GENETICS
GREGOR MENDEL
Father of modern genetics
 Studied pea plants


variety of distinct heritable features, or characters



character variations are called traits
Mating of plants can be controlled
used varieties that were “true-breeding”
plants that produce offspring of the same variety when they
self-pollinate
 When cross pollination occurs offspring showed a mix of
characteristics of both parents

LE 14-2
Removed stamens
from purple flower
Transferred spermbearing pollen from
stamens of white
flower to eggbearing carpel of
purple flower
Parental
generation
(P)
Carpel
Stamens
Pollinated carpel
matured into pod
Planted seeds
from pod
First
generation
offspring
(F1)
Examined
offspring:
all purple
flowers
TERMS

Trait


specific characteristic that changes from one individual to
another
Gene

chemical factors that determine traits



Allele


Found in DNA
Genes for various traits are found on chromosomes
different forms of a gene
Example
Trait = plant height
 Alleles (different forms of gene)



Short
Tall
DOMINANCE

Principle of Dominance

some alleles are dominant and others are recessive





Ex. Plant height
 Dominant = tall (T)
 Recessive = short (t)
Each gene is coded for using 2 alleles


Dominant alleles
 are generally expressed using a capital letter
Recessive alleles
 are usually expressed using a lower case letter
One allele from each parent
Dominant alleles control the appearance of the trait
If two recessive alleles are present the recessive
condition will be expressed
ALLELES

Homozygous

individual with 2 of the same allele for a given trait
Ex. TT, RR = homozygous dominant
 Ex. tt,, rr = homozygous recessive



Can “true breed” = create offspring like self in self
polination
Heterozygous

individual with 2 different alleles for a given trait

Ex. Tt, Rr, Ww

Are not “true breeders”

Genotype

The combination of alleles for a given trait


Ex. TT, Tt, tt
Phenotype

The physical appearance caused by the interaction of
alleles
In general, if a dominant allele is present then the
dominant condition will be shown
 Ex. Tall, Short


Parent


F1 (first filial)


First generation studied
Offspring of 2 individuals from the parent generation
F2 (second filial)

Offspring of 2 individuals from the first filial generation
THE LAW OF SEGREGATION
Each individual contains 2 alleles for each gene (one
from each parent)
 During meiosis, when the number of chromosomes are
reduced (diploid to haploid), gametes are produced that
contain one allele for each trait
 Segregation = the separation of alleles for a certain trait
 Each gamete only carries a single allele for each gene
 During fertilization, two gametes fuse forming a diploid
cell.


This cell contains 2 alleles for each gene
LE 14-3
P Generation
(true-breeding
parents)
Purple
flowers
White
flowers
F1 Generation
(hybrids)
F2 Generation
All plants had
purple flowers
LE 14-4
Allele for purple flowers
Locus for flower-color gene
Allele for white flowers
Homologous
pair of
chromosomes
LE 14-5_2
P Generation
Appearance:
Genetic makeup:
Purple
flowers
PP
White
flowers
pp
P
p
Gametes
F1 Generation
Appearance:
Genetic makeup:
Purple flowers
Pp
1
Gametes:
2
1
P
p
2
F1 sperm
P
p
PP
Pp
Pp
pp
F2 Generation
P
F1 eggs
p
3
:1
LE 14-6
3
Phenotype
Genotype
Purple
PP
(homozygous
Purple
Pp
(heterozygous
1
2
1
Purple
Pp
(heterozygous
White
pp
(homozygous
Ratio 3:1
Ratio 1:2:1
1
PROBABILITY

The likelihood that a particular event will occur.
Ex. If you flip a coin you have a probability of ½ that
you will end up with heads.
 What is the probability of flipping 3 heads in a row?



½ x ½ x ½ = 1/8
The principles of probability can be used to
predict outcomes of genetic crosses.

Punnet Squares
Tt X Tt Cross
Section 11-2
Go to
Section:
Tt X Tt Cross
Section 11-2
Go to
Section:
THE TESTCROSS

breeding the mystery individual with a homozygous
recessive individual
LE 14-7
Dominant phenotype,
unknown genotype:
PP or Pp?
Recessive phenotype,
known genotype:
pp
If Pp,
then 2 offspring purple
and 1 2 offspring white:
If PP,
then all offspring
purple:
p
1
p
P
p
p
Pp
Pp
pp
pp
P
Pp
Pp
P
P
Pp
Pp
THE LAW OF INDEPENDENT ASSORTMENT

Genes for different traits can segregate
independently during the formation of gametes.

