Heredity
Biology 30i
Early Theories of Inheritance
Aristotle (384-322 B.C.E.)
  proposed the first widely accepted theory of inheritance
•  called pangenesis
• egg and sperm consist of particles called pangenes
that come from all parts of the body.
• upon fertilization the pangenes develop into the
parts of the body from which they are derived.
•  egg and sperm consist of particles called
pangenes that come from all parts of the
body.
•  upon fertilization the pangenes develop
into the parts of the body from which they
are derived.
Antony van Leeuwenhoek (1632-1723)
  discovered sperm in semen.
•
•
he believed he thought he saw a complete
miniature person called a homunculus inside
the head of the sperm.
other people of Antony’s time thought that the
egg contained the entire person.
Mendelian Genetics
Gregor Mendel (1822-1884)
  an Augustinian monk in Brunn (Czech
Republic), Austria
  his research laid the foundation for
modern genetics and the science of
inheritance.
  for seven years he bred pea plants
(Pisum sativum) and analyzed the
results.
Mendelian Genetics

Mendel focuses on seven different traits of pea plants..
Mendelian Genetics

Mendel let plants self-pollinate to ensure they were true
breeding.
•  true breeding plants exhibit the same characteristics
generation after generation.
•  Mendel called
•  true breeding plants the parental or P generation
•  the first offspring first filial or F1 generation
•  If the F1 generation were to pollinate the offspring
would be called the second filial or F2 generation
Mendelian Genetics
•
•
Mendel called the first offspring first filial or F1 generation
If the F1 generation were to pollinate the offspring would be called the
second filial or F2 generation
•  because all of Mendel’s initial crosses only involved
one trait we call them monohybrid crosses.
•  Mendel observed that:
• for every trait crossed the F1 generation only
showed one of the two parental traits.
ie. if plants with round seeds were crossed with
plants of wrinkled seeds the F1 generation
would only have plants of round seeds.
Mendelian Genetics
•
Mendel observed that:
•  for every trait crossed the F1 generation only showed one of the two
parental traits.
ie. if plants with round seeds were crossed with plants of wrinkled
seeds the F1 generation would only have plants of round seeds.
• even though the F1 generation had a copy of both
genes only one was expressed.
• Mendel called this characteristic dominant.
allele: one of alternative forms of a gene.
the gene for wrinkled and the gene for round
peas are alleles.
Mendelian Genetics
•  even though the F1 generation had a copy of both genes only one
was expressed.
•  Mendel called this characteristic dominant.
allele: one of alternative forms of a gene.
the gene for wrinkled and the gene for round peas are alleles.
dominant trait: a characteristic that is
expressed when one or both alleles in an
individual are the dominant form
~ dominant alleles are indicated by an
uppercase letter (R)
Mendelian Genetics
dominant trait: a characteristic that is expressed when one or
both alleles in an individual are the dominant form
~ dominant alleles are indicated by an uppercase letter (R)
•  Mendel called the characteristic that was not
expressed recessive
recessive trait: a characteristic that is expressed only
when both alleles in an individual are the recessive
form.
•  Mendel concluded that one form showed complete
dominance.
•  an individual with one dominant and one
recessive (Rr) had the same characteristics as one
with two dominant forms (RR)
Mendelian Genetics
•  Mendel concluded that one
form showed complete
dominance.
•  an individual with one
dominant and one
recessive (Rr) had the
same characteristics as
one with two dominant
forms (RR)
Mendel’s Traits
Trait
Dominant
Recessive
Stem Length
Tall (T)
Short (t)
Pod Shape
Inflated (I)
pinched (i)
Seed Colour
Yellow (Y)
Green (y)
Flower
Position
Axial (A)
Terminal (a)
Flower Colour
Purple (P)
White (p)
Seed Shape
Round (R)
Wrinkled (r)
Pod Colour
Green (G)
Yellow (g)
Mendelian Genetics
Important Definitions
Homozygous: having identical alleles for the same gene
Heterozygous: having different alleles for the same gene.
Genotype: the genetic complement of an organism
Phenotype: the observable characteristics of an organism
Segregation: the separation of alleles during meiosis.
Mendelian Genetics
Genotype: the genetic complement of an organism
Phenotype: the observable characteristics of an organism
Segregation: the separation of alleles during meiosis.
Law of Segregation
  Mendel’s First Law
•  All individuals have two copies of each factor (gene).
These copies segregate (separate) randomly during
gamete formation, and each gamete receives one copy
of every gene.
  in 1909 Danish Botanist Wilhem Ludwig Johannsen called
Mendel’s “factors” genes
Mendelian Genetics
Analyzing Genetic Crosses
Reginald Punnett (1875-1967)
•  devised a visual way to analyze the results of crosses, called a
Punnett’s square.
Mendelian Genetics
Trait
Stem Length
Pod Shape
Seed Shape
Flower Colour
Dominant
Phenotype
Tall
Inflated
Round
Purple
Genotype(s)
TT (homozygous)
Tt (heterozygous)
II (Homozygous)
Ii (hetorozygous)
RR (Homozygous)
Rr (Heterozygous)
PP (Homozygous)
Pp (herozygous)
Recessive
Phenotype
Genotype(s)
Short
tt (homozygous)
Pinched
ii (homozygous)
Wrinkled
rr (homozygous)
White
pp (homozygous)
In order to see recessive phenotypes the genotype must be homozygous
Mendelian Genetics
Punnett Squares
• are used to predict the genotype and phenotype of potential off-spring
• very useful when producing economically important cattle and plants.
P Generation
♀
♂
Phenotypic Ratio
Genotypic Ratio
Mendelian Genetics
Punnett Squares
•  are used to predict the genotype and phenotype of potential off-spring
•  very useful when producing economically important cattle and plants.
P Generation
F1 Generation
♀
♂
♀
♂
Phenotypic Ratio
Phenotypic Ratio
Genotypic Ratio
Genotypic Ratio
Test Cross
  a test cross of an individual of unknown genotype to an
individual that is fully recessive
  the phenotypes of the F1 generation of the test cross
reveals whether the unknown genotype is homozygous or
heterozygous
  example:
•  you have a white ram (white is dominant “W” and
black is recessive “w”) and want to know if it is
heterozygous or homozygous for breeding purposes.

