View
218
Download
0
Category
Preview:
Citation preview
8/17/2019 Lecture06 Patterns Inheritance Sv.ppt
1/23
1
Introduction to Genetics
Lecture 6
Outline
• How are traits passed from parents to offspring?
• Testing hypotheses
• Mendel’s experiments
• Mendel’s principles
• More complex patterns of inheritance
Questions Mendel Was Trying to
Answer?
What are the basic patterns in the transmission of traits
from parents to offspring?
H1: blending inheritance
H2: inheritance of acquired characteristics
8/17/2019 Lecture06 Patterns Inheritance Sv.ppt
2/23
2
Concept of “blending of inherited traits”
X
Progeny inter-
mediate in size
Once trait is
“blended,”
can’t get the
original types
back
offspring: blended combination of parents
Mechanism to explain
evolution:
Jean-Baptiste de Lamarck
(1744-1829)
The inheritance of
acquired characters
Gregor Mendel
• Heredity: the transmission of traits from parents tooffspring
(1822-1884)
•
Question: why offspringresemble their parents and howtransmission of traits occurs
• studied heredity in a scientificway
• “particulate” hypothesis ofinheritance
8/17/2019 Lecture06 Patterns Inheritance Sv.ppt
3/23
3
TerminologySELF-POLLINATION:
pollen from stamen fertilizes egg in carpel of same flower
CROSS POLLINATION:
fertilization of one plant by pollen from another plant
Terminology
character: heritable feature that varies among individuals
trait: a variant for a character
gene: hereditary determinant for a trait
allele: different version of a gene
genotype: the alleles found in an individual
phenotype: physical appearance that expresses the genotype
homozygous: an organism with a pair of identical alleles for
a character
heterozygous: an organism with 2 different alleles for a
character
Mendel’s Experiments
• true-breeding: individuals that produce offspring
like themselves over many generations of self-
pollination
• hybridization: created by mating two true-
breeding varieties
P – parental generation (true-breeding parents)
F1 – first generation
F2 – second generation
8/17/2019 Lecture06 Patterns Inheritance Sv.ppt
4/23
4
Peas Can Be Self-Pollinated or Cross-Pollinated
Self-pollination
Female organ
(receives pollen)
Eggs
Male organs(produce pollengrains, whichproduce malegametes)
Cross-pollination
CROSS-POLLINATION
3. Transfer pollen to thefemale organs of the
individual whose maleorgans have beenremoved.
2. Collect pollen from adifferent individual.
1. Remove male organsfrom one individual.
SELF-
POLLINATION
true-breeding white X true-breeding red hybrid
Mendel Studied Seven Characters That Were Variable in
Pea PlantsTrait Phenotypes
Seed shape
Seed color
Pod shape
Pod color
Flower color
Flower andpod position
Stem length
Tall Dwarf
Terminal (at tip)Axial (on stem)
Purple White
YellowGreen
Inflated Constricted
Green Yellow
Round Wrinkled
Character /Trait
Cross Pollination
8/17/2019 Lecture06 Patterns Inheritance Sv.ppt
5/23
5
Traits in First and Second Generations
Variations in Inherited Characters
Mendel’s Law of Segregation
8/17/2019 Lecture06 Patterns Inheritance Sv.ppt
6/23
6
Phenotype vs Genotype
Does the type of gamete affect the inheritance of seed shape?
