Transcript CHAPTER 11

Patterns of Inheritance
The science of genetics has ancient roots
 (400 BCE) An early explanation for inheritance
suggested that particles called pangenes came from all
parts of the organism and were incorporated into eggs or
sperm to be passed to offspring.
 (1800s)The idea that hereditary materials mix in
forming offspring, called the blending hypothesis,
was suggested but later rejected because it did not
explain how traits that disappear in one generation
can reappear in later generations.
Experimental genetics began in an abbey
garden (mid 1800s)
 Heredity is the transmission of traits from one
generation to the next.
 Genetics is the scientific study of heredity.
 Gregor Mendel (1860s)
– Father of modern genetics
– Experimented with garden peas
– Found patterns to inheritance of traits
– Inheritance could be predicted
Experimental genetics began in
an abbey garden
Mendel used pea plants.
Why are they a good organism for genetic studies?
Mendel found evidence to support that
– parents pass on to their offspring discrete “heritable factors”
and
– the heritable factors (today called genes), retain their
individuality generation after generation.
White
1 Removal of
stamens
Stamens
Carpel
Parents
(P)
2 Transfer
Purple of pollen
3 Carpel matures
into pea pod
4 Seeds from
pod planted
Offspring
(F1)
Experimental genetics began in
an abbey garden
 True-breeding varieties result when self-fertilization
produces offspring all identical to the parent.
 The offspring of two different varieties are hybrids.
 The cross-fertilization is a hybridization, or genetic
cross.
 Parental plants are the P generation.
 Offspring/Filial = F1 generation.
 A cross of F1 plants produces an F2 generation.
Genetics Vocabulary
 Gene-segment of DNA that had the coding for a
particular trait.
 Allele- one of several varieties of a gene.
 Locus- location on a chromosome where the
gene is located
 Testcross-mating between an individual of
unknown genotype and a known(homozygous
recessive individual)
 … dominant, recessive, homozygous,
heterozygous, phenotype, genotype
The Experiment
P generation
(true-breeding
parents)

Purple
flowers
F1 generation
White
flowers
All plants have
purple flowers
Fertilization
among F1 plants
(F1  F1)
F2 generation
3
4
1 of plants
of plants
4
have purple flowers have white flowers
Traits
Character
Dominant
Recessive
Purple
White
Axial
Terminal
Yellow
Green
Round
Wrinkled
Inflated
Constricted
Green
Yellow
Tall
Dwarf
Flower color
The seven pea
characteristics
studied by
Mendel
Flower position
Seed color
Seed shape
Pod shape
Pod color
Stem length
Mendel’s Discoveries
• Principle of Dominance
• Law (Principle) of Segregation
• Law (Principle) of Independent Assortment
Mendel’s
Theory of Segregation
An individual inherits a unit
of information (allele)
about a trait from each
parent
During gamete formation, the
alleles segregate from
each other (homologous
chromosomes separate
from each other, one allele
of each gene from each
parent is passed to
offspring)
Alleles
• Different molecular forms of a gene
• Arise by mutation
Homologous chromosomes bear the alleles for
each character
 A locus (plural, loci) is the specific location of a
gene along a chromosome.
 For a pair of homologous chromosomes, alleles of
a gene reside at the same locus.
The law of independent assortment is revealed
by tracking two characters at once
 A dihybrid cross is a mating of parental varieties
that differ in two characters.
 Mendel performed the following dihybrid cross with
the following results:
– P generation: round yellow seeds  wrinkled green seeds
– F1 generation: all plants with round yellow seeds
– F2 generation:
– 9/16 had round yellow seeds
– 3/16 had wrinkled yellow seeds
– 3/16 had round green seeds
– 1/16 had wrinkled green seeds
RrYy
F1 generation
Sperm
1
4
1
4
RY
1
4
rY
RY
RRYY
RrYY
Eggs
1
4
1
4
1
4
rY
RrYY
rrYY
1
4
Ry
RRYy
RrYy
1
4
ry
RrYy
rrYy
Ry
RRYy
RrYy
RRyy
Rryy
RrYy
rrYy
Rryy
rryy
ry
9
16
Yellow
round
3
16
Green
round
3
16
Yellow
wrinkled
1
16
Green
wrinkled
The hypothesis of independent assortment
Actual results; hypothesis supported
F1 generation
R
r
All yellow round seeds
(RrYy)
y
Y
r
R
Y
R
y
Metaphase I
of meiosis
r
R
Y
y
r
r
R
Y
y
Anaphase I
Y
y
Metaphase II
R
r
r
R
Y
y
Y
y
Gametes
Y
Y
R
R
1
4
RY
y
y
r
r
1
4
Y
Y
r
r
ry
F2 generation 9
Fertilization
:3
:3
:1
1
4
rY
y
y
R
R
1
4
Ry
Mendel’s
Law of Independent Assortment
Mendel concluded that the two “units” for a trait were to be
assorted into gametes independently of any other “units”
for the other traits
Members of each pair of homologous chromosomes are
sorted into gametes at random during meiosis
1/4 AB
Allelic
combinations
possible in
gametes
1/4 ab
1/4 Ab
1/4 aB
Genetics is more complicated
than Mendel’s experiments lead
him to understand….
