Transcript Document

Classical
Genetics
Humans have a long history of
animal and plant breeding…
but without an understanding of
the underlying process
Gregor Mendel
Mendel
conducted
experimental
crosses
Classical Mendelian Genetics has a limitation: The requirement for
observable phenotypic differences in different genotypes
Mendel chose single
gene mutants with
extreme phenotypes
to study.
This made different
genotypes recognizable
and countable.
Terminology
• Genes and alleles
• Genotype and Phenotype
• Homozygote, Heterozygote, Hemizygote
• Dominance
• Meiosis and Syngamy (Fertilization)
• Parents, Gametes, Offspring
Genes and Alleles
• A gene is a nucleotide sequence of a DNA
molecule that codes for the primary structure
of a protein or RNA molecule
• Alleles are gene variants. They differ in their
nucleotide sequences.
Genotype and Phenotype
•
Genotype: An individual’s genetic constitution
AA, Aa, aa are diploid genotypes
•
Phenotype: An organism’s appearance, reflecting genotypic and
environmental influences
blue
yellow
white
Dominance
• Many alleles are mutations whose gene products (proteins) work
poorly or not at all (e.g., allele a). These alleles are recessive to
normal alleles in the sense that they affect the phenotype only when
there are no functional alleles present, i.e., in the homozygous
recessive genotype aa.
• Both homozygotes (e.g., AA) for the normal allele and heterozygotes
(e.g., Aa) share the functional allele (A) and exhibit the normal
phenotype. However, aa individuals are unable to perform the
function that this gene is responsible for and they will have a different
phenotype.
• Operationally, one allele is said to be dominant over another if the
heterozygote has the same phenotype as a homozygote (e.g., Aa and
AA look alike).
Homozygous for
two normal alleles
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Homozygous for
two non-functional alleles
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Heterozygous for
a normal and a non-functional allele
Dominance
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Siamese Cats:
An enzyme that catalyzes
pigment synthesis is
denatured under warmer
physiological conditions,
like warmer parts of the
cat’s body. Only cooler
extremities reveal intense
pigmentation.
Similarly, the enzyme
can be deactivated not
only under conditions
that are too warm
(below), but also
under conditions that
are cooler (above).
Mendel figured out how to start a breeding experiment:
A Classical Mendelian Research Program
true breeding
line “A”
true breeding
line “a”
PA
generation
x aa
Mendelian AA
Research
F1 generation
all Aa
Program
backcross used
as a test cross
1/2 Aa, 1/2 aa
monohybrid cross
F2 generation
1/4 AA, 2/4 Aa, 1/4 aa
backcross used
as a test cross
The Three Steps of
Classical Genetic Analysis
Classical genetic analysis involves 3 steps
based on the structure of a eukaryotic life cycle
gametes
Syngamy
(fertilization)
Meiosis
multicellular body
(parents and offspring)
Classical genetic analysis involves 3 steps
based on the structure of a eukaryotic life cycle
gametes
Syngamy
(fertilization)
Meiosis
1. Parental Genotypes
Offspring
Classical genetic analysis involves 3 steps
based on the structure of a eukaryotic life cycle
2. Meiotic products = gametes
Syngamy
(fertilization)
Meiosis
1. Parental Genotypes
Offspring
Rules for step 2:
Diploid parents making haploid gamete genotypes
AA parents produce all A gametes
aa parents produce all a gametes
but
Aa parents produce 1/2 A and 1/2 a gametes
MENDEL’S FIRST LAW
Genetic Segregation is Based on Chromosomal Segregation
Classical genetic analysis involves 3 steps
based on the structure of a eukaryotic life cycle
2. Meiotic products = gametes
Syngamy
(fertilization)
Meiosis
1. Parental Genotypes = start
3. Fertilization products = Offspring
Fertilization: Sperm, Egg, and Zygote
Predicting products of fertilization
Step 1 parental genotypes
Steps 2-3 predict gametes and combine them randomly
haploid gametes
haploid
gametes
diploid
offspring
Predicting products of fertilization:
AA x AA
Step 1 AA x AA
Steps 2-3 predict gametes and combine them randomly
All A gametes
All A
gametes
All AA
diploid
offspring
Genotypic ratio: all AA
Phenotypic ratio: all “A”
Predicting products of fertilization:
AA x Aa
Step 1 AA x Aa
Steps 2-3 predict gametes and combine them randomly
1/2 A
All A
1/2
AA
1/2 a
1/2
Aa
Genotypic ratio: 1/2 AA and 1/2 Aa; 1:1
Phenotypic ratio: all “A”
Predicting products of fertilization:
aa x aa
Step 1 aa x aa
Steps 2-3 predict gametes and combine them randomly
All a gametes
All a
gametes
All aa
diploid
offspring
Genotypic ratio: all aa
Phenotypic ratio: all “a”
Predicting products of fertilization:
AA x aa
Step 1 AA x aa
Steps 2-3 predict gametes and combine them randomly
All A gametes
All a
gametes
All Aa
diploid
offspring
Genotypic ratio: all Aa
Phenotypic ratio: all “A”
Predicting products of fertilization:
Aa x aa
Step 1 Aa x aa
Steps 2-3 predict gametes and combine them randomly
