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SPECIAL TYPES OF CROSSES

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1. Incomplete 1. Incomplete Dominance:Dominance:

When the phenotype of When the phenotype of offspring result in a offspring result in a BLENDING of the trait.BLENDING of the trait.

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The RED (RR) color of a four The RED (RR) color of a four o’clock flower is o’clock flower is incompletely incompletely dominantdominant to the WHITE (rr) to the WHITE (rr) colored flower…hybrid colored flower…hybrid offspring will be PINK (Rr).offspring will be PINK (Rr).

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Incomplete DominanceIncomplete DominanceExample:Example: four o’clock flowersfour o’clock flowersred (RR) x white (WW)

RR = red flowerRR = red flowerWW = white flower

R

R

W W

RW

RW

RW

RW

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Incomplete DominanceIncomplete Dominance

RWRW

RWRW

RWRW

RWRW

RR

RR

WW

All RW =All RW = pink pink(heterozygous pink)(heterozygous pink)

produces theproduces theFF11 generation generation

W

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2. Codominance2. CodominanceWhen BOTH alleles for a trait are When BOTH alleles for a trait are

represented in the heterozygous represented in the heterozygous offspring. offspring.

The RED (RR) color of hair in horses is COdominant to white hair (WW)…hybrid offspring will have a ‘roan’ coat (RW) color… BOTH red and white hairs (“ROAN”)!

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Red x White: RR x WW Roan x Roan: RW x RW R R R WW R W WRW RW

RW RWRR RWRW WW

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Genotype: RW Genotype: RR, RW, WW Genotypic Ratio: 4 RW : 0 Genotypic Ratio: 1 RR : 2 RW : 1 WWPhenotype: Roan Phenotype: Red, Roan, WhitePhenotypic Ratio: 4 Roan : 0 Phenotypic Ratio: 1 Red: 2 Roan: 1 White

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3. Multiple Alleles – genes for which more than two different alleles exist.

Ex: skin color, eye color, hair color, blood type

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Example:Example: blood type blood type

A (dominant), B (dominant)A (dominant), B (dominant) AB (codominant), O (recessive) AB (codominant), O (recessive)

IIA A - blood type- blood type I IBB – blood type – blood type I IAAIIBB – blood type – blood type

ii – blood typeii – blood type

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GENOTYPES PHENOTYPESGENOTYPES PHENOTYPES

OO Type OOO Type O

AO Type AAO Type A

AB Type ABAB Type AB

BB Type BBB Type B

BO Type BBO Type B

AA Type AAA Type A

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Codominance ProblemCodominance Problem

Example: A woman heterozygous for type B blood marries a man who is heterozygous type A. What are the possible genotypes and phenotypes of their children? heterozygous female Type B (IBi) x

heterozygous male Type A (IAi)

IAIB IBi

IAi ii

IB

IA i

i

Geno – 1 IA IB :

1 IA i :

1 IB i :

1 ii

Pheno – 1 Type A : 1 Type B: 1 Type AB: 1 Type O

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Another Codominance Another Codominance ProblemProblem

• Example:Example: male Type O (ii) x female type AB (IAIB)

IAi IBi

IAi IBi

1/2 = IAi1/2 = IBi

i

IA IB

i

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CodominanceCodominanceQuestionQuestion::

If a boy has a blood type O and If a boy has a blood type O and his sister has blood type AB, his sister has blood type AB, what are the genotypes and what are the genotypes and phenotypes of their parents?phenotypes of their parents?

boy - boy - type O (ii) type O (ii) X girl - X girl - type type AB (IAB (IAAIIBB))

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CodominanceCodominance

Answer:Answer:

IAIB

ii

Parents:Parents:genotypesgenotypes = IAi and IBiphenotypesphenotypes = A and B

IB

IA i

i

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4. Sex-linked Traits4. Sex-linked TraitsGenes carried only on the Genes carried only on the sex sex

chromosomes.chromosomes. Ex: color blindness or hemophiliaEx: color blindness or hemophilia

XXXX genotype for females genotype for femalesXYXY genotype for males genotype for males

1. Most 1. Most sex-linked traitssex-linked traits are are carried on carried on XX chromosome chromosome

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1. Most 1. Most sex-linked traitssex-linked traits are are carried on carried on XX chromosome chromosome2. The Y chromosome in males is 2. The Y chromosome in males is small and cannot carry/mask any small and cannot carry/mask any traits.traits.3. Therefore, males express sex-3. Therefore, males express sex- linked traits more often than linked traits more often than females.females.** Males that are affected don’t usually live past a certain age to pass on their

traits to offspring.

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Genotypes PhenotypesXX Normal FemaleXX* Carrier FemaleX*X* Affected FemaleXY Normal MaleX0Y Affected Male

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Sex-linked TraitsSex-linked Traits

Sex ChromosomesSex Chromosomes

XX chromosome - female Xy chromosome - male

fruit flyeye color

Example: Example: Eye color in fruit Eye color in fruit fliesflies

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Sex-linked Trait Sex-linked Trait ProblemProblem

Example: Eye color in fruit flies (red-eyed male) x (white-eyed female)

XRY x XrXr

Remember: the Y chromosome in males does not carry traits.

RR = red eyedRr = red eyedrr = white eyedXY = maleXX = female

XR

Xr Xr

Y

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Sex-linked Trait Solution:Sex-linked Trait Solution:

XR Xr

Xr Y

XR Xr

Xr Y

50% red eyed female

50% white eyed male

XR

Xr Xr

Y

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A woman who is a carrier for colorblindness marries a man who is colorblind. What are the possible genotypes and phenotypes of their children?

Genotypes:

Phenotypes:

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Female CarriersFemale Carriers