At metaphase I, the cell has 16 duplicated chromosomes, each with two sister chromatids. After meiosis I, each cell has 8 duplicated chromosomes and 16 chromatids. Meiosis II separates the sister chromatids, leaving each daughter cell with 8 single-chromatid chromosomes.

Hardy–Weinberg equilibrium assumes a large population, no gene flow, no mutation, random mating, and no natural selection.

The allele frequencies satisfy p + q = 1, so q = 0.4. The dominant-phenotype frequency is p² + 2pq = 1 − q² = 1 − (0.4)² = 0.84, or 84%.

The woman is a carrier, XX^a, because her affected father passed X^a to her. In these diagrams, X represents the normal-vision allele and X^a the recessive color-blindness allele.

Her sons receive Y from their father and either X or X^a from her with equal probability. Therefore, 50% of her sons are expected to be affected. An affected son represents 25% of all births if male and female births are equally likely; that is a different probability.

An unidentified transforming material from the heat-killed S bacteria converted live R bacteria into pathogenic S bacteria. Avery, MacLeod, and McCarty later identified DNA as the transforming material. The Hershey–Chase experiment supported DNA as the genetic material in bacteriophage T2.

For short intervals between linked genes, greater separation generally gives a higher recombination frequency. Recombination frequency can estimate genetic map distance, but multiple crossovers can cause it to underestimate longer distances.

Each Okazaki fragment is synthesized 5′→3′ away from the advancing replication fork. The lagging strand is synthesized discontinuously, and DNA ligase joins the fragments. DNA polymerase III elongates the fragments in E. coli.

Exon and intron sequences are transcribed into pre-mRNA, and rRNA is also transcribed from DNA. RNA processing adds the 5′ cap and poly-A tail and removes introns. Amino acids are the building blocks used in translation, not products of transcription.

Each amino acid is specified by a three-nucleotide codon. Thus, 120 × 3 = 360 nucleotides in one coding DNA strand encode the 120 amino acids.

A nonsense mutation changes an amino-acid codon to a stop codon. A missense mutation changes a codon so that it specifies a different amino acid. Thus, A and B are incorrectly matched.

PCR amplifies a target DNA sequence. Dideoxy chain-termination (Sanger) sequencing determines a DNA sequence. X-ray diffraction provides information about molecular structure. DNA profiling helps identify individuals, and gel electrophoresis separates nucleic acids or proteins.

I, II, and III are true. A typical duplicated chromosome has two sister chromatids from G2 through metaphase of mitosis. Normal meiosis halves the chromosome number. G1 precedes DNA replication, but its duration varies among cells. Homologous chromosome pairs align together in meiosis I, not mitosis.

DNA replication during S phase doubles the DNA content without changing the chromosome number. After S phase, a typical diploid cell of this organism has 8 duplicated chromosomes and 16 chromatids.

There are 100 − 64 − 32 = 4 recessive homozygotes. The recessive-allele frequency is q = (32 + 2 × 4)/(2 × 100) = 0.2. The genotype frequencies also fit Hardy–Weinberg proportions, so the square-root calculation shown below gives the same result.

There are three heterozygous loci: Aa, Cc, and Dd. Each contributes two possible alleles to a gamete, while BB and EE each contribute one. Therefore, 2 × 1 × 2 × 2 × 1 = 2³ = 8 gamete types.

I. Hemophilia is X-linked recessive in this cross. Every son of an affected mother and an unaffected father receives the affected X allele from his mother and is affected.

II. Red-green color blindness is X-linked recessive in this cross. Daughters of a homozygous unaffected mother and an affected father are heterozygous carriers and are expected to be unaffected.

III. Sickle-cell disease is autosomal recessive. Children of an affected mother (aa) and a homozygous unaffected father (AA) are heterozygotes (Aa) and do not have sickle-cell disease.

IV. Two parents with sickle-cell disease (aa × aa) have children with genotype aa, so the listed daughter is affected. Thus, only II and III are expected to be unaffected.

Under the stated typical-karyotype assumption, X inactivation indicates an XX animal. The inactive X forms condensed chromatin called a Barr body, and X inactivation can produce mosaic gene expression. Barr bodies depend on X-chromosome number, not sex alone; for example, an XXY male can have a Barr body.

Nondisjunction in meiosis II is the failure of sister chromatids to separate. Resulting abnormal gametes may have n + 1 or n − 1 chromosomes. Fertilization by a normal gamete can produce trisomic (2n + 1) or monosomic (2n − 1) zygotes. Turner syndrome (45,X) is one possible sex-chromosome monosomy; not all monosomies cause Turner syndrome, and not all trisomies cause Klinefelter syndrome.

In double-stranded DNA, G = C and A = T. If G = C = 26%, then A + T = 48%. Therefore, T = 24%.

Spliceosomes are complexes of small nuclear ribonucleoproteins (snRNPs) and other proteins. They remove introns from pre-mRNA and join exons to form mature mRNA. Exons can include both protein-coding sequences and untranslated regions.

A one-nucleotide insertion or deletion shifts the reading frame, changing downstream codon groupings and often introducing a premature stop codon. The severity of a mutation depends on its location and effect; a frameshift is not always the most severe type.

