Exceptions to Mendelian Genetics Codominance Multiple Alleles Incomplete

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

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

Codominance Not 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

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

Multiple Alleles 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?

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

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

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

Your Turn WHAT IS THE PROBABILITY OF HAVING A MALE OFFSPRING? X Y 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 Genotype Phenotype Example Problems: ◦ Red-green colorblindness is a recessive Xlinked trait ***XNXn is a carrier. She does NOT exhibit the trait, but she CAN pass it on to her children. X NY Male – Not colorblind X n. Y Male Colorblind Female – Not colorblind X NX N X NX n X n Female – Not colorblind*** Female Colorblind

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

Remember: X n = female XNY = male Children = Female carriers (XNXn), Colorblind Males (Xn. Y) XN Y Xn Xn X NX n X n. Y

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

Remember: X NX N = female Xn. Y = male Children = Female carriers (XNXn), Normal Males (XNY) Xn Y XN XN X NX n X NY

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?

Remember: XNXn = female XNY = male XN Children = Female carrier (XNXn), Normal Female (XNXN) Normal Male (XNY), Colorblind Male (Xn. Y) Y XN Xn X NX N X NX n X NY X n. Y