Download Mendelian Genetics

Mendelian Genetics
Honors Biology
Pre-Mendelian Theory of
Heredity
Blending Theory—hereditary material
from each parent mixes in the offspring


Individuals of a population should reach a
uniform appearance after many
generations
Once traits are blended, they can no longer
be separated out to appear in later
generations
Pre-Mendelian Theory of
Heredity
Problems—inconsistent with
observations:


Individuals of a population don’t reach
uniform appearance
Traits can skip generations
Modern Theory of Heredity
Based on Gregor Mendel’s fundamental
principles of heredity


Parents pass on discrete inheritable factors
(genes) to their offspring
These factors remain as separate factors
from one generation to the next
Useful Genetic Vocabulary
Homozygous—having 2 identical alleles for a
given trait (PP or pp)
Heterozygous—having 2 different alleles for a
trait (Pp); ½ gametes carry one allele (P) and
½ gametes carry the other allele (p)
Phenotype—an organism’s expressed traits
(purple or white flowers)
Genotype—an organism’s genetic makeup
(PP, Pp, or pp)
Mendel’s Principles of Heredity
First Law of Genetics: Law of Segregation




alternate forms of genes are responsible for
variations in inherited traits
for each trait, an organism inherits 2 alleles, one
from each parent
If 2 alleles differ, one is fully expressed (dominant
allele); the other is completely masked (recessive
allele)
2 alleles for each trait segregate during gamete
production
Mendel’s Discoveries
Developed true-breeding
lines—populations that
always produce offspring
with the same traits as
the parents when parents
are self-fertilized
Counted his results and
kept statistical notes on
experimental crosses
Crosses Tracking One
Characteristic: Flower Color
x
Ratio
3.15:1
x
3.14:1
x
3.01:1
x
2.96:1
x
2.95:1
x
2.82:1
x
2.84:1
3:1
Genotype versus Phenotype
1
PP
(homozygous)
2
Pp
(heterozygous)
Pp
(heterozygous)
1
pp
(homozygous)
Genotypic Ratio 1:2:1
Purple
3
Purple
Purple
1
White
Phenotypic Ratio 3:1
The Testcross
The cross of any individual
to a homozygous
recessive parent
Used to determine if the
individual is homozygous
dominant or heterozygous
CAUTION:
Must perform many, many
crosses to be statistically
significant
Mendel’s Principles of Heredity
Second Law of Genetics: Law of
Independent Assortment


During gamete formation, the segregation
of the alleles of one allelic pair is
independent of the segregation of another
allelic pair
Law discovered by following segregation of
2 genes
Dihybrid Cross
Mendelian Inheritance Reflects
Rules of Probability
Rules of Multiplication: The probability
that independent events will occur
simultaneously is the product of their
individual probabilities.
Mendelian Inheritance Reflects
Rules of Probability
Question: In a Mendelian cross between pea
plants that are heterozygous for flower color
(Pp), what is the probability that the offspring
will be homozygous recessive?
Answer:
 Probability that an egg from the F1 (Pp) will
receive a p allele = ½
 Probability that a sperm from the F1 will
receive a p allele = ½
 Overall probability that 2 recessive alleles will
unite at fertilization: ½ x ½ = ¼
Mendelian Inheritance Reflects
Rules of Probability
Works for Dihybrid Crosses:
Question: For a dihybrid cross, YyRr x YyRr,
what is the probability of an F2 plant having
the genotype YYRR?
Answer:
 Probability that an egg from a YyRr parent
will receive the Y and R alleles = ½ x ½ = ¼
 Probability that a sperm from a YyRr parent
will receive the Y and R alleles = ½ x ½ = ¼
 Overall probability of an F2 plant having the
genotype YYRR: ¼ x ¼ = 1/16
Mendelian Inheritance Reflects
Rules of Probability
Rules of Addition: The probability of an
event that can occur in two or more
independent ways is the sum of the
separate probabilities of the different
ways.
Mendelian Inheritance Reflects
Rules of Probability
Question: In a Mendelian cross between pea
plants that are heterozygous for flower color
(Pp), what is the probability that the offspring
will being a heterozygote?
Answer:
 There are 2 ways in which a heterozygote
may be produced: the dominant allele may be
in the egg and the recessive allele in the
sperm, or the dominant allele may be in the
sperm and the recessive allele in the egg.
Mendelian Inheritance Reflects
Rules of Probability
Probability that the dominant allele will be in
the egg with the recessive in the sperm is ½
x½=¼
Probability that the dominant allele will be in
the sperm with the recessive in the egg is ½
x½=¼
Therefore, the overall probability that a
heterozygote offspring will be produced is ¼
+¼=½
Pedigree Analysis
Analysis of existing populations
Studies inheritance of genes in humans
Useful when progeny data from several
generations is limited
Useful when studying species with a
long generation time
Symbols:
= female
= male
= affected individual
= mating
I
II
= offspring in birth order
I and II are generations
= Identical twins
= Fraternal twins
Dominant Pedigree:
I
II
III
For dominant traits:
•Affected individuals have at least one affected parent
•The phenotype generally appears every generation
•2 unaffected parents only have unaffected offspring
Recessive Pedigree:
I
II
III
For recessive traits:
•Unaffected parents can have affected offspring
•Affected progeny are both male and female
Recessive Human Disorders
Sickle-cell anemia; autosomal recessive