Reciprocal Cross
What Mendel Observed
• For all seven pairs of contrasting traits:
1. The F1 generation showed only one of the two parental traits
• dominant trait (trait expressed)
• recessive trait (not expressed)
•
In genetics, the terms dominant and recessive identify onlywhich phenotype is observed in individuals carrying two
different genetic traits
2. The F2 generation showed an ~ 3:1 ratio of the
dominant:recessive parental traits
8/17/2019 Lecture06 Patterns Inheritance Sv.ppt
7/23
7
Mendel Used the Scientific Method
Mendel repeated his experiments for all the traits of pea plants
Mendel repeated his experiments for all the traits of pea plants
Mendel’s Principle of Segregation
Gametes
p P
P
p
Gametes
pp Pp
Pp PP
Explaining the 3:1 ratioof phenotypes in the F2 generation
– 2 alleles of each gene
must segregate intodifferent gamete cellsduring gamete (eggand sperm) formation,then come backtogether when an eggis fertilized by a spermto form a zygote
8/17/2019 Lecture06 Patterns Inheritance Sv.ppt
8/23
8
A Cross Between 2 Homozygotes and 2 Heterozygotes
Probabilityis 100%
50%
25%
Probabilityis 25%
Mendel’s Hypothesis
• Particulate Inheritance -
heritable factors (genes)
retain their individuality
from one generation to the
next
contradicts blending
inheritance and
contradicts inheritance of
acquired characteristicsGenes on homologous chromosomes
We now know that alleles are distributed to gametes during me iosis
Mendel’s Model
1. peas have two versions of each gene – different
versions of genes are called alleles (phenotype, genotype)
2. alleles do not blend together
3. each gamete contains one allele of each gene
4. males and females contribute equally to the genotype
of their offspring (homozygous, heterozygous)
5. Some alleles are dominant to other alleles
8/17/2019 Lecture06 Patterns Inheritance Sv.ppt
9/23
9
Testcross
• used to determine an
individual’s actual geneticcomposition
• A purple-flowered plant can behomozygous (PP) orheterozygous (Pp)
in a TEST CROSS, the knownindividual is a ___________
Dihybrid Cross: A Cross between Pure
Lines for Two Traits
YyRr x YyRr
Chromosome Theory of Inheritance
• genes are located on chromosomes
• links meiosis with patterns of inheritance
1903: Walter Sutton and Theodor Boveri carefully observed meiosis;
noted that chromosome number was reduced by half and
hypothesized that chromosomes are composed of Mendel’s
hereditary determinants, which we now call genes
- meiosis explains Mendel's principle of segregation and independent
assortment
8/17/2019 Lecture06 Patterns Inheritance Sv.ppt
10/23
10
Separation of alleles during anaphase of meiosis 1 is responsible for
Mendel’s Principle of Segregation
Meiosis IAlleles segregate
Recessive allelefor seed shape
G a m e t e s
Chromosomes replicate
Rr parent
Dominant allelefor seed shape
Meiosis II
Principle of segregation: Each gamete carries onlyone allele for seed shape, because the alleles havesegregated during meiosis.
Meiosis and the Principle of Independent Assortment
Meiosis I
Replicated chromosomesprior to meiosis
G a m e t e s
Alleles for seed shape
Meiosis II
Principle of independent assortment: The genes for seed shape and seed colorassort independently, because they are located on different chromosomes.
Meiosis II
Meiosis I
Alleles for seed color
1/4 RY 1/4 ry 1/4 Ry 1/4 rY
Chromosomes can line up intwo ways during meiosis I
R
R y y r
Y Y
R R
R R
R R
r r
r r
r r
r
Y Y
Y Y
Y Y
y y
y y
y y y
y
y y
y y
R R
R R
R R
r r
r r
r r
Y Y
Y Y
Y Y
Law of Probability
Probability of r being present
in both gametes (homozygous)
at fertilization is
! (egg has r) x ! (sperm has r)
! x ! = " for rr in F2! x ! = " for RR in F2
8/17/2019 Lecture06 Patterns Inheritance Sv.ppt
11/23
11
Law of Probability
Heterozygous example
F1 gametes can combine
to produce Rr offspring in
2 mutually exclusive ways:
dominant allele can come
from either egg or sperm,
but NOT both
-for F2 heterozygote
" + " = ! for Rr
Probability in a Dihybrid Cross
Fig. 14.8
YyRr x YyRr
Y = yellow
R = round
y = green
r = wrinkled
Probability: Dihybrid Cross
YyRr x YyRr
1)
Seed color
Punnett square: Yy x Yy
YY homozygous yellow = "
Yy heterozygous yellow = !
yy = homozygous green"
Y y
Y
y
Yy
Yy
YY
yy
8/17/2019 Lecture06 Patterns Inheritance Sv.ppt
12/23
12
Probability: Dihybrid Cross
YyRr x YyRr
1) Seed shape
Punnett square: Rr x Rr
RR = homozygous round"
Rr = heterozygous round !
