CSIR NET Life Sciences Inheritance Biology Syllabus

What is crossing over and when does it occur?

Crossing over is the reciprocal exchange of genetic material between non-sister chromatids of homologous chromosomes at chiasmata during pachytene of prophase I of meiosis, producing recombinant chromosomes.

How is recombination frequency related to genetic map distance?

1% recombination frequency = 1 map unit = 1 centimorgan (cM). Map distance (cM) = (number of recombinant offspring / total offspring) x 100, valid for closely linked genes.

Why does recombination frequency saturate at 50% for distant genes?

With multiple crossovers between far-apart loci, even and odd numbers of exchanges cancel out, so the genes assort as if unlinked; observed recombination cannot exceed 50%, underestimating true map distance.

What is the coefficient of coincidence and interference?

Coefficient of coincidence (c.o.c.) = observed double crossovers / expected double crossovers. Interference = 1 - c.o.c.; positive interference means one crossover reduces the chance of a nearby second crossover.

What is sex linkage?

Sex linkage is the inheritance of genes located on the sex chromosomes (usually the X), producing characteristic crisscross inheritance patterns that differ between males and females. Example: red-green colour blindness, hemophilia.

Distinguish sex-limited from sex-influenced characters.

Sex-limited traits are expressed in only one sex though genes are present in both (e.g., milk yield, beard); sex-influenced traits are expressed in both sexes but behave as dominant in one sex and recessive in the other (e.g., pattern baldness — dominant in males).

In a three-point test cross, how do you identify gene order?

Compare the parental (most frequent) gametes with the double-crossover (least frequent) gametes; the gene whose allele has switched in the double-crossover class relative to the parental class is the middle gene.

What is tetrad analysis and in which organisms is it used?

Tetrad analysis examines all four meiotic products held together in an ascus (e.g., Neurospora, yeast, Chlamydomonas) to study segregation, recombination, and gene-centromere distance directly.

How is gene-to-centromere distance calculated in ordered tetrad (Neurospora) analysis?

Distance (cM) = (1/2 x number of second-division segregation [SDS] asci / total asci) x 100; the 1/2 accounts for only two of four chromatids being recombinant.

Define parental ditype (PD), nonparental ditype (NPD), and tetratype (T) in tetrad analysis.

PD = both spore types are parental; NPD = both spore types are recombinant; T = tetratype contains all four genotypes (two parental, two recombinant). Genes are linked when PD >> NPD.

How are molecular markers used in genetic mapping?

Polymorphic DNA markers (RFLP, RAPD, AFLP, SSR/microsatellites, SNPs) are scored as codominant/dominant loci in a mapping population, and their co-segregation with traits is used to compute linkage and build genetic maps.

How does somatic cell hybridization map genes to chromosomes?

Human-rodent hybrid cells randomly lose human chromosomes; by correlating the presence of a specific human chromosome with the expression of a gene product across hybrid clones (synteny/concordance analysis), the gene is assigned to that chromosome.

Name common mapping populations developed in plants for QTL/linkage mapping.

F2, backcross (BC), recombinant inbred lines (RILs), doubled haploids (DH), and near-isogenic lines (NILs).

What is special about inheritance of mitochondrial and chloroplast genes?

They are cytoplasmically (extranuclear) inherited, generally uniparental/maternal, do not follow Mendelian ratios, show non-reciprocal cross results, and produce variegation/heteroplasmy patterns.

What is maternal inheritance? Give a classic example.

Maternal inheritance is transmission of a trait through the cytoplasm of the egg, so offspring resemble the female parent regardless of male genotype. Example: leaf variegation in Mirabilis jalapa (Correns) and shell coiling in Limnaea (maternal effect).

List the three main methods of genetic transfer in bacteria.

Transformation (uptake of naked DNA), conjugation (cell-to-cell transfer via pilus/F factor), and transduction (transfer of bacterial DNA by a bacteriophage — generalized or specialized).

How does interrupted mating map bacterial genes?

In Hfr x F- conjugation, mating is interrupted at timed intervals (e.g., by blending