Caused by single amino acid substitution in
hemoglobin
Abnormal hemoglobin packs together to
form rods creating crescent-shaped cells
Reduces amount of
oxygen hemoglobin
can carry
Genetic Testing & Counseling
Genetic counselors can help determine
probability of prospective parents
passing on deleterious genes

Genetic screening for various known
diseases alleles (gene markers)
Genetic Testing & Counseling
Fetal testing
Amniocentesis

needle inserted into uterus and amniotic
fluid extracted
 Test for certain chemicals or proteins in
the fluid that are diagnostic of certain
diseases
 Karyotype-can see chromosome
abnormalities
Genetic Testing & Counseling
Fetal testing
Chorion Villus Sampling
 Suctions off a small amount of fetal tissue from
the chorionic villus of placenta
 Karyotype-can see chromosome abnormalities
Ultrasound at 12 weeks
--can see any physical abnormalities
Variations to Mendel’s First
Law of Genetics
Incomplete dominance—pattern of
inheritance in which one allele is not
completely dominant over the other

Heterozygote has a phenotype that is
intermediate between the phenotypes of
the 2 homozygous dominant parent and
homozygous recessive parent
Incomplete Dominance in Snapdragon Color
F2
Genotypic ratio:
1 CRCR: 2 CRCW: 1 CWCW
Phenotypic ratio:
1 red: 2 pink: 1 white
Variations to Mendel’s First
Law of Genetics
Codominance—pattern of inheritance in
which both alleles contribute to the
phenotype of the heterozygote
Multiple Alleles
Some genes may have more than just 2
alternate forms of a gene.
Example: ABO blood groups

A and B refer to 2 genetically determined
polysaccharides (A and B antigens) which are
found on the surface of red blood cells (different
from MN blood groups)
 A and B are codominant; O is recessive to A and B
Multiple Alleles for the ABO Blood Groups
3 alleles: IA, IB, i
Pleiotropy
The ability of a single gene to have multiple
phenotypic effects (pleiotropic gene affects
more than one phenotype)
Example:


In tigers and Siamese cats, the gene that controls
fur pigmentation also influences the connections
between a cat;s eyes and the brain. A defective
gene cause both abnormal pigmentation and
cross-eye condition
Sickle-cell disease—impact of abnormal
hemoglobin can affect other organs
Epistasis
Interaction between 2 nonallelic genes
in which one modifies the phenotypic
expression of the other.
If epistasis occurs between 2 nonallelic
genes, the phenotypic ratio resulting
from a dihybrid cross will deviate from
the 9:3:3:1 Mendelian ratio
CC, Cc = Melanin deposition
cc = Albinism
BB, Bb = Black coat color
bb = Brown coat color
A cross between
heterozygous black mice
for the 2 genes results
in a 9:3:4 phenotypic
ratio
9 Black (B_C_)
3 Brown (bbC_)
4 Albino (__cc)
Polygenic Traits
Skin pigmentation in humans
--3 genes with the dark-skin
allele (A, B, C) contribute
one “unit” of darkness to the
phenotype.
These alleles are
incompletely dominant over
the other alleles (a, b, c)
--An AABBCC person would
be very dark; an aabbcc
person would be very light
--An AaBbCc person would
have skin of an intermediate
shade
Chromosome Theory of
Inheritance
Based on Mendel’s observations and
genetic studies and cytological evidence


