Download Patterns of Inheritance Understanding the Chromosome A History of

Patterns of Inheritance
Understanding the Chromosome
Gene: a unit of hereditary
information (nucleotide sequence).
Locus: physical location of a gene.
Allele: an alternate form of a
gene (green vs. brown eyes)
Chapter 10
A History of Genetics
• Gregor Mendel, a monk who studied at
the University of Vienna in the mid-1800s.
– Plant breeding
– Mathematics
• His experiments became the
foundation of the modern
science of genetics.
Pair of Chromosomes
During meiosis, each gamete
produced receives one copy
of each chromosome.
Mendel’s Experimental System
• Mendel experimented on ordinary peas.
• The pea flower is a perfect flower:
– Petal, sepal, stamen, carpel are easily accessible
and distinguishable.
– Can self-fertilize.
– Can be crossfertilized by hand.
• Flower color is
found in one gene.
Mendel’s Experiments
Mendel’s Experiments
• Mendel chose two “truebreeding” pea plants, plants that
would always produce one color
of flower, purple and white.
• He found that all of
the first generation
offspring were
purple.
• He cross pollinated them to
produce the first generation (F1).
Where did the white color go?
To try to determine what happened, he
created a second generation, F2, by
self-pollinating the fist generation.
Mendel’s Experiments
Cross
Pollination
• In the F2 generation of
plants, Mendel found
F
that 1/4 of the flowers
First Generation
Offspring
were white and 3/4 were
purple.
Self-Pollination
• The white trait
had been
“hidden” in
the F1
F
generation.
Second Generation
Offspring
F1
First Generation
Offspring
P
Parental Generation
1 Purple, 1 White
F1
First Generation Offspring
All Purple
1
2
Cross
Pollination
Self-Pollination
F2
Second Generation
Offspring
3/ Purple, 1/ White
4
4
Mendel’s Conclusions
• Mendel was the first to perform controlled breeding
experiments with the same plant for a period of time
while taking accurate notes.
• He came up with 5 theories to explain his
inheritance results:
1. Each trait is determined discrete physical units.
2. Certain traits have dominance over others.
3. Traits are segregated from each other during meiosis.
4. Chance determines which trait will go to each gamete.
5. True-breeding organisms have two copies of the same
trait.
Mendel’s Conclusions
2. Certain traits have dominance over others.
- The purple flower color seemed to “hide” the
white flower color during the first generation
of offspring.
- The purple color was the dominant allele.
- It blocked expression of the white color gene.
- The white color was the recessive allele.
- It could not be seen if the purple allele was
present in a chromosome copy.
Mendel’s Conclusions
1. Each trait is determined discrete physical
units.
– We now know these “physical units” are genes.
– Mendel was lucky since the pea plant has only
one gene for flower color.
– Each plant of the parental population had a
different allele (purple or white) for the flowercolor gene.
Mendel’s Conclusions
3. Traits are segregated from each other
during meiosis.
- Mendel’s Law of Segregation: two alleles
of a gene segregate or separate from one
another during meiosis.
- Each gamete produced receives only one
allele.
- When the sperm fertilizes the egg, the
offspring receives one allele from its
mother and one from its father.
Mendel’s Conclusions
4. Chance determines which trait will go
to each gamete.
-
Homologous chromosomes separate at
random during meiosis.
The distribution of alleles in gametes is
also random.
Purple Flower Allele
Mendel’s Conclusions
5. True-breeding organisms have two
copies of the same trait.
-
True-breeding plants are homozygous for
flower color :
-
-
They have two copies of the the same allele
for the flower color gene.
The first generation of offspring were
heterozygous for flower color:
-
They had one copy of each allele,
one purple and one white.
White Flower Allele
Parental “True-Breeding”
Generation
Purple color is the dominant allele.
We will call the allele “P”.
Since the purple parent is homozygous for
purple color, it has two copies of the allele,
or “PP” .
White color is the recessive allele.
We will call the allele “p”.
Since the white parent is homozygous for
white color, it is “pp” .
