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Sylvia S. Mader
Immagini e
concetti
della biologia
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
A7 - Mendel:
laws of
inheritance
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Mendel and the laws of inheritance
Before Gregor Mendel (1822-1884) a “blending
model of inheritance” was accepted.
Mendel performed his experiments very carefully
using the common pea plant (Pisum sativum).
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Why the pea plant?
• Easy to grow and mature quickly
• Have a short generation time
• Self-pollinate
• Easy to control the reproduction
• Produce many offsprings
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
How did he performed the first
experiments?
To begin his experiments, Mendel used plants that
showed the same trait generation by generation, also
called pure lines.
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
How did he performed the first
experiments?
Mendel examined seven different traits. Each trait had
only two possible variations.
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
How did he performed the first
experiments?
Mendel crossed two pure-lines that differed for only one
trait (P generation)
The result was the F1
generation
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
How did he performed the first
experiments?
Mendel then crossed the individuals of the F1 generation
Three out of four offsprings in the
F2 generation showed one trait
phenotypic ratio 3:1
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Law of Segregation
“Inheritance of one trait is determined by pairs
of ‘factors’ that segregate in the gamete
formation”
Each gamete contains only one factor from each pair
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Mendel’s factors
Mendel’s “factors” are the genes and a gene can occur in
alternate “variations” called alleles.
The position of a gene on the
chromosome is called locus.
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Dominant or recessive?
When two different alleles are present for one trait, one is
expressed (dominant) while the other one is not (recessive).
A homozygous organism has two copies of the same allele,
a heterozygous organism has one of each type of allele at
the gene locus.
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Genotype Vs phenotype
The gene composition and arrangement of an organism
is its genotype.
The expression of the genes into a trait is referred to as
the phenotype.
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
The inheritance of one trait influences
a different trait?
II experiment: Mendel selected plants that differed
for two traits.
P generation
DIHYBRID CROSS
Tall plants
Green peas
Short plants
Yellow peas
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Dihybrid cross
F1 gametes
F1
F1 gametes
P
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Law of Independent Assortment
“Each pair of factors assort independently:
inheritance of alleles for one trait does not
affect the inheritance for another trait”
Each gamete can contain all possible factor combinations
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Mendel’s and the laws of probability
The probability of genotypes and phenotypes in the final
generation can be predicted using a Punnett square.
Alleles for earlobe shape:
Attached (E)
Free (e)
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Mendel’s and the laws of probability
Genotypic probabilities
EE
=
½x½
=
¼
Ee
=
½x½
=
¼
eE
=
½x½
=
¼
ee
=
½x½
=
¼
Phenotypic ratio
Free earlobe
3
Attached earlobe
1
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Testcrosses
A one-trait testcross determine whether a dominant
phenotype is homozygous dominant or heterozygous.
homozygous dominant
heterozygous
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Mendel’s laws apply to humans
Genetic pedigrees can reveal the patterns of inheritance.
How to create
a pedigree
= female
= unaffected
= male
= affected
= union
Autosomal
recessive
disorder
Autosomal
dominant
disorder
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Mendel’s laws apply to humans
Autosomal recessive disorder
Ex.
Tay-Sachs disease
Cystic fibrosis
Phenylketonuria
Sickle-cell disease
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Mendel’s laws apply to humans
Autosomal dominant disorder
Ex.
Neurofibromatosis
Huntington disease
Achondroplasia
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Genetic disorders
Autosomal disorders can be detected early on.
Amniocentesis, embryonic test
and egg test can be used to detect
genetic disorders.
Egg test used before in vitro fertilization
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Complex inheritance patterns
Incomplete dominance: phenotype of the heterozygous is
an intermediate between its homozygous parents.
1. Pink flowers =
Incomplete dominance
2. In F2 generation all
three genotypes appear
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Complex inheritance patterns
Multiple alleles: most genes exist in a large number of
allelic forms.
A,B,0 blood groups:
comprises three sets of
alleles at the I locus.
IA
Antigens A on RBC
Dominant
IB
Antigens B on RBC
Dominant
i
No antigens on RBC Recessive
Phenotype
Genotype
A
IAIA , IAi
B
IBIB , IBi
AB
IAIB
0
ii
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Polygenic inheritance
Several genes and the environment
can influence a single multifactorial
traits.
Continuous variations of phenotypes
result in a bell-shaped curve.
Black points = dominant alleles
Color intensity = environmental influence
Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
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Pleiotropy
A single gene influences multiple phenotypic traits
Sickle-cell anemia (SCA)
Autosomal recessive blood disorder characterized by
red blood cells that assume an abnormal, sickle shape.
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Genes are carried by chromosomes
One gene influences multiple phenotypic traits
Genes position in fruit fly Drosophila
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Genes of the X chromosome have a
unique inheritance pattern
X-linked inheritance means that the gene causing the trait or
the disorder, i.e. white eyes, is located on the X chromosome.
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
X-linked inheritance
X-linked alleles have a unique inheritance pattern as they
do not have a corresponding allele on the Y chromosome.
Human X-linked disorders:
• Color blindness
• Muscular dystrophy
• Hemophilia
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Genetic linkage group
Genetic linkage is the tendency of genes
that are located proximal to each other on a
chromosome to be inherited together.
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Mapping the chromosomes
A linkage map is a genetic map of a species that shows
the position of its genes relative to each other in terms of
recombination frequency.
Gene map of chromosome 2 in Drosophila
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012
Mapping the chromosomes
A direct relation exists between recombinant phenotypes
and the distance between alleles
1% recombinants = 1 map unit
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Sylvia S. Mader Concepts of Biology © Zanichelli editore, 2012