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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
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
• 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
Gregor Mendel
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
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
• 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
Some phases of meiosis
• 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
• 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
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
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
Some phases of meiosis
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
A few more points
• Testcross
• Backcross
• Incomplete dominance
• Codominance
• The trihibrid cross
• 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.
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
© 2003 John Wiley and Sons Publishers
Incomplete dominance: Crosses between red and white varieties of snapdragons.
© 2003 John Wiley and Sons Publishers
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
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
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
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
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
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)
Forked-Line Method(phenotypic or genotypic ratio)
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
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
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
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
Brachydactyly
Wooly hair - genodermatosis
Ehler-Danlos syndrome – fragile tissue, scars easily, elastic joints
Marfan syndrome – tall thin structure, rupture prone artery
Achondroplasia
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)
Albinism
Xeroderma pigmentosum
Sickle cell anemia