Inheritance Patterns
Dominance Relationships:
– Complete dominance (Simple dominance)
– Incomplete dominance
– Codominance
Multiple Alleles
X-linked
Gene Interactions:
- Pleiotrophy
- Polygenic
Incomplete
DominanceExample snapdragons
X
Incomplete
Homozygous
Homozygous
parent
parent
Dominance
All F1 are
heterozygous
Another example of incomplete
dominance in humans is
hypercholesterolemia,
dangerously high levels of
cholesterol occur in the blood heterozygotes have
intermediately high cholesterol
levels.
X
F2 shows three phenotypes in 1:2:1 ratio
Many genes have more than two alleles
in the population (multiple alleles)
Human ABO blood group phenotypes involve three alleles for a single gene.
Blood
Group
(Phenotype)
Genotypes
Carbohydrates Present
on Red Blood Cells
Carbohydrate A
A
IAIA
or
I Ai
Carbohydrate B
B
IBIB
or
IBi
AB
IAIB
Antibodies
Present
in Blood
Reaction When Blood from Groups Below Is Mixed
with Antibodies from Groups at Left
O
A
B
AB
Anti-B
Anti-A
Carbohydrate A
and
Carbohydrate B
None
Anti-A
O
ii
Neither
Anti-B
No reaction
Clumping reaction
Chromosomes determine sex in many species
 Many animals have a pair of sex chromosomes,
• designated X and Y,
• that determine an individual’s sex.
 In mammals,
• males have XY sex chromosomes,
• females have XX sex chromosomes,
• the Y chromosome has the SRY gene for the development of
testes, and
• an absence of the Y allows ovaries to develop.
Chromosomes determine sex in many species
 Some organisms lack sex chromosomes altogether.
 In bees, sex is determined by chromosome number.
• Females are diploid.
• Males are haploid.
 In some species sex is determined by the temperature at
which the eggs are incubated. (some crocodiles and
turtles)
Sex-linked genes exhibit a unique
pattern of inheritance
 Sex-linked genes are located on either of the sex
chromosomes.
 The X chromosome carries many genes unrelated to
sex. These genes are called X-linked.
Ex- The inheritance of white eye color in the fruit fly
illustrates an X-linked recessive trait.
Sex-linked traits
Examples- red-green colorblindness, Hemophilia, Duchenne’s
muscular dystrophy
Cross a colorblind male with a female that is a carrier for the trait.
_______x _______
X- Inactiviation
Nondisjunction
A single character may be influenced
by many genes
 Many characteristics result from polygenic
inheritance, in which a single phenotypic character
results from the additive effects of two or more genes.
A single gene may affect many
phenotypic characters
 Pleiotropy occurs when one gene influences many
characteristics.
 Sickle-cell disease is a human example of pleiotropy. This
disease
• affects the type of hemoglobin produced and the shape of red
blood cells and
• causes anemia and organ damage.
• Sickle-cell and nonsickle alleles are codominant.
• Carriers of sickle-cell disease are resistant to malaria.
Non-Nuclear Inheritance
The environment affects many characters
 Many characters result from a combination of heredity
and the environment. For example,
• skin color is affected by exposure to sunlight,
• susceptibility to diseases, such as cancer, has hereditary and
environmental components, and
• identical twins show some differences.
 Only genetic influences are inherited.
 Nature v. Nuture ( Genes v. Environment)
Environmental Effects on Plant
Phenotype
• Hydrangea macrophylla
• Action of gene responsible for floral color is
influenced by soil acidity
• Flower color ranges from pink
to blue
Environmental Effects on Plant Phenotype
(Yarrow, Achillea millefolium)
Temperature Effects
on Phenotype
• Rabbit is homozygous for an
allele that specifies a heatsensitive version of an enzyme
in melanin-producing pathway
• Melanin is produced in cooler
areas of body
Genetic traits in humans can be tracked through
family pedigrees
 In a simple dominant-recessive inheritance ; one
allele is dominant and the other is recessive
 P= purple p= white
Cross a heterozygous purple flowering plant with a
plant that had white flowers.
 Wild-type traits, those prevailing in nature, are
not necessarily specified by dominant alleles.
