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Genetics (patterns of inheritance)
MENDELIAN GENETICS
branch of biology that studies how
genetic characteristics are inherited
MENDELIAN GENETICS
Gregory Mendel, an
Augustinian monk (18221884), was the first who
systematically studied
how genes work
GENE • Portion of DNA with meaningful
information  determines a characteristic
(eyes color, type of hair, height, etc.)’
Transcription
Translation
Proteins: characteristic
• Genes are passed from one generation to
another by means of reproduction  offspring
 Mendel published his
work in 1866, but his
ideas were not accepted
until 1900
• In organisms with sexual reproduction
Fertilization and Meiosis (crossing-over,
independent assortment) ensure genetic
variability
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Genetics (patterns of inheritance)
Why was Mendel successful?
A) He used pea plants, whose sexual reproduction he could easily control. Pea plants…
 Are easy to grow
 Come in many readily distinguishable
varieties
 Are easily manipulated
 Can self-fertilize
B) He studied simple well-defined
characteristics (or traits), such as flower color,
and he varied one trait at a time. Previous
investigators had tried to study many complex
traits, such as human height or intelligence
C) He repeated his crosses multiple times and
applied statistical tests to his results
Some genetics terms to know…
LOCUS (pl. loci) Specific place where a gene is located in a chromosome
Sickle cell
hemoglobin
Normal
hemoglobin
Locus
Free or
Unattached
earlobes
Attached
earlobes
Blood type A
ALLELES
Blood type O
Homologous chromosomes:
Diploid organism
Alleles for blood type
•Two or more forms of a gene may be present in the population
•Alleles are different “versions” of a gene located at the same locus in homologous
chromosomes
•In diploid organisms, only two alleles may be present in a given individual
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Genetics (patterns of inheritance)
GENOTYPE
 Combination of alleles present in an individual from mother and
father. The genotype is always diploid
Green
Yellow
(e.g. blood type: AO, AB, etc)
pods (GG)
pods (gg)
 Letters in uppercase and lowercase are used to
gametes: g
name the genes present in one individual
gametes: G
Alleles… G = Green pods, g = yellow pods
Homozygous
Genotypes
Heterozygous
Genotype
GG, gg
Gg
Two identical alleles for
a trait or characteristic
Breeding GG x gg
Two different
alleles for a trait
Phenotype: all Green pods
Genotype: Gg
PHENOTYPE
 The way each combination of alleles expresses in the
organism as a result of the genetic interaction. What we “see”
in an organism as a result of genes interaction
 The “outcome” of the genotype, the observable
characteristic or trait
Carrier: Any individual
who is heterozygous opr
hybrid for a characteristic
Then…what allele expresses (is visible) in the individual?
Mendel’s laws of heredity
1) Law of dominance
 If two different alleles are present in an
individual… the dominant allele overshadows or
masks the recessive allele. In heterozygous organisms,
only the dominant version of the trait is observed
Green
pods (GG)
Yellow
pods (gg)
gametes: G
gametes: g
Breeding GG x gg
A Punnett square shows the possible combinations
of parents’ gametes and the possible offspring
genotypes that result from each cross.