Parental “True-Breeding”
Generation
The First Generation Offspring
PP
Cross Pollination
pp
The First Generation Offspring
The Second
Generation
Offspring
Since the F1 generation
offspring receive one allele
from each parent, they get
one purple allele (P) and
one white allele (p).
They are heterozygous for
flower color, “Pp”.
The Second
Generation
Offspring
PP
Homozygous
for purple.
Pp
Heterozygous
for purple.
Pp
Heterozygous
for purple.
pp
Homozygous
for white.
Pp
The F1 generation flowers were
self-pollinated to obtain the F2
generation.
Each F2 offspring received any
combination of the two alleles
P (purple) and p (white).
The Punnett
Square
Pp
A system for predicting
genotypes and phenotypes
of offspring.
P
p
The Second
Generation
Offspring
PP
Pp
Three types of offspring
are produced: PP, Pp,
and pp.
Pp
Pp
These are the three
possible genotypes, the
allele combinations.
There are only two phenotypes, physical or
observable properties, white and purple.
The Punnett
Square
Pp
A system for predicting
genotypes and phenotypes
of offspring.
P
P
Pp
Pp
p
1. Determine genotypes of each
parent and label the square with
the possible gametes.
p
P
PP
Pp
pp
p
2. Fill in the genotypes of each
offspring by adding gametes from
each column and row.
Pp
pp
The Punnett
Square
Pp
A system for predicting
genotypes and phenotypes
of offspring.
P
The Punnett
Square
A system for predicting
genotypes and phenotypes
of offspring.
p
P
3. Determine phenotypes by
assessing dominant alleles.
Pp
pp
The Punnett
Square
Pp
A system for predicting
genotypes and phenotypes
of offspring.
P
p
PP 1/4
Pp 1/4
Pp 1/4
1
pp /4
P
Pp
p
1/
4
Purple
Pp
1/ + 1/ = 1/
4
4
2
Purple
pp
1/
4
White
Pp
1/
4
Pp
p
PP
p
PP 1/4
Pp
Pp
Phenotype
P
P
PP
Genotype
Ratio
Pp
Phenotype
Ratio
1/
4
+ 1/2= 3/4
1/
4
Genotype Ratio = 1:2:1
Phenotype Ratio = 3:1
4. Predict probabilities
of each phenotype and
genotype by determining
the fraction of the offspring that
possesses each unique trait.
p
Pp
1/
4
1
pp /4
Example
Problems
Example 1
SS
Homozygous Smooth
(SS) seed shape vs.
Homozygous Wrinkled
(ss) shape
Yy
1. Determine possible gametes.
Heterozygous Tall (Tt)
plant vs. Homozygous
Dwarf (tt) plant
1. Determine possible gametes.
Tt
Example 4
Heterozygous Green
(Gg) seed pod color
vs. Heterozygous
Green (Gg) pod color
tt
Gg
1. Determine possible gametes.
YY
Homozygous Yellow
(YY) seed color vs.
Heterozygous Yellow
(Yy) color
ss
Example 3
Example 2
1. Determine possible gametes.
Gg
Mendel’s Second Experiments
Mendel’s Second Experiments
• After determining the hereditary statistics
of a single gene, Mendel investigated the
inheritance of two independent genes.
SSYY
All F1 generation seeds
were heterozygous
yellow and smooth
(SsYy).
Cross Pollination
• He cross-pollinated a true-breeding
yellow (YY), smooth seeded (SS) pea
plant with a true-breeding green (yy),
wrinkled seeded (ss) plant.
Second
Generation
Seeds
To predict the F2
generation genotypes
and phenotypes we
must work out the
Punnett Square.
SsYy
Ss Yy
First, determine possible gametes.
SsYy
ssyy
For the F2 generation, Mendel
self-pollinated the heterozygous
F1 generation pea plants.
Second
Generation
Seeds
To predict the F2
generation genotypes
and phenotypes we
must work out the
Punnett Square.
SsYy
SY
SY
Sy
SsYy
sY
Possible Gametes:
SY, Sy, sY, and sy.
sy
Sy
sY
sy
Second
Generation
Seeds
To predict the F2
generation genotypes
and phenotypes we
must work out the
Punnett Square.
