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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
genes of an organism determines all biological
processes
Genetics
• Genes are discrete units of inherited information
consisting of a specific nucleotide sequences within
molecules of DNA
• Each gene provides information related to a particular
trait (protein) that the organism exhibits
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) of
pea plants each with its own distinctive trait (variant of
that character)
• He created true-breeding lines of peas that
demonstrated only 1 variation of a particular trait over
many generations
– for example, a true breeding purple pea plant had
only purple flowers, 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 the opposite trait (purple
vs white, round vs wrinkled, tall vs dwarf)
• 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 happens when you cross a true breeding plant
that has purple flowers with a true breeding plant that
has white flowers?
• 100% of the plants following the cross have purple
flowers (no plant had white flowers)
• What happens when you cross these 2nd generation
purple flowered plants with one another?
• 75% of the plants following the cross have purple
flowers and 25% of the plants following the cross have
white flowers
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 over the other
– the dominant allele is more common in the
population
• Dominant alleles are designated with a capital letter,
whereas their counterpart recessive allele is
designated with the same letter, but lowercase
Genetic makeup (alleles)
pp
PP
P plants
Gametes
All p
All P
F1 plants
(hybrids)
All Pp
Gametes
1–
2
F2 plants
Phenotypic ratio
3 purple : 1 white
Genotypic ratio
1 PP : 2 Pp : 1 pp
1–
2
P
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 of the same alleles for a trait
is said to be homozygous
• An organism that has 2 different alleles for a trait is
said to be heterozygous
Mendel’s Law of Dominance
• If the 2 alleles of an inherited pair differ
(heterozygous), 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
Mendel’s Law of Segregation
• A sperm or egg carries only
1 allele (haploid) for each
inherited trait
• 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’ (diploid) to the
offspring
P
a
B
P
a
b
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
– explains why the shape of a pea (round vs wrinkled)
is independent of its color (green vs yellow)
• peas can be green and round or yellow and
round or green and wrinkled or yellow and
wrinkled
• 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
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 location 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
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
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 or 100%
• An event that is certain not to occur has a
probability of 0 or 0%
• When you flip a coin, the probability of getting
heads (or tails) is 1 in 2 (50%) 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
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
ww
W_
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
Dd
Sperm
D
• Does this mean
that deaf parents
always have
deaf children?
D
Normal
Dd
Parents
d
DD
Normal
Dd
Normal
(carrier)
Dd
Normal
(carrier)
dd
Deaf
Eggs
d
Offspring
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
Incomplete Dominance
• In some allele combinations, dominance does
not exist
• Instead, 2 traits are blended together to form a
3rd trait
• In snapdragons when a red flowering plant is
crossed with a white flowering 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
AB
Carbohydrate B
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