Mendelian factors (genes) are located on
chromosomes
It is the chromosomes that segregate and
independently assort
Genes on the same chromosome
tend to be inherited together
Experiment
Purple flower

Certain genes are linked
 They tend to be inherited
together because they
reside close together on
the same chromosome
 PpLI
PpLI
Observed
offspring
284
21
21
55
Phenotypes
Purple long
Purple round
Red long
Red round
Long pollen
Prediction
(9:3:3:1)
215
71
71
24
Explanation: linked genes
PL
Parental
diploid cell
PpLI
PI
Meiosis
Most
gametes
PL
PI
Fertilization
Sperm
Most
offspring
PL
PI
PL
PL
PL
PI
PI
PI
PL
PI
PL
Eggs
PI
3 purple long : 1 red round
Not accounted for: purple round and red long
Figure 9.19
Crossing over produces new
combinations of alleles

Crossing over can separate linked alleles
 Producing gametes with recombinant
chromosomes
A
B
a
b
A
b
a
B
A B
a
Tetrad
Figure 9.20 A
b
Crossing over
Gametes
Thomas Hunt Morgan
 Performed some of the early studies of
crossing over using the fruit fly Drosophila
melanogaster
Experiments with Drosophila revealed
linkage traits. Why Drosophila?
 Easily cultured
 Prolific breeders
 Short generation times
 Only 4 pairs of chromosomes, visible under
microscope
Figure 9.20 B
Morgan’s experiments
Experiment
Black body,
vestigial
wings
Gray body,
long wings
(wild type)
Demonstrated the role
of crossing over in
inheritance

GgLI
ggll
Male
Female
Offspring
Gray long
Black vestigial Gray vestigial Black long
965
944
206
Parental
phenotypes
Recombinant
phenotypes
Recombination frequency =
Explanation
391 recombinants
= 0.17 or 17%
2,300 total offspring
GL
g l
g l
gl
GgLI
(female)
GL
g l
Gl
gL
Eggs
GL
gl
gl
gl
Offspring
Figure 9.20 C
185
Gl
gl
ggll
(male)
gl
Sperm
gL
gl
Geneticists use crossover data to
map genes

Morgan and his students
 Used crossover data to map genes in
Drosophila
Figure 9.21 A
One of Morgan’s students, Alfred Sturtevant,
used crossing over of linked genes to
develop a method for constructing a genetic
map.
This map is an ordered list of the genetic loci
along a particular chromosome.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Sturtevant hypothesized that the frequency of
recombinant offspring reflected the distances
between genes on a chromosome.
The farther apart two genes are, the higher the
probability that a crossover will occur between
them and therefore a higher recombination
frequency.
 The greater the distance between two genes,
the more points between them where crossing
over can occur.
Sturtevant used recombination frequencies from
fruit fly crosses to map the relative position of
genes along chromosomes, a linkage map.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Recombination frequencies
Can be used to map the relative positions of
genes on chromosomes. Mutant phenotypes
Short
aristae
Chromosome
g
Black
body
(g)
Cinnabar
eyes
(c)
Vestigial
wings
(l)
Brown
eyes
Red
eyes
(C)
Normal
wings
(L)
Red
eyes
l
c
17%
9%
9.5%
Recombination
frequencies
Long aristae
(appendages
on head)
Gray
body
(G)
Wild-type phenotypes
Figure 9.21 B
Figure 9.21 C
Fig. 15.5b
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Sturtevant used the testcross design to map the
relative position of three fruit fly genes, body
color (b), wing size (vg), and eye color (cn).
 The recombination frequency between cn and
b is 9%.
 The recombination frequency between cn and
vg is 9.5%.
 The recombination frequency between b and
vg is 17%.
 The only possible
arrangement of these
three genes places
the eye color gene
between the other two. Fig. 15.6
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Sturtevant expressed the distance between
genes, the recombination frequency, as map
units.
 One map unit (sometimes called a
centimorgan) is equivalent to a 1%
recombination frequency.
What is the sequence of these
three genes on the chromosome?
A series of matings shows that the
recombination frequency between the
black-body gene (b) and the gene for
short wings (s) is 36%. The
recombination frequency between
purple eyes (p) and short wings is 41%.
The recombination frequency between
black-body gene and purple eyes is 6%.
Answer
B 6% P
B
36%
S
P
41%
P 6% B B
36%
P
41%
S
S 6% + 36% = 42%
S
You may notice that the three recombination
frequencies in our mapping example are not quite
additive: 9% (b-cn) + 9.5% (cn-vg) > 17% (bvg).
This results from multiple crossing over events.
 A second crossing over “cancels out” the first
and reduces the observed number of
recombinant offspring.
 Genes father apart (for example, b-vg) are
more likely to experience multiple crossing over
events.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Some genes on a chromosome are so far apart that a
crossover between them is virtually certain.
In this case, the frequency of recombination reaches is its
maximum value of 50% and the genes act as if found on
separate chromosomes and are inherited independently.
 In fact, several genes studies by Mendel are located on
the same chromosome.
 For example, seed color and flower color are far
enough apart that linkage is not observed.
 Plant height and pod shape should show linkage,
but Mendel never reported results of this cross.
•If the recombination frequency is 50% or greater, the
genes are not linked
•If the recombination frequency is less than 50%, the
genes are linked
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Genes located far apart on a chromosome
are mapped by adding the recombination
frequencies between the distant genes and
intervening genes.
Sturtevant and his
colleagues were able
to map the linear
positions of genes in
Drosophila into four
groups, one for each
chromosome.
Fig. 15.7
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
A linkage map provides an imperfect picture of a
chromosome.
Map units indicate relative distance and order, not precise
locations of genes.
 The frequency of crossing over is not actually uniform
over the length of a chromosome.
Combined with other methods like chromosomal banding,
geneticists can develop cytological maps.
 These indicated the positions of genes with respect to
chromosomal features.
More recent techniques show the absolute distances
between gene loci in DNA nucleotides.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
SEX CHROMOSOMES AND SEXLINKED GENES
Chromosomes determine sex in many species