Dominant Traits
Examples of
single-gene
inherited
traits in
humans
Recessive Traits
Freckles
No freckles
Widow’s peak
Straight hairline
Free earlobe
Attached earlobe
Genetic traits in humans can be tracked through
family pedigrees
A pedigree
Many inherited disorders in humans are controlled
by a single gene
 The most common genetic disease in the United
States is cystic fibrosis (CF), resulting in excessive
thick mucus secretions. The CF allele is
– Recessive
– Carried by about 1 in 31 Americans.
– Shortened life expectancy (40-50 years)
 Dominant human disorders include
– Achondroplasia, resulting in dwarfism, and
– Huntington’s disease, a degenerative disorder of the
nervous system.
New technologies can provide insight into one’s
genetic legacy
 New technologies offer ways to obtain genetic
information
– before conception,
– during pregnancy, and
– after birth.
 Genetic testing can identify potential parents who
are heterozygous carriers for certain diseases.
New technologies can provide insight
into THE OFFSPRING’s genetic legacy
 Several technologies can be used for detecting
genetic conditions in a fetus.
– Amniocentesis extracts samples of amniotic fluid
containing fetal cells and permits
– karyotyping and
– biochemical tests on cultured fetal cells to detect other
conditions, such as Tay-Sachs disease.
– Chorionic villus sampling removes a sample of
chorionic villus tissue from the placenta and permits
similar karyotyping and biochemical tests.
Amniocentesis
Amniotic fluid
extracted
Ultrasound
transducer
Fetus
Chorionic Villus Sampling (CVS)
Tissue extracted
from the
Ultrasound
chorionic villi
transducer
Fetus
Placenta
Chorionic
villi
Placenta
Uterus
Cervix
Cervix
Uterus
Centrifugation
Amniotic fluid
Fetal cells
Several
hours
Cultured
cells
Several
weeks
Several
weeks
Karyotyping
Biochemical
and genetics
tests
Fetal cells
Several
hours
Several
hours
New technologies can provide insight into THE
OFFSPRING’s genetic legacy
 Blood tests on the mother at 14–20 weeks of
pregnancy can help identify fetuses at risk for
certain birth defects.
 Fetal imaging, the most common procedure is
ultrasound imaging, uses sound waves to
produce a picture of the fetus.
 Newborn screening can detect diseases that can
be prevented by special care and precautions.
New technologies can provide insight into one’s
genetic legacy
 New technologies raise ethical considerations that
include
– the confidentiality and potential use of results of
genetic testing,
– time and financial costs, and
– determining what, if anything, should be done as a
result of the testing.
Genes on the same chromosome
tend to be inherited together
 Linked genes, which
– are located close together on the same chromosome and
– tend to be inherited together.
Crossing over produces
new combinations of alleles
 Crossing over between homologous
chromosomes produces new combinations of
alleles in gametes / new chromosomes!.
Geneticists use crossover data to map genes
 When examining recombinant frequency (crossing
over), the greater the distance between two genes
on a chromosome, the more points there are
between them where crossing over can occur.
 Recombination frequencies can thus be used to
map the relative position of genes on
chromosomes.
•
Queen
Victoria
Albert
Alice
Louis
Alexandra
Czar
Nicholas II
of Russia
Alexis
Female Male
Hemophilia
Carrier
Normal
Most compounds are synthesized by a sequence
of metabolic steps involving many enzymes. If
the enzymes (proteins) are not present or altered
in some way (as in- the instructions coded for in
the gene are altered) the compound will be
affected.
In addition to simple gene inheritance;
environmental factors, interactions between
genes, mutations, even nutrition can have an
effect on how a gene is expressed. This makes
genetics a difficult and complex field of study.
You should now be able to
1. Define and distinguish between these terms: the P
generation, the F1 generation, and the F2 generation.
2. Define and distinguish between the following pairs of
terms: homozygous and heterozygous; dominant
allele and recessive allele; genotype and phenotype.
Also, define a monohybrid cross and a Punnett
square.
You should now be able to
3. Name and explain Mendel’s laws.
4. Describe the structure of homologous
chromosomes.
5. Explain how family pedigrees can help determine
the inheritance of many human traits.
6. Explain how recessive and dominant disorders are
inherited. (be able to use a Punnett square)
7. Describe the types and use of fetal testing:
amniocentesis, chorionic villus sampling, and
ultrasound imaging.
You should now be able to
8. Describe the inheritance patterns of incomplete
dominance, multiple alleles, codominance,
pleiotropy, and polygenic inheritance. Be able to
do crossed involving incomplete dominance and
ABO blood group.
You should now be able to
10. Define the term: linked genes.
11. Explain how sex is genetically determined in humans
and the significance of the SRY gene.
12. Describe patterns of sex-linked inheritance and
examples of sex-linked disorders. Be able to do a
Punnett Square.