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Chapter 10 Genetics
Heritable Traits
Heredity is the tendency for traits (features) to
be passed from parent to offspring
Gregor Mendel
• Gregor Mendel
performed
experiments with
garden peas
• Peas are ideally suited
to the study of
heredity
Purple
White
Self-fertilize
Mendel’s Experimental Design
 First he established true-breeding
varieties and named these pure lines the P
generation
Mendel’s Experimental Design
 Next he crossed two different varieties
and named the resulting offspring the F1
generation
The trait expressed
in the F1 generation
known as the
dominant trait
Mendel’s Experimental Design
 Finally he allowed the plants from the F1
generation to self-fertilize and named the
resulting offspring the F2 generation
The trait not
expressed in the F1
generation, but
reemerges in the F2
generation is known as
the recessive trait
Seven Characteristics Studied
Patterns Emerged
 Mendel found a consistent
proportions of expressed
traits in the F2 generation
 3/4 expressed the
dominant trait while 1/4
expressed the recessive
trait
 the dominant : recessive
ratio 3:1
F3 Generation
• 1/4 of plants from the F2
that were dominant were
true-breeding dominant
• 1/2 of plants showed
both traits
• 1/4 of the plants from
the F2 that were
recessive were truebreeding recessive
True Ratio
• He determined that the 3:1 ratio that he observed in
the F2 generation was, in fact, a disguised 1:2:1 ratio
Mendel Proposes Theories
• Hypothesis 1
 parents do not transmit traits directly to their
offspring
 parents transmit information about the trait in the
form of genes
• Hypothesis 2
 each parent contains two copies of a gene
 the two copies of the gene may or may not be the
same
Mendel Proposes Theories
• Hypothesis 3
 alternative forms of a gene lead to alternative
traits
 the alternative forms of a gene are known as
alleles
 An individual with 2 identical alleles is said to be
homozygous (homo = same)
 An individual with 2 different alleles is said to be
heterozygous (hetero = different)
Genotype vs Phenotype
• Genotype indicates the alleles an individual possesses
 a dominant trait is expressed as a capital letter
eg. P = purple flowers
 a recessive trait is expressed as a lower case letter
eg. p = white flowers
• Phenotype is the physical appearance of an individual
depending on their genotype
 a homozygous dominant individual has genotype PP, and
phenotype purple flowers
 a homozygous recessive individual has genotype pp, and
phenotype white flowers
 a heterozygous individual has genotype Pp, and phenotype
purple flowers
Genotype vs Phenotype
Genotype
PP
Pp
Pp
pp
Phenotype
Homozygous
Dominant
Heterozygous
Homozygous
Recessive
Punnett Square
• The results from a cross between a true-breeding,
white-flowered plant (pp) and a true breeding, purpleflowered plant (PP) can be visualized with a Punnett
square
• Lists the possible gametes from one individual with
the possible gametes from the other individual on the
opposite side
• The genotypes of potential offspring are represented
within the square
A Punnett square analysis
Punnett Square of Flower Color
Testcross
• Mendel devised the testcross with a
homozygous recessive individual in order to
determine the genotype of unknown
individuals in the F2 generation
Homozygous
dominant, all
offspring
dominant
phenotype
Heterozygous,
½ offspring
dominant
phenotype,
½ recessive
phenotype
Mendel’s Laws
• Mendel’s First Law: Segregation
 the two alleles of a trait separate from each other
during the formation of gametes, so that half of
the gametes will carry one copy and half will carry
the other copy
Dihybrid Cross
• Investigates the inheritance pattern
for 2 different characteristics
 when crossing individuals who are truebreeding for two different characters, the
F1 individual that results is a dihybrid
 after the dihybrid individuals self-fertilize,
there are 16 possible genotypes of
offspring
Analysis of a
dihybrid cross
Independent Assortment
• Mendel’s Second
Law: Independent
Assortment
 genes located on
different
chromosomes are
inherited
independently of
one another
Why Some Traits Don’t Show
Mendelian Inheritance
• Continuous variation
 Characteristics can show a
gradation in phenotypes
 This type of inheritance is
called polygenic
 The gradation in
phenotypes is called
continuous variation
Why Some Traits Don’t Show
Mendelian Inheritance
• Pleiotropic effects
 an allele that has more than one effect on a
phenotype is considered pleiotropic
 these effects are characteristic of many
inherited disorders, such as cystic fibrosis
and sickle-cell disease
Pleiotropic effects of the cystic
fibrosis gene, cf
Why Some Traits Don’t Show
Mendelian Inheritance
• Environmental effects
 the degree to which
many alleles are
expressed depends on
the environment
 some alleles are heatsensitive
 Arctic foxes only
produce fur pigment
when temperatures are
warm
Why Some Traits Don’t Show
Mendelian Inheritance
• Incomplete dominance
 not all alternative alleles are either fully
dominant or fully recessive
 incomplete dominance produces a
heterozygous phenotype that is
intermediate between those of the parents
 each allele is represented, but essentially
only at 50%
Incomplete dominance
Why Some Traits Don’t Show
Mendelian Inheritance
• Codominance
 a gene may have more than two alleles in a
population
• both alleles are expressed equally
• these alleles are said to be codominant
Codominance and Blood Type
• The gene that determines ABO blood type in humans
exhibits more than one dominant allele
 the gene that encodes blood type, designated I,
has three alleles: IA,IB, and i
 different combinations of the three alleles
produce four different phenotypes, or blood types
(A, B, AB, and O)
 both IA and IB are dominant over i and also
codominant
Multiple alleles controlling the
ABO blood groups
Chromosomes Are the Vehicles
• The chromosomal theory of inheritance –
chromosomes carry the information for traits (genes)
• Supported by evidence:
 similar chromosomes pair with one another during
meiosis
 reproduction involves the initial union of only eggs
and sperm
• each gamete contains only copy of the genetic
information and combines to form a diploid cell
Linkage
• Linkage is defined as the tendency of close-together
genes to segregate together
 the farther apart two genes are from each other
on the same chromosome, the more likely crossing
over is to occur
 this would lead to independent segregation
 the closer that two genes are to each other on the
same chromosome, the less likely that crossing
over will occur between them
Linkage and Crossing Over
These two chromosomes could
segregate independently into 2
separate cells as is, or….