SsYy
Next, determine genotypes.
SsYy
SY
SsYy
SSYY
sY
sy
SSYy
SsYY
SsYy
SsYy
Ssyy
Sy
SSYy
SSyy
To predict the F2
generation genotypes
and phenotypes we
must work out the
Punnett Square.
SsYY
SsYy
ssYY
ssYy
SsYy
Ssyy
ssYy
ssyy
sY
sy
SSYy
SsYY
SsYy
SSYy
SSyy
SsYy
Ssyy
SsYY
SsYy
ssYY
ssYy
SsYy
Ssyy
ssYy
ssyy
sY
sy
SY
Sy
sY
1/
16
1/
16
1/
16
SSYY
SSYy
SsYY
1/
16
1/
16
1/
16
SSYy
sY
Sy
Sy
Third, determine phenotypes.
Second
Generation
Seeds
SsYy
Sy
SY
SSYY
sy
SY
SsYy
SY
SsYy
sY
sy
Fourth, predict probabilities.
Sy
SY
Second
Generation
Seeds
To predict the F2
generation genotypes
and phenotypes we
must work out the
Punnett Square.
Second
Generation
Seeds
1/
16
SSyy
1/
16
SsYy
1/
16
SsYY
SsYy
ssYY
1/
16
1/
16
1/
16
Ssyy
ssYy
SsYy
sy
Genotype
Ratio
Phenotype
SSYY
1/
16
Smooth, Yellow
SSYy
2/
16
Smooth, Yellow
SsYY
2/
16
Smooth, Yellow
SsYy
4/
16
Smooth, Yellow
Ssyy
2/
16
Smooth, Green
1/
16
SsYy
1/
16
Ssyy
1/
16
ssYy
1/
16
ssyy
SSyy
1/
16
Smooth, Green
ssYy
2/
16
Wrinkled, Yellow
ssYY
1/
16
Wrinkled, Yellow
ssyy
1/
16
Wrinkled, Green
SsYy
SY
SY
Phenotype
Ratio
9/
16
Sy
sY
sy
3/
16
3/
16
1/
16
Sy
sY
sy
1/
16
1/
16
1/
16
SSYY
SSYy
SsYY
1/
16
1/
16
1/
16
1/
16
SSYy
SSyy
SsYy
Ssyy
1/
16
1/
16
1/
16
1/
16
ssYy
SsYY
SsYy
ssYY
1/
16
1/
16
1/
16
SsYy
Ssyy
ssYy
1/
16
SsYy
1/
16
ssyy
Phenotype Ratio = 9:3:3:1
Mendel’s Second Conclusions
• Different traits can be inherited
independently.
– Mendel’s Law of Independent Assortment.
– During meiosis, chromosomes are randomly
pulled to either pole during anaphase I.
– Genes on separate chromosomes (like pea
seed shape and seed color) will be randomly
sorted into gametes.
Example
Problems
Independent
Assortment of
Alleles on
Different
Chromosomes
Example 5
Homozygous Green
and Constricted
(GGii) seedpod vs.
Heterozygous Green,
Homozygous Inflated
(GgII) seedpod
GgII
1. Determine possible gametes.
GGii
Inheritance Strategies
Example 6
ppll
Homozygous White
flowers at tips of
branches (ppll) vs.
Heterozygous Purple
flowers at leaf junctions
(PpLl)
Linked Genes
• Genes that are inherited together.
– If they are on the same chromosome, they will be
inherited together.
– If genes are on different chromosomes, they may
or may not be inherited together.
Flower Color
Purple
allele (C)
PpLl
Red
allele (c)
1. Determine possible gametes.
Pollen Shape
Purple
allele (C)
Long
allele (S)
Red
allele (c)
Round
allele (s)
color
shape
Long
allele (S)
Round
allele (s)
Inheritance Strategies
Linked Genes
Flower Color
Pollen Shape
If NOT linked:
Inheritance of alleles
is linked if genes are
found on the same
chromosome.
color
shape
OR
Chromosome sorting into gametes decides inheritance patterns.
