Gregor Johann Mendel (1822 - 1884) Gregor Johann Mendel (1822 - 1884)
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Transcript of Mendel
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Mendelian Genetics
• Genetics - the scientific study of heredity
• Heredity - the process by which physical or mental qualities, abilities, or illness pass from parents to children
• Mendel - the first person (1866) to attempt to explain the principles of heredity
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Key Points
• The basic principles of inheritance
• Characters are inherited through genes
• Alleles are paired
• Some alleles are dominant
• Some alleles are recessive
• During gamete formation, alleles segregate from each other
• Different genes assort independently
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• Gregor Mendel - The Father of Genetics• Came as a poor boy to the Augustinian monastery
in Brunn , Austria in 1843• Ordained priest in 1847• Studied natural sciences at the University in
Vienna in 1851• Made science teacher in 1854• Carried out experiments in the monastery gardens
for 7 years• In 1866, Mendel published his ideas under the title
‘Experiments in Plant Hybridization’ in an attempt to explain the principles of heredity
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Gregor Mendel
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Mendel’s Laws
• Mendel’s 1st Law syn. Mendel’s Law of Segregation
• Members of each pair of alleles separate when gametes are formed
• Mendel’s 2nd Law syn. Mendel’s Law of Independent Assortment
• Two or more pairs of alleles segregate independently of one another during gamete formation
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787 Tall
277 Dwarf
All tall
Tall Dwarf
P
F1
F2
X Tall DwarfP TT x tt Gametes T t
F1 Tt Tall
F1 x F1 Tt x Tt
Gametes T t T t
F2 T t T TT Tt t Tt tt
3 : 1
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• Traits are determined by a pair of factors - genes
• Organism inherit genes from parents
• There are dominant and recessive genes
• Alleles separate during gamete formation
• Genes are transmitted unchanged generation to generation
Tall DwarfP TT x tt Gametes T t
F1 Tt Tall
F1 x F1 Tt x Tt
Gametes T t T t
F2 T t T TT Tt t Tt tt Dwarf
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Some phases of meiosis
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• Pure Breeding - homozygous
• Homozygous - having identical alleles
• Heterozygous - having dissimilar alleles
• Dominant - an allele that manifest in the homozygous or heterozygous state
• Recessive - an allele which is masked by its dominant allele
• Genotype - the genetic constitution of an organism
• Phenotype - the observable properties of an organism
Tall DwarfP TT x tt Gametes T t
F1 Tt Tall
F1 x F1 Tt x Tt
Gametes T t T t
F2 T t T TT Tt t Tt tt
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• Trait - character, any detectable phenotypic property of an organism
• Monohybrid cross - a cross between parents differing with respect to a single specified pair of alleles
• Gene - hereditary unit that occupies a fixed chromosomal locus
• Allele - a mutational form of a given gene
• Filial generation - daughter generation
• Homologous chromosome - similar chromosomes that pair during meiosis
Tall DwarfP TT x tt Gametes T t
F1 Tt Tall
F1 x F1 Tt x Tt
Gametes T t T t
F2 T t T TT Tt t Tt tt
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315 Y/R 108 G/R
All Yellow/Round
Yellow/Round Green/Wrinkled
P
F1
F2
X Yellow Green Round WrinkledP YYRR x yyrr Gametes YR yr
F1 YyRr Yellow/Round
F1 x F1 YyRr x YyRr
Gmt YR Yr yR yr
F2 101 Y/W 32 G/W
9 3 3 1
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F2 YR Yr yR yr
YR YYRR YYRr YyRR YyRr
Yr YYRr YYrr YyRr Yyrr
yR YyRR YyRr yyRR yyRr
yr YyRr Yyrr yyRr yyrr
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Some phases of meiosis
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Key Points
• The basic principles of inheritance
• Characters are inherited through genes
• Alleles are paired
• Some alleles are dominant
• Some alleles are recessive
• During gamete formation, alleles segregate from each other
• Different genes assort independently
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A few more points
• Testcross
• Backcross
• Incomplete dominance
• Codominance
• The trihibrid cross
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• Testcross: A cross between an individual whose genotype may be unknown and an individual who is homozygous recessive for the genes in question.
• Backcross: A cross involving an F1 heterozygote and one of the P1 parents.
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787 Tall
277 Dwarf
All tall
Tall Dwarf
P
F1
F2
X Tall DwarfTest 1 TT x tt Gametes T t
Progeny Tt Tall
Test 2 Tt x tt
Gametes T t t
Progeny t T Tt Tall t tt Dwarf
3 : 1
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© 2003 John Wiley and Sons Publishers
Incomplete dominance: Crosses between red and white varieties of snapdragons.
