Genetics&

The Work of Mendel

HOMOLOGOUS CHROMOSOME

Pairs of matching chromosomes, with one being inherited from each parent.

ALLELES- alternative forms of the same gene.  When 1 allele masks (hides) the effect of another, that allele is

called DOMINANT and the hidden allele is called RECESSIVE.

Dominant alleles are represented by a CAPITAL letterRecessive alleles are represented by a LOWERCASE letter

A b C d e

A B c d E

Chromosome from MOM

Chromosome from DAD

ALLELES

What are Dominant Genes?

Hereford: white face is dominant

Horned is recessive to polled.

PHENOTYPE (what the organism LOOKS like)Examples of phenotypes: blue eyes, brown fur, striped fruit,

yellow flowers

GENOTYPE the genes present in the DNA of an organism 1 gene from MOM + 1 gene from DAD= 2 genes (2 letters) for

offspring

3 possible GENOTYPES:"TT" – Homozygous Dominant "Tt" - Heterozygous "tt" – Homozygous Recessive

Genotype Symbol

Genotype Vocabulary

Phenotype

TThomozygous DOMINANT

or purebred tall

tall

Ttheterozygous

or hybrid

tall

tthomozygous RECESSIVE 

or purebred

short

short

Gregor Mendel (1822-1884)

Austrian monk

Called the “Father of Genetics" for his study of the inheritance of 7 traits in pea plants.

Mendel collected data for 7 pea traits

Pollen transferred from white flower to stigma of purple flower

all purple flowers result

Mendel’s work

?

self-pollinate

• Bred pea plants– cross-pollinate

true breeding parents– raised seed & then

observed traits– allowed offspring

to self-pollinate & observed next generation

2nd

generation

3:175%purple-flower peas

25%white-flower peas

Looking closer at Mendel’s work

Parents

100%1st

generation(hybrids)

100%purple-flower peas

Xtrue-breedingpurple-flower peas

true-breeding white-flower peas

self-pollinate

Making crosses

• Can represent alleles as letters– flower color alleles P or p– true-breeding purple-flower peas PP– true-breeding white-flower peas pp

PP x pp

PpF1

P X

purple white

all purple

P P

p Pp Pp

p Pp Pp

all the F1 (1st filial generation) offspring

are purple colored hybrids

100% purple offspring

P p

P

p

Pp x Pp

Heterozygous - hybrid

Heterozygous - hybrid

Punnett squares

Pp x Pp

P pmale / sperm

P

p

fem

ale

/ eg

gs

PP

75%

25%

3:1

25%

50%

25%

1:2:1

%genotype

%phenotype

PP Pp

Pp pp pp

Pp

Pp

1st

generation(hybrids)

Aaaaah,phenotype & genotype

can have different ratios

Monohybrid Cross

• Example:Cross between two heterozygotes for brown eyes (Bb)

BB = brown eyesBb = brown eyesbb = blue eyes

B

b

B b

Bb x Bb

malegametes

female gametes

Monohybrid Cross

BB

Bb

Bb

bb

B

b

B b

Bb x Bb

1/4 = BB - brown eyed1/2 = Bb - brown eyed1/4 = bb - blue eyed

1:2:1 genotype 3:1 phenotype

Dihybrid cross

• Other of Mendel’s experiments followed the inheritance of 2 different characters – seed color and

seed shape– dihybrid crosses

Dihybrid cross

true-breedingyellow, round peas

true-breedinggreen, wrinkled peasx

YYRR yyrrP

100%F1generation(hybrids)

yellow, round peas

Y = yellowR = round

y = greenr = wrinkled

self-pollinate

9:3:3:19/16

yellowround peas

3/16greenround peas

3/16yellow

wrinkledpeas

1/16green

wrinkledpeas

F2generation

YyRr

Dihybrid Cross

• Example:cross between round and yellow heterozygous pea seeds.

R = roundr = wrinkledY = yellowy = green

RY Ry rY ry RY Ry rY ry x RY Ry rY ryRY Ry rY ry possible gametes produced

RrYyRrYy x RrYyRrYy

Dihybrid cross

YyRr YyRr

YR Yr yR yr

YR

Yr

yR

yr

YYRR

x

YYRr YyRR YyRr

YYRr YYrr YyRr Yyrr

YyRR YyRr yyRR yyRr

YyRr Yyrr yyRr yyrr

9/16yellowround

3/16greenround

3/16yellow

wrinkled

1/16green

wrinkled

YyRr

Yr yRYR yr

YyRr

YR yr

or

Incomplete Dominance

• Incomplete dominance is when both alleles

are expressed and results in an intermediate

phenotype. The hybrid is a blend of both

alleles.

• When expressing incomplete dominant

alleles, both alleles are represented by

different capitalized letters.

Incomplete dominance

• Hybrids have “in-between” appearance– RR = red flowers– rr = white flowers– Rr = pink flowers

• make 50% less color

RR Rr rr

RRWW

RW

Incomplete dominance

true-breedingred flowers

true-breeding white flowers

XP

100%

100% pink flowers1st

generation(hybrids)

self-pollinate

25%white

2nd

generation

25%red 1:2:1

50%pink

R R

W

W

Homozygous

HomozygousIncompleteDominanceP (parental)generation

RW RW

RW RW

R R

W

W

Incomplete Dominance F1 (First Filial)

generation

100% pink offspring

R W

R

W

Heterozygous

Heterozygous

Incomplete Dominance

F1 x F1

Incomplete dominance

R Wmale / sperm

R

Wfem

ale

/ eg

gs RR

RW WW

RW

25%

1:2:1

25%

50%

25%

1:2:1

%genotype

%phenotype

RR

RW

RW

WW

25%

50%

RW x RW

Co-dominance

• Some alleles are equally strong and neither are masked

by the other. Alleles which are equally expressed are said

to be "co-dominant".

• The hybrid is a mixture of both alleles.