Principles of Inheritance and
First, a bit of Review…
• Remember the structure of DNA?
• And how it relates to a GENE?
• Check out the next slides to get a
different perspective from your
textbook on these concepts…
DNA : deoxyribonucleic acid
• DNA is found in the nucleus of
a cell and carries the genetic
instructions for making living
• The structural arrangement of
DNA, which looks something
like an immensely long ladder
twisted into a helix, or coil.
• The sides of the "ladder" are
formed by a backbone of sugar
and phosphate molecules, and
the "rungs" consist of
nucleotide bases joined weakly
in the middle by hydrogen
Watson and Crick
What is a Gene?
• A gene is the basic unit
of heredity passed from
parent to offspring. Genes
are pieces of DNA, and
most genes contain the
information for making a
specific protein or coding
for a particular trait.
• Humans are estimated to
A Mode of Inheritance:
the missing component of Darwin’s Idea
Pangenesis? Acquired traits?
Pangenesis and the Blending of Bloods: An
early hypothesis of how traits are passed from
one generation to the next. Pre-Mendel, this was
a popular idea, and Darwin even considered it a
This idea depends on the true mixing of
characteristics from each parent.
• A Moravian monk who
discovered the principles of
particulate inheritance through Gregor Mendel
carefully conducted plant (1822- 1884)
breeding experiments. His
work still stands today.
Mendel’s work showed inheritance to be
PARTICULATE—no mixing or blending of
the traits, but a COMBINATION OF EFFECTS!
Mendel’s breeding experiments
• Mendel created “pure
breeding” strains for
• He noted that in 1st
generation (F1) cross
of these strains, certain
• However, in the F2
• This was strong
The ¾ to ¼ ratio in
A AA Aa
a Aa aa
A Punnett Square shows how the traits
are inherited (A=dominant trait; a=recessive trait).
Mendel’s Principle of Segregation
• Traits exist in discrete (or particulate) units;
we call these alleles.
• An offspring gets one allele from each
parent for a particular trait.
• His experiment also revealed the presence
of dominant and recessive traits (see next
slide for explanation).
Mendel Found Different Expressions of Genes…
• Dominant: alleles that are always
expressed. Example: TT or Tt—in
Mendel’s work, the tall trait is
visible as long as the allele T is
• Recessive: alleles that are expressed
when a dominant allele is NOT
present. Example: tt—in Mendel’s
work, short plants were only present
when they had the recessive
• Codominant: when both alleles are
expressed. Example: AB blood—the Tall and short pea plants
individual has both A and B types in
Mendel’s Principle of
• In a situation where two traits are examined,
and when each trait exists on a different
• The traits will act INDEPENDENTLY
because the chromosomes are inherited
independently of one another.
• So, from Mendel’s work, you should be
getting the idea that:
– He created our modern ideas of heredity,
though we did not know about them until after
– There are TWO principles that explain the
particulars of heredity.
– We can predict the traits of offspring with a
And now…a survey of Mendelian
• Have some fun checking out the variety of traits
that are inherited in Mendelian fashion (this means
it is one gene that controls the trait—one mistake,
and you have it!) ☺
• These are separated into traits that exist in
DOMINANT form and RECESSIVE form.
Remember, just because the trait is Dominant,
does not make it MORE frequent in a population!
• Which traits do you have?
Some traits with dominant inheritance
• Bent little finger
• Short big toe
• Mid-digital hair Mid-digital hair Sort Big Toe
Darwin’s tubercle Bent Little finger
• Refers to shortening of the
fingers and toes of the
hands and feet.
• There are many different
forms of this condition.
• Dominant inheritance
Dominant Inheritance of Pedigree of brachydactyly
• Ring fingers longer than middle
fingers have been associated
• The prenatal development of
fingers and gonads is controlled
by the same gene.
• In men, relatively long fingers
seem to be linked to low fetal
• Robert Lewis Stephenson was
said to be moody. This painting
suggests that he had a long ring
finger…a genetic link?
• A trait with dominant
• Sometimes associated
with defective hearing
and other problems
• Notice that dominant
traits such as this don’t
• A dominant trait.