During meiosis I, crossing over
could occur, separating far genes
from on another, but having no
effect on genes close to each other
These two hybrid chromosomes could
then segregate independently into 2
separate cells (gametes)
Human Chromosomes
• Each human somatic cell normally has 46
chromosomes, or 23 pairs
 22 of the 23 pairs are perfectly matched in both
males and females and are called autosomes
 1 pair are the sex chromosomes
• females are designated XX while males are designated XY
• the genes on the Y chromosome determine “maleness”
• a female passes on an X to offspring, males may pass X
(50%) or Y (50%)
Nondisjunction
• Sometimes errors occur during meiosis
• Nondisjunction is the failure of
chromosomes to separate correctly
during either meiosis I or meiosis II
 this leads to aneuploidy, an abnormal
number of chromosomes
 most of these abnormalities cause a failure
to develop or an early death before
adulthood
 in contrast individuals with an extra copy of
chromosome 21
Nondisjunction
Nondisjunction
Sperm
Ovum (egg)
Sperm
meiosis
0
M P
Monosomy
M P
Sperm
Trisomy
OR
Ovum
Trisomy 21 or Down Syndrome
1
2
6
7
8
13
14
15
19
(a)
20
3
4
9
16
21
5
10
11
17
22
12
18
X Y
(b)
Nondisjunction of Sex
Chromosomes
• Nondisjunction of the X chromosome
creates three possible viable conditions
• XXX female (triple X)
– usually taller than average but other symptoms vary
• XXY male (Klinefelter syndrome)
– sterile male with many female characteristics and
diminished mental capacity
• XO female (Turner syndrome)
– sterile female with webbed neck and diminished stature
Nondisjunction of the X
chromosome
Nondisjunction
Sperm
Ovum
X
Ova (eggs)
meiosis
M P
Turner
X
XX
M P
Sperm
XX
OR
XX
Y
Klinefelter
Ovum
XXY
Nondisjunction of Sperm
• Nondisjunction of the Y chromosome
also occurs
 in such cases, YY gametes are formed,
leading to XYY males
 these males are fertile and may be
aggressive
Studying Pedigrees
• To study human heredity, scientists
examine crosses that have already been
made
 they identify which relatives exhibit a trait
by looking at family trees or pedigrees
 often one can determine whether a trait is
sex-linked or autosomal and whether the
trait’s phenotype is dominant or recessive
A general pedigree
A pedigree of albinism
Generation
I
II
III
IV
KEY:
V
Male
Female
Affected
Carrier
1. Is the trait sex-linked or autosomal?
2. Is the trait dominant or recessive?
3. Is the trait by a single gene or several?
Unaffected
Pedigree of color blindness
Generation
I
II
III
IV
1. Is the trait sex-linked or autosomal?
2. Is the trait dominant or recessive?
3. Is the trait by a single gene or several?
Mutations
• Accidental changes in genes are called
mutations
 mutations occur only rarely and almost
always result in recessive alleles
 in some cases, produce harmful effects
called genetic disorders
Some Important Genetic Disorders
Hemophilia: Sex-linked Trait
• Hemophilia is a recessive, blood-clotting
disorder (do not make the protein
clotting factor VIII)
• This type of hemophilia is sex-linked
Sex-linked Genetic Disorder
Sickle-cell Trait: Recessive Trait
• Affected individuals are homozygous recessive and
carry two copies of mutated gene that produces a
defective version of hemoglobin
• the hemoglobin sticks together and forms rodlike structures that produce a stiff red blood
cell with a sickle shape
• the cells cannot move through the blood vessels
easily and tend to clot
• incomplete dominance occurs, affected
individual makes 50% of defective gene, not
enough to cause disease, but resistant to
malaria, prevalent in African-decent
Sickle-cell disease
The sickle-cell allele confers
resistance to malaria
Huntington’s Disease: Dominant
Trait
 it causes progressive deterioration of brain
cells
 every individual who carries the allele
expresses the disorder but most persons
do not know they are affected until they
are more than 30 years old
Huntington’s disease is a
dominant genetic disorder
Genetic Screening
• Genetic screening can allow prenatal
diagnosis of high-risk pregnancies
 amniocentesis is when amniotic fluid is
sampled and isolated fetal cells are then
grown in culture and analyzed
• 1 in 600 tests may result in miscarriage
 chorionic villus sampling is when fetal cells
from the chorion in the placenta are
removed for analysis
• 1 in 100 tests may result in miscarriage
Amniocentesis
Inquiry & Analysis
•
Is woolly hair sex-linked or
autosomal? Is woolly hair
dominant or recessive?
Why Woolly Hair Runs in Families