1. Chapter 4

2. MULTIPLE ALLELES When a given gene has several alleles, not just two
A diploid individual still has a maximum of 2 alleles, one on each homologous chromosome

3. MULTIPLE ALLELES ABO Blood Groups Discovered in early 1900s
Important when considering tranfusions
4 types; 3 alleles

4. MULTIPLE ALLELES ABO Blood Groups A: IAIA; IAi B: IBIB; IBi AB: IAIB
O: ii

6. MULTIPLE ALLELES ABO Blood Groups
Antibody – protein molecule that recognizes and binds to foreign material
Antigen – molecule that is recognized as foreign and stimulates antibody production
Don’t stimulate antibody formation in organism expressing them (exception: Autoimmune diseases)

7. MULTIPLE ALLELES ABO Blood Groups
IA : specifies the ‘A’ antigen; antibodies against ‘B’ and will clump onto the IB
IB : specifies the ‘B’ antigen; antibodies against ‘A’ and will clump onto the IA
AB : have both antigens, but no “anti-” antibodies
O (ii) : have no antigens and no “anti-” antibodies

8. MULTIPLE ALLELES ABO Blood Groups Safe Transfusions:
A (IAIA /IAi) – can receive A or O
Can give to A or AB
B (IBIB /IBi) – can receive B or O
Can give to B or AB
AB (IAIB) – can receive AB or O
Can give to only AB
O (ii) – can receive only O
Can give to any blood group, A, B or O

9. Figure 4.4 Antigenic reactions that characterize the human ABO blood types. Blood serum from each of the four blood types was mixed with blood cells from the four types in all possible combinations. In some cases, such as a mix of B serum with A cells, the cells become clumped.

10. MULTIPLE ALLELES What does this have to do with molecular genetics?
The base pair sequence of a gene specifies amino acid sequence of a protein…this protein function depends on the sequence of amino acids
So, a simple change in the base sequence can drastically change the protein function

11. Modifications of Dominance
Complete Dominance
One allele is dominant to another, so the heterozygous individual shows the dominant phenotype
Incomplete Dominance
One allele is not completely dominant to another (partial dominance), heterozygous individual shows a new intermediate phenotype

12. Modifications of Dominance
Incomplete Dominance
Ex. Plumage color in chickens
Cross a true-breeing black (CBCB) with a true-breeding white (CWCW) and the heterozygous F1 offspring (CBCW) exhibits a bluish-grey plumage
C : color
B : black
W : white
Can’t be true-breeders…why?

13. Modifications of Dominance
Incomplete Dominance
Explanation: believed to occur for this reason…
CB : produces color gene expression
CW : produces no gene expression
So a heterozygous individual produces “half” a dose of gene expression
Insufficient
Heterozygotes that produce a “normal” dominant appearance are described as haplosufficient

14. Modifications of Dominance
Codominance
one allele is not dominant to another, instead the phenotype produced exhibits both dominant phenotypes
Ex. ABO blood grouping (AB)
Explanation…believed to occur because
Both alleles for competing phenotypes are expressed

15. In Review Complete dominance Incomplete dominance & Codominance
A/A & A/a produce the same phenotype and can be written as A/- because the second allele does not change the expression of the gene
Incomplete dominance & Codominance
A/A & A/a do not produce the same phenotype so they must be written out as they appear

16. Modified Mendelian Ratios
Production of NEW Phenotypes
Ex. Comb Shape in Chickens (may be true-breeders)
a) R/- p/p
b) R/- P/-
c) r/r P/-
d) r/r p/p
Assuming recessives
do not take any action,
we can assume the
single comb is a
product of other genes
while the others are due
to the activity of the
R and P alleles

17. Modified Mendelian Ratios
Production of NEW Phenotypes
Ex. Fruit Shape in Summer Squash – available in long, sphere, and disk-shaped
Sphere – A dominant allele of either gene and homozygous recessive of the other
Disk-shaped – A dominant allele of both genes
Long – double homozygous recessive

18. Modified Mendelian Ratios
Produces NO new phenotype
Epistasis
Involves a gene masking or modifying the phenotypic expression of another gene
Interaction between 2 or more genes to control a single phenotype
Confined to dihybrid crosses where two pairs of alleles assort independently
Does not produce a new phenotype, only masks
Epistatic gene – the gene that masks another
Hypostatic gene – the gene that is masked

19. Modified Mendelian Ratios
Epistasis
Recessive – must be homozygous
Ex: coat color in rodents – natural coat color in wild rodents is a greyish color (produced by alternating bands of black and yellow – agouti pattern)
Aids in camouflage
Found in mice, squirrels, etc
Other colorations exist, but are recessive to agouti
(A/– agouti; a/a nonagouti)
(C/– pigment; c/c albino)
(B/– black; b/b brown)
c is epistatic when homozygous (recessive gene)
A is hypostatic

