1. Chapter 5 – Extensions and Modifications of Basic Principles

2. Dominance revisited Mendelian concept Incomplete dominance
In the heterozygous condition, only one allele (dominant) is expressed
Incomplete dominance
Heterozygote has phenotype intermediate to homozygous phenotypes

3. Incomplete dominance Heterozygote has intermediate phenotype
Does NOT have to be phenotype “right in the middle”
Lighter shade of red to very light shade of pink

4. Codominance Heterozygote expresses both alleles//both phenotypes
MN locus
Codes of antigen on red blood cells
Does not cause significant immune response like ABO or Rh groups
LM allele = M antigen; LN allele = N antigen
LMLN individual has both antigens present

5. Dominance “Dominance” can depend on which level you are looking at
Cystic Fibrosis – autosomal recessive disorder
Normal allele produces carrier protein in plasma membrane that allows Cl- passage in/out of cell
Mutant allele produces defective protein that prohibits Cl- from exiting cell
Carriers of Cystic Fibrosis
Physiological level – recessive
Carriers have enough normal channels for unaffected phenotype
Molecular level – codominant
Carriers have both normal and mutant channel proteins

6. Incomplete penetrance
Genotype does not always produce expected phenotype
Polydactyly
Dominant allele
Individuals with dominant allele can occasionally have normal number of digits, but have affected children
Penetrance
% of individuals with a particular genotype that express expected phenotype
42 individuals have polydacylous allele; 38 express polydactyly
38/42 = 90% penetrance

7. Variable Expressivity
Degree to which trait is expressed
Polydactyly
Some extra digits are fully functional; others are just small skin tags
Penetrance and expressivity are due to other genes and environmental factors
Mere presence of allele does not guarantee expression, or standard “one size fits all” expression

8. Lethal alleles
Cause death at an early age of development (usually before or shortly after birth) so some genotypes are appear among progeny
Recessive – need to be homozygous to be lethal; heterozygote will have different phenotype
Dominant – lethal in both homozygotes and heterozygotes
Only transmissible when lethal after individual has passed reproductive age

9. Multiple alleles One gene may have more than 2 possible alleles
Regardless of possible alleles in a population, an individual can only have a maximum of 2 different alleles
ABO blood type
Codes for antigens of surface of red blood cells
3 possible alleles
IA – puts “A” antigen
IB – puts “B” antigen
i – puts no antigen
i is recessive to both IA and IB; IA and IB are co-dominant
Served as primitive means of paternity testing

11. Gene interaction More than one gene contributes to a single phenotype
Polygenic inheritance

12. Epistasis One gene masks the effects of another gene
Can be dominant or recessive
Albinism
Lack of pigment melanin
Very light skin and hair; pink or very light blue eyes

13. Albinism cont Duplicate recessive epistasis
Since pigment production is a multi-step process requiring multiple enzymes, different genes can each result is albinism
P generation aaBB (albino) x AAbb (albino)
F1 AaBb (normal pigmentation)
F2 9A_B_:3aaB_:3A_bb:1aabb
9 normal pigmentation:7 albino

14. Complementation test
Test to determine whether two different mutations are at the same locus or different loci
Cross homozygous individuals with different mutations
D. melanogaster
both apricot (a) eye color and white (b) eye color is recessive to normal wild-type red

15. Complementation test cont
If same locus, all F1 will have a mutant phenotype
If different loci, F1 will have wild-type phenotype

16. Interaction between sex and heredity
Sex-influenced traits
Autosomal Mendelian inheritance, but expressed differently in sexes
Beards on goats
Dominant trait in males
Expression requires only one allele
Recessive trait in females
Must be homozygous to have a beard

17. Sex influenced traits cont
Human male pattern baldness
NOT x-linked
Dominant in male; recessive in females
Affected males can be homozygous or heterozygous
Affected females must be homozygous
Usually results in “thinning” – variable expressivity
Difference due to presence of male sex hormones
Males castrated prior to puberty do not exhibit pattern baldness, even with genotype

18. Sex-limited traits Autosomal inheritance
Trait is only expressed in one sex; zero penetrance in other sex
Domestic chickens
H = hen plumage; h = cock plumage
Male hh = cock feather tail
Female hh = hen feather
Cock plumage never expressed in females

19. Cytoplasmic inheritance
Inheritance of DNA in cytoplasm (mitochondria or chloroplasts)
Inherited from mother only
Sperm contributes nucleus, but no cytoplasm
Characteristics exhibit extensive phenotypic variation
Each cell can contain hundreds of mitochondria, and may not have same genetic information
Homoplasmy – all the same
Heteroplasmy – different genetic information
Ratio of “normal” to “mutant”

20. Genetic maternal effect
Genotype inherited from both parents, but phenotype is determined by MOTHER’S genotype
Limnaea peregra
Dextral coiling (to the right) is dominant over sinistral (to the left) coiling
Phenotype determined by mother’s genotype (not her phenotype)

21. Genomic Imprinting
Differential expression of gene depending of whether it was inherited from mother or father
Due to different methylation patterns of DNA
Microdeletion of 15p
Deleted from father – Prader-Willi syndrome
Deleted from mother – Angelman syndrome

22. Anticipation
Genetic trait becomes either more strongly expressed or expressed at an earlier age as it is passed from generation to generation
Due to an unstable region of DNA that tends to increase in size in next generation

23. Environmental Effects
Himalayan allele in rabbits
Produces dark fur – nose, feet, ears
Develops at temperatures less than 20°C
Enzyme is inactivated at temperatures over 30°C

24. Phenocopy
Environmental factors produce a phenotype that mimics the phenotype of another genotype
PKU – phenylketonuria
Autosomal recessive
Phenylalanine can not be broken down; build-up causes brain damage
Affected child put on restricted diet - prevents retardation
Can go off diet after nervous system is fully formed (early 20s)
Affected woman when pregnant must be diet restricted
If not, excess phenylalanine can cross placenta and give child PKU phenotype, even if genotypically unaffected

25. Inheritance of Continuous Characteristics
Discontinuous – few, distinct phenotypes
Continuous – wide range of phenotypes
Often form bell-shaped curve when plotted
Height, skin color
Usually due to multiple genes contributing to a single trait
Polygenic inheritance

26. Pleiotropy One gene affects multiple characteristics PKU
Mental retardation, light skin and eye color