Biology 12

Unit 3: HeredityMendelian Genetics

Genes and Heredity

• Mendel’s Experiments

• Alleles

• Punnett Square

• Monohybrid & Dihybrid Crosses

Mendelian Genetics

• Traits are controlled by genes

• Law of Dominance: dominant and recessive

• Law of Segregation: genes separate during meiosis

• Law of Independent Assortment: genes for one trait will separate independently of how other traits separate

More Mendelian

• Mendel worked with pea plants• True breeding tall plants are homozygous for

the tall (T) allele• True breeding dwarf plants are homozygous

for the dwarf (t) allele

• When the parental generation (P1) was made up of true breeding tall and true breeding dwarf plants all offspring were tall

P1 phenotype: Tall X Dwarf

genotype: TT X tt

gametes: T t

F1 phenotype: Tall X Tall

genotype: Tt X Tt

gametes: T t T t

F2 phenotype: 3 Tall : 1 dwarf

genotype: 1 TT : 2 Tt : 1 tt

Punnett Square

• This F1 generation of tall plants were all heterozygous: one tall allele and one dwarf allele

• When F1 individuals were crossed Mendel found a 3:1 ratio of tall to short plants

T

t

T

Tt

Tt

Tt

Tt

t

T

T

t

Tt

Tt

tt

TT

t

Possible Crosses

D

D

d

Dd

Dd

dd

DD

d

E

e

e

Ee

ee

ee

Ee

e

C

c

C

Cc

Cc

Cc

Cc

c

A

A

A

AA

AA

AA

AA

A

B

B

B

Bb

BB

Bb

BB

b

f

f

f

ff

ff

ff

ff

f

Monohybrid Cross

Test Cross

• When an individual showing the dominant phenotype is crossed with a homozygous recessive individual to determine the genotype

t

T

t

?t

Tt

?t

Tt

?

If dwarf individuals are observed in the offspring, then the unknown genotype is heterozygous

Dihybrid Crosses

• Mendel looked at two traits at a time to determine the Law of Independent assortment

• True breeding Yellow Round Seed cross with true breeding green wrinkled seed

Let Y be yellow, y is greenLet R be round, r is wrinkled

P1: genotype YYRR X yyrr gametes YR yrF1: genotype YyRr X YyRr gametes YR Yr yR yr

F2:YR

YR Yr yR yr

yr

yR

Yr

Dihybrid Crosses

YR

YR Yr yR yr

yr

yR

Yr

YYRR

YYRr

YYRr

YYrr

YyRR

YyRR YyRr

YyRr yyrryyRr

yyRryyRRYyRr

Yyrr

Yyrr

YyRr

Mendel’s First & Second LawThe Law of Segregation &

The Law of Independent Assortment

Non-Mendelian Inheritance• Multiple Alleles

– Example: Eye colour in Drosophelia

– Wild Type > Apricot > Honey > White

• Incomplete Dominance– Example: Snap dragons, red X white => pink

• Co-Dominance– Example: Shorthorn cattle, red X white => roan

CR

CR

CW

CRCW

CRCW

CWCW

CRCR

CW

E2

E1

E3

E2E4

E1E3

E3E4

E1E2

E4

Hr

Hr

Hw

HrHw

HrHw

HwHw

HrHr

Hw

• Polygenic Inheritance– Gene interaction: Example Chicken Comb

Single (rrpp)Rose (R-pp)Pea (rrP-)Walnut (R-P-)

• Pleiotropic genes– One gene with multiple effects– Example: Sickle-cell anemia

• Lethal Genes– When a specific genotype is not viable– eg. if AA is lethal, a heterozygous cross would

give a 2:1 ratio (2 Aa : 1 aa – the AA is dead)

Non-Mendelian Genetics

Clip 1

Clip 2

Linkage

• Mendel demonstrated his Law of Independent Assortment using simple traits.

• Some traits do not follow this Law.

• When genes are on the same chromosome they do not separate independently