REAL Comprehensive Genetics Test 2: Non-Mendelian Inheritance, Chromosome Structure, and Transposable Elements

Maternal effect

Offspring phenotype depends on the mother’s genotype rather than its own or the father’s.

Function of maternal effect

Maternal mRNAs and proteins in the egg determine early embryonic development.

Molecular mechanism of maternal effect

Nurse cells load maternal RNAs/proteins into the oocyte during oogenesis.

Nurse cells

Cells surrounding the oocyte that supply nutrients and developmental molecules.

Epigenetic inheritance

Heritable gene regulation changes not involving DNA sequence alterations.

Epigenetic vs genetic variation

Epigenetic alters expression; genetic changes alter DNA sequence.

Epigenetic changes vs mutations

Epigenetic marks are reversible; mutations are permanent.

Dosage compensation

Equalization of X-linked gene expression between sexes.

Dosage compensation in mammals

One X chromosome is randomly inactivated in females (Barr body).

Dosage compensation in marsupials

Paternal X chromosome is always inactivated.

Dosage compensation in Drosophila

Male X chromosome expression doubles.

Dosage compensation in C. elegans

Hermaphrodites reduce transcription from each X by half.

Dosage compensation in birds

Partial/incomplete compensation between sexes.

XIST gene

Noncoding RNA that coats and silences one X chromosome in females.

TSIX gene

Antisense RNA to XIST that keeps the active X chromosome from being silenced.

Establishment of X inactivation

Randomly occurs early in development through XIST RNA coating.

Maintenance of X inactivation

Preserved by DNA methylation and histone modification.

Example of X inactivation

Calico cats show mosaic fur due to random X inactivation.

Barr bodies

Condensed inactive X chromosomes visible in female nuclei.

Active X chromosomes

All individuals retain only one active X chromosome.

Genomic imprinting

A DNA region is silenced in a parent-of-origin-specific manner via methylation.

Inheritance of imprinting

Imprints are established during gametogenesis before fertilization.

Mechanism of imprinting

DNA methylation marks silence one parental allele.

Timing of imprinting

marks occur during spermatogenesis or oogenesis.

Igf2 gene

Expressed only from the paternal allele due to imprinting.

Angelman syndrome

Maternal deletion on 15q11–q13 causing neurological defects.

Prader-Willi syndrome

Paternal deletion on 15q11–q13 causing developmental and metabolic issues.

Monoallelic expression

Only one parental allele is expressed in imprinted genes.

Extranuclear inheritance

Genes in mitochondria or chloroplasts passed maternally.

Egg vs sperm cytoplasm

Egg provides nearly all cytoplasm and organelles; sperm contributes little.

Maternal inheritance patterns

Mothers transmit mitochondrial traits to all offspring.

Heteroplasmy

Mixture of normal and mutant mtDNA affecting phenotype severity.

Endosymbiosis

Theory that mitochondria and chloroplasts evolved from engulfed prokaryotes.

Mitochondrial and chloroplast genomes

Circular and compact

Mitochondrial diseases

Caused by mtDNA or nuclear mutations impacting energy production.

Maternal inheritance

Transmission of organelle genomes through the egg cytoplasm.

Sex-linked inheritance

X-linked traits affect males more; females may be carriers.

Sex-influenced inheritance

Autosomal traits expressed differently in males and females.

Sex-limited inheritance

Traits expressed in only one sex (e.g.

Genetic linkage

Genes close on the same chromosome are inherited together.

Recombination frequency

Map distance = recombinants ÷ total × 100.

Genetic mapping

Determines gene order and distance on chromosomes.

Mapping accuracy

Decreases with gene distance due to multiple crossovers.

Testcross

Heterozygote × homozygous recessive reveals linkage patterns.

Independent assortment

Unlinked genes assort randomly (1:1:1:1 ratio).

Linked genes

No crossover yields 1:1 parental types.

Far apart genes

Crossovers restore independent assortment ratios.

Interference

One crossover reduces the chance of another nearby.

Recombinants

Offspring with new allele combinations from crossing over.

Double crossover logic

Double crossovers may restore parental types

Crossing over

Exchange between homologous chromosomes during meiosis I.

Mitotic recombination

Rare exchange producing mosaic tissue patches.

Karyotype

Chromosome set visualized by size and centromere position.

Deletions

Chromosome segment loss causing gene absence.

Interstitial deletion

Central fragment lost after two breaks and rejoining ends.

Duplication

Repetition of a chromosome segment.

Inversion

Segment flipped in orientation and reinserted.

Inversion loop

Structure allowing pairing between normal and inverted chromosomes in meiosis.

Translocation

Segment moved to another nonhomologous chromosome.

Effects of translocations

Can disrupt or misregulate genes near breakpoints.

Breakage and repair

Double-strand breaks cause rearrangements during repair.

Misaligned crossovers

Repetitive sequences mispair → unequal exchange.

Gene families

Related genes derived from ancestral duplication.

Homologs

Genes sharing a common ancestor.

Orthologs

Homologs in different species due to speciation.

Paralogs

Homologs within the same species from duplication.

Balanced translocation

No net DNA loss or gain; carriers usually normal.

Unbalanced translocation

Gain/loss of DNA causes developmental abnormalities.

Semisterility

Reduced fertility due to abnormal segregation in rearrangements.

Euploidy

Normal complete chromosome set.

Aneuploidy

Abnormal number of individual chromosomes.

Polyploidy

Extra complete sets of chromosomes.

Autopolyploidy

Extra sets from nondisjunction within one species.

Allopolyploidy

Chromosome sets combined from different species.

Mitotic nondisjunction

Post-zygotic error leading to mosaic individuals.

Gynandromorphy

Mosaic with male and female tissues from X loss in early mitosis.

Chimerism

Individual formed from fusion of two embryos.

Patau syndrome

Trisomy 13 causing severe malformations and early death.

Edwards syndrome

Trisomy 18 causing multiple organ defects and early death.

Down syndrome

Trisomy 21 causing developmental and facial features.

Klinefelter syndrome

XXY males; tall

Jacobs syndrome

XYY males; tall with normal fertility.

Trisomy X

XXX females; tall

Turner syndrome

XO females; short

Prokaryotic chromosomes

Circular

Eukaryotic chromosomes

Linear

Prokaryotic gene number

About 1

Eukaryotic gene number

Tens of thousands.

Prokaryotic proteins

Histone-like proteins compact supercoiled DNA.

Eukaryotic chromatin

DNA wrapped around histones forming nucleosomes.

Prokaryotic coding DNA

Mostly coding with few introns.

Eukaryotic coding DNA

Mostly noncoding with many introns.

Replication origin in prokaryotes

Single origin enabling fast replication.

Replication origin in eukaryotes

Multiple origins per chromosome.

Centromeres/telomeres in prokaryotes

Absent.

Centromeres/telomeres in eukaryotes

Present for segregation and protection.

DNA supercoiling

Over- or under-winding compacting DNA.

Topoisomerases

Enzymes managing DNA supercoiling tension.

Type I topoisomerase

Cuts one strand to relieve tension.

Type II topoisomerase

Cuts both strands and passes another segment through.