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Chapter 5 – Extensions and Modifications of Basic Principles

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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

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