rr = homozygous wrinkled"
R r
R
r
Rr
Rr
RR
rr
Probability: Dihybrid Cross
Question:
What is the probability of obtaining a pea,
yellow in color and round in shape,
homozygous for both traits? (refer to previous two
slides)
YYRR = " (YY) x " (RR) = 1/16
Check results with Fig. 14.8
Probability: Dihybrid Cross
Question:
What is the probability of obtaining a pea,
yellow in color (heterozygous) and round in
shape, (homozygous)?
YyRR = ! (Yy) x " (RR) = 1/8
Check results with Fig. 14.8
8/17/2019 Lecture06 Patterns Inheritance Sv.ppt
13/23
13
More Complex Patterns of
Inheritance
– Incomplete dominance
– Codominance
– Pleiotropic effects
– Epistasis
– Polygenic inheritance
– Environmental effects
Incomplete Dominance
the heterozygotes have an intermediate phenotype
since alleles of a gene are not always clearly
dominant or recessive
– Example:
• color in snapdragons and four o’clock flowers
Incomplete Dominance in Snapdragons
F1: appears to support blending hypothesis
8/17/2019 Lecture06 Patterns Inheritance Sv.ppt
14/23
14
Incomplete Dominance(snapdragons)
Explanation?
R allele: has functional enzyme for
producing ____ pigment
r allele: has a non-functional
enzyme
Codominance
– produces a
heterozygote
phenotype that is a
combination of that of
the two homozygotes
– Ex.
Roan color in horses
Unlike incompletedominance, both alleles
are expressed
Frequency of Dominant Alleles
Dominant alleles are not always the
more common alleles in a
population
• Polydactyly
–
8/17/2019 Lecture06 Patterns Inheritance Sv.ppt
15/23
15
Multiple Alleles
Pleiotropic Effects
• Single geneinfluences many traits
(can have multiplephenotypic effects)
– Cystic fibrosis
– Marfan syndrome
Epistasis• one gene affects the action of another gene
in many cases, the alleles at one gene mask or reduce the effectsof alleles at a different gene
E – determines if color pigment will be deposited in fur,
B gene determines how dark pigment is; b = brown, B = black
8/17/2019 Lecture06 Patterns Inheritance Sv.ppt
16/23
16
Epistasisone gene affects the action of another gene
Epistasis• Interaction between two genes where one modifies the
phenotypic expression of the other
– F1 generation: all purple
– F2 generation: ratio of 9 purple:7 white
– Mendelian genetics predicts a 9:3:3:1 ratio
–
Why is the ratio modified?
• There are two genesthat contribute to
kernel color
– B! production of
pigment
– A! deposition of
pigment
•
gene expression – to produce pigment a
plant must possess at
least one functional
copy of each gene
8/17/2019 Lecture06 Patterns Inheritance Sv.ppt
17/23
17
Genes at Different Loci Can Interact to Influence a TraitCrosses between chickens with different comb phenotypes giveodd results
Parentalgeneration
Rose comb Pea comb
All Walnut combs
F1
Walnut combs Rose combs Pea combs Single combs
F2
New combphenotype
Another newcomb phenotype
!
9 : 3 : 3 : 1
Different Genes Can Interact to Influence a Trait
Parentalgeneration
A genetic model to explain the results
F2
F1
Rose combRRpp
Pea combrrPP
All Walnut combsRrPp
rrppSingle comb
R_ppRose comb
R_P_Walnut comb
rrP_Pea comb
Two genes (R and P )interact to producecomb phenotype
Blanks in a genotypemean that either allele canbe present
!
