Genetics
Genetics
• Genetics is the science of heredity
• Genetics explains how genes bring about
characteristics in living organisms and how
those characteristics are transmitted from
parents to offspring
• Genetics is at the center of all biology because
gene activity underlies all biological processes!
Genetics
• Remember, genes are discrete units of genetic
(hereditary) information consisting of a
specific nucleotide sequence in DNA
Experimental Genetics
• The modern science of genetics began in the
1860’s when Gregor Mendel, an Austrian
monk studied the principles
of genetics by breeding
garden peas
– Available in a wide variety of
shapes and colors
– Cheap and abundant
– Short generation times with
large amounts of offspring
Experimental Genetics
• Mendel studied 7 characters (heritable
features) each with its own distinctive trait
(variant of that character)
• He created true-breeding lines; lines of peas
that were homologous for each trait
– A true breeding line had only the genes that
coded for that trait, both chromosomes had the
same version of the gene
– For example, a true breeding purple pea plant had
only ‘purple’ genes, not white
Flower color
Purple
White
Axial
Terminal
Seed color
Yellow
Green
Seed shape
Round
Wrinkled
Pod shape
Inflated
Constricted
Pod color
Green
Yellow
Tall
Dwarf
Flower position
Stem length
Experimental Genetics
• Mendel wanted to see what happened when
he crossed true-breeding lines for one trait
with true-breeding lines for another trait
• His results led to the establishment of several
principles:
– Mendel’s Law of Dominance
– Mendel’s Law of Segregation
– Mendel’s Law of Independent Assortment
• What would
happen if you
cross a purple
flower with a
white flower?
• Mendel’s results
indicated that 3
of the 4 flowers
produced had
purple flowers,
while 1 had white
• How?
Mendel’s Law of Dominance
• The white and purple flowers of the pea
plants are two versions of a gene for flower
color
• Alternative versions of a gene are called
alleles
• Mendel’s law of dominance states that when
an organism has 2 different alleles for any
given character, 1 allele will dominate
Genetic makeup (alleles)
pp
PP
P plants
Gametes
All p
All P
F1 plants
(hybrids)
All Pp
Gametes
1–
2
F2 plants
1–
2
P
Sperm
Phenotypic ratio
3 purple : 1 white
Genotypic ratio
1 PP : 2 Pp : 1 pp
P
p
P
PP
Pp
p
Pp
pp
p
Mendel’s Law of Dominance
• For each character, an organism inherits 2
alleles, 1 from each parent
• These alleles may be the same or different
• An organism that has 2 identical alleles for a
gene is said to be homozygous
• An organism that has 2 different alleles for a
gene is said to be heterozygous
Mendel’s Law of Dominance
• If the 2 alleles of an inherited pair differ, then
one allele will determine the organism’s
appearance over the other, and is called the
dominant allele
• The other allele has no noticeable effect on
the organism’s appearance and is called the
recessive allele
– We use upper and lower case letters to describe
the dominant and recessive alleles, respectively
Mendel’s Law of Segregation
• A sperm or egg carries only 1 allele for each
inherited character
• This is because allele pairs segregate (separate)
during gamete formation (meiosis!)
• When sperm and egg unite
during fertilization, they
each contribute their own
allele, restoring the paired
‘condition’ to the offspring
Mendel’s Law of Independent
Assortment
• The alleles of a gene pair separate from one
another independently of the other alleles of
another gene pair during segregation
(meiosis)
• The origin of any particular allele will be
randomly selected from paternal or maternal
chromosomes via the process of crossing-over
(why, for example, a cat’s color is independent
of its tail length)
Mendel’s Law of Independent
Assortment
• For example, Aa will segregate from Bb, or in
other words, the color of the flower is
independent from the inheritance of the
height of the plant
• In this example,
yellow and green
are 2 traits for the
color character
(indicated by Y and
y, respectively) and
round and
wrinkled are 2
traits of another
character
(indicated by R and
r, respectively)
rryy
RRYY
ry
Gametes RY
RrYy
Sperm
1
–
4
1
–
4
RY
1
–
4
rY
1
–
4
Ry
1
–
4
RY
1
–
4
rY
1
–
4
Ry
1
–
4
ry
RRYY
RrYY
RRYy
RrYy
RrYY
rrYY
RrYy
rrYy
9
––
16
RRYy
RrYy
RRyy
Rryy
RrYy
rrYy
Rryy
rryy
ry
3
––
16
3
––
16
1
––
16
Yellow
round
Green
round
Yellow
wrinkled
Green
wrinkled
Genetics terminology
• The complete genetic make-up of an organism
is called its genotype
• The physical expression of the genotype is its
phenotype
Genotype:
P
a
B
P
a
b
PP
Homozygous
for the
dominant allele
aa
Bb
Homozygous
Heterozygous
for the
recessive allele
Phenotypes can reveal genotypes
• Chocolate labs are labrador retrievers that are
homozygous recessive for coat color
• Black labs have at least 1 copy of the
dominant allele; but their genotype can be Bb
or BB
B_
bb
Phenotypes can reveal genotypes
• How can you determine your dog’s genotype
(without a blood test)?