Sister chromatids separate during anaphase of mitosis and anaphase II of meiosis. Homologous chromosomes separate during anaphase I of meiosis.

Homologous chromosomes pair during prophase I of meiosis. Crossing over between their nonsister chromatids produces recombinant chromatids that can later enter gametes. The exchange occurs during meiosis, not between gametes.

The founder effect occurs when a new population is established by a small number of individuals carrying a subset of the source population’s genetic variation. A bottleneck is a sharp reduction in population size. Gene flow transfers alleles between populations through individuals or gametes. Hybridization involves genetically distinct populations or species. A cline is a gradual geographic change in a trait or allele frequency.

Codominance means that both alleles are distinctly expressed in a heterozygote. With complete dominance, the heterozygote has the dominant homozygote’s phenotype; with incomplete dominance, it has an intermediate phenotype. In epistasis, one gene masks or modifies the effect of another gene. Pleiotropy occurs when one gene affects multiple traits.

If the mother is I^Bi and the father is I^Ai, their children can have genotypes I^AI^B, I^Ai, I^Bi, or ii. Therefore, AB, A, B, and O are all possible blood types.

Duplication creates an extra copy of a chromosome segment. Deletion removes a segment. Inversion reverses a segment’s orientation within a chromosome. Translocation moves a segment to a nonhomologous chromosome.

The nuclear envelope breaks down during late prophase or prometaphase and reforms during telophase. DNA replication occurs during S phase, before mitosis.

The G2 checkpoint checks that DNA replication is complete and helps prevent entry into mitosis with damaged or incompletely replicated DNA.

ABO blood type is determined primarily by multiple alleles at one locus, with codominance between I^A and I^B, rather than by polygenic inheritance. Polygenic traits are influenced by multiple genes; skin pigmentation is an example. Epistasis is interaction in which one gene masks or modifies another gene’s effects.

With complete dominance, homozygous dominant and heterozygous individuals have the same phenotype, while homozygous recessive individuals express the recessive phenotype. A typical XY male is hemizygous for most X-linked genes because there is only one copy. Heterozygous describes two different alleles at a locus.

Klinefelter syndrome typically involves a male with a 47,XXY karyotype arising from sex-chromosome nondisjunction. Features vary, and reduced fertility is common. A 45,X karyotype is associated with Turner syndrome; trisomy 21 causes Down syndrome.

Primase synthesizes short RNA primers that initiate DNA synthesis. In eukaryotes, RNA polymerase II transcribes pre-mRNA.

DNA is transcribed into RNA, which is translated into protein. Proteins contribute to phenotypes, which can also be influenced by environmental factors.

The anticodons pair antiparallel with the mRNA sequence 5′-UAU CUA CAU-3′. The complementary DNA template is 3′-ATA GAT GTA-5′. Thus, choice C gives the template bases in the requested direction.

G0 is a nondividing state, not a checkpoint. The G1 checkpoint evaluates conditions before DNA replication, the G2 checkpoint precedes mitosis, and the M checkpoint monitors spindle attachment. All four descriptions are correct, so none is incorrectly matched.

Mitosis normally produces somatic cells. Germline precursors also undergo mitosis, while gamete production involves meiosis. Independent assortment of 23 homologous chromosome pairs can produce 2²³ chromosome combinations in gametes, even before accounting for crossing over.

PP × pp produces purple-flowered Pp offspring. Self-pollination of the F1 heterozygotes gives expected F2 genotypes PP:Pp:pp = 1:2:1 and purple:white phenotypes = 3:1. Pink F1 flowers would indicate incomplete dominance.

A testcross investigates the genotype of an individual with a dominant phenotype by crossing it with a homozygous recessive tester. Statement I is false; II, III, and IV are true. AA × aa is expected to produce only dominant-phenotype Aa offspring, while Aa × aa gives an expected 1:1 dominant:recessive ratio. A finite sample of all dominant offspring alone does not prove that the unknown individual is homozygous.

The two original strands remain, while the total number of strands doubles each round. Their fraction is 1/2 after one round, 1/4 after two rounds, and 1/8 = 12.5% after three rounds.

Only I is true. Pyrimidines have one ring, and purines have two. Standard Watson–Crick pairs are A–T and G–C. The DNA backbone consists of deoxyribose sugars and phosphate groups. For comparable molecules under the same conditions, GC-rich DNA generally has a higher melting temperature than AT-rich DNA. G–C pairs have three hydrogen bonds and A–T pairs have two; base stacking, sequence, and solution conditions also affect melting temperature.

The growing polypeptide is transferred from the P-site tRNA to the amino acid on the A-site tRNA when a peptide bond forms. After translocation, the peptidyl-tRNA occupies the P site, and the uncharged tRNA leaves through the E site. Elongation factors use GTP.

Unaffected parents in generation I have an affected child, supporting recessive inheritance. The affected daughter in generation III has an unaffected father, which excludes X-linked recessive inheritance under the stated assumptions. Autosomal recessive inheritance fits the pedigree.

Wobble allows flexible base pairing at the third position of the codon, which pairs with the first position of the anticodon. Codons still contain three nucleotides. Inosine is a modified base found in some tRNA anticodons and can pair with more than one codon base.