1. Patterns of Inheritance
A Summary

2. Autosomal Dominant & Recessive
Autosomes are all chromosomes other than the pair that determines sex.
Dominant alleles are expressed in the phenotype.
Recessive alleles are not expressed in the phenotype unless they are homozygous.

5. Incomplete Dominance
In incomplete dominance, the phenotype of a heterozygous individual is intermediate between the phenotypes of the two homozygous phenotypes.

6. CR CW Cw CR CR CR CW CW CW White CWCW Red CRCR Pink CRCW 1⁄2 Sperm
P Generation
F1 Generation
F2 Generation
Red
CRCR
Gametes CR CW 
White
CWCW
Pink
CRCW
Sperm Cw
1⁄2
Eggs
CR CR
CR CW
CW CW

7. Whippet Phenotypes
Normal Whippet
M+M+
“Fast” Whippet
M+M-
“Bully” Whippet
M-M-

8. Normal, “Fast”, and “Bully” Whippets

9. Wendy, the Bully Whippet

10. “Fast” Whippet x “Fast” Whippet
M+M- x M+M- M+ M-
M+M+
M+M-
M-M-
Genotype ratio 1 : 2 : 1
Phenotype ratio 1 : 2 : 1
(Normal : Fast : Bully)

11. Autosomal Codominance
In codominance, two different alleles are equally expressed in the phenotype.

13. Polygenic Traits Some traits are affected by many genes.
Such traits are said to be polygenic.
Polygenic traits often vary continuously, instead of in an “either / or” fashion.
Body size, skin color, hair color, and eye color are polygenic.

14. A Simplified Model of Polygenic Control of Skin Color
AaBbCc
aabbcc
Aabbcc
AaBbcc
AABbCc
AABBCc
AABBCC
20⁄64
15⁄64
6⁄64
1⁄64
Fraction of progeny

15. X-Linked Inheritance
Sex is determined by the last pair of chromosomes, designated X and Y
XX genotype = female
XY genotype = male
Male parents pass the X chromo to female children, the Y to male children.
Females pass one or the other X to both male and female children.

20. Some Additional Patterns
(not covered in our course)

21. Mitochondrial DNA Mitochondria have their own DNA chromosome.
Mitochondrial traits can be passed through the mother only, because the zygote contains only mitochondria from the ovum.

23. Affected Mother XY
XcY
XXc
XcXc XX

24. Epistasis One gene affects the expression of a second gene  BC bC Bc
1⁄4
BBCc
BbCc
BBcc
Bbcc
bbcc
bbCc
BbCC
bbCC
BBCC
9⁄16
3⁄16
4⁄16 
Sperm
Eggs
One gene affects the expression of a second gene