genetics final exam

four rules of Mendelian inheritance?

1. Genes directly influence traits

2. Genes are passed unchanged across generations (except rare mutations)

3. Genes obey segregation

4. Genes obey independent assortment.

What is a maternal effect?

mother’s genotype directly determines the offspring’s phenotype.

What is epigenetic inheritance?

change in gene/chromosome that affect how gene work and passed when cell divide

What is extranuclear inheritance?

Inheritance of genes outside the nucleus,  (e.g. mitochondria or chloroplasts)

What is linkage

when crosses involve 1+ genes it obeys the law of independent assortment (2+ genes are close on the chromosome)

Do the father’s or offspring’s genotypes affect the phenotype in maternal effect?

No, only the mother’s genotype matters.

Why does maternal effect occur?

mom provide gene products (RNAs or proteins) to their eggs that control early development.

Who discovered the first maternal effect gene and when?

A.E. Boycott in the 1920s, while studying water snail coiling.

What are the two possible orientations of snail shells?

Right-handed (dextral, dominant) and left-handed (sinistral, recessive).

If the mother is DD or Dd, what phenotype will her offspring have?

All offspring will be dextral (right-handed).

If the mother is dd, what phenotype will her offspring have?

All offspring will be sinistral (left-handed).

What process explains maternal effect in snails?

Oogenesis, maternal nurse cells deposit RNAs and proteins into the egg

What does the D allele’s products cause during cleavage?

Right-handed (dextral) body plan.

What does the d allele’s products cause during cleavage?

Left-handed (sinistral) body plan.

Why is the sperm genotype irrelevant for snail coiling?

expressed too late to influence early embryo development.

What roles do maternal effect genes play in development?

Cell division, cleavage pattern, and body axis orientation.

epigenetic inheritance can pass to which type of cell

daughter cells, but without changing DNA sequence.

Is epigenetic inheritance permanent across generations?

No, it is reversible and does not permanently alter DNA.

What is the purpose of dosage compensation?

offset differences in the number of active sex chromosome

How do placental mammals achieve dosage compensation?

By inactivating one X chromosome in females somatic cells

How do Drosophila males achieve dosage compensation?

doubling the expression of genes on their single X chromosome.

How do C. elegans hermaphrodites achieve dosage compensation?

reducing expression of each of their two X chromosomes by 50%.

Do birds have a global mechanism for dosage compensation?

No, some Z-linked genes are compensated, but many are not.

What is a Barr body?

highly condensed, inactivated X chromosome in female mammal cells

Who proposed the X-inactivation (Lyon hypothesis) and when?

Mary Lyon in 1961

What are the three phases of X-chromosome inactivation?

Nucleation, spreading, and maintenance.

what is nucleation phase

occur during development, Xic are counted and one chromosome is left active

what is spreading phase

happen during development. start at Xics and wont stop until whole chromosome is inactive

what is maintenance phase

happen from embryonic development through adult life and still there after cell division

What gene region is crucial for X inactivation?

The X-inactivation center (Xic), including the Xist gene.

Do all X-linked genes get inactivated?

No, some escape inactivation, like pseudo autosomal genes.

What is genomic imprinting?

epigenetic process where DNA is marked so only one parental allele is expressed.

What is monoallelic expression?

only one allele (maternal or paternal) is expressed, while the other is silenced.

What does the Igf2 gene encode?

Insulin-like growth factor 2, a hormone needed for normal growth

In Igf2 imprinting, which allele is expressed?

The paternal allele; the maternal allele is silenced.

What happens if the paternal Igf2 allele is mutant (Igf2–)?

The offspring may be dwarf, since no functional Igf2 protein is made.

What are the three stages of imprinting?

1. Establishment during gametogenesis

2. Maintenance during embryogenesis/adulthood

3. Erasure & reestablishment in germ cells

What molecular process usually controls imprinting?

DNA methylation at the imprinting control region (ICR).

What is the imprinting control region (ICR)?

A DNA region near imprinted genes that gets methylated in sperm or egg, controlling expression.

Which two human disorders are caused by imprinting defects on chromosome 15?

Prader-Willi syndrome (PWS) and Angelman syndrome (AS).

If the deletion on chromosome 15 is inherited from the father, which syndrome results?

Prader-Willi syndrome (PWS).

If the deletion on chromosome 15 is inherited from the mother, which syndrome results?

Angelman syndrome (AS).

What gene is missing in Angelman syndrome?

UBE3A, which regulates protein degradation.

What genes are missing in Prader-Willi syndrome?

Several genes, including SNRNP, NDN, and snoRNA clusters, important for neuron function and gene splicing.

What is extranuclear inheritance?

Inheritance of genetic material outside the nucleus, mainly from mitochondria and chloroplasts

Where is mitochondrial and chloroplast DNA located

nucleoid

What is the structure of organelle genomes?

Single circular chromosomes of double-stranded DNA.

Which organelle genome is typically larger: mitochondria or chloroplasts?

Chloroplast genomes (about 10 times larger).

What is the function of mitochondria?

Oxidative phosphorylation to make ATP.

How large is human mitochondrial DNA (mtDNA)?

17,000 base pairs, encoding 13 polypeptides, rRNAs, and tRNAs.

What is the main function of chloroplasts?

Photosynthesis

What does chloroplast DNA (cpDNA) contain?

110–120 genes for rRNAs, tRNAs, and photosynthesis proteins.

Who discovered maternal inheritance of chloroplasts?