In mammals, a male has one X chromosome and
one Y chromosome
 And a female has two X chromosomes

The Y chromosome
 Has genes for the development
of testes

The absence of a Y chromosome
 Allows ovaries to develop
Figure 9.22 A
(male)
44
Parents’
+
diploid
XY
cells
22
+
X
(female)
44
+
XX
22
+
Y
Sperm
22
+
X
44
+
XX
44
+
XY
Offspring
(diploid)
Egg

Other systems of sex determination exist
in other animals and plants
22
+
XX
22
+
X
76
+
ZW
76
+
ZZ
32
16
Figure 9.22 B
Figure 9.22 C
Figure 9.22 D
Sex-linked genes exhibit a unique pattern of
inheritance

All genes on the sex chromosomes
 Are said to be sex-linked

In many organisms
 The X chromosome carries many genes unrelated to sex
For genes on X chromosomes, females have 2
copies of gene—can have 2 different alleles
For genes on X chromosomes, males have only
one allele; the allele they express



Males’ X comes from mom (dad contributes Y)
Males are said to be hemizygous
If allele is recessive, it will be expressed

A male receiving a single X-linked allele
from his mother
 Will have the disorder

A female
 Has to receive the allele from both parents
to be affected

In Drosophila
 White eye color is a sex-linked trait
Figure 9.23 A
The inheritance pattern of sex-linked
genes

 Is reflected in females and males
Female

XR XR
Male
Female
Xr Y
XR Xr

Eggs XR
Y
XR Xr
XR Y
Female
XR Y
XR X r
XR
F
Figure 9.23 B
All red eyes
Xr Y
Sperm
XR
Y
XR XR
XR Y
XR
Xr
Y
XR Xr
XR Y
Xr Xr
Xr Y
Eggs
Eggs
R = red-eye allele
r = white-eye
allele
1
Male

Sperm
Sperm
Xr
Male
F2
Xr
Xr XR
Xr Y
Figure 9.23 C
Xr
Figure 9.23 D
All
red
eyes
and
½
red
eyes and ½
CONNECTION
Sex-linked disorders affect mostly males

Most sex-linked human disorders
 Are due to recessive alleles
 Are mostly seen in males
Queen
victoria
Albert
Alice
Louis
Alexandra
Figure 9.24 A
Figure 9.24 B
Czar
Nicholas II
of Russia
Alexis
The End
Nature versus Nature
Environmental conditions can influence the
phenotypic expression of a gene, so that a
single genotype may produce a range of
phenotypes
One may have a history of heart disease in
their family and thus be at risk of heart
disease themselves. If this person watches
his/her diet, exercises, doesn’t smoke, etc.
his/her risk of actually developing heart
disease decreases