Mendel

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

P

X

Tall TT

P Dwarf

Tall

Gametes T F1

F1

All tall

F1 x F1

F2

3:1 787 Tall

277 Dwarf

t Tt Tall

Tt

Gametes T t F2

Dwarf x tt

T T TT t Tt

x

Tt T

t t Tt tt

Tall Dwarf TT x tt

P

Gametes T F1

Tt Tall

F1 x F1

Tt

Gametes T t F2

t

T T TT t Tt

x

Tt T

t t Tt tt

Dwarf

• 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

Some phases of meiosis

Tall TT

P

Dwarf x tt

Gametes T F1

Tt Tall

F1 x F1

Tt

Gametes T t F2

t

T T TT t Tt

x

Tt T

t t Tt tt

• 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 TT

P

Dwarf x tt

Gametes T F1

Tt Tall

F1 x F1

Tt

Gametes T t F2

t

T T TT t Tt

x

Tt T

t 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

P

X Yellow/Round

Green/Wrinkled

P

Yellow Round YYRR

Green Wrinkled x yyrr

Gametes YR F1

F1 All Yellow/Round 9

F2

3

3

1

YyRr Yellow/Round

F1 x F1 YyRr Gmt YR

315 Y/R 108 G/R 101 Y/W 32 G/W F2

yr

Yr

x

YyRr yR

yr

F2

YR

YR

YYRR YYRr

Yr

yR

yr

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.

P

X

Tall TT

Test 1 Dwarf

Tall

Dwarf x tt

Gametes T

t

Progeny

F1

All tall

Test 2

Tt Tall Tt

Gametes T t F2

3:1 787 Tall

277 Dwarf

Progeny T t

x

tt t

t Tt Tall tt Dwarf

Incomplete dominance: Crosses between red and white varieties of snapdragons. © 2003 John Wiley and Sons Publishers

© 2003 John Wiley and Sons Publishers

F2 Genotypic Ratio

Monohybrid Aa x Aa

Gametes: A

a

F2 (Punnett Square) A

a

A

AA

Aa

a

Aa

aa

Genotype

Frequency

AA

1/4

Aa

1/2

aa

1/4

F2 Phenotypic Ratio Genotype

Frequency

A_

3/4

aa

1/4

F2

Dihybrid F1 x F1

Genotype Frequency

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

AABB

1

AABb

2

AAbb

1

AaBB

2

AaBb

4

Aabb

2

aaBB

1

aaBb

2

aabb

1

F2 Genotype Frequency

Phenotype Frequency

AABB

1

A_B_

9

AABb

2

A_bb

3

AAbb

1

aaB_

3

AaBB

2

aabb

1

AaBb

4

Aabb

2

aaBB

1

aaBb

2

Aabb

1

Trihybrid Cross P F1

AABBCC x aabbcc 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 Method F1 RrYyCc Gametes 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

The Probability Method (phenotypic or genotypic ratio) 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

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