Beyond Mendelian Genetics Incomplete Dominance, Codominance, Multiple Alleles, & Polygenic Traits

Beyond Dominant & Recessive What will happen if some alleles are neither dominant nor recessive? What will happen if some traits are controlled by more than one gene?

Beyond Dominant & Recessive incomplete dominance – one allele is not completely dominant over the other e.g., white flower crosses with a red flower = pink flower codominance – both alleles contribute to the phenotype e.g., red cow crosses with a white cow = roan cow multiple alleles – > 2 alleles control the phenotype e.g., coat color of rabbits polygenic traits – several genes control the trait e.g., skin color in humans

Types of Dominance

Incomplete Dominance

Incomplete Dominance neither allele is dominant or recessive organisms with two different phenotypes produces offspring with a third phenotype that is a blending of the parents; an intermediate phenotype is shown

Incomplete Dominance R R Rr r r e.g., cross a red flower (RR) with a white flower (rr) & the offspring will be pink (Rr)! genotype(s) of offspring: Rr genotypic ratio: 4:0 phenotype(s) of offspring: pink phenotypic ratio: R R Rr pink r r

Incomplete Dominance when dealing with incomplete dominance, doesn’t matter what letter you use, as long as heterozygous condition always denotes the intermediate trait. below R is used, but could also use W or P. Ww = pink, Pp = pink if these letters are used

Incomplete Dominance R r RR Rr rr R r e.g., in another flower, if red (Rr) & blue (rr) flowers are crossed, they produce a 3rd purple (Rr) flower what would be the genotypic ratio and phenotypic ratio if you crossed two purple flowers? genotype(s) of offspring: RR, Rr, rr genotypic ratio: 1:2:1 phenotype(s) of offspring: red, purple, blue phenotypic ratio: R r RR red Rr purple rr blue R r

CoDominance

CoDominance: neither allele is really dominant or recessive some equally dominant parents with different phenotypes produce an offspring with a 3rd phenotype 3rd phenotype will show both parental phenotypes simultaneously (at same time)

CoDominance: e.g., in cattle & horses, if you cross a pure red (RR) with a pure white (WW), you get (RW) which produces a 3rd phenotype, a roan color these cattle or horses actually have both red & white hairs intermixed, or are spotted R R RW roan W W

CoDominance: What will the offspring be like if you cross a roan colored horse with a white colored horse (RW X WW)? genotypes? genotypic ratio? phenotypes? phenotypic ratio? R W W W

CoDominance: R W RW roan WW white W W What will the offspring be like if you cross a roan colored horse with a white colored horse (RW X WW)? genotypes: RW, WW genotypic ratio: 2:2 or simplified to 1:1 phenotypes: roan, white phenotypic ratio? R W RW roan WW white W W

Which are these Examples of? + = = +

Multiple Alleles 2 or more possible alleles for the same gene within a population however, individuals within the population are only able to hold two of them an individual can not have more than 2 alleles for a trait, but there are more than 2 allele possible for that trait in the population non-human examples: rabbit fur color, mice skin color, eye color in flies, wing size in flies

Multiple Alleles e.g., blood type - single gene, but 4 phenotypes Blood Types in Humans AA or Ao  Type A BB or Bo  Type B AB (codominant)  Type AB oo (recessive)  Type O

Polygenic Traits show a wide range of phenotypes many traits are produced by interaction of several genes traits controlled by 2 or more genes (1 gene has 2 alleles) show a wide range of phenotypes phenotype is produced by the interaction of more than 1 pair of alleles e.g., in humans… eye color, skin color, & height

Examples of Polygenic Traits

Examples of Polygenic Traits

Examples of Polygenic Traits