Exam Two Genetics

Complete Dominance

Heterozygote has the same phenotype as the dominant homozygote; AA = Aa

Complete Dominance Ratio

Aa × Aa gives a 3 dominant : 1 recessive phenotypic ratio

Structural Mutation

Mutation that changes the structure or function of a protein or molecule

Regulatory Mutation

Mutation that changes how much, when, or where a gene is expressed

Loss-of-Function Mutation

Mutation that reduces or eliminates normal gene activity

Null / Amorphic Mutation

Complete loss-of-function mutation that produces no functional gene product

Hypomorphic / Leaky Mutation

Partial loss-of-function mutation that produces reduced but not zero gene activity

Dominant-Negative Mutation

Mutant protein interferes with the normal protein, causing a mutant phenotype even with a WT allele

Gain-of-Function Mutation

Mutation that increases normal activity or gives a gene product an altered function; usually dominant

Hypermorphic Mutation

Gain-of-function mutation that causes too much normal gene activity

Neomorphic Mutation

Gain-of-function mutation that gives the gene product a new function

Haplosufficiency

One functional WT allele makes enough gene product for the normal phenotype

Haploinsufficiency

One functional WT allele does not make enough gene product for the normal phenotype

Mutation Type vs. Dominance

A mutation's dominance depends on how its gene product affects phenotype, not simply whether it is loss- or gain-of-function

Incomplete Dominance

Heterozygote has a phenotype intermediate between the two homozygotes

Incomplete Dominance Ratio

Heterozygote × heterozygote gives a 1 : 2 : 1 phenotypic ratio

Codominance

Both alleles are distinctly and detectably expressed in the heterozygote

Codominance vs. Incomplete Dominance

Codominance shows both allele products; incomplete dominance gives an intermediate phenotype

Multiple Alleles

More than two alleles of one gene exist in a population, although each diploid individual has only two

ABO Blood Group System

Multiple-allele system where IA and IB are codominant and both are dominant over i

ABO Genotypes

IAIA or IAi = A; IBIB or IBi = B; IAIB = AB; ii = O

Antigen

Molecule recognized by antibodies; ABO alleles determine red-blood-cell surface antigens

H Antigen

Red-blood-cell carbohydrate that is modified by the A or B enzyme

IA Allele

Produces an enzyme that modifies H antigen to make A antigen

IB Allele

Produces an enzyme that modifies H antigen to make B antigen

i Allele

Produces no functional enzyme to modify H antigen, giving type O when ii

Allelic Series

Set of alleles of one gene with an ordered hierarchy of effects or dominance

Rabbit C-Locus Allelic Series

Coat-color alleles generally follow C > cch > ch > c

Temperature-Sensitive Mutation

Mutation whose phenotype changes depending on environmental temperature

Himalayan Rabbit Allele

Temperature-sensitive hypomorphic allele; pigment forms in cooler body regions where the enzyme functions

Gene-Environment Interaction

Environmental conditions change how a genotype is expressed as a phenotype

Lethal Allele

Allele that causes death when present in a particular genotype

Recessive Lethal Allele

Allele that causes death when homozygous but survives in heterozygotes

Recessive Lethal Ratio

Heterozygote × heterozygote gives 1:2:1 at conception but usually 2:1 among surviving offspring

Agouti AY Allele

Gain-of-function regulatory allele that is dominant for yellow coat color but recessive lethal when homozygous

Delayed Age of Onset

Genetic phenotype appears later in life, sometimes after the individual has reproduced

Pleiotropy

One gene influences multiple different phenotypic traits

Sickle-Cell Pleiotropy

One β-globin mutation affects hemoglobin, RBC shape, cell density, disease symptoms, and malaria resistance

Overdominance

Heterozygote has higher relative fitness than either homozygote

Sickle-Cell Overdominance

In malaria environments, AS heterozygotes can have higher fitness than AA or SS individuals

Balancing Selection

Natural selection that maintains multiple alleles in a population, often because heterozygotes have an advantage

Penetrance

Whether individuals with a particular genotype actually express the expected phenotype

Complete Penetrance

Every individual with the relevant genotype expresses the phenotype

Incomplete Penetrance

Some individuals carry the genotype but do not show the expected phenotype

Nonpenetrant Individual

Individual with a phenotype-associated genotype who does not express that phenotype

Penetrance Formula

Penetrance (%) = individuals with genotype who show phenotype / total individuals with genotype × 100

Variable Expressivity

Same genotype produces different degrees or forms of the phenotype in different individuals

Penetrance vs. Expressivity

Penetrance asks whether the phenotype appears; expressivity asks how strongly or in what form it appears

Environment and Penetrance/Expressivity

Environmental conditions can affect whether a phenotype appears and how strongly it is expressed

Sex-Limited Trait

Trait expressed in only one sex even though both sexes can carry the genes

Sex-Limited vs. Sex-Linked

Sex-limited describes which sex expresses a trait; the gene does not have to be on a sex chromosome

Sex-Influenced Trait

Trait where the same genotype can produce different phenotypes or dominance relationships in males and females

Pleiotropy vs. Variable Expressivity

Pleiotropy = one gene affects multiple traits; expressivity = one genotype varies in severity or form among individuals

