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

2. 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

3. 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

4. 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

5. 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

6. 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

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

8. 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

9. 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?

10. 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

11. 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

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

13. 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

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

15. 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

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

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

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

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

20. 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?

21. 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