1/2 A 1/2 a
Test
Cross
All a
gametes
1/2
Aa
1/2
aa
Genotypic ratio: 1/2 Aa 1/2 aa
Phenotypic ratio: 1/2 “A” 1/2 “a”
Predicting products of fertilization:
Aa x Aa
Step 1 Aa x Aa
Steps 2-3 predict gametes and combine them randomly
1/2 A 1/2 a
Monohybrid
1/2 A
1/4 AA
1/4 Aa
Cross
1/2 a
1/4 Aa
1/4 aa
Genotypic ratio: 1/4 AA 2/4 Aa 1/4 aa
Phenotypic ratio: 3/4 “A” 1/4 “a”
Summary of the six diallelic crosses
(with dominance)
Mendel’s Experimental Results - Single
Genes
A Classical Mendelian Research Program
true breeding
line “A”
true breeding
line “a”
PA
generation
x aa
Mendelian AA
Research
F1 generation
all Aa
Program
backcross used
as a test cross
1/2 Aa, 1/2 aa
monohybrid cross
F2 generation
1/4 AA, 2/4 Aa, 1/4 aa
backcross used
as a test cross
Only monohybrid and test crosses
produce patterns in the progeny
red
1/2 red
x
blue
1/2 blue
red
3/4 red
x
red
1/4 blue
Only monohybrid and test crosses
produce patterns in the progeny
Aa
red
1/2 Aa red
x
aa
blue
1/2 aa blue
Aa
red
3/4 A_ red
x
Aa
red
1/4 aa blue
Brain Teasers
•
Mother and father both find the taste of phenylthiourea very bitter, but
three of their four children find it tasteless. Assuming that this difference
is caused by a single gene with two alleles, is the non-taster phenotype
dominant or recessive (circle the correct answer)? What kind of cross
is this? Be prepared to explain with a diagram of the cross that
identifies phenotypes and their genotypes.
•
Mother finds the taste of phenylthiourea very bitter, but father and three
of their four children find it tasteless. Assuming that this difference is
caused by a single gene with two alleles, is the non-taster phenotype
dominant or recessive (circle the correct answer) )? What kind of cross
is this? Be prepared to explain with a diagram of the cross that identifies
phenotypes and their genotypes.
Remember
Monohybrid crosses provide the most information:
Informing about both dominance and the number of genes
...and the parents in monohybrid crosses look alike
Test crosses also produce different progeny phenotypes, but
...whereas the parents in test crosses look different
Hints:
• Each family produced both phenotypes in their
children, so the matings must be either test crosses
or monohybrid crosses.
• Parents look alike in monohybrid crosses, but not in
test crosses.
Brain Teasers
•
Mother and father both find the taste of phenylthiourea very bitter, but
three of their four children find it tasteless. Assuming that this difference
is caused by a single gene with two alleles, is the non-taster phenotype
dominant or recessive (circle the correct answer)? What kind of cross
is this? Be prepared to explain with a diagram of the cross that
identifies phenotypes and their genotypes.
Two progeny phenotypes, parents alike:
Therefore a monohybrid cross, taster dominant:
Taster(Aa) x Taster(Aa)
3 Non-taster (aa) and Taster (AA, Aa)
Brain Teasers
•
Mother finds the taste of phenylthiourea very bitter, but father and three
of their four children find it tasteless. Assuming that this difference is
caused by a single gene with two alleles, is the non-taster phenotype
dominant or recessive (circle the correct answer) )? What kind of cross
is this? Be prepared to explain with a diagram of the cross that identifies
phenotypes and their genotypes.
Two progeny phenotypes, parents not alike:
Therefore a test cross, but can’t resolve
dominance relationships:
Aa x aa
1/2 Aa and 1/2 aa
No Dominance
• Some heterozygotes have phenotypes unlike either
homozygote. The alleles of these heterozygotes are
said not to exhibit dominance.
• In this case, each genotype has a unique phenotype.
Incomplete Dominance
white
Summary of the six diallelic crossses
(no dominance)
Mendel’s Second Law
Independent Assortment
Two genes will be inherited independently of one another
Classical genetic analysis involves 3 steps
based on the structure of a eukaryotic life cycle
2. Meiotic products = gametes
Syngamy
(fertilization)
Meiosis
1. Parental Genotypes = start
3. Fertilization products = Offspring
Mendel’s Second Law
1
1/4 AB
1/4 aB
1/4 Ab
1/4 ab
2
3
Dihybrid Cross - Peas
Dihybrid cross - Eye color
BIG
1/4 AB
1/4 aB
1/4 Ab
1/4 ab
and
MESSY
Using punnet
squares can get
cumbersome
Forking Diagram
27/64
Sex Linkage
The Human Chromosome Complement:
22 autosomes and a heteromorphic pair of sex chromosomes
X
X
Human Y Chromosome
Homogametic and Heterogametic Genotypes
XX
XY
In our species
XX = female, XY = male
Other species
XY = female, XX = male
Sex Linkage
Figur e 1. Se x Link age
A
centromer e
B
X chromosome
Y chromosome
centromer e
C
pair ing
reg ion
differential
reg ions
Y Li nkage
X Li nkage
pseudo- autosomal
Practice
Hemophelia
Victoria’s Clan
Color Blindness
normal color vision:
XCXC, XCXc
XCY
color blindness:
XcXc
XcY
Inheritance of White Eye
wild type (red) eye
white eye
Inheritance of White Eyes
Inheritance of White Eyes
White eye revisted
Some species have heterogametic females
homogametic
males
heterogametic
females