example:
•  you have a white ram (white is dominant “W” and black is recessive “w”)
and want to know if it is heterozygous or homozygous for breeding
purposes.

do a test cross by crossing your unknown ram with one
showing a recessive phenotype.
•  it must have a recessive genotype (ww)
♂
Ww
Test 1
Test 2
♀
♀
ww
Ww
Ww
ww
ww
♂
WW
ww
Ww
Ww
Ww
Ww
If the ram is Heterozygous it will
produce:
If the ram is Homozygous it will
produce:
Phenotypic ratio: 50% white 50%
black, or 2:2 or 1:1
Phenotypic ratio: 100 % White
Genotypic ratio:
2:2 or 1:1 hetero:homo recessive
Genotypic ratio:
100% heterozygous
Analyze:
Analyze:
Heterozygous Seed shape
crossed with a recessive
seed shape
What are the predicted
phenotypes and genotypes?
Test 1
♀
Phenotypic Ratio
• 50% round 50% wrinkled or 2:2
or 1:1, ½, 2/4,
Rr
Genotypic Ratio
♂
rr
Rr
rr
Rr
rr
• 50% hetero: 50% homo
recessive, 2:2 or 1:1, hetero 2/4
or ½ homo recessive 2/4 or ½
Example Problem
Offspring Numbers
Phenotype
  A horticulturist has seeds from
a cross but does not know the
round-seed
5472
genotype of the phenotype of
peas
the parents. Use the following
wrinkle1850
information to figure out the
seed peas
parental phenotype and
genotype
Solution
  because there is two different phenotypes one the
parents must not be homozygous dominant
Offspring
Phenotype
Numbers
round-seed
peas
5472
wrinkleseed peas
1850
Solution
  because there are two
different phenotypes one
the parents must not be
homozygous dominant
Solution
  5472/1850 = 2.96
2.96/1 or 2.96 : 1
~3:1
to get a 3:1 ratio both parents
must be heterozygous
Tester
♀
♂
R
R
??
R?
R?
R?
R?
Mendelian Genetics
Proof
♀
♂
Rr
Proof
Rr
♀
RR
Rr
Rr
rr
3:1 Phenotypic ratio
♂
Rr
rr
Rr
Rr
rr
rr
1:1 (50%) Phenotypic ratio
Mendel’s Second Law
•  The Law of Independent Assortment
  Mendel also crossed plants of two traits.
•  because two traits are involved in these crosses they
are called a dihybrid cross.
  Mendel crossed true breeding tall plants that had green
pods (TTGG) with true breeding short plants that had
yellow pods (ttgg) to produce the F1 generation
•