DOMINANT
and RECESSIVE ALLELES
Masks the effects of other
alleles for the trait
(characteristic) when an
heterozygous genotype is
present. The upper case
(E) is designated for the
dominant,
Will not express unless it
is present in a recessive
homozygous (gg)
genotype. The lower case
is used for the recessive
alleles (e)
Phenotype: all Green pods
Genotype: Gg
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Genetics (patterns of inheritance)
During the production of sex cells or gametes in meiosis,
the DNA information from mother and father segregate
(separate) in order to become haploid (N)
2) Law of
segregation
What Mendel did
•Monohybrid cross: only one trait is
involved in the analysis
What happens during meiosis
Diploid parents (2n)
Interphase (S) DNA replication
Anaphase I:
Different traits end up
in different sex cells
(gametes):
Segregation
What Mendel observed
Phenotype 3:1
75% Green pod
25% White pod
Genotype:
50% Gg
25% GG
25% gg
3) Law of independent assortment
What Mendel did
•Dihybrid cross: two traits are analyzed together
During the production of sex cells
or gametes in meiosis, the
assortment (left and right) of each
pair of chromosomes is random
compared to the other pairs
F1 x F1
Phenotype 9:3:3:1
9 (G pod / Y seed)
3 (G pod / y seed)
3 (g pod / Y seed)
1 (g pod / y seed)
What happens during meiosis
Meiosis I
Metaphase:
G Independent g
assortment
G
Y y
Y
Option 1
y
g
Y y
Y
Meiosis II
Option 2
y
Y
y
Y
Possible
gametes
G
G
GY
y
Gy
g
g
gY
gy
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Genetics (patterns of inheritance)
Mendelian patterns of inheritance in human traits
 Mendelian genetics allows to explain many inherited traits…
Important!
 Dominant traits are not necessarily “normal” or more common
 Recessive traits do not always mean “bad” traits
 Example: blue eyes are recessive!
Human Disorders Controlled by a Single Gene
 Genetic disorders are anyways normally caused by recessive alleles…
Why?
 Most dominant disorders would kill the individual before reproduction
or severely reduce chances of mating!
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Genetics (patterns of inheritance)
Variations of Mendel’s Laws
A) X-Linked or Sex linked Genes
2 alleles, but located in the X chromosome…
 X and Y chromosomes are non-homologous: different size, different genes!
 Because each chromosome
has many genes, these genes
are inherited as a linkage
group.
 Genes found on Xchromosomes are called X-linked
 The Y is much smaller than the
X chromosome and has less
genes.
 In men, because there is
only one X, genes on the Xchromosomes will be
expressed.
Example: HEMOPHILIA is a X-linked trait in humans.
Let N=normal allele and n=hemophilia allele
If a woman who is heterozygous (carrier) for the trait has children with a man who is normal
Genotype woman=XN Xn and male=XN Y
As meiosis separates alleles for the trait, only one allele can be present for the trait in the sex
cells, the gametes (sex cells) this couple can produce are…
Gametes woman=XN or Xn and male=XN or Y
sex cells male
sex cells female
XN
Y
XN
XN XN
XN Y
Xn
XNXn
Xn Y
For this couple, every baby girl will be normal, but half of their sons will be hemophilic!
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Genetics (patterns of inheritance)
B) Incomplete Dominance
2 alleles, but 3 phenotypes are possible!
 1 gene with 2 alleles, but…
 Homozygous RR is red
 Homozygous rr is white
 With a heterozygous condition,
both alleles express themselves
 The final phenotype in the
heterozygous is a “mix” of the
two alleles
 Hypercholesterolemia (dangerously high levels of
cholesterol in the blood) is incompletely dominant
C) Multiple alleles
3 alleles exist, which leads to multiple phenotypes
 1 gene, that has 3 alleles with different
genetic interactions
 Human blood type has 3 alleles: A, B, O
 A and B are codominant
 Both are expressed in the
heterozygous AB condition
 A and B are dominant over 0
 Genotype A0 will be
phenotype A (blood type A)
 Genotype B0 will be
phenotype B (blood type B)
 As O is recessive to the
other alleles, only the
genotype OO produces
phenotype O (blood type O)
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Genetics (patterns of inheritance)
D) Polygenic Inheritance
 Some traits are determined by several genes located at several different loci (places) on
the same or different chromosomes.
 Polygenic inheritance is the additive or cumulative effects of many
genes on a single phenotype
Skin color determination
 Because there are several loci
for skin color, there are many
possible variations depending
on the number of dominant
genes present in the individual
Aabbccdd
 eye color, hair color, and
height are examples of
polygenic traits
AaBBCCDD
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