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

Maternal effect

Offspring phenotype depends on mothers genotype, not offspring or father's.

Function of maternal effect

Occurs during gametogenesis or early development in the egg.

Molecular mechanism of maternal effect

Maternal gene products in egg (proteins/RNAs) guide embryogenesis.

Nurse cells

As an animal oocyte matures, many surrounding cells, called nurse cells, provide the oocyte with nutrients and other materials.

Epigenetic inheritance

Modifications to DNA/chromosomes that alter gene expression without changing sequence.

Difference between epigenetic changes and genetic variations

Epigenetic changes differ from genetic variations by altering the gene without changing the sequence while genetic variations permanently change the DNA sequence.

Difference between epigenetic changes and mutations

Epigenetic changes affect gene activity and expression by altering the structure of DNA or histones, without changing the underlying DNA sequence, while mutations are permanent changes to the DNA sequence itself.

Dosage compensation

Balances expression of sex-linked genes between sexes.

Dosage compensation in placental mammals

One X in females randomly inactivated -> Barr body.

Dosage compensation in marsupials

Paternal X inactivated.

Dosage compensation in Drosophila

Male X doubled in activity.

Dosage compensation in C. elegans

Hermaphrodites reduce expression of each X by 50%.

Dosage compensation in birds

Inconsistent dosage compensation.

Establishment of X-inactivation in mammals

X-inactivation in mammals is established early in embryonic development (blastocyst stage) when one X chromosome in each female somatic cell is randomly inactivated through XIST-mediated silencing.

Maintenance of X-inactivation

It is then stably maintained throughout life by DNA methylation, histone modifications, and continued XIST activity.

Example of phenotypic effect of X-inactivation

The patchy coat of a calico cat arises because of random X-inactivation in a heterozygous female (XᴼXᵇ).

Phenotype resulting from X-inactivation

The patchy coat of a calico cat arises because of random X-inactivation in a heterozygous female (XᴼXᵇ).

Barr bodies

Highly condensed structures in the interphase nuclei of somatic cells in female cats but not found in male cats.

Active X chromosomes

In females, males, female with Turner syndrome and Triple X syndrome, and male with Klinefelter syndrome, there is 1 active X chromosome.

Genomic imprinting

An analogous situation in which a segment of DNA is marked and that mark is retained and recognized throughout the life of the organism inheriting the marked DNA.

Inheritance of genomic imprinting

Happens prior to fertilization; involves a change in a single gene or chromosome during gamete formation.

Molecular mechanism of genomic imprinting

Marked epigenetically, meaning they are chemically modified in a way that affects gene expression without changing the DNA sequence itself.

DNA methylation

The main mechanism by which imprinted genes are marked.

Timing of imprinting

Established during gametogenesis; spermatogenesis for males and oogenesis for females.

Igf2 gene

Codes a protein called insulin-like growth factor 2; imprinting results in expression of the Igf2 allele from the male parent but not the female parent.

Angelman syndrome

Caused by deletion inherited from the female parent, leading to abnormalities on chromosome 15, specifically in the 15q11-q13 region.

Prader-Willi syndrome

Caused by deletion inherited from the male parent, leading to abnormalities on chromosome 15, specifically in the 15q11-q13 region.

Monoallelic expression

In genomic imprinting, only 1 allele is expressed based on epigenetic silencing, while in dominant/recessive, both alleles are expressed based on functional differences.

Extranuclear inheritance

Genes outside the nucleus, also called cytoplasmic inheritance, organelle inheritance, or maternal inheritance.

Egg cells vs sperm cells

Egg cells contain a huge amount of cytoplasm and mitochondria, contributing almost all the cytoplasm to the zygote, while sperm cells have very little cytoplasm.

Maternal inheritance patterns

Non-Mendelian patterns where mothers pass traits to all children, but fathers do not pass it on.

Heteroplasmy

A single cell that contains a mixture of more than one type of mitochondrial DNA (mtDNA); its effect is dependent on the ratio of mutants to normal mitochondria.

Endosymbiosis

The process by which eukaryotic cells evolved from partnerships between different prokaryotic cells.

Mitochondrial and chloroplast genomes

General features of these genomes include their unique structures and functions related to energy production and photosynthesis.

Mitochondrial diseases

Results from mutations in mitochondrial DNA or nuclear genes that encode mitochondrial proteins.

Maternal inheritance

Is about mitochondria (and chloroplasts for plants). Everyone has them, but normally only mom passes them on.

X-inactivation

Is placental mammal's method for sex chromosome dosage compensation, where one X-chromosome is selected and randomly condensed in every cell of early female embryos.

Sex-linked inheritance

Has to do with sex chromosomes; females can be heterozygous carriers for X-linked genes, while males cannot be carriers and are more commonly affected by X-linked conditions.

Sex-influenced inheritance

Where heterozygotes for an autosomal gene show either the dominant or recessive trait depending on their sex.

Sex-limited inheritance

Another term for sexual dimorphism, traits that are usually expressed only or mainly in one sex.

Genetic linkage

When genes close together are inherited together.

Recombination frequency

Calculated as Map distance = number of recombinant offspring / total number of offspring x 100.

Genetic mapping

Traditionally done using genetic linkage mapping to determine the order and distance of genes on chromosomes.

Accuracy of genetic mapping

Less accurate for widely separated genes because multiple crossovers can occur and these recombinations are undetected.

Testcross

A cross between a parent who is heterozygous for two or three traits and a parent who is homozygous recessive for the same traits.

Independent Assortment

If the genes are not linked, the expected phenotype ratios in the offspring are 1:1:1:1 for 2 genes and 1:1:1:1:1:1:1:1 for 3 genes.