• If two achondroplastic
people marry, about 2/3 of
their children dwarf and 1/3
of normal height.
• There is also a higher than
normal frequency of
spontaneous abortions and
stillbirths, so being
homozygous for the trait is
probably incompatible with
Some traits with recessive inheritance
• Hitchhiker’s Thumb
• Counter clockwise hair whorl
Alopecia Albinism Counter clockwise whorl
• A recessive trait.
• The hair of people with
Pedigree of Alopecia universalis
the condition falls out,
but otherwise they are
• Tay-Sachs disease caused by a recessive allele
that results in the absence hexosaminidase, an
enzyme necessary for the breakdowns of a
lipid called GM2 ganglioside accumulates
abnormally in nerve cells.
• This causes progressive destruction of the
central nervous system of afflicted children
who usually die before they are 5 years old.
• Population Differences in Prevalence
– People of eastern European (Ashkenazi)
Jewish descent. About one in every 27
American Jews is a carrier.
– Some French Canadians and Cajuns also
have a high frequency.
This EM photo shows
– The U.S. population in general population numerous membrane bound
and Sephardic Jews have a carrier myelin figures in a patient with
frequency of 1 in 250. Tay-Sachs
• The most common form is
an autosomal recessive
• Afflicted individuals lack
the enzyme (tyrosinase)
necessary for producing the
• characterized by absence of
pigment in hair, skin, and
• Eye problems and sun
sensitivity are common
Sickle Cell Anemia
• Normal red blood cells are fat and
donut shaped—they carry lots of
oxygen to our organs.
• People heterozygous individuals
have some normal and some
sickled (see photo below right) red
• People homozygous for the
condition have severely sickle-
shaped red blood cells that cannot
carry oxygen efficiently.
• We will learn more about the
variation in SSA around the world
in the upcoming weeks.
There are OTHER types of traits!
• Mendelian traits are controlled by a single
gene, so are easily observed and studied.
• OTHER traits include:
– Sex-linked traits (found on the sex
– Polygenic traits (controlled by multiple genes)
• Here are some examples of non-Mendelian
• Inheritance involving genes on the X and Y chromosomes.
• Recessive traits on the the X chromosome of females are
expressed in males (when there is no allele for the trait on Male
the Y chromosome). Parent
• Traits on the Y chromosome are only expressed in males
(since females have two Xs!). XY
Y XX XY
Sex-Linked Traits: Color Blindness
Normal Color Blind
• Red-green color blindness
is a sex-linked trait carried
on the X chromosome.
• Color receptors in the eyes
(cones) are abnormal in
Retina showing rods and cones Can you see the square?
Sex-Linked Traits: Hairy Ears
• Probably a Y-
linked trait as
females do not
exhibit the trait.
• Found in high
frequency in some
(who marry within
Sex-Linked Traits: Hemophilia
• Females (circles
on the diagram) can
carry the allele;
males (squares on
the diagram) are
affected with the
Sex-Linked Traits: Male Pattern Baldness
• Early baldness of the
ordinary type has been
thought to be autosomal
dominant in males and to be
autosomal recessive in
females who transmit the
trait if heterozygous but are
bald only if homozygous.
• The transmission through
generations, as in the
descendants of President
John Adams, suggests the
operation of a single major Four generations of male pattern
gene. baldness in the Adams family
• Most traits are controlled
by more than one gene,
• For some polygenic
traits, a few different
alleles can produce a
large number of different
• The example of skin
color at the left,
Polygenic Inheritance: Eye Color
• Despite what we all learned in high school
biology, eye color is likely to be a polygenic
trait. The early view that blue is a simple
recessive has been repeatedly shown to be
wrong by observation of brown-eyed offspring
of 2 blue-eyed parents.
• Blue-eyed offspring from 2 brown-eyed
parents is a more frequent finding.
• At least two separate genes, each with two
incompletely dominant alleles, govern human
• A man and a woman, each heterozygous for
both genes, could have children with five
different eye colors, ranging from light blue
(no dominant alleles) through light brown (two
dominants) to almost black (all four alleles