Phenotypic Variation

Same genotype can produce different phenotypes because of genes, environment, penetrance, or expressivity

PKU Gene-Environment Interaction

Phenylketonuria is genetic, but a low-phenylalanine diet can greatly reduce disease effects

Extranuclear Inheritance

Inheritance involving genetic information or effects outside the nuclear chromosomes

Organellar / Cytoplasmic Inheritance

Inheritance of genes located in mitochondria or chloroplasts rather than the nucleus

Why Cytoplasmic Inheritance Is Non-Mendelian

Parents contribute unequal amounts of cytoplasm, so organellar genes do not follow normal Mendelian ratios

Uniparental Inheritance

Organelles are inherited from only one parent, usually the mother

Biparental Organellar Inheritance

Both parents contribute organelles to the offspring

Maternal Mitochondrial Inheritance

Mammalian mitochondrial DNA is normally inherited from the mother because the egg supplies most cytoplasm

Nuclear vs. Organellar Genes

Nuclear genes usually have two copies in diploids; cells can contain many copies of organellar genes

Organellar Genome

Mitochondria and chloroplasts contain their own DNA, often as multiple circular genome copies

Organellar Nucleoid

DNA-protein structure containing organellar DNA

Homoplasmy

All copies of an organellar gene in a cell have the same allele

Heteroplasmy

A cell contains a mixture of different mitochondrial or chloroplast genome alleles

Homoplasmy vs. Homozygosity

Homoplasmy describes organellar genome copies; homozygosity describes two identical nuclear alleles

Heteroplasmy vs. Heterozygosity

Heteroplasmy describes mixed organellar genomes; heterozygosity describes two different nuclear alleles

Replicative Segregation

Random distribution of organelles and their genomes into daughter cells

Effect of Replicative Segregation

Heteroplasmic cells can produce daughter cells with different proportions of WT and mutant organelles

Heteroplasmy and Disease Severity

Higher proportions of mutant mitochondrial DNA can produce stronger phenotypes, depending on tissue and mutation

Baur and Correns

Researchers whose plant experiments demonstrated cytoplasmic inheritance of chloroplast traits

Mirabilis jalapa

Four-o'clock plant used to demonstrate maternal chloroplast inheritance

Variegated Plant

Plant with green and white patches caused by mixtures of functional and mutant chloroplasts

Green Plant Cells

Cells with functional chloroplasts that can produce chlorophyll

White Plant Cells

Cells with mutant chloroplasts that cannot produce normal chlorophyll

Variegated Plant Cells

Cells containing a mixture of functional and mutant chloroplasts

Human Mitochondrial Pedigree

Affected mothers may transmit mtDNA traits to sons or daughters; affected fathers do not transmit them

Why Mitochondrial Siblings Can Differ

Heteroplasmic mothers can produce eggs with different proportions of mutant mitochondrial DNA

LHON

Leber hereditary optic neuropathy; mitochondrial disorder with maternal inheritance and incomplete penetrance

Chlamydomonas Chloroplast Inheritance

Chloroplast DNA is usually inherited from the mt+ parent

Chlamydomonas Mitochondrial Inheritance

Mitochondrial DNA is usually inherited from the mt- parent

Chlamydomonas Nuclear vs. Organelle Segregation

Nuclear mating-type genes segregate 2:2, while chloroplast genes can show a 4:0 pattern

Petite Mutant

Yeast mutant with defective mitochondrial respiration that forms small colonies

Grande Yeast

Wild-type yeast with functional mitochondrial respiration and larger colonies

Segregational / Nuclear Petite

Petite caused by a nuclear mutation; follows Mendelian 2:2 segregation

Neutral Petite

Petite usually lacking mitochondrial DNA; crossing with WT generally gives WT offspring

Suppressive Petite

Petite with altered mtDNA that tends to become predominant when crossed with WT

Endosymbiosis

Relationship in which one organism lives inside another

Endosymbiosis Theory

Mitochondria and chloroplasts evolved from free-living bacteria that entered ancestral eukaryotic cells

Evidence for Endosymbiosis

Mitochondria/chloroplasts resemble bacteria in size, membranes, DNA, gene sequences, transcription, and translation

Mitochondrial Ancestor

Mitochondria are thought to descend from an alphaproteobacterial ancestor

Chloroplast Ancestor

Chloroplasts are thought to descend from a cyanobacterial ancestor

Secondary Endosymbiosis

Process where a eukaryote acquires a photosynthetic organelle by engulfing another photosynthetic eukaryote

Organelle Genome Reduction

Many ancestral organelle genes were lost or transferred to the nuclear genome during evolution

NUMTs

Mitochondrial-derived DNA sequences that have moved into the nuclear genome

NUPTs

Chloroplast/plastid-derived DNA sequences that have moved into the nuclear genome

Nuclear Control of Organelles

Most proteins used by mitochondria and chloroplasts are encoded by nuclear genes and imported into the organelles

Organelle Targeting Sequence

Protein sequence that directs a nuclear-encoded protein to a mitochondrion or chloroplast

Maternal Effect

Offspring phenotype is determined by the mother's nuclear genotype rather than the offspring's own genotype