Linked Genes

If the genes are linked, but no crossing over occurs, the expected phenotype ratio is 1:1 for 2 genes.

Close Genes

If the genes are close together on the same chromosome, the expected phenotype ratio is 1:1 for 2 genes.

Far Apart Genes

If the genes are far apart on the same chromosome, the expected phenotype ratio is 1:1:1:1.

Interference

The phenomenon where the occurrence of one crossover in a region of a chromosome reduces the likelihood of another crossover occurring nearby.

Recombinants

In practice, the number of recombinants is never more than about 50% because crossing over only occurs between homologous chromosomes during meiosis.

Crossing Over

A normal part of meiosis that occasionally happens during mitosis, creating daughter cells with slightly different genotypes.

Mitotic Recombination

A recombination event that can cause visible phenotypic mosaics when it creates homozygous patches for recessive alleles.

Karyotype

A method to tell chromosomes apart by their structure: metacentric, submetacentric, acrocentric, telocentric.

Deletions

Chromosomal breakage and loss of a fragment; terminal deletion involves a single break piece without centromere eventually lost.

Interstitial Deletion

Breakage in 2 places with the central fragment lost but 2 outer pieces reattach.

Duplication

A segment of a chromosome is copied and inserted again.

Inversion

A chromosome segment breaks, flips 180 degrees, and reattaches.

Translocation

A segment is moved from one chromosome to another or to a different part of the same chromosome.

Breakage and Repair

Chromosomal structural alterations start with DNA double-strand breaks, often from physical, chemical, or recombination stress.

Misaligned Crossovers

Happen when repetitive DNA sequences cause non-homologous alignment during recombination, leading to unequal exchange of DNA.

Gene Families

Sets of related genes formed by duplication of an ancestral gene, contributing to evolutionary innovation.

Homologs

Genes derived from a common ancestor.

Orthologs

Genes in different species that arose via speciation.

Paralogs

Genes within the same species that arose via gene duplication.

Balanced Translocation

A translocation with no net gain or loss of genetic material, where the carrier is usually healthy but faces reproductive risks.

Unbalanced translocation

Gain or loss of material leading to abnormal gene dosage and possible developmental or health effects.

Semisterility

Condition caused by inversions and balanced translocations that disrupt normal chromosome pairing during meiosis, resulting in some viable gametes.

Euploidy

The usual number of chromosomes for a species.

Aneuploidy

Variation in the number of particular chromosomes within a set.

Polyploidy

Variation in the number of complete sets of chromosomes, which is uncommon and most lethal in animals.

Patau syndrome

Trisomy 13 genotype with a frequency of 1/15,000, causing mental & physical disabilities, organ malformations, and stillbirth/early death.

Edward syndrome

Trisomy 18 genotype with a frequency of 1/6000, causing mental & physical disabilities, organ malformations, and stillbirth/early death.

Down syndrome

Trisomy 21 genotype with a frequency of 1/800, causing mental disabilities, characteristic facial features, and short stature.

Klinefelter syndrome

XXY genotype with a frequency of 1/650, causing underdeveloped testes, tall stature, and breast development.

Jacobs syndrome

XYY genotype with a frequency of 1/1000, causing tall stature.

Trisomy X

XXX genotype with a frequency of 1/1000, causing tall stature and early menopause.

Turner syndrome

X0 genotype with a frequency of 1/2500, causing underdeveloped ovaries, short stature, webbed neck, and heart problems.

Autopolyploidy

The increase in the number of sets within a single species due to complete nondisjunction.

Allopolyploidy

The increase in the number of chromosome sets in an alloploid, combining autopolyploidy and alloploidy.

Mitotic nondisjunction

A process occurring after fertilization during mitotic divisions that can produce phenotypic mosaicism.

Gynandromorphy

A type of phenotypic mosaicism where one sister chromatid does not migrate to a pole and is later lost.

Chimerism

A condition where a single individual has different cells with genotypes as different as those from two individuals, resulting from the complete fusion of early dizygotic twin embryos.

Prokaryotic chromosomes

Characterized by circular DNA, small size, and one main chromosome.

Eukaryotic chromosomes

Characterized by linear DNA, larger size, and multiple chromosomes per nucleus.

Number of genes in prokaryotes

Typically small, with about 1,000 to 5,000 genes.

Number of genes in eukaryotes

Typically larger, with tens of thousands of genes.

Associated proteins in prokaryotes

Include some histone-like proteins, with chromatin organized in supercoiled loops.

Eukaryotes

DNA wrapped around histones to form nucleosomes highly organized into chromatin.

Prokaryotes coding DNA

Very high coding DNA and introns are rare/absent.

Eukaryotes coding DNA

Much lower coding DNA and introns are common.

Origin of replication in prokaryotes

One origin of replication per chromosome with faster replication due to smaller genome size.

Origin of replication in eukaryotes

Multiple origins per chromosome to allow fast replication of large genomes but slower per origin.

Centromeres and telomeres in prokaryotes

Absent.

Centromeres and telomeres in eukaryotes

Present.

DNA supercoiling

Refers to the overwinding or underwinding of the DNA double helix beyond its normal relaxed state.

Topoisomerases

Enzymes that regulate DNA supercoiling.

Type 1 topoisomerase

Cuts one strand of DNA that allows rotation and then reseals it.

Type 2 topoisomerases

Cuts both strands and passes through another segment before resealing.

Importance of DNA supercoiling

Important for compaction of DNA, facilitates DNA replication and transcription, regulation of gene expression, and prevents damage.