1. Allele Expression
Allele expression not always as simple as dominant alleles overriding recessive ones.
Alleles of a single gene may interact together and give rise to phenotypes that are dissimilar to both of the parents.
This may be because:
The genes are sex-linked.
Incomplete dominance
Codominance
Multiple alleles
Incomplete dominance in snapdragons produces pink flowers from red and white parents
Roan coat color in cattle is a result of codominance between red and white alleles

2. Incomplete Dominance
In cases of incomplete dominance, neither allele dominates and the heterozygote is intermediate in phenotype between the two homozygotes.
Examples of incomplete dominance include flower color in snapdragons (right) and sweet peas, where red and white flowered plants cross to produce pink flowered plants.
CrCr
CrCw
CwCw

3. Flower Color in Snapdragons
Red flower
White flower
Parents
CrCr X
CwCw
Gametes Cr Cw
Possible fertilizations
Pink
F1 offspring
CrCw

4. Example problems (answer by showing Punnet squares):
1. If a white flowered plant is crossed with a red flowered plant, what are the genotypic and phenotypic ratios of the F1? 2. If two of the F1 offspring were crossed, what genotypes and phenotypes ratios would appear in the F2?

5. Codominance
In cases of codominance, both alleles are independently and equally expressed in the heterozygote.
Roan (stippled red and white) coat color in cattle.
AB human blood groups.
Black and tan tabby cats
White cow
Red bull
CRCR
CWCW
Parents X CR CW
Gametes
Possible fertilizations
Roan
CRCW
F1 offspring

6. Example problem 1. Cross two heterozygous (roan) shorthorn cattle X
Roan cow
Roan bull
CRCW
Parents X CW CR
Gametes
Possible fertilizations
White
Roan
Red
CRCR
CRCW
CWCW
Offspring

7. Example Problem
2. A true breeding red parent is crossed with a roan parent X
Roan cow
Red bull
Parents
Gametes CW CR
CRCR
CRCW
Possible fertilizations
Offspring
Roan
Red
CRCR
CRCW

8. Multiple Alleles in Blood
Humans have 4 blood group phenotypes: A, B, AB and O
The four common blood groups of the human ABO blood group system are determined by three alleles: IA, IB, i (however any one individual can possess only two alleles)
IA & IB are co-dominant and i is recessive.

9. Blood type IA IA IA i type A IB IB IB i type B IA IB type AB i i
genotype
phenotype
status
IA IA IA i
type A
type A oligosaccharides on surface of RBC __
IB IB IB i
type B
type B oligosaccharides on surface of RBC
IA IB
type AB
both type A & type B oligosaccharides on surface of RBC
universal recipient
i i
type O
no oligosaccharides on surface of RBC
universal donor

10. Multiple Alleles in BloodX
Blood group: AB
Parent genotypes
EXAMPLE 1: Cross two parents, both with AB blood type AB
Gametes IB IA
Possible fertilizations
Children's
genotypes
IAIB
IBIB
IAIA B
Blood groups AB A

11. Multiple Alleles in Blood
Blood group: B
Blood group: A X
Parent genotypes
EXAMPLE 2: Two parents with blood groups A and B respectively, both heterozygous
IBi
IAi
Gametes i IB IA
Possible fertilizations
Children's' genotypes
IAi ii
IBi
IAIB A O B
Blood groups AB

12. XY Sex Determination
In the XY type, sex determination is based on the presence or absence of the Y chromosome; without it, an individual will develop into a female.
XY sex determination occurs in:
Mammals (including humans)
Fruit fly Drosophila
Some dioecious (separate male and female) plants such as kiwifruit.
Females are homogametic with two similar sex chromosomes (XX). The male has two unlike chromosomes (XY) and is heterogametic.
Primary sex characteristics are initiated by genes on the X. ‘Maleness’ is determined by the Y.
Female
Male XX XY X
Parents X X X Y
Gametes
Possible fertilizations XX XY XX XY
Offspring
Sex:
Female
Male
Female
Male

13. Sex Linkage Genes located on the X chromosomes are called X-linked
Because the Y chromosome is small and does not contain many genes, few traits are Y- linked and Y-linked diseases are rare. X Y
Note the size differences between the X and Y chromosomes. The Y lacks alleles for many of the genes present on the X.

14. Examples:
X-linked traits are denoted XD for a dominant allele and Xd for a recessive allele
For most X-linked genes, the dominant form is the healthy form.
Hemophilia Xh Normal blood clotting XH
Muscular dystrophy Xm Non-muscular dystrophy XM
Color blindness Xĉ Non-color blindness XC

16. A human female can be homozygous or heterozygous with respect to sex-linked genes.
Female heterozygous for X-linked alleles are called carriers, because they don’t have the disease (they have one good copy of the gene) but they do “carry” the bad allele.
Since (normally) males only have one X chromosome, males only have one copy of genes located on the X chromosome.

17. Sex Linkage Sex-linked traits show a distinct pattern of inheritance.
Fathers pass sex-linked alleles to all their daughters but not to their sons.
Mothers can pass sex-linked alleles to both sons and daughters.
In females, sex-linked recessive traits will be expressed only in the homozygous condition.
In contrast, any male receiving the recessive allele from his mother will express the trait.
Carrier mother X X Y
Unaffected father
Carrier daughter X
Affected son X Y
Unaffected son Y X
Unaffected daughter X

18. Practice Problems
Suppose a color blind man fathers children with a woman of the genotype XC XC. What proportion of daughters would be color blind? What proportion of sons would be color blind?
One of the daughters from the above problem marries a color blind man.
What proportion of their sons will be color blind?

19. Pleiotropy Most genes are pleiotropic
one gene affects more than one phenotypic character
wide-ranging effects due to a single gene:
dwarfism (achondroplasia)
gigantism (acromegaly)
The genes that we have covered so far affect only one phenotypic character, but most genes are pleiotropic

20. Acromegaly: André the Giant

21. Pleiotropy
It is not surprising that a gene can affect a number of organism’s characteristics
consider the intricate molecular & cellular interactions responsible for an organism’s development
cystic fibrosis
mucus build up in many organs
sickle cell anemia
sickling of blood cells

22. Polygenic Inheritance Patterns
Traits determined by 2 or more genes

23. Polygenic inheritance
Some phenotypes determined by additive effects of 2 or more genes on a single character
phenotypes on a continuum
human traits
skin color
height
weight
eye color

24. Prevalence of dominance
Because an allele is dominant does not mean…
it is better
it is more common
Polydactyly:
dominant allele

25. Polydactyly individuals are born with extra fingers or toes
dominant to the recessive allele for 5 digits
recessive allele far more common than dominant
 399 individuals out of have only 5 digits
 most people are homozygous recessive (aa)

26. Hound Dog Taylor