Genes for different traits are not necessarily linked.
LE 14-8
P Generation
YYRR
yyrr
Gametes YR
yr
YyRr
F1 Generation
Hypothesis of
dependent
assortment
Hypothesis of
independent
assortment
Sperm
1
Sperm
1
2
YR
1
2
yr
1
1
2
2
1
4
Yr
1
4
yR
1
4
yr
YR
4
YYRR
YYRr
YyRR
YyRr
YYRr
YYrr
YyRr
Yyrr
YyRR
YyRr
yyRR
yyRr
YyRr
Yyrr
yyRr
yyrr
YR
YYRR
1
YR
Eggs
Eggs
F2 Generation
(predicted
offspring)
4
YyRr
1
Yr
4
yr
YyRr
3
4
yyrr
1
1
yR
4
4
1
Phenotypic ratio 3:1
yr
4
9
16
3
16
3
16
3
16
Phenotypic ratio 9:3:3:1
THE SPECTRUM OF DOMINANCE

Complete dominance


Codominance


phenotypes of the heterozygote and dominant homozygote are
identical
two dominant alleles affect the phenotype in separate,
distinguishable ways
Incomplete Dominance

phenotype of F1 hybrids is somewhere between the
phenotypes of the two parental varieties
LE 14-10
P Generation
Red
CRCR
White
CWCW
CR
Gametes
CW
Pink
CRCW
F1 Generation
Gametes
1
1
F2 Generation
2
CR
2
CR
1
2
1
CW
Sperm
2
CW
Eggs
1
1
2
2
CR
CRCR
CRCW
CRCW
CWCW
CW
FREQUENCY OF DOMINANT ALLELES

Dominant alleles are not necessarily more common in
populations than recessive alleles
Polydactyl individuals
 1/400 in the U.S.

MULTIPLE ALLELES

more than two allelic forms

Example – blood type
four phenotypes: A, B, AB, O
A B
 Three alleles: I , I , and i.

PLEIOTROPY

multiple phenotypic effects

Ex. multiple symptoms of certain hereditary diseases, such as
cystic fibrosis and sickle-cell disease
POLYGENIC TRAITS

Traits controlled by 2 or more genes

Ex. Eye color, hair color, skin color
SKIN COLOR
AaBbCc
aabbcc
20/64
Fraction of progeny
15/64
6/64
1/64
Aabbcc
AaBbCc
AaBbcc AaBbCc AABbCc AABBCc AABBCC
EPISTASIS

a gene at one locus alters the phenotypic expression of a
gene at a second locus

Ex. Coat color of mice
One gene determines the pigment color (with alleles B for black and b
for brown)
 The other gene (with alleles C for pigment color and c for no pigment
color ) determines whether the pigment will be deposited in the hair

LE 14-11
BbCc
BbCc
Sperm
1
1
1
1
1
4
BC
1
4
bC
1
4
1
Bc
4
bc
4
BC
BBCC
BbCC
BBCc
BbCc
4
bC
BbCC
bbCC
BbCc
bbCc
4
Bc
BBCc
BbCc
BBcc
Bbcc
4
bc
BbCc
bbCc
Bbcc
bbcc
9
16
3
16
4
16
NATURE AND NURTURE:
THE ENVIRONMENTAL IMPACT ON PHENOTYPE

phenotype for a character depends on environment as
well as genotype

Ex. hydrangea flowers
same genotype range from blue-violet to pink
 depends on soil acidity

HUMAN INHERITANCE

Humans are not good subjects for genetic research
generation time is too long
 parents produce relatively few offspring
 breeding experiments are unacceptable

Personal Pedigree
PEDIGREE ANALYSIS

Pedigree


family tree that describes the interrelationships of parents
and children across generations
Can be used to make predictions about future offspring
LE 14-14A
Ww
ww
ww
Ww ww ww Ww
WW
or
Ww
Ww
Ww
ww
Dominant trait (widow’s peak)
Second generation
(parents plus aunts
and uncles)
Third
generation
(two sisters)
ww
Widow’s peak
First generation
(grandparents)
No widow’s peak
LE 14-14B
First generation
(grandparents)
Second generation
(parents plus aunts
and uncles)
Ff
FF or Ff ff
Third
generation
(two sisters)
Attached earlobe
Recessive trait (attached earlobe)
Ff
ff
ff
Ff
Ff
ff
FF
or
Ff
Ff
ff
Free earlobe
DOWN’S SYNDROME
 Occurs
in 1/800-1,000 births
 Caused by nondisjunction
 Trisomy 21 = three copies of chromosome 21
Frequency of Down Syndrome
Per Maternal Age
Frequency of Fetuses with Down
Syndrome to Normal Fetuses
at 16 weeks of pregnancy
Frequency of Live Births of
Babies with Down Syndrome
to Normal Births
15 - 19
----
1 / 1250
20 - 24
----
1 / 1400
25 - 29
----
1 / 1100
30 - 31
----
1 / 900
32
----
1 / 750
33
1 / 420
1 / 625
34
1 / 325
1 / 500
35
1 / 250
1 / 350
36
1 / 200
1 / 275
37
1 / 150
1 / 225
38
1 / 120
1 / 175
39
1 / 100
1 / 140
40
1 / 75
1 / 100
41
1 / 60
1 / 85
42
1 / 45
1 / 65
43
1 / 35
1 / 50
44
1 / 30
1 / 40
45 and older
1 / 20
1 / 25
Age (years)
DOWN’S SYNDROME
 The
image shows a karyotype of a person
with Down’s Syndrome, Trisomy 21
SEX CHROMOSOME DISORDERS