20. Figure 4.11 Recessive epistasis: generation of an F2 9 agouti : 3 black : 4 white ratio for coat color in rodents.

21. Modified Mendelian Ratios
Epistasis
Recessive - must be homozygous
Ex: coat color in labrador retrievers – available in black, yellow, & chocolate
One gene specifies black pigment (B/-) or brown (b/b)
An independent gene either allows (E/-) or hides (e/e) the expression of the Black / brown gene
Black: B/- E/-
Chocolate: b/b E/-
Yellow: -/- e/e (B/- black noses; b/b brown noses)
e is epistatic when homozygous (recessive gene)
B is hypostatic

23. Modified Mendelian Ratios
Epistasis
Dominant
Ex: fruit color in summer squash – available in white, yellow, or green
W/-, -/- white
w/w, Y/- yellow
w/w, y/y green
W – epistatic (homozygous or heterozygous)
y - hypostatic

24. Modified Mendelian Ratios
Epistasis
Dominant
Ex: Greying in horses
It doesn’t matter what color the horse’s base is (sorrel, black, bay, etc) over time the Grey gene will mask that phenotype
It is a progressive process
Grey does not affect skin or eye color, only hair

26. Figure 4.15 Dominant epistasis: Dominant greying allele in the horse causes the coat to turn grey as the horse matures. A horse (my Lipizzaner) is shown at age 4 (top) and age 7 (bottom).

27. Figure 4.15 Dominant epistasis: Dominant greying allele in the horse causes the coat to turn grey as the horse matures. A horse (my Lipizzaner) is shown at age 4 (top) and age 7 (bottom).

28. Figure 4.15 Dominant epistasis: Dominant greying allele in the horse causes the coat to turn grey as the horse matures. A horse (my Lipizzaner) is shown at age 4 (top) and age 7 (bottom).

29. Modified Mendelian Ratios
Epistasis
Duplicate Genes
When a gene at one locus produces a phenotype identical to that produced at another locus
Ex: sweet peas flower colors
C: colored
c: no color
P: purple
p: white
Purple flowers: C/- P/-
White flowers: c/c -/- OR C/- p/p
(duplicate recessive epistasis OR complementary gene action) when 1 or both loci are homozygous recessive

30. Modified Mendelian Ratios
Essential & Lethal Genes
Mutations not only change phenotypes, they can also cause death (which I guess technically does change the phenotype)
Alleles resulting in death are lethal alleles, caused by essential genes (essential to the normal functioning of the organism)
When caused by a dominant lethal allele both the heterozygous and homozygous individuals will show the lethal phenotype
When caused by a recessive lethal allele, only the homozygous individual will show the lethal phenotype

31. Modified Mendelian Ratios
Essential & Lethal Genes
Lethal alleles
Ex: Yellow body color in mice
Acts dominant in determining body color, but acts recessive in determining lethality (only heterozygotes survive to birth)
Ex: Huntington’s disease in humans
Autosomal dominant (can’t be studied until reproductive age)
Onset doesn’t appear until early-thirties, and death in forites
Ex: Hemophilia in humans
X-linked recessive

32. Modified Mendelian Ratios
Gene Expression
Penetrance – frequency with which a gene manifests itself in individuals in the population
Depends on genotype and environment
Expressivity – degree to which a gene or phenotype are expressed in an individual
Environment –
Age of onset: creates internal environmental changes
Genes are not “on” all the time; genes can be activated or deactivated over time
Pattern baldness
Muscular Dystrophy

33. Modified Mendelian Ratios
Gene Expression
Environment –
Sex – expression of genes are influenced by gender
Sex-limited traits –autosomal genes that affect only 1 gender and not the other
Ex: milk production
Ex: appearance of horns in some species
Ex: facial hair
Temperature – Reactions are catalyzed by enzymes, which function in a certain range.
Ex: fur color in Himalayan rabbits
>30 C – all white
<25 C – typical coloration (black paws, ears, nose tail) and anywhere it is artificially cooled

34. Modified Mendelian Ratios
Gene Expression
Environment –
Chemical – can have significant effect on an organism
EX: Phenylketonuria (PKU): autosomal recessive, trouble metabolizing amino acid phenylalanine, diet determines severity (proteins)
Nature vs Nurture
What are the relative contributions of genes and the environment to the phenotype?
Ex: Height – influenced by genes (potential) and environment (diet, overall health, hormones)
Ex: Alcoholism – influenced by genes (susceptibility) and environment (choice)
Ex: Intelligence – influenced by genes (potential) and environment (learning, challenges)