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Mendelian Inheritance. ... the outward physical manifestation of internally coded, inheritable, information. Genetics: expression of genes. Gregor Mendel. Father of Genetics 1823-1884. Gregor Mendel. Austrian monk Studied science and mathematics at University of Vienna - PowerPoint PPT Presentation
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Mendelian Inheritance
... the outward physical manifestation of internally coded, inheritable, information.
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Genetics: expression of genes
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Gregor MendelFather of
Genetics1823-1884
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Gregor MendelAustrian 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
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One-Trait InheritanceMendel performed cross-breeding
experiments with pea plants– 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
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Why Peas?Either or
traitsEasy to growMany
offspringEasy to
regulate pollination
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Mendel’s method
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ShortTall
Tall
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Mendel’s HypothesesEach parent has two factors
(alleles)Each parent gives one of
those factors to the offspringTall has TTShort has ttTall is dominantShort is recessive
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Law of SegregationEach 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
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TT tt
T tTt
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TT TT
T TTT
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tt tt
t ttt
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Modern Genetics ViewEach 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
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Law of Segregation and Random Fertilization…
genetic variationAlleles separate during
gamete productionGametes have one allele for
each traitDuring fertilization gametes
combine at random to form individuals of the next generation
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Discovery of Chromosomes in 1900 Confirms Law of Segregation
Chromosomes are in pairs
Each chromosome has one of the allele pair
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Homologous Chromosomes
Chromosomes of the same pair
Each homologue will have one allele for a paired gene
Homologous chromosomes pair up during meiosis
Only one of each homologue will be in each gamete
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Chromosomes line up in a double row.
Meiosis I Metaphase
Assume a T allele on each red chromatid and a t allele on each green chromatid
T T t t
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Chromosomes separateEach each daughter cell gets doubled chromosomes
T T t t
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Doubled Chromosomes
Separate in Second Meiotic Division
T T t t
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Each gamete will have a T allele or a t allele
T T t t
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AlleleMember of a paired gene
– One allele comes from each parent
Represented by a single letter
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Dominant & Recessive AllelesDominant alleles are
expressedRecessive alleles are not
expressed in the presence of a dominant allele– Recessive alleles are only
expressed if both alleles are present
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HomozygousBoth alleles alikeAA or aa
A A a a
or
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HeterozygousAlleles are differentAa
A a
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GenotypeGenetic make upRepresented by allelesTT & Tt are genotypes for
tall pea plantsThis is the "internally coded, inheritable information" carried by all living organisms.
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PhenotypeA traitGenotype
determines the phenotype
Tall is a phenotype
“Think adjective!”
Descriptive
This is the "outward, physical manifestation" of the organism.
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Phenotype: red flowers
Cells contain red granules
Enzymes help convert colorless pigment into red pigment
Most enzymes are proteins
Most traits are produced by the action of proteins.
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Summary: Genotype Versus PhenotypeGenotype
– Refers to the two alleles an individual has for a specific trait
– If identical, genotype is homozygous
– If different, genotype is heterozygousPhenotype
– Refers to the physical appearance of the individual
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Punnett SquareTable 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
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Monohybrid TestcrossIndividuals with recessive phenotype always
have the homozygous recessive genotypeHowever, Individuals with dominant
phenotype have indeterminate genotype
– May be homozygous dominant, or
– HeterozygousTest cross determines genotype of
individual having dominant phenotype
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One-Trait Test CrossUnknown is Heterozygous
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One-Trait Test CrossUnknown is Homozygous Dominant
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Human Genetic DisordersAutosome - Any chromosome other than a sex
chromosomeGenetic 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
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AlbinismLack of
pigment– Skin– Hair– Eyes
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Amino Acids
Melanin Pigment
Enzyme
A a
AA = Normal pigmentationAa = Normal pigmentationaa = Albino
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A man & woman are both carriers (heterozygous) for albinism. What is the chance their children will inherit albinism?
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AA = Normal pigmentationAa = Normal pigmentation (carrier)aa = Albino
Man = Aa
Woman = Aa
A
a a
A
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A
a
a
A AA
Aa
Aa
aa
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AA
Aa
Aa
aa
Genotypes1 AA, 2Aa, 1aa
Phenotypes3 Normal1 Abino
Probability25% for albinism
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Dwarfism = DNormal height = d
DD = DwarfismDd = Dwarfismdd = Normal height
Dwarfism
Dwarf Band
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A man with heterozygous dwarfism marries a woman who has normal height. What is the chance their children will inherit dwarfism? Dwarfism is dominant.
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d
d
D Dd
dd
Dd
dd
d
DD = DwarfDd = Dwarf dd = Normal
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Dd
dd
Dd
dd
Genotypes2 Dd, 2dd
Phenotypes2 Normal2 Dwarfs
Probability50% for Dwarfism
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Polydactyly (more than the normal amount of fingers or toes) is a dominant trait!
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Law of Independent AssortmentThe inheritance of one gene does
not influence the inheritance of another gene if they are on separate chromosomes.
The gene for albinism does not affect the gene for dwarfism
When in the Course of human events it becomes necessary for one genome to dissolve the temporary bonds which have connected them with another and to assume among the powers of the earth, the separate and equal station to which the Laws of Nature and of Nature's God entitle them, a decent respect to the variation that mankind requires that they should declare the causes which impel them to the separation.
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Two Trait ProblemA heterozygous tall plant that is
also heterozygous for yellow seeds is crossed with another plant with the same genotype
Tall and yellow seeds are dominant to short and green seeds.
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Tall YellowTtY
y
TY
What gametes can each parent produce?
TytYty
TtYy
TYTytYty
Tall Yellow
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9 Tall-Yellow
Match gametes on a Punnent Square
TY
Ty
tY
ty
TY Ty tY ty
TtYy
TtYy
TTYY TTYy TtYY
TTYy TTyy Ttyy
TtYY TtYy ttYY ttYy
TtYy Ttyy ttYy ttyy
3 Tall-Green3 Short-Yellow
1 Short-Green
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A man with blue eyes and normal height marries a woman with heterozygous brown eyes and heterozygous dwarfism. What are the possible phenotypes of their children? Dwarfism & brown eyes are dominant.
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Normal height-Blue
ddbb
db
What gametes can each parent produce?
DdBb
DBDbdBdb
Dwarf-Brown
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DB
DbdB
db
Match gametes on Punnent Square
db
DdBb
Ddbb
ddBb
ddbb
Dwarf-Brown eyes
Dwarf-blue eyes
Normal height-Brown eyes
Normal height-Blue eyes
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