MANDELIAN GENETICS

Crosses that deviate from Mandelian

inherintance

TO THE STUDENTS…

Explain codominant alleles.

Calculate the genotypic and phenotypic ratio (1:2:1).

Explain incomplete dominant alleles.

Calculate the genotypic and phenotypic ratio (1:2:1).

Explain multiple alleles

Explain polygenes/polygenic inheritance

CODOMINANT ALLELE

INCOMPLETE DOMINANT ALLELES

MULTIPLE ALLELES

LETHAL ALLELES

POLYGENES/POLYGENIC INHERITANCE

LINKED GENES

SEX-LINKED GENES

Monohybrid

No. Character One Flower colour

Genes/ character One Gene for flower

colour

Alleles/gene Two Dominant: Red (R)

Recessive: White (r)

Phenotype Two Red (RR, Rr) and

White (rr)

Genotype Three RR, Rr, rr

Dihybrid

No.

Character

Two Seed colour

Seed shape

Genes/

character

One/

character

Gene of seed colour

Gene for seed shape

Alleles/gene Two/gene Seed colourDominant: Green

Recessive: Yellow

Seed Shape

Dominant: Round

Recessive: Wrinkle

both alleles of a pair are fully expressed in a heterozygous form

It occurs when both of the contributions of both alleles are visible and do not over power each other in the phenotype.

Eg :MN blood group

CODOMINANT ALLELE

MM

NN

MN

--->x--->x

Mendel Complete dominance

Codominance

The MN blood system is a third (in addition to the ABO and the Rh) system of blood antigens also related to proteins of the red blood cell plasma membrane.

CODOMINANT ALLELE

Two allele = LM and LN

•Ability to produce antigen M •Ability to produce antigenN

•LM LM

•LM LN

•LN LN

have only the M antigen on their red

blood cells, and will be type M.

have only the N antigen on their red

blood cells, and will be type N.

Produce both antigen M and N,

are type MN

Inheritance of MN blood group

P : LMLM X LNLN

TRUE BREEDING

CODOMINANT ALLELE

self cross

G : LM LM LN LN

F1 : LM LN

Inheritance of MN blood group

F1xF1: LM LN X LM LN

G: LM LN LM LN

F2 : LM LM LM LN LMLN LNLN

1 12

CODOMINANT ALLELE

CODOMINANT ALLELE

x

Chestnut Colored Horse Cremello colored horse

Palomino colored Horse

golden colored coat with

a white mane and tail

I remember codominance in the form of an example like so:

CODOMINANT ALLELE

red x white ---> red & white spotted

--->x

a cross between organisms with two different phenotypes produces offspring

with a third phenotype in which both of the parental traits appear together.

A cattle can be red (RR = all red hairs), white (WW = all white hairs), or roan (RW = red & white hairs together).

a) Predict the phenotypic ratios of offspring when a homozygous white cow is crossed with a roan bull.

b) What should the genotypes & phenotypes for parent

if a farmer wanted only cattle with red fur?

A cross between a black cat & a tan cat produces a tabby

pattern (black & tan fur together).

a) What pattern of inheritence does this illustrate?

b) What percent of kittens would have tan fur if a tabby

cat is crossed with a black cat?

INCOMPLETE DOMINANT ALLELE

Eg : Colour ofAntirrhinum (snapdragon)

flower

a form of intermediate inheritance in which one allele for a specific trait is not completely

dominant over the other allele

x --->

--->x--->x

Mendel Complete dominance

Incomplete dominance

1:2:1 ratio of

red:pink:white flowers.

INCOMPLETE DOMINANT ALLELE

The effect of heterozygous

can be detected in the

phenotype

I remember Incomplete Dominance in the form of an example like so:

RED Flower x WHITE Flower ---> PINK Flower

With incomplete dominance, a cross between organisms with two different phenotypes produces offspring with a third phenotype that is a blending of the parental traits.

INCOMPLETE DOMINANT ALLELE

A cross between a blue blahblah bird & a white blahblahbird produces offspring that are silver. The color ofblahblah birds is determined by just two alleles.

a) What are the genotypes of the parent blahblahbirds in the original cross?

b) What is/are the genotype(s) of the silver offspring?

c) What would be the phenotypic ratios of offspring produced by two silver blahblah birds?

