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Transcript
Chapter 11
Mendelian
Inheritance
Outline
Blending Inheritance
Monohybrid Cross
Law of Segregation
Modern Genetics
Genotype vs. Phenotype
Punnett Square
Dihybrid Cross
Law of Independent Assortment
Human Genetic Disorders
2
3
Gregor Mendel
Austrian monk
Studied science and mathematics at University
of Vienna
Conducted breeding experiments with the
garden pea Pisum sativum
Carefully gathered and documented
mathematical data from his experiments
Formulated fundamental laws of heredity in
early 1860s
Had no knowledge of cells or chromosomes
Did not have a microscope
Gregor Mendel
4
Fruit and Flower of the
Garden Pea
5
Garden Pea Traits
Studied by Mendel
6
Blending Inheritance
Theories of inheritance in Mendel’s
time:
Based on blending: parents of
contrasting appearance produce
offspring of mixed appearance
Mendel’s findings were in contrast
with this
He formulated the particulate theory
of inheritance
Inheritance involves reshuffling of
genes from generation to generation
7
8
One-Trait Inheritance
Mendel performed cross-breeding
experiments
Used “true-breeding” (homozygous) plants
Chose varieties that differed in only one
trait (monohybrid cross)
Performed reciprocal crosses
- Parental generation = P
- First filial generation offspring = F1
- Second filial generation offspring = F2
Formulated the Law of Segregation
Mendel’s Monohybrid Crosses:
An Example
9
10
Law of Segregation
Each individual has a pair of factors
(alleles) for each trait
The factors (alleles) segregate
(separate) during gamete (sperm &
egg) formation
Each gamete contains only one
factor (allele) from each pair
Fertilization gives the offspring two
factors for each trait
Modern Genetics View
Each trait in a pea plant is controlled by
two alleles (alternate forms of a gene)
Dominant allele (capital letter) masks
the expression of the recessive allele
(lower-case)
• Alleles occur on a homologous pair of
chromosomes at a particular gene locus
Homozygous = identical alleles
Heterozygous = different alleles
11
Homologous Chromosomes
12
Genotype Versus Phenotype
Genotype
Refers to the two alleles an individual
has for a specific trait
If identical, genotype is homozygous
If different, genotype is heterozygous
Phenotype
Refers to the physical appearance of
the individual
13
Punnett Square
Table listing all possible genotypes
resulting from a cross
All possible sperm genotypes are
lined up on one side
All possible egg genotypes are lined
up on the other side
Every possible zygote genotypes
are placed within the squares
14
Punnett Square Showing
Earlobe Inheritance Patterns
15
Monohybrid Testcross
Individuals with recessive phenotype always
have the homozygous recessive genotype
However, Individuals with dominant
phenotype have indeterminate genotype
May be homozygous dominant, or
heterozygous
Test cross determines genotype of individual
having dominant phenotype
16
One-Trait Test Cross
Unknown is Heterozygous
17
One-Trait Test Cross
Unknown is Homozygous Dominant
18
Two-Trait Inheritance
Dihybrid cross uses true-breeding plants
differing in two traits
Observed phenotypes among F2 plants
Formulated Law of Independent Assortment
- The pair of factors for one trait segregate
independently of the factors for other traits
- All possible combinations of factors can occur
in the gametes
19
Two-Trait (Dihybrid) Cross
20
Two-Trait Test Cross
21
Human Genetic Disorders
Autosome - Any chromosome other than a sex
chromosome
Genetic disorders caused by genes on
autosomes are called autosomal disorders
Some genetic disorders are autosomal
dominant
- An individual with AA has the disorder
- An individual with Aa has the disorder
- An individual with aa does NOT have disorder
Other genetic disorders are autosomal
recessive
- An individual with AA does NOT have disorder
- An individual with Aa does NOT have disorder,
but is a carrier
- An individual with aa DOES have the disorder
22
Autosomal Recessive Pedigree Chart
23
Autosomal Dominant Pedigree Chart
24
Incomplete Dominance
Heterozygote has phenotype
intermediate between that of either
homozygote
Homozygous red has red phenotype
Homozygous white has white phenotype
Heterozygote has pink (intermediate)
phenotype
Phenotype reveals genotype without test
cross
25
Incomplete Dominance
26
Multiple Allelic Traits
27
Some traits controlled by multiple alleles
The gene exists in several allelic forms (but each individual
only has two)
ABO blood types
The alleles:
 IA = A antigen on red cells, anti-B antibody in plasma
 IB = B antigen on red cells, anti-AB antibody in plasma
 I = Neither A nor B antigens, both antibodies
Phenotype
(Blood Type)
A (actually AA or AO)
B (actually BB or BO)
AB
O (actually OO)
Genotype
IAIA or IAi
IBIB or IBi
IAIB
ii
Inheritance of Blood Type
28
29
Polygenic Inheritance
Occurs when a trait is governed by two
or more genes having different alleles
Each dominant allele has a quantitative
effect on the phenotype; these effects
are additive
Result in continuous variation of
phenotypes
Height in Human Beings
30
Frequency Distributions in
Polygenic Inheritance
31
Terminology
Pleiotropy
A gene that affects more than one
characteristic of an individual
Sickle-cell (incomplete dominance)
Codominance
More than one allele is fully expressed
ABO blood type (multiple allelic traits)
Epistasis
A gene at one locus interferes with the
expression of a gene at a different locus
Human skin color (polygenic inheritance)
32
Environment and Phenotype:
Himalayan Rabbits
33
Review assignment
p.201 Reviewing the chapter (#1-11)
p.202 Testing Yourself (#1-17)
DUE TUESDAY
34