• You can testcross your dog; mating your dog
with a homozygous recessive dog (bb; a
chocolate lab)
• If the black lab was BB, all of its offspring will
be black (Bb)
• If the black lab was Bb, half would be black
(Bb) and half would be brown (bb)
Testcross:
B_
Genotypes
bb
Two possibilities for the black dog:
BB
B
Gametes
b
Offspring
Bb
or
Bb
All black
b
B
b
Bb
bb
1 black : 1 chocolate
Geneticists use the testcross to
determine unknown genotypes
• Mendel used testcrosses to verify that he had
true-breeding lines of pea plants
• Mendel performed his experiments nearly 100
years before the discovery of DNA!
• The testcross continues to be an important
tool of geneticists for determining genotypes
Mendel’s laws reflect the rules of
probability
• Mendel’s strong background in mathematics
(and physics and chemistry…) served him well
in his studies of inheritance
• He knew he needed large sample sizes
• The laws of inheritance reflect the probability
of an event occurring
– The probability of having a girl: 1 in 2
– The probability of rolling a 5 on a dice: 1 in 6
– The probability of drawing a queen from a deck of
cards: 4 in 52 (1 in 13)
Probability
• An event that is certain to occur has a
probability of 1
• An event that is certain not to occur has a
probability of 0
• When you flip a coin, the probability of getting
heads (or tails) is 1 in 2 every time you toss
the coin; independent of previous tosses!
Segregation and
fertilization as
chance events
Bb male
Formation of sperm
Bb female
1
–
2
Formation of eggs
B
1
–
2
B
1
–
2
b
1
–
2
B
B
b
B
1
–
4
1
–
4
b
b
B
1
–
4
F2 genotypes
b
b
1
–
4
Extra credit opportunity!!!
• Try it at home…
• Toss a coin 100 times and record the
outcomes; your answer should be close to ½
for heads and ½ for tails (if you are using a fair
coin…)
• Submit your answers (and perhaps some
photographic/video proof) for extra credit!!!
Genetic traits may be tracked
• Individuals exhibiting a recessive trait would
be homozygous recessive (carry 2 copies of
the recessive allele)
• Individuals exhibiting a dominant trait,
however, could be homozygous dominant
(carry 2 copies of the dominant allele) or be
heterozygous (carry 1 copy of the dominant
and 1 copy of the recessive allele)
Genotype
Dominant Traits
Recessive Traits
F_
Genotype
ff
Freckles
No freckles
W_
ww
Widow’s peak
Straight hairline
ee
E_
Free earlobe
Attached earlobe
Genetic disorders
• Genetic disorders may be inherited as a
recessive or dominant trait
• Most human genetic disorders are recessive;
most people who have recessive disorders are
born to normal parents who are both
heterozygous for the allele controlling the
disorder
• In this way, the parents are carriers of the
recessive allele, but are phenotypically normal
Offspring produced by parents who
are carriers for a recessive
trait
Normal
Normal
Parents
Dd
Sperm
D
• Does this mean
that deaf
parents always
have deaf
children?
D
Dd
d
DD
Normal
Dd
Normal
(carrier)
Dd
Normal
(carrier)
dd
Deaf
Eggs
d
Offspring
It is said that everything should be
tried once, except square-dancing and
inbreeding….
• It is relatively unlikely for 2 carriers of a rare,
harmful allele will meet and mate
• However, the probability increases greatly if
close relatives marry and have children
• A mating of close relatives, called inbreeding,
is more likely to produce offspring
homozygous for recessive traits
Genetic disorders
• Let’s take a non-human example…
• Dog breeds that have been inbred for
appearance frequently exhibit serious genetic
disorders, such as weak hip joints, eye
problems, etc.
• Endangered species frequently suffer from
inbreeding (reduced numbers increase
chances of close matings)
A case study: The Florida Panther
• The Florida panther population once
numbered in the 30’s in the 1990’s
• Close matings resulted in reduced sperm
counts, heart defects, and low survival rates
among kittens
• Introduction of the Texas Panther in recent
years has yielded hybrids with a
higher survival rate
(controversial!)
www.bigcatrescue.org/catswild/florida_panther.htm
Genetic disorders
• Why are most genetic disorders recessive?
• Dominant alleles that cause lethal diseases are
much less common than lethal recessives
• This is because the dominant allele cannot be
carried by heterozygotes without it affecting
them (and subsequently kill the embryo)
• In contrast, recessive alleles are continually
carried from generation to generation by
healthy (unaffected) heterozygous carriers
Genetic disorders
• Most dominant genetic disorders can be
eliminated when it causes the death of an
individual before he/she has a chance to mate
(and pass along his/her alleles)
• A lethal dominant allele, however, can escape
elimination when it does not cause death until
a relatively advanced age
– Huntington’s Disease – degenerative disease of
the nervous system does not appear until 35-40
years of age (50% chance of inheriting it)
Three’s a crowd…
• Mendel was fortunate in that he chose
characters for which there were only 2 alleles
• Many genes, however, have more than 2
alleles in the population
• More often than not, the
inheritance patterns of a
particular trait are more
complex
www.flickr.com/photos/27887160@N02/2601298345/
Incomplete dominance
• In some allele combinations, dominance does
not exist
• Instead, 2 traits are blended together to form
a 3rd trait
• In snapdragons (a plant, not a cool dragon,
unfortunately) when a red plant is crossed
with a white plant, some offspring are red,
some are white and some are pink!
