Exceptions to Mendelian Genetics Codominance, Multiple Alleles, Incomplete dominance, and Sex-linked Inheritance

Incomplete Dominance Some alleles are not completely dominant over the other allele and an intermediate phenotype exists These alleles are said to have incomplete dominance When both alleles are present in a heterozygote, they are both expressed in a combined phenotype A blending of the two alleles results When expressing incomplete dominant alleles, both alleles are represented by the same capital letters and lower case letters Example: Pink carnations = neither white or red flowers are dominant so a blending occurs and the flowers are pink

Your Turn: Incomplete dominance If a pink carnation was crossed with a red carnation, will any of the offspring be pink? Use the following alleles: R for red and r for white

R r R R RR Rr Remember: Rr = pink flower RR = red flower offspring = Red (RR) and Pink (Rr) R r RR Rr R R

Codominance Not all alleles are dominant and recessive Some alleles are equally strong and neither are masked by the other These alleles are said to be codominant When both alleles are present in a heterozygote, they are both expressed in the phenotype When expressing codominant alleles, both alleles are represented by different capital letters Example: Roan Horses= both red and white hairs are expressed, one is not dominate over the other

Your Turn: Codominance A male roan horse is crossed with a female red horse. What colors will their foals be? Use the following alleles: CR for red and CW for white

CR CW CR CR CRCR CRCW Remember: CRCR = Female CRCW = male Foals = Roan (CRCW) and Red (CRCR) CR CW CRCR CRCW CR CR

Multiple Alleles Phenotype Genotype A IA IA or IA i B IB IB or IB i AB Multiple alleles: traits that are controlled by 3 or more alleles Example: human blood types 4 different blood types: A, B, AB, and O 3 different alleles: IA, IB, i Codominance is also seen in blood types: when both phenotypes are expressed equally Example: AB blood type Phenotype Genotype A IA IA or IA i B IB IB or IB i AB IA IB O ii

Your Turn: Blood Type Problem Cross a heterozygous blood type A man with a heterozygous blood type B women. What blood types could be found in their children?

IB i IA i IAIB IAi IBi ii Remember: IBi = female IAi = male Offspring= blood type A (IAi), blood type B (IBi), blood type AB (IAIB), and blood type O (ii) IB i IAIB IAi IBi ii IA i

Sex-Linked Traits The presence of a gene on a sex chromosome is called sex-linkage. X-linked genes = genes found on the X chromosome Can be found in BOTH males and females Y-linked genes = genes found on the Y chromosome Will be found ONLY in males

XY = male (Y makes the guy!) Sex-Linked Traits X Y Remember: XX = female XY = male (Y makes the guy!) X X

WHAT IS THE PROBABILITY OF HAVING A MALE OFFSPRING? Your Turn X Y WHAT IS THE PROBABILITY OF HAVING A MALE OFFSPRING? XX XY X X

Sex-Linked Traits Examples of sex-linked traits: Red-green colorblindness Male pattern baldness Hemophilia Eye color in fruit flies

Sex-Linked Traits XNY XnY XNXN XNXn XnXn Genotype Phenotype Male – Not colorblind Male - Colorblind Female – Not colorblind Female – Not colorblind*** Female - Colorblind Example Problems: Red-green colorblindness is a recessive X-linked trait XNY XnY XNXN ***XNXn is a carrier. She does NOT exhibit the trait, but she CAN pass it on to her children. XNXn XnXn

Your Turn: Sex-Linked Traits: Colorblindness 1 PROBLEM 1: Normal vision father X Colorblind mother Will any of the children be colorblind?

Xn Xn XN Y XNXn XnY Remember: XnXn = female XNY = male Children = Female carriers (XNXn), Colorblind Males (XnY) Xn Xn XNXn XnY XN Y

Sex-Linked Traits: Colorblindness PROBLEM 2: Colorblind father X Normal vision mother (not a carrier) Will any of the children be colorblind?

XN XN Xn Y XNXn XNY Remember: XNXN = female XnY = male Children = Female carriers (XNXn), Normal Males (XNY) XN XN XNXn XNY Xn Y

Sex-Linked Traits: Colorblindness PROBLEM 3: If one of the daughters from Problem 2 marries a normal vision husband, will her sons have normal vision?

XN Xn XN Y XNXN XNXn XNY XnY Remember: XNXn = female XNY = male Children = Female carrier (XNXn), Normal Female (XNXN) Normal Male (XNY), Colorblind Male (XnY) XN Xn XNXN XNXn XNY XnY XN Y