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© 2003 John Wiley and Sons Publishers
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Monohybrid
Aa x Aa
Gametes: A a
F2 (Punnett Square)
A a
A AA Aa
a Aa aa
F2 Genotypic Ratio
Genotype Frequency
AA 1/4
Aa 1/2
aa 1/4
F2 Phenotypic Ratio
Genotype Frequency
A_ 3/4
aa 1/4
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Dihybrid
F1 x F1 AaBb x AaBb
Gametes: AB Ab aB ab
F2 (Punnett Square)
AB Ab aB ab
AB AABB AABb AaBB AaBb
Ab AABb AAbb AaBb Aabb
aB AaBB AaBb aaBB aaBb
ab AaBb Aabb aaBb aabb
F2
Genotype Frequency
AABB 1
AABb 2
AAbb 1
AaBB 2
AaBb 4
Aabb 2
aaBB 1
aaBb 2
aabb 1
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F2
Genotype Frequency
AABB 1
AABb 2
AAbb 1
AaBB 2
AaBb 4
Aabb 2
aaBB 1
aaBb 2
Aabb 1
Phenotype Frequency
A_B_ 9
A_bb 3
aaB_ 3
aabb 1
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Trihybrid Cross
P AABBCC x aabbcc
F1 AaBbCc
F1 x F1 AaBbCc x AaBbCc
F2
Three methods in obtaining F2 ratios
1. The Punnett Square method
2. The Probability method
3. The Forked-Line method
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The Punnett Square MethodF1 RrYyCcGametes RYC RYc RyC Ryc rYC rYc ryC ryc
Gamete RYC RYc RyC Ryc rYC rYc ryC ryc
RYC
RYc
RyC
Ryc
rYC
rYc
ryC
ryc
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RrYyCc x RrYyCc
Rr x Rr Yy x Yy Cc x Cc
RR Rr rr YY Yy yy CC Cc cc
¼ ½ ¼ ¼ ½ ¼ ¼ ½ ¼
Example 1: Frequency of genotype RRYy cc
Answer: ¼ x ½ x ¼ = 1/32 or 2/64
Example 2: Phenotype Wrinkled–Yellow–Red
Answer: ¼ x (¼ + ½) x (¼ + ½ ) = 9/64
The Probability Method (phenotypic or genotypic ratio)
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Forked-Line Method(phenotypic or genotypic ratio)
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F2 Genotypes (27)
AABBCC AaBBCC aaBBCC
AABBCc AaBBCc aaBBCc
AABBcc AaBBcc aaBBcc
AABbCC AaBbCC aaBBcc
AABbCc AaBbCc aaBbCc
AABbcc AaBbcc aaBbcc
AAbbCC AabbCC aabbCC
AAbbCc AabbCc aabbCc
AAbbcc Aabbcc aabbcc
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F2 Phenotypes
27 A_B_C_
9 A_B_cc
9 A_bbC_
9 aaB_C_
3 aabbC_
3 aaB_cc
3 A_bbcc
1 aabbcc
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Round-Yellow-Red x Wrinkled-Green-White
P RRYYCC x rryycc
F1 RrYyCc
Gametes RYC RYc RyC Ryc rYC rYc ryC ryc
F2 27 R_Y_C_ Round-Yellow-Red
9 R_Y_cc Round-Yellow-White
9 R_yyC_ Round-Green-Red
9 R_yycc Round-Green-White
3 rrY_C_ Wrinkled-Yellow-Red
3 rrY_cc Wrinkled-Yellow-White
3 rryyC_ Wrinkled-Green-Red
1 rryycc Wrinkled-Green-White
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Mendelian Traits in Human (Dominant)• Brachydactyly (short fingers)• Congenital night blindness• Ehler-Danlos syndrome (fragile tissue, elastic
joints)• Achondroplasia (dwarfism)• Huntington’s disease (a neurological disorder)• Marfan syndrome (tall, gangly stature)• Neurofibromatosis (tumorlike growth)• Phenylthiocarbamide (PTC) tasting• Widow’s peak (pointed hairline)• Wooly hair
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Brachydactyly
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Wooly hair - genodermatosis
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Ehler-Danlos syndrome – fragile tissue, scars easily, elastic joints
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Marfan syndrome – tall thin structure, rupture prone artery
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Achondroplasia
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Mendelian Traits in Human (Recessive)• Albinism (lack of pigment)• Alkaptonuria (amino acid disorder)• Hemophilia (sex-linked)• Cystic fibrosis (a respiratory disorder)• Duchenne muscular dystrophy (sex-linked)• Galactosemia• Xeroderma pigmentosum• Phenylketonuria (amino acid disorder)• Sickle cell anemia• Tay-Sachs disease (a lipid storage disorder)• Thalassemia (Cooley’s anemia)
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Albinism
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Xeroderma pigmentosum
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Sickle cell anemia
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