SPECIAL TYPES OF CROSSES

1. Incomplete Dominance: When the phenotype of offspring result in a BLENDING of the trait.

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

Incomplete Dominance Example: four o’clock flowers Mendelian Genetics 4/27/2017 Incomplete Dominance Example: four o’clock flowers red (RR) x white (WW) RR = red flower WW = white flower W RW RW R RW RW

Incomplete Dominance W R W All RW = pink (heterozygous pink) Mendelian Genetics 4/27/2017 Incomplete Dominance W R W All RW = pink (heterozygous pink) produces the F1 generation RW

Mendelian Genetics 4/27/2017 2. Codominance When BOTH alleles for a trait are represented in the heterozygous 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”)!

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

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

3. Multiple Alleles – genes for which more than two different alleles exist. Ex: skin color, eye color, hair color, blood type

Example: blood type A (dominant), B (dominant) AB (codominant), O (recessive) IA - blood type IB – blood type IAIB – blood type ii – blood type

GENOTYPES PHENOTYPES AA Type A AO Type A AB Type AB BB Type B BO Type B OO Type O

Codominance Problem IB IA i IAIB IBi IAi ii Mendelian Genetics 4/27/2017 Codominance 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) 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 IB IA i IAIB IBi IAi ii

Another Codominance Problem Mendelian Genetics 4/27/2017 Another Codominance Problem Example: male Type O (ii) x female type AB (IAIB) i IA IB IAi IBi 1/2 = IAi 1/2 = IBi

Mendelian Genetics 4/27/2017 Codominance Question: If a boy has a blood type O and his sister has blood type AB, what are the genotypes and phenotypes of their parents? boy - type O (ii) X girl - type AB (IAIB)

Codominance Answer: IB IA i IAIB ii Parents: genotypes = IAi and IBi Mendelian Genetics 4/27/2017 Codominance Answer: IB IA i IAIB ii Parents: genotypes = IAi and IBi phenotypes = A and B

4. Sex-linked Traits Genes carried only on the sex chromosomes. Mendelian Genetics 4/27/2017 4. Sex-linked Traits Genes carried only on the sex chromosomes. Ex: color blindness or hemophilia XX genotype for females XY genotype for males 1. Most sex-linked traits are carried on X chromosome

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

Genotypes Phenotypes XX Normal Female XX* Carrier Female X*X* Affected Female XY Normal Male X0Y Affected Male

Sex-linked Traits Example: Eye color in fruit flies Sex Chromosomes Mendelian Genetics 4/27/2017 Sex-linked Traits Example: Eye color in fruit flies Sex Chromosomes XX chromosome - female Xy chromosome - male fruit fly eye color

Sex-linked Trait Problem Mendelian Genetics Sex-linked Trait Problem 4/27/2017 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 eyed Rr = red eyed rr = white eyed XY = male XX = female XR Xr Y

Sex-linked Trait Solution: Mendelian Genetics 4/27/2017 Sex-linked Trait Solution: XR Xr Y 50% red eyed female 50% white eyed male XR Xr Xr Y

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:

Mendelian Genetics 4/27/2017 Female Carriers