Crossing-over can alter inheritance
patterns.
Inheritance Strategies
Inheritance Strategies
Crossing-over can alter inheritance patterns.
Crossing-over can alter inheritance patterns.
Genes have been
rearranged. Genetic
recombination has
occurred.
Chromosomes
sorted into
gametes have
a new set of
linked genes.
Inheritance of Traits
• Traits in the pea plant observed by Mendel
exhibited simple dominance.
– One allele is dominant over the other, completely
blocking its expression.
• Traits can exhibit more complex dominance
strategies:
–
–
–
–
–
–
Incomplete dominance.
Codominance.
Multiple Alleles.
Polygenic Inheritance.
Pleiotrophy.
Sex-linked.
Dominance Strategies
CRCR
Incomplete Dominance
• Heterozygous produces an
in-between phenotype.
• The sweet-pea exhibits this
trait:
Cross between a red flower (dominant)
and a white flower (recessive) produces a
heterozygous pink flower (intermediate).
CwCw
CR
Cw
CRCw
F1 Generation:
All CRCw, pink.
Dominance Strategies
CRCR
Incomplete Dominance
• Heterozygous produces an
in-between phenotype.
• The sweet-pea exhibits this
trait.
CR
CR
Cw
Dominance Strategies
Cw
F1
CRCw
F1 x F1
CRCw
CwCw
CRCw
Multiple Alleles
• There may be more
than two alleles for a
given gene.
• In fruit fly, there can be
many eye colors
(yellow, orange, pink,
brown, or red).
CR
taputea.lbl.gov
Cw
Dominance Strategies
Dominance Strategies
Codominance
Sickle-Cell Anemia
• Both phenotypes are expressed.
• Heterozygous individuals
for the sickle-cell
mutation have immunity
to the parasite that
causes malaria.
Ex: Sickle Cell Anemia
• Single amino acid is defective in
hemoglobin.
– O2 carrying protein in red blood cell.
• Blood cells change shape & clump.
• Heterozygous (HRHr) would have half
normal and half abnormal hemoglobin.
• Homozygous recessive person would
have sickle-cell anemia.
HRHr x HRHr
Pg. 239
Distribution of
Sickle Cell Anemia
2.5-5.0%
5.0-7.5%
7.5-10.0%
10.0-12.5%
>12.5%
• Homozygous recessive
have sickle-cell disease.
A large percentage of
the population in
regions where malaria
is common carry the
sickle-cell mutation.
Distribution of Malaria
HR
Hr
HR HRHR HRHr
Hr
HRHr HrHr
Dominance Strategies
Dominance Strategies
Blood Types
Blood Types
Multiple alleles & co-dominance
- More than two alleles present: IA, IB, i
- Heterozygous gives you both phenotypes: type AB blood
Multiple alleles & co-dominance
- More than two alleles present: IA, IB, i
- Heterozygous gives you both phenotypes: type AB blood
Blood Type
Genotype
Proteins
O
A
ii
No proteins on the
surface of the cell.
IAIA or IAi
B
IBIB or IBi
A proteins on the
exterior of the cell.
B proteins on the
exterior of the cell.
AB
IAIB
Antibody
Blood Type
Genotype
Antibody
Frequency
ii
Anti-A; Anti-B
4%
Anti-B
O
A
IAIA or IAi
Anti-B
40%
Anti-A
B
IBIB or IBi
Anti-A
10%
none
AB
IAIB
none
46%
Anti-A; Anti-B
Both A and B proteins on
the exterior of the cell.
Dominance Strategies
Polygenic inheritance
• Several genes contribute to a single phenotype.
Ex: Skin Pigmentation
– More than 3 genes as well as environmental
factors produce a wide range of skin colors.
Dominance Strategies
Polygenic Inheritance
Ex: Skin Pigmentation
– Very pale and very
dark are rare alleles
for skin color.
Universal
Donor
Universal
Recipient
Human Disorders
m
Human Disorders
M
Albinism
•
•
•
•
M
Follows simple dominance.
m
Recessive disorder.
A mutation causes enzyme (tyrosinase) not to function properly.