because two traits are involved in these crosses they are called a dihybrid
cross.
Mendel crossed true breeding plants tall plants that had green pods (TTGG)
with true breeding short plants that had yellow pods (ttgg) to produce the F1
generation
P1 cross
TTGG X ttgg
predicted phenotypic
ratio
♀
♂
TG
TG
tg
tg

in this case the true breeding plants will produce only one type of
gametes
TTGG → will produce gametes with the TG genes
ttgg → will produce gametes with the tg genes
♀
♂
TG
TG
tg
tg
TtGg
TtGg
TtGg
TtGg


the phenotypic ratio of the F1
generation:
100% tall and green pods
the genotypic ratio of the F1
generation
100% heterozygous


Mendel then crossed the F1 generation to produce an F2
generation
in this case the plants of the F1 generation produce four
different types of gametes
TtGg → will produce gametes with the:
TG genes (tall, green)
Tg genes (tall, yellow)
tG genes (short, green)
tg genes (short, yellow)
TtGg → will produce gametes with the:
TG genes
Tg genes
tG genes
tg genes
♀
♂
TG
Tg
tG
tg
TG
TTGG TTGg TtGG
TtGg
Tg
TTGg
TTgg
TtGg
Ttgg
tG
TtGG
TtGg
ttGG
ttGg
tg
TtGg
Ttgg
ttGg
ttgg
♀
♂
TT = tall
GG = green
Tt = tall
Gg = green
tt = short
gg = yellow
TG
Tg
tG
tg
Phenotypes
Tally
TG
TTGG TTGg TtGG
TtGg
Tall & Green Pods
9
Tg
TTGg
TTgg
TtGg
Ttgg
Tall & Yellow
Pods
3
tG
TtGG
TtGg
ttGG
ttGg
Short & Green
Pods
3
tg
TtGg
Ttgg
ttGg
ttgg
Short & Yellow
Pods
1

for every dihybrid cross that Mendel carried he got the
9:3:3:1 ratio (when he crossed the F1 generation).
•  this ratio is what is expected if the segregation of
alleles for one gene had no influence on the
segregation of alleles of another gene.
Law of Independent Assortment
•  The two alleles of one gene segregate (assort)
independently of the alleles for other genes during
gamete formation
Law of Independent Assortment
•  The two alleles of one gene segregate (assort) independently of the alleles
for other genes during gamete formation
Pleiotropic Genes
•  a gene that affects more than one characteristic
•  example: Sickle-cell anemia
•  the normal hemoglobin is produced by the allele HbA
•  in sicke-cell anemia the individual has two copies of the
mutated allele Hbs
Pleiotropic Genes
•  a gene that affects more than one characteristic
•  example: Sickle-cell anemia
•  the normal hemoglobin is produced by the allele HbA
•  in sicke-cell anemia the individual has two copies of the mutated allele Hbs
•  the mutation cause abnormally shaped hemoglobin that
cannot deliver oxygen to the cells.
• causes fatigue, enlarged spleen, pneumonia and
major organ damage.
•  a heterozygous individual has resistance to malaria but
an increased chance of having homozygous recessive
offspring.
Beyond
Mendel
Beyond Mendel
Incomplete and Co-dominance
  there are patterns of inheritance that do not
follow the same patterns that Mendel
observed.
•  they still follow the same rules as laid out
by Mendel’s laws
  Incomplete dominance
•  occurs when neither of the two alleles for
the same gene can completely conceal the
presence of the other.
• example: Mirabilis jalapa (Four
o’clock Plant)
Beyond Mendel

Incomplete dominance
•  example: Mirabilis jalapa (Four o’clock Plant)
•  a cross between a true breeding red-flowered plant and a
true-breeding white-flower produces offspring with pink
flowers.
•  when representing incomplete dominance upper and
lower case letters are not used.
•  all upper case letters are used with subscripts to
denote the alleles.
R1R1 → red flower
R2R2 → white flower
R1R2 → pink flower
Beyond Mendel
Beyond Medel