Turner’s Syndrome (XO)

underdeveloped ovaries, short stature, webbed
neck, and broad chest. Individuals are sterile, and
lack expected secondary sexual characteristics.
Mental retardation typically not evident.
SEX CHROMOSOME DISORDERS

Klinefelter’s Syndrome (XXY)


Nondisjunction in males
Some development of breast tissue, little body hair
is present; typically tall, with or without evidence of
mental retardation. Males with XXXY, XXXXY, and
XXXXXY karyotypes have a more severe
presentation, and mental retardation is expected.
RECESSIVELY INHERITED DISORDERS
Only expressed in individuals that are homozygous
recessive
 Carriers


heterozygous individuals


carry the recessive allele
phenotypically normal
Albinism
CYSTIC FIBROSIS

most common lethal genetic disease in the



1/2,500 people of European descent
results in defective or absent chloride transport channels
in plasma membranes
Symptoms:


mucus buildup in some internal organs
abnormal absorption of nutrients in the small intestine
SICKLE-CELL DISEASE

1/400 African-Americans

Incompletely recessive


caused by the substitution of a single amino acid in the
hemoglobin protein in red blood cells
Symptoms:
physical weakness
 Pain
 organ damage
 even paralysis

Sickle Cell Anemia
DOMINANTLY INHERITED DISORDERS

Achondroplasia
form of dwarfism
 lethal when
homozygous for the
dominant allele

Achondroplasia

Huntington’s disease


degenerative disease of the nervous system
no obvious phenotypic effects until about 35 to 40 years
of age
SOME AUTOSOMAL DISORDERS IN HUMANS
Type of Disorder
Disorder
Major Symptoms
Disorders caused
by recessive
alleles
Albinism
Lack of pigment in hair, skin, and
eyes
Cystic Fibrosis
Excess mucus in lungs, digestive
tract, liver; increased susceptibility to
infections; death in childhood unless
treated
Phenylketonuria
Accumulation in brain cells; lack of
normal pigment; mental retardation
Tay-Sachs Disease
Lipid accumulation in brain cells;
mental deficiency; blindness; death in
early childhood
Disorders Caused
by dominant
alleles
Achondroplasia
Dwarfism (one form)
Huntington’s Disease
Mental deterioration and uncontrolled
movements; appears in middle age
Disorders caused
by codominant
Sickle Cell Anemia
Sickled red blood cells; damage to
many tissues
alleles
MULTIFACTORIAL DISORDERS
Genetic factors
 Environmental factors


Ex. Cancer, heart disease
GENETIC TESTING AND COUNSELING

Genetic counselors can provide information to
prospective parents concerned about a family history for
a specific disease
COUNSELING BASED ON MENDELIAN GENETICS AND
PROBABILITY RULES

Using family histories, genetic counselors help couples
determine the odds that their children will have genetic
disorders
TESTS FOR IDENTIFYING CARRIERS

For a growing number of diseases, tests are available
that identify carriers and help define the odds more
accurately
FETAL TESTING

Amniocentesis


chorionic villus sampling (CVS)


liquid that bathes the fetus is removed and tested
sample of the placenta is removed and tested
ultrasound and fetoscopy

allow fetal health to be assessed visually in utero
LE 14-17A
Amniocentesis
Amniotic
fluid
withdrawn
Fetus
A sample of
amniotic fluid can
be taken starting at
the 14th to 16th
week of pregnancy.
Centrifugation
Placenta
Uterus
Cervix
Fluid
Fetal
cells
Biochemical tests can be
performed immediately on
the amniotic fluid or later
on the cultured cells.
Fetal cells must be cultured
for several weeks to obtain
sufficient numbers for
karyotyping.
Biochemical
tests
Several
weeks
Karyotyping
LE 14-17B
Chorionic villus sampling (CVS)
A sample of chorionic villus
tissue can be taken as early
as the 8th to 10th week of
pregnancy.
Fetus
Suction tube
inserted through
cervix
Placenta Chorionic villi
Fetal
cells
Biochemical
tests
Several
hours
Karyotyping
Karyotyping and biochemical
tests can be performed on
the fetal cells immediately,
providing results within a day
or so.
NEWBORN SCREENING

Tested at birth

Ex. Phenylketonuria (PKU)