This document discusses different types of non-Mendelian genetics including polygenic traits, multiple alleles, codominant traits, incomplete dominant traits, and sex-linked traits. It provides examples for each type including codominant flower color inheritance where heterozygotes express both traits together, incomplete dominance in snapdragons where heterozygotes show a blended trait, and sex-linked traits like baldness that are inherited through the X chromosome. Worked examples are given for genetic crosses involving these different non-Mendelian inheritance patterns.

1. Biology
Unit 5 Notes:
NON-Mendelian
Genetics

2. (1) What does Non-Mendelian mean?
• Not all traits are as simple as dominant and
recessive.
• Some traits…
– Have more than just 2 alleles or variations.
– Have 2 dominant alleles.
– Are only passed on via sex chromosomes.
– Are the result of 100’s of genes being
expressed.

3. (2) Non-Mendelian Types
• Polygenic Traits
• Multiple Allele
• Codominant Traits
• Incomplete Dominant Traits
• Sex-Linked Traits

4. (3) Polygenic & Multiple Allele Traits
• Polygenic = A trait which is the result of
many genes being expressed at one time.
– Examples:
• Body shape and structure
• Hair / Skin / Eye color
• Multiple Allele = A trait which has more than
just 2 types of alleles that can be inherited.
– Example:
• Blood Type

5. (4) Codominant Traits
• Co = Together
• Codominant = When 2 dominant traits are
expressed equally together (at the same
time).
• Example:
– Some flowers can be homozygous dominant
and RED.
– Some flowers can be homozygous dominant
and WHITE.
– Some flowers can be heterozygous, they look
RED AND WHITE (but NOT blended!!).

6. Homozygous Dominant Red
AA
Homozygous Dominant White
BB
Heterozygous
AB

9. (5) Setting Up Codominant Problems
• Use all capital letters.
• Use 2 different letters, one per variation.
• The heterozygote will show both
colors/traits at the same time, in patches.
• Example:
– Homozygous Dominant #1 = AA
– Homozygous Dominant #2 = BB
– Heterozygous = AB

10. (6) Codominant Example #1
• A certain species of pansies are
codominant for flower color. You cross a
homozygous red flower, with a homozygous
white flower:

11. (7) Codominant Example #2
• Now you take 2 offspring from the last cross
(2 heterozygous pansies) and cross them:

12. (8) Incomplete Dominant Traits
• Incomplete = not fully
• Incomplete Dominant = When 2 dominant traits are
partially expressed together at the same time,
resulting in a blending appearance.
• Example:
– Some flowers can be homozygous dominant and RED.
– Some flowers can be homozygous dominant and
WHITE.
– Some flowers can be heterozygous, they look PINK (like
a blending of red + white).

13. Homozygous Dominant Red
AA
Homozygous Dominant White
BB
Heterozygous
AB

16. (9) Setting Up Incomplete Dominant Problems
• Use all capital letters.
• Use 2 different letters, one per variation.
• The heterozygote will look blended.
• Example:
– Homozygous Dominant #1 = AA
– Homozygous Dominant #2 = BB
– Heterozygous = AB

17. (10) Incomplete Dominant Example #1
• Snapdragon flowers show incomplete
dominance for their petal color. You cross
a homozygous red flower with a
homozygous white flower:

18. (11) Incomplete Dominant Example #2
• Now cross 2 heterozygous snapdragons
from the last problem:

19. (12) Sex-Linked Traits
• Sex-Linked = Traits only found on the sex
chromosomes.
• REMEMBER:
– Male = X Y
– Female = X X
• These are often the types of traits that
appear to “skip” generations or genders, or
often show up more frequently in males.

21. (13) Setting Up Sex-Linked Problems
• First write the sex chromosomes per the gender of
the individual.
• Then figure out the genotype for the trait (AA, Aa,
or aa).
• Write the allele-letters like exponents on the sex
chromosomes.
• “X-Linked” means it’s only on the x chromosome.
• “Carrier” refers to a female who is heterozygous.
She “carries” the recessive trait, but doesn’t show
it.

22. (14) Sex-Linked Example #1
• Hair loss is a recessive gene connected to
the x-chromosome.
• Write out the genotypes for:
– Normal Female =
– Female Carrier =
– Bald Female =
– Normal Male =
– Bald Male =

23. (15) Sex-Linked Example #2
• Hair loss is a recessive gene connected to
the x-chromosome. A man with normal hair
and a woman who is a carrier decide to
have children:

24. (16) Sex-Linked Example #3
• Dealing with the same hair-loss trait…. A
bald man and a normal female decide to
have children: