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8/11/2019 4. Mendelian Inheritance
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Principles of Mendelian
Inheritance
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Gregor Mendel
Experiments in Plant
Hybrids (1866)
particulateinheritance
1822-1884
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Seven Characteristics
Studied by Mendel
Form of ripe seed
Smooth or wrinkled Color of seed albumen
Yellow or green
Color of seed coat
Gray or white
Form of ripe pods
Inflated or constricted
Color of unripe pods Green or yellow
Positions of flowers on
stems
Axial or terminal
Length of stem
Tall or short
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Carefully controlled experiments
(minimal chance of accidental
pollination) with large samplesizes (28,000 plants over 8 years);
either/or expression traits
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1. Principle of Segregation Different expressions of a trait are controlled by
discrete particles that occur in pairs
Offspring inherit one particle from each parent no blending
During fertilization the particles reunite todetermine the expression of the trait in the
offspring.
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Allele, Gene, and Locus
GENEsegment of DNA that directs the
production of a specific protein
ALLELE- Alternate forms of a gene that
can direct the cell to produce slightly
different forms of the same protein and
ultimately, different expressions of traits
LOCUSlocation of a gene on a
chromosome
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Heterozygous and Homozygous
Homozygous
Two copies of the
same allele present at
a locus
For example, TT
Heterozygous
Two different alleles at
a locus
For example, Tt
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Genotype and Phenotype
Genotype
The specific genetic
makeup of a trait
Represented by letters
For example, Tt
Phenotype
The physical
expression of the
genes, or what we see
For example, tall or
short plant
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2. Dominance and
Recessiveness Some traits are masked and therefore not
expressed.
Dominant: a trait governed by an allele that isexpressed in the presence of another, different
allele.
Recessive: a trait that is not expressed in thepresence of another allele.
There must be two alleles of the same recessive trait
to express that trait.
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F1Generation
F2Generation
Yellowparents
Green
parents
Allyellow
F0GenerationMendels Pea Plant
Experiments
showed that in theF2Generations there wasa 3:1 phenotypic ratio.
Yellow seeds are dominant
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F1Generation
F2Generation
Yellow
parents
Green
parents
Allyellow
F0Generation
yy YYx
Yy
Yy Yy YYyy
Hybrid cross
Self-fertilizeYellow seeds are dominant
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yy
Y
Yy Yy
Y
Y
Y
Male gametes
Fe
malegametes
F0
Generation
100% of F1generation
plants are yellow
Yy = yellow
y y
Punnett Square/Mating Matrix
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3. Law of Independent
Assortment
Traits governed by different systems of
inheritance, e.g., smooth pea pods and seed
color, show no pattern of interdependence.
In other words, alleles for different traits sortindependently in the F1and subsequent
generations.
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LinkageAn exception to Independent Assortment
Genes whose loci are close together, on
the same chromosome, are often linked
(inherited together)
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F1Generation
F2Generation
Wrinkled,
greenparents
Smooth,
yellowparents
Allsmooth,yellow
F0Generation
Yellow is dominant YY
SSYY ssyyx
SsYy
Smooth,green
Wrinkled,yellow
Wrinkled,green
Smooth,yellow
Hybrid cross
Self-fertilize
Smooth is dominant SS
9 3 3 1
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sySY
Male gametes
Femalegamete
s
F1GenerationSelf-fertilizationSy sY
sy
SY
Sy
sY
SSYY SSYy SsYY SsYy
SSYy
SsYY SsYy ssYY ssYy
SsYy Ssyy ssYy ssyy
SSyy SsYy Ssyy
Yellow is dominant
Smooth is dominant
Smooth, yellow 9
Smooth, green 3
Wrinkled, yellow 3
Wrinkled, green 1
Phenotypic ratio 9:3:3:1
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Genetics Problem
AA Aa
Aa aa
A
a
a
A
The probability of the woman having normal height
is 25%, because the phenotypic ratio is:
3 abnormal : 1 normal, or 3/4
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Mendelian Inheritance in Humans
Monogenic vs.Polygenic
Online Mendelian
Inheritance in Man
http://www.ncbi.nlm.nih.gov/Omim/
http://www.ncbi.nlm.nih.gov/Omim/http://www.ncbi.nlm.nih.gov/Omim/http://www.ncbi.nlm.nih.gov/Omim/http://www.ncbi.nlm.nih.gov/Omim/8/11/2019 4. Mendelian Inheritance
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ABO blood system
Governed by ABO locus
on chromosome 9
Alleles determine which
antigens are present onblood cells
Genotype Phenotype
AO, AA A
BO, BB B
AB AB
OO O
Codominant
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Common misconceptions
Eye color
Recessive alleles arecompletely hidden
Dominant alleles are mostcommon
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Tongue folding
Darwins tubercle
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Pedigree analysis
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Modes of Mendelian Inheritance
1. Autosomal dominant
2. Autosomal recessive
3. X-linked recessive4. X-linked dominant
5. Y-linked
6. Mitochondrial
} Sex-linked
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Autosomal dominant
Governed by (carried on) autosomesno
sex bias
If you inherit just one copy of the allele, the
trait will be expressed
Aa = dominant achondroplasia allele (A) is
expressed over the recessive normal (a)
allele
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Short stature, torso & head normal in
size, but
Short limbs, lordosis
Carried on Chromosome pair 4
QUESTION: Are these brothers from
one zygote? (identical or fraternal
twins?)
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The dwarf, Seneb,
and his family,
c. 2600 BC.
What is his genotype?
What genotype are
his children?
WHY?
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Autosomal recessive
MUST be homozygousto be affected
Heterozygous will not beaffected
If both parents have thecondition, all offspringwill be affected
Albinism
QUESTION: If bothparents are carriers canthey have albinochildren?
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Sex-linked traits
Controlled by loci on sex chromosomes
X or Y
Hemophilia
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Males can be HEMIZYGOUS for a trait, such as hemophilia
- they can have only one copy of the allele AND express the
trait because the X and Y chromosomes are not truly
homologous
(These are
chromosomes that have
replicated during cell
division.)
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Mutation
Alice
Alexandria and Tzar Nicholas
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Orange = 75% are male
Tortoiseshell and calico =always female; if male are
sterile
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Non-Mendelian inheritance
Pleiotropy one gene influences more than one trait
Polygenic inheritance (NON-monogenic)
multiple genes for one trait includes most traits of the skeleton
Genotype = phenotype
strong environmental effects
Also: Mitochondrial inheritance
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Pleiotropy
A single geneinfluences more than
one phenotypic
expression
Phenylketonuria
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Environmental influence Many polygenic traits are influenced by the environment
Most Mendelian traits will not be influenced by theenvironment
Can human culture affect
gene expression?
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Mitochondrial inheritance
Mitochondria in cytoplasm of cells
mtDNA directs the conversion of energy
within the cell
Mother to offspring transmission only
Mitochondrial Eve
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Modern Synthesis
A synthesis of Mendels and
Darwin/Wallaces views was not
achieved until the mid-1930s
Modern Synthesisevolution defined as
a two stage process
1. The production and redistribution of variation
2. Natural selection acting on this variation andaffecting reproductive success