Chapter 5 – Extensions and Modifications of Basic Principles

1. Chapter 5 – Extensions and Modifications of Basic Principles

2. Dominance revisited • Mendelian concept • 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