Polygenic Inheritance
Characters in a population vary in gradations along a continuum;
referred to as quantitative characters
8/17/2019 Lecture06 Patterns Inheritance Sv.ppt
18/23
18
Human Height
•
polygenic
–
produced by actions of many
genes
– Quantitative characters
usually indicate polygenic
inheritance; an additive
effect of 2 or more genes
– each gene adds a small
amount to the value of the
phenotype
Extremes are muchrarer than the
intermediate values
Skin pigmentation
is under polygenic control
continuous variation
According to this model, 3 genes
affect color of skin
A,B,C: dark skin alleles
a, b, c: light skin alleles
Alleles have cumulative effect:
AaBbCc or AABbcc will have the
same genetic contribution to skin
darkness
Environmental influences/exposure to sun?
The Environment and Phenotype
• Norm of Reaction
8/17/2019 Lecture06 Patterns Inheritance Sv.ppt
19/23
19
Environmental Effects
•
expression of some
genes is influenced byenvironmental factors:temperature
Color resemblessnowy background
in winter
Color resemblestundra background
in summer•
Some alleles are heat-
sensitive
– Arctic foxes
Summary
Traits determined by polygenes may givepatterns of inheritance resembling concept of
“blending of traits”
Environmental variables: temperature, nutrition,
light affect gene action
Multifactorial characters: many factors, genetic
and environmental, collectively influence
phenotype
Patterns of Inheritance
Applying Mendel
s rules to humans
• Mutations cause genetic disorders
•
The inheritance of disorders is studied by looking at
pedigrees (family trees)
8/17/2019 Lecture06 Patterns Inheritance Sv.ppt
20/23
20
Pedigree analysis:
dominant allele
Pedigree analysis:
recessive allele
Pedigree of a Family with an Autosomal Recessive
Disease
Carrier male Carrier female
Affectedmale
Affectedfemale
I
II
III
IV
E a c h r o w r
e p r e s e n t s a g e n e r a t i o n
Carriers(heterozygotes)are indicatedwith half-filledsymbols
Albinism: Recessive Trait
8/17/2019 Lecture06 Patterns Inheritance Sv.ppt
21/23
21
Albinism: Recessive Trait
•
allele is rare in the
population
• both parents must
be heterozygous
carriers to have an
affected child
Sickle-Cell Anemia: Recessive Trait
Normal red blood cell Sickled red blood cell
Smooth shape allows foreasy passage through
capillaries
Irregular shapecauses blockage of
capillaries
Sickle-Cell Anemia: Recessive Trait
•
Autosomal inheritance: autosomal recessive trait in
which the protein hemoglobin is defective
8/17/2019 Lecture06 Patterns Inheritance Sv.ppt
22/23
22
Tay-Sachs Disease: Recessive Trait
Enough enzymeto prevent CNS
deterioration – affected
individuals
cannot break
down specific
lipids
– lipids
accumulate in
brain cells
– children die
when young
Absence of enzyme, hexosamidase A: without hexA, fatty substances build up on
nerve cells in body and brain progressive degeneration of central nervous
system
Affected femaleI
II
III
IV
Unaffected male
If a child shows the trait,then one of the parentsshows the trait as well
Huntington’s Disease: autosomal dominant trait that causes
progressive deterioration of brain cells: fatal
gene sequenced in 1993: Huntington allele is near the tip ofchromosome 4
Family with an Autosomal Dominant Disease
Recognizing autosomal dominant disorders:
Achondroplasia: A Dominant Trait
People without achondroplasia: homozygous for recessive allele
Recessive allele: much more prevalent than dominant allele
heterozygotes have
the dwarf phenotype
8/17/2019 Lecture06 Patterns Inheritance Sv.ppt
23/23
Phenylketonuria (PKU)
•
autosomal recessive genetic disorder• individuals lack the enzyme, phenylalanine hydroxylase,
that converts phenylalanine to tyrosine
• phenylalanine and phenylpyruvic acid accumulate in the
body and interfere with the development of the nervoussystem, causing seizures and mental retardation
Newborn Screening
Newborn screening:
determine the concentration of Phe and
the ratio of Phe to tyrosine, both of whichwill be elevated in PKU
Changing the environmentalfactor, by consuming a diet low
in phenylalanine, usually results
in normal development
newborn screening
Genetic Testing and Counselling
Tests for identifying carriers for autosomal recessive diseases:
sickle-cell anemia, cystic fibrosis, Tay-Sacks, etc
Recommended