No melanin is produced.
Human Disorders
Huntington’s disease
• A degenerative disease of the nervous
system, which is fatal.
• Dominant disorder
• On the 4th chromosome
Woody Guthrie
American folk singer
who wrote
"This Land Is Your Land"
Cystic Fibrosis
• Follows simple
dominance.
• Recessive disorder.
• Defective transport
protein.
• “Salty” forehead.
• Mucus in lungs.
Dominance Strategies
Pleiotropy
• One gene with multiple phenotypes
Ex: SRY gene on Y chromosome
– One gene, activates many other genes to make
"male traits" in an embryo.
The SRY gene activates all
the genes required for
male embryonic growth.
SRY or
“Sex Determining
Region of the
Y-Chromosome”
Dominance Strategies
Dominance Strategies
w+
Sex-linked traits
Sex-linked traits
• Trait is carried on the X-chromosome
white
eyed
male
Ex: Drosophila (fruit fly) studies.
– Thomas Morgan (early 20th century)
found a fruit fly w/ white eyes
w
– Red = Wild type
ww
all
red
eyed
w+w
w
w+
w
F2
But all the fruit
flies with white
eyes were male.
Males are more likely to inherit
a disease on the X chromosome
because they only have one.
• X chromosome has many traits
unrelated to the sex of the
individual.
w+w+
=
w+
Phenotype Ratio = 3:1
– White = Mutant phenotype
Sex-linked traits
red
eyed
female
F1
• Most common in natural population.
Dominance Strategies
x
F1 x F1
What number do you see?
X chromosome
XWXw crossed with XWY
Healthy Carrier Female x Healthy Male
XW
XW
Y
Xw
W = normal
w = disorder
Y chromosome
http://www.kcl.ac.uk/teares/gktvc/vc/lt/colourblindness/cblind.htm
Dominance Strategies
Trace the path between the X’s.
Sex-linked traits
• Colorblindness
Normal,
Carrier Female,
XCXc
XC
Xc
C
Normal X
Male,
XCY
Y
The gene for colorblindness is recessive on the X-chromosome.
• What % of females will be colorblind?
• What % of males will be colorblind?
• What % children will be colorblind?
http://www.kcl.ac.uk/teares/gktvc/vc/lt/colourblindness/cblind.htm
Tracing Genetic Traits
• Pedigrees are diagrams that show the
genetic relationships between
individuals.
– A genetic family tree.
– Useful for determining disease inheritance.
Family Pedigree
Human Disorders
Human Disorders
Hemophilia
Hemophilia
• Alexis had hemophilia, because his mother was
a carrier.
• Sex-linked.
• Abnormal allele for blood clotting
factors.
• Bleed excessively when injured.
• Pedigree (family tree) uses
phenotypes to predict genotypes of
members of a family.
Queen Victoria was a
carrier for hemophilia.
Human Disorders
Nondisjunction: Error during meiosis where
chromosomes do not segregate properly.
Human Disorders
Nondisjunction
– Error during meiosis where chromosomes do not
segregate properly.
– Results in abnormal number of chromosomes.
Human Disorders
Nondisjunction
• Down Syndrome (Trisomy 21)
1 in 700
children
born
in the
– Caused by 3 copies of chromosome #21
United States
(47 chromosomes total, instead of 46).
– Results from both copies of the chromosome entering the
new gamete during meiosis.
Homework
Calculate the genotypes, genotype ratios, phenotypes, and
phenotype ratios of the following breeding experiments:
a. Heterozygous brown eyes crossed with Homozygous
Recessive blue eyes.
b. Heterozygous brown hair and Homozygous Recessive blue
eyes with Homozygous Recessive blonde hair and
Heterozygous brown eyes.
c. Homozygous Recessive blonde hair and Heterozygous wavy
hair with Heterozygous Brown hair and Heterozygous wavy
hair.
(Note that hair texture exhibits incomplete dominance with
curly hair dominant and straight hair recessive)
Human Disorders
Nondisjunction
• Down Syndrome (Trisomy 21)
– Frequency increases with maternal age.