Incomplete dominance
•  two human examples of incomplete dominance are
sickle cell anemia and familial hypercholesterolemia
Sickle Cell Anemia
HbA HbA → normal red blood cells
Hbs Hbs → sickle shaped red blood cells
HbA Hbs → have the sickle trait
•  this is called heterozygous advantage because if you
have one copy of the mutation you don’t have the
disease and you are resistant to malaria.
Beyond Mendel

Incomplete dominance
Familial Hypercholesterolemia
•  a genetic condition that prevents the tissues from
removing low-density lipoproteins (bad cholesterol)
from the blood.
•  if you are homozygous for the trait you have six times
the amount of cholesterol in your blood.
•  most have a heart attack by the age 2
•  heterozygous individuals have about twice as much
cholesterol in their blood and may have a heart attack
by the age 35.
Beyond Mendel

Co-dominance
•  occurs when both alleles are fully expressed.
•  example: Blue Roan Horses
• a heterozygous animal where both the base colour
and white are expressed.
• both black and white hairs grow on the body
creating a blue appearance
Beyond Medel
Chromosomal Theory
Walter Sutton and Theodor Boveri (1902)
  observed chromosomes came in pairs and segregated
during meiosis.
•  chromosomes formed new pairs when the egg and
sperm united.
• this supported Mendel’s observations on inheritance
and his “factors” became alleles of a gene.
Beyond Medel

humans have 44 autosomal chromosomes and 2 sex
chromosomes.
•  humans have thousands of different traits.
•  Sutton hypothesized that each chromosome carries
multiple genes
• genes that are located on the same chromosome are
said to be linked genes.
Beyond Medel

the Chromosomal theory of inheritance:
•  chromosomes carry genes, the units of heredity
•  paired chromosomes segregate during meiosis. Each
sex cell or gamete has half the number of
chromosomes found in the somatic cells. This explains
why each gamete has one one of each of the paired
alleles.
Morgan’s Experiment
  studied the principles of inheritance using Drosophila
melanogaster, fruit flies
  fruit flies a great animals to study because:
•  they reproduce rapidly (in 10 to 15 days)
• offspring can mate shortly after leaving the egg
• females produce over 100 eggs
• they are small and easy to take care of.
• males can be easily distinguished from females.
• males have smaller-rounded abdomen, females
have a pointed abdomen.
Beyond Medel
Beyond Medel



Morgan observed a white-eyed phenotype and after a
number of test crosses figured out it only occurred in
males.
•  a sex-linked trait is one that is determined by genes
located on the sex chromosomes
Morgan first crossed a white eyed male with a red eyed
female (red eyed being dominant)
•  all members of the F1 generation had red eyes
Morgan then crossed to members of the F1 generation
•  he observed ¾ red eyes and ¼ whites eyes.
Beyond Medel



Morgan then crossed two members of the F1 generation
•  he observed ¾ red eyes and ¼ whites eyes in the F2
generation
•  he noticed all the females had red eyes and the white
eyed phenotype only appeared in the males.
because the sex chromosomes in males are not
homologous they contain different genes.
Morgan concluded that the Y chromosome does not carry
the gene to determine eye colour.
•  we now know the gene for eye colour in fruit flies is on
the X chromosome.
Beyond Medel
Punnett Squares for Sex Linked Inheritance
F1 Generation
♀
♂
XrY
F2 Generation
XRXR
X RX r
X RY
X RX r
X RY
•  4/4 red eyed fruit flies
♀
♂
XRY
XRXr
X RX R
X RX r
X RY
XrY
•  3/4 red eyed fruit flies
(2 female and 1 male)
• ¼ white eyed, 1 male
Beyond Medel


in humans it is estimated that
•  the X chromosome carries between 100 and 200 genes
•  the Y chromosome carries less than 100 genes
disorders that require two recessive alleles, one on each X
chromosome only need to be present once in males.
•  this is why some sex linked disorders occur more frequently in
males.
•  examples: colour blindness, hemophilia, near-sightedness
(myopia), night-blindness.
•  recessive lethal X-linked disorders also occurs more frequently
in males.
•  example: infantile spinal muscular atrophy