P generation
Red
RR
White
rr
r
R
Gametes
F1 generation
Pink
Rr
Gametes
1–
2
R
1–
2
r
Sperm
1–
2
F2 generation
R
1–
2
r
1–
2
R
RR
rR
1–
2
r
Rr
rr
Eggs
Incomplete dominance
• In this case, heterozygous individuals exhibit a
third phenotype, pink.
• The resulting pink flowers are Rr and can
produce red, white or pink offspring of their
own
• In the case of incomplete dominance, the
phenotype does reveal the genotype for all
traits!
Multiple alleles
• Multiple alleles exist for most genes
• For example, the ABO blood group in humans
involves 3 alleles of a single gene: A, B, and O
• An individual can have type A, B, O, or AB
blood
• The A and B alleles are co-dominant; both
alleles are expressed in heterozygous
individuals
Blood
Group
(Phenotype)
O
Genotypes
Red Blood Cells
OO
AO or AA
Carbohydrate A
A
B
AB
BO or BB
Carbohydrate B
AB
Safe a life, give blood
• Blood group AB can receive blood from any
blood type, but can only donate to AB
• Blood group A can receive blood from only A
or O, but can donate to A or AB
• Blood group B can receive blood from only B
or O, but can donate to B or AB
• Blood group O can receive blood only from O,
but can donate to A, B, O or AB!
Blood type complications
• In addition to blood ‘type’, our red blood cells
may (or may not) have a protein known as the
Rh factor
• Individuals without this factor have a
“negative” blood type, while those with this
factor have a “positive” blood type
• Problems can occur when an Rh- mother
carries an Rh+ child, especially for children
conceived after the birth of an Rh+ child
Multiple alleles
• No matter how many alleles for a given gene
are in a population, a diploid individual will
only have 2 alleles, one on each homologous
chromosome
Genotype:
P
a
B
P
a
b
PP
Homozygous
for the
dominant allele
aa
Bb
Homozygous
Heterozygous
for the
recessive allele
The chromosome basis of
inheritance
• Mendel established his principles (laws) of
inheritance long before mitosis and meiosis
were understood, and longer still before
chromosomes were ‘discovered’
• The chromosome theory of inheritance states
that genes occupy specific loci, or positions,
on chromosomes and it is the chromosomes
that undergo segregation and independent
assortment during meiosis
F1 generation
All round yellow seeds
(RrYy)
R
r
y
Y
r
R
y
Y
R
Y
y
Y
y
R
R
Y
y
Anaphase I
of meiosis
r
Y
R
r
R
Y
Metaphase I
of meiosis
(alternative
arrangements)
r
Metaphase II
of meiosis
r
Y
y
r
R
Y
y
y
Y
Y
r
r
r
1
– ry
4
1
– rY
4
Fertilization among the F1 plants
F2 generation
R
Gametes
y
1
– RY
4
r
9
:3
:3
:1
y
y
R
R
1
–
4
Ry
Genes on the same chromosome
tend to be inherited together
• The number of genes in a given cell is far
greater than the number of chromosomes
• Each chromosome contains hundreds or
thousands of genes
• Genes located close together on the same
chromosome tend to be inherited together
and are called linked genes
• Linked genes do not follow Mendel’s law of
independent assortment
Experiment
Purple flower
PpLl
Phenotypes
Purple long
Purple round
Red long
Red round
PpLl
Observed
offspring
284
21
21
55
Long pollen
Prediction
(9:3:3:1)
215
71
71
24
Explanation: linked genes
PL
Parental
diploid cell
PpLl
pl
Meiosis
Most
gametes
pl
PL
Fertilization
Sperm
Most
offspring
PL
pl
PL
PL
PL
pl
pl
pl
PL
pl
PL
Eggs
pl
3 purple long : 1 red round
Not accounted for: purple round and red long
Sex chromosomes and sex-linked
genes
• The X-Y system of sex chromosomes is only 1
of several sex-determining systems
• Insects have an X-O system; females have 2 X
chromosomes, while males have only 1 (XO)
• Some organisms lack sex chromosomes
altogether and sex is instead determined by
chromosome number
• Other organisms have temperaturedependent sex determination!
Sex chromosomes and sex-linked
genes
• In addition to bearing genes that determine
sex, sex chromosomes contain genes for
characters unrelated to gender
• A gene located on a sex chromosome is called
a sex-linked gene
• Sex-linked disorders typically affect human
males, since they have only one copy of each
chromosome
Sex-linked disorders
• Hemophilia is a sex-linked recessive trait
• Hemophiliacs lack 1 or more proteins required
for blood clotting, and bleed excessively when
injured as a result
• Hemophilia is a caused by a recessive allele on
the X chromosome
• Woman can be carriers, but rarely suffer from
the condition……Why?