Carl Correns, studying Mirabilis jalapa (four o’clock plant).

In Mirabilis, what determines offspring leaf color?

Only the maternal parent, not the father.

Why does maternal inheritance occur in chloroplasts?

Because chloroplasts are inherited through the egg cytoplasm, not pollen

What is heteroplasmy?

condition where a cell contains both normal and mutant organelles

In most animals, how are mitochondria inherited?

Maternal inheritance.

What is paternal leakage?

Occasional inheritance of mitochondria from sperm.

In yeast, how are mitochondria inherited?

Biparentally

In angiosperms (flowering plants), how are organelles inherited?

Usually maternal, though biparental inheritance sometimes occurs.

How are mitochondrial diseases passed to offspring?

maternal inheritance of mtDNA.

What is another way mitochondrial mutations arise?

somatic cells, often increasing with age

Why are mitochondria prone to mutations?

High oxygen usage creates free radicals and mtDNA has limited repair.

What cells are most affected by mitochondrial diseases?

Cells requiring high ATP, like nerve and muscle cells.

What is Leber hereditary optic neuropathy?

mitochondrial disease caused by mutations in respiratory chain proteins, leading to vision loss.

What is mitochondrial myopathy?

disease caused by mutations in tRNA genes for leucine, leading to muscle weakness.

What is heteroplasmy’s role in mitochondrial disease?

Disease symptoms appear when mutant mitochondria exceed a threshold ratio.

What does the endosymbiosis theory explain?

The evolutionary origin of mitochondria and chloroplasts in eukaryotes.

How did mitochondria and chloroplasts originate?

free-living bacteria engulfed by early eukaryotic/archaeal cells

What was the bacterial ancestor of mitochondria?

Gram-negative nonsulfur purple bacteria.

What was the bacterial ancestor of chloroplasts?

Cyanobacteria

What evidence supports the endosymbiosis theory?

• Mitochondria and chloroplasts have circular chromosomes like bacteria.

• Their genes are more similar to bacterial genes than to nuclear genes.

• They reproduce independently inside cells.

During evolution, what happened to most bacterial genes?

lost or transferred to the nuclear genome

What advantages did endosymbiosis give eukaryotic cells?

• Chloroplasts enabled photosynthesis.

• Mitochondria allowed more efficient ATP production.

What benefit might bacteria have gained from the relationship?

stable, nutrient-rich environment inside the host cell.

What were Frederick Griffith’s experiments with Streptococcus pneumoniae designed to show?

demonstrated that a substance from dead virulent bacteria could transform live non-virulent bacteria into a virulent form

What did Griffith call the unknown substance responsible for transformation?

The "transformation principle."

What did Avery, MacLeod, and McCarty’s experiment prove?

showed that DNA, not protein or RNA, is the genetic material responsible for transformation.

What enzyme destroyed the transforming ability in Avery’s experiment?

DNase (which breaks down DNA).

What did Hershey and Chase use to confirm DNA as the genetic material?

Radioactive labeling—³²P to label DNA and ³⁵S to label protein—in bacteriophage T2 infections of E. coli.

What was the conclusion of the Hershey and Chase experiment?

DNA enters the bacterial cell and directs viral reproduction, proving DNA is the genetic material

What did Gierer and Schramm discover using Tobacco Mosaic Virus (TMV)?

That RNA can serve as the genetic material in some viruses.

What are the four levels of nucleic acid structure?

(1) Nucleotides, (2) Linear strand, (3) Double helix, (4) 3D folding and protein interaction.

What are the three components of a nucleotide?

phosphate group, a pentose sugar, and a nitrogenous base.

What is the difference between purines and pyrimidines?

Purines (A, G) have double rings; pyrimidines (C, T, U) have single rings.

What is the difference between DNA and RNA nucleotides?

DNA has deoxyribose and thymine; RNA has ribose and uracil.

How are nucleotides linked together?

By phosphodiester bonds between the 5′ carbon of one sugar and the 3′ carbon of another.

Who discovered the double-helix structure of DNA?

James Watson and Francis Crick in 1953.

What were Rosalind Franklin’s contributions?

used X-ray diffraction to show that DNA was helical with 10 base pairs per turn.

What was Chargaff’s rule?

The amount of adenine equals thymine, and cytosine equals guanine (A=T, C=G).

What are the main features of the DNA double helix?

Two antiparallel strands, right-handed helix, 10 bases per turn, stabilized by hydrogen bonding and base stacking.

What is the difference between major and minor grooves in DNA?

spaces where proteins can bind to specific base sequences

What are the three forms of DNA?

A-DNA (right-handed, 11 bp/turn), B-DNA (right-handed, 10 bp/turn, most common), and Z-DNA (left-handed, 12 bp/turn).

What is Z-DNA associated with?

Gene regulation and chromosomal structure compaction.

What is triplex DNA?

A three-stranded DNA structure where a synthetic strand binds to the major groove of double-stranded DNA.

How is DNA compacted in cells?

By winding around histone proteins to form nucleosomes, then fibers, loops, and chromosomes.

How does RNA differ from DNA?

RNA is usually single-stranded, uses ribose sugar, and replaces thymine with uracil.

What are common secondary structures of RNA?

Bulge loops, internal loops, multibranched junctions, and stem-loops (hairpins).

What are the two key functional sites in tRNA?

The anticodon (base pairs with mRNA) and the acceptor site (binds amino acid).

What interactions contribute to RNA’s tertiary structure?

Base-pairing, base stacking, and interactions with ions, molecules, and proteins.