Chapter 5 – Extensions and Modifications of Basic Principles

Dominance revisited

• Mendelian concept– In the heterozygous condition, only one allele

(dominant) is expressed

• Incomplete dominance– Heterozygote has phenotype intermediate to

homozygous phenotypes

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

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

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

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

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

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

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

Gene interaction

• More than one gene contributes to a single phenotype

• Polygenic inheritance

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

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:1aabb9 normal pigmentation:7 albino

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

Complementation test cont• If same locus, all

F1 will have a mutant phenotype

• If different loci, F1 will have wild-type phenotype

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

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

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

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”

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)

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

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

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

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

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

Pleiotropy

• One gene affects multiple characteristics

• PKU– Mental retardation, light skin and eye color