1. Extensions of Mendelian Genetics
Mendelian genetics seems to be relevant to only a small set of heritable features
For only a few characters there are…
Only 2 versions of an allele (green or yellow)
1 gene codes for a single external character
1 allele is completely dominant to the other
The basic patterns of segregation & independent assortment apply to more complex patterns of inheritance

2. Different Types of Dominance
Complete dominance
Phenotype of heterozygote & HomoD are indistinguishable
The pattern with which you are already familiar
Incomplete dominance
Phenotype of heterozygote is in between the 2 Homo phenotypes
Example: pink snapdragons

3. Snapdragons – Incomplete Dominance

4. So incomplete dominance does NOT provide evidence for “blending” theories

5. Codominance Codominance
Phenotype of heterozygote is separate & distinguishable from Homozygous Dominant & Homozygous Recessive
Example: AB blood type or Rhododendron flower

6. Dominance & Phenotype
The observed dominance/recessiveness of alleles depends on the level of the investigation
Consider Tay-Sachs disease
Brain cells of the baby do not metabolize certain lipids
As lipids accumulate, seizures, blindness, and mental degeneration
Death occurs within a few years of conception

7. Tay-Sachs Disease At the Organismal level, the disease is recessive
Only children with 2 copies of the recessive trait will have the malady
Heterozygote is not afflicted – they produce some lipid- metabolizing enzyme, though not as much as in HomoD
So intermediate enzyme production
This suggests the biochemical level, the disease is an example of incomplete dominance
Which is Tay-Sachs: dominance or incomplete dominance?

8. Prevalence & Dominance
Polydactyly
Extra fingers or toes
1 of 400 in the US
The allele for polydactyly is dominant, but rarely present
Recessive homozygotes (HomoR) are found 399 out of 400 instances

9. Blood Typing
Only 2 alleles existed for Mendel’s peas, but this is not typical for most traits
Consider ABO blood group in humans
A refers to the “A” membrane carbohydrate & type A blood
B refers to (seriously, I’m not writing this down)
O means neither A or B carbohydrate is found
AB means both A & B are found
BUT the A and B alleles are codominant and are both expressed if an individual inherits both alleles

11. Epistasis
A gene at one locus alters the phenotypic expression of a gene at a second locus
Example: Mouse fur color
Bb or BB = Black bb = brown
If HomoR for (C) gene [cc], then no fur color (albino or white)
Regardless of fur color specified by brown-black gene
If NOT HomoR for (c) gene [Cc or CC], then can be brown (bb) or black (Bb or BB)

12. What is the phenotype of…
BBcc?
BbCc?
bbCC?
Bbcc?
BBCC?
bbcc?

13. Pleiotrophy Single gene has multiple effects
Should be unsurprising given intricate molecular and cellular interactions for development of an organism
Phenylketonuria
Mental Retardation
Reduced skin and hair pigmentation

14. Polygenic Inheritance
The additive effect of 2 or more genes on 1 phenotypic character
Called quantitative characters since there is a continuum of gradations
Normal curve of phenotypes
Example: human skin pigmentation is determined by at least 3 separately inherited genes
AABBCC = Dark
AaBbCc = Intermediate
aabbcc = Light

16. Pedigree Analysis
Family tree describing the interrelationships of parents & children across the generations

17. Recessively Inherited Disease
Requires 2 copies of the recessive allele (Homozygous Recessive) to express the mality
Heterozygotes are called carriers
Normal phenotype, but may transmit disease to offspring
Examples:
Cystic Fibrosis
Tay Sachs disease
Sickle-cell disease

18. Cystic Fibrosis Recessive autosomal disease
Common in those of European descent
1 of 2,500 affected, but 1 of 25 are carriers
Affects Chloride ion transport between a cell and extracellular fluid
If untreated, most die before 5th birthday
Typically, patients live until their 20s or 30s with efficacious treatment

19. Sickle-Cell Anemia Recessive autosomal disease African descent
Affects Hemoglobin protein in RBCs
Low blood oxygen = hemoglobin molecules clump together forming sickle shaped RBCs
Sickle-celled RBCs clump together creating chronic vascular occlusion of small vessels
Example of incomplete dominance
Heterozygotes are usually normal but will show some symptoms during prolonged periods of reduced blood oxygenation

20. Dominant Alleles
Most harmful alleles are recessive, but some human diseases are due to dominant alleles
Only require one copy of the allele to be expressed
Hypothesis: if there is a lethal disease carried on a dominant allele, it would have burned out its carriers by now.
UNLESS, the lethal disease carried by a dominant allele is one that affects organisms of advanced age
Like Huntington’s disease

21. Examples of Dominant Allele Disease
Achondroplasia – form of dwarfism
Heterozygous individual = dwarf
1 in 25,000 have achondroplasia, so 99% of the population are HomoR
Huntington’s disease
Caused by a lethal dominant allele
Degenerative disease of nervous system
Usually only affects those > 40 yrs old

22. Genetic Testing
Pedigree analysis gives some info about risk to offspring
There are also tests to identify carriers of certain genetic diseases
Amniocentesis – amniotic fluid is removed and then cells contained in the fluid are cultured to identify certain chromosomal defects via karyotype
Chorionic villus sampling (CVS) – placental tissue is removed for same purpose as amnio, but results are available far faster, & can be performed earlier in pregnancy
Karyotype can be immediately obtained