’s Deductions
Proposed that “unit factors” exist
in pairs to explain these results
Each parent has two unit factors but
contributes only one to every progeny
in the form of gametes
Designated upper-case as Dominant
and lower-case as Recessive
Mendel’s Deductions
Seed coat color
YY
yy
Dominant
Recessive
Yy
All offspring will be yellow
and will be heterozygotes
Genetic Language:
Gene - Discrete “unit factors” of inheritance
Allele - Different forms of a gene (e.g. Y or y)
Genotype - Allelic composition of a trait
(e.g. YY, Yy, or yy)
Phenotype - Physical manifestation of a trait
(e.g. Yellow or green seed)
Genetic Language:
Homozygous – Individuals with two
identical copies of a gene
Same allele (yy)
Heterozygous - Individuals with two
different copies of a gene
Two different alleles (Yy)
Genetic Language:
Parental
Y
YY
“Pure-breeding
Homozygous
Lines”
F1
“Hybrid”
Yy
Heterozygous
yy
Homozygous
Punnett Squares:
Yy
F1 monohybrid
self-fertilization
Y
Y
y
YY
Yy
F2
Yy
y
Yy
Phenotype
yy
3:1
BUT
Genotype 1 : 2 : 1
Genotype vs. Phenotype
YY
Yy
yy
Homozygous
dominant
Heterozygous
hybrid
Homozygous
recessive
Same phenotype
How do you distinguish
between the two?
Test Cross:
yy
Y
yy
y
y
Yy
Yy
YY
y
y
Y
Yy
Yy
y
yy
yy
Yy
Y
Yy
Yy
If homozygous, all
progeny are Yellow
If heterozygous,
progeny 1 : 1
Yellow : Green
Mendel’s 1st Law:
• Principle
of segregation
Hereditary traits are determined by discrete factors (now
called genes) that appear in pairs. During sexual development,
these pairs are separated (segregated) into gametes and only
one factor from each parent is passed to the offspring.
Discrete factors explained how a characteristic could persist
through generations without blending and why it could
“disappear and reappear” in subsequent generations
Practice Your Punnetts!
• Draw the punnett squares
• Calculate # of each genotype and
phenotype
– Yy cross yy (Y = yellow, y = green)
– Yy cross Yy
– Rr cross rr (R = round, r = wrinkled)
– RR cross rr
– BB cross Bb (B = brown, b = blue)
– bb cross bb
Mendel Understood Probability
Probability: The number of times an event is expected to
occur divided by the number of trials during which that
event could have happened
The probability of rolling a 2 with one roll of one die:
1 event / 6 possible outcomes =
1/6
Mendel Understood Probability
The Multiplication Rule: The probability of two or more
independent events occurring simultaneously is the
product of their individual probabilities.
The probability of rolling two 2’s with a pair of dice:
The probability of rolling a 2 =
1/6
So rolling two 2’s =
1/6 x 1/6 =1/36
Mendel Understood Probability
In the cross Yy x Yy , what is the
probability of yielding 3 yy offspring?
The probability is ½ that a y will be contributed by
one parent
p(y) = ½
The probability is ½ that a y will be contributed by the
other parent
p(y) = ½
The probability of having one yy offspring
½x½=¼
p(yy) = ¼
The probability of having three yy offspring
¼ x ¼ x ¼ = 1/64
Mendel Understood Probability
The Addition Rule: The probability that an event can
occur in two or more alternative ways is the sum of the
separate probabilities of the different ways.
(Used to answer “either / or” questions only)
The probability of rolling a 2 or a 5 =
1/6 + 1/6 = 1/3
Mendel Understood Probability
In the cross Yy x Yy , what is the
probability of yielding yellow seeded
offspring (Yy or YY)?
The probability of being YY
p(YY) = ¼
The probability of being Yy
p(Yy) = ½
The probability of being either YY or Yy:
¼+½=¾
Probability
One more thing to remember:
p(a mutually exclusive event) = 1 – p(all the other events)
Practice Probability
•
What is the probability that you will roll
one dice and see:
1. A 3?
2. A odd number?
3. A 3 or a 4?
•
Rolling two dice what is the probability to
see:
1. Two 3’s (one on each dice)?
2. A 3 and a 4?
Mode of Inheritance
The pattern that the trait follows in families:
Four Mendelian:
• Autosomal (non-sex chromosome) Recessive
• Autosomal Dominant
• X-linked Recessive
• X-linked Dominant
Also complex inheritance
• will be covered later
Autosomal Traits:
Both Males and Females affected, and both
transmit to both sexes of offspring
• Recessive – usually rare in population
– Skips Generations
– Inbreeding increases risk of recessive traits
• Dominant – more common
– Doesn’t skip generations
• Complex
X-Linked Traits:
Gene on X chromosome is carrying trait.
• Recessive
– Only males are affected
– Passed from unaffected mothers to sons
– Affected fathers will only transmit to
heterozygous, unaffected daughters
• Dominant
– Males and females both affected
– Can be passed to both offspring, however
often see more females affected because of
male lethality
– Affected fathers to every single daughter
Two genes
• Now lets examine what happens
when we look at more than one gene
at a time:
– Two Traits
– Two different genes
– Two alleles per gene
– Genes are each on separate
chromosomes
Mendel’s Next Experiment:
Dihybrid cross
YYRR
Parental
Pure-breeding
lines
for two traits Homozygous
yyrr
Homozygous
Yellow or Green Seed Color (Y or y)
Round or Wrinkled Shape (R or r)
Mendel’s Next Experiment:
Dihybrid cross
yyrr
YYRR
P
X
Let’s check
the F2
Yy R r
F1
Were the two traits transmitted together
or independently?
Mendel’s Next Experiment:
Dihybrid cross
YyRr
YyRr
YR
(½)
yr
(½)
YR (½)
yr (½)
YYRR
YyRr
¼
Two Phenotypes
¼
YyRr
¼
yyrr ¼
3 : 1
yellow green
round wrinkled
Traits transmitted together
Mendel’s Next Experiment:
Dihybrid cross
In Reality F2 Looks Like:
Four Phenotypes:
315
9 :
108
3
101
:
3
32
:
1
Mendel’s Next Experiment:
Dihybrid cross
F2 offspring of Dihybrid cross
Four Phenotypes:
new phenotypes
recombinants
original phenotypes
parental or non-recombinant
Mendel’s Next Experiment:
Dihybrid cross
YyRr
4 different
possible gametes
YR
(¼)
Yr
+
yR
(¼) + (¼)
yr
+
(¼) = 1
Mendel’s Next Experiment:
Dihybrid cross
YR(¼) Yr (¼) yR (¼) yr (¼)
YR
(¼)
YYRR
YYRr
YyRR
YyRr
YYRr
YYrr
YyRr
Yyrr
Yr
(¼)
yR
(¼)
YyRR
YyRr
yyRR
YyRr
Yyrr
yyRr
yyRr
12
4
:
round wrinkled
9:3:3:1
12
:
yellow
4
green
yr
(¼)
yyrr
3:1
Therefore traits must be transmitted independently
Mendel’s 2nd Law:
Independent Assortment
Inheritance of a pair of factors for one trait is
independent of the simultaneous inheritance of
factors for another trait
Two genes will assort independently and randomly
Mendel’s 3rd Experiment:
Trihybrid cross
yyrrtt
YYRRTT
Parental
X
Tall
plants
Short
plants
YyRrTt
Independent
Assortment
F1
Tall
plants
F2
27:9:9:9:3:3:3:1
Mendel’s Laws
1. Principle of Segregation
Two alleles segregate randomly during
formation of gametes
2. Independent Assortment
Two genes will assort independently and
randomly from each other
Practice Your Punnetts!
• Draw the punnett squares for two genes
• Calculate # of each genotype and
phenotype
(Y = yellow, y = green)
(R = round, r = wrinkled)
– YyRr x YyRr
– YYRr x Yyrr
Pedigree Analysis
Pedigrees are visual ways to examine a
family’s inheritance pattern for any trait of
interest
• Identify:
– Relationships between family members
– Who has trait of interest (phenotype)
Mode of inheritance
Pedigree Analysis
Insert Figure 4.13
Autosomal Recessive
Autosomal Dominant
Complex Inheritance
Next Class:
• Read Chapter Five
• Homework – Chapter Four Problems;
– Review: 1,3,5, 7
– Applied: 1,2,4,5, 8, 9,10, 11,15
• Pedigree Assignment –
Due October 11th