INCOMPLETE DOMINANT ALLELE

The color of fruit for plant "X" is determined by two alleles. When two plants with orange fruits are crossed the following phenotypic ratios are present in the offspring: 25% red fruit, 50% orange fruit, 25% yellow fruit. What are the genotypes of the parent orange-fruited plants?

INCOMPLETE DOMINANT ALLELE

Inheritance Definition Example Phenotype -

Genotype

Incomplete

dominant allele

Codominant allele

Multiple allele

Polygenic

inheritance

Lethal gene

Linked gene

Sex-linked gene

One gene having more than two alleles.

All alleles control the same character

Eg: ABO blood type ( IA , IB , IO )

MULTIPLE ALLELES

Monohybrid

No. Character One Flower colour

Genes/ character One Gene for flower

colour

Alleles/gene Two Dominant: Red (R)

Recessive: White (r)

Phenotype Two Red (RR, Rr) and

White (rr)

Genotype Three RR, Rr, rr

No. Character One Human ABO blood

group

Genes/ character One Gene for human

blood group

Alleles/gene

Phenotype

Genotype

Three IA , IB , IO

•Four •A, B, O, AB

IA IO, IA IA , IO IB ,

IB IB , IA IB , IO IO•Six

MULTIPLE ALLELES

TRY THIS…

P: TYPE A X TYPE AB

IA IA IA IB

F1: IA IA IA IB IA IB IA IA

A AB AB A

MULTIPLE ALLELES

Brandon is blood Type O. He has an olderbrother with blood types A and a sister withblood group B.

a) What are the genotypes of his parents with respect to this trait?

b) If Brandon’s sister married with a man with blood group AB, What is the percentage of their child having blood group B?

MULTIPLE ALLELES

an additive effect of two or more geneson a single phenotypic character.

e.g: height

POLYGENES/POLYGENIC INHERITANCE

Quantitative Variation

• AABBCC - very dark

• AaBbCc - intermediate

• Aabbcc - very light

POLYGENES

Differences between multiple alleles and polygenes

Multiple Alleles Polygene Not influence by

environmental factors

genes that have multiple alleles

discontinuous variation

Influence by environmental factors

more than one gene

continuous variation/ quantitative variation

• the interaction between two or more genes to control a single phenotype

Phenotypes Genotypes

Walnut R_P_

Rose R_pp

Pea rrP_

Single rrpp

types of combs on chickens

• One gene masks another

– coat color in mice =

2 genes

• pigment (C) or

no pigment (c)

• more pigment (black=B)

or less (brown=b)

• cc = albino,

no matter B allele

• 9:3:3:1 becomes 9:3:4

•How would you

know that differencewasn’t random chance?

•Chi-square test!

2005-2006

Epitasis in Labrador retrievers

• 2 genes: E & B

– pigment (E) or no pigment (e)

– how dark pigment will be: black (B) to brown (b)

QUESTIONS

1. A dominant allele W confers black fur for mice. A mice that is homozygous recessive (ww) has white fur. David would like to know whether his pet black-furred mice is homozygous dominant (WW) or heterozygous (Ww). How might he determine his pet’s genotype?

1. Two pairs of genes affect comb type in chickens.

When both genes are recessive, a chicken has a

single comb. A dominant allele of one gene, P , gives

rise to pea comb. Yet a dominant allele of the other

(R) gives rise to rose comb. An epistatic interaction

occurs when a chicken has at least one of both

dominants, P_R_, which gives rise to walnut comb.

Predict the phenotypic ratios resulting from a cross

between two walnut-combed chickens that are

heterozygous for both genes (PpRr).

Given information:

• Single comb = pprr

• Pea comb = P_ _ _

• Rose comb = _ _ R _

• Walnut comb = P_R_

P : PpRr X PpRr

G :

F1 : Punnet square

•PR •Pr •pR •pr •PR •Pr •pR •pr

PR Pr pR pr

PR PPRR

walnut

PPRr

walnut

PpRR

walnut

PpRr

walnut

Pr PPRr

walnut

PPrr

pea

PpRr

walnut

Pprr

pea

pR PpRR

walnut

PpRr

walnut

ppRR

rose

ppRr

rose

pr PpRr

walnut

Pprr

pea

ppRr

rose

pprr

single

•9 walnut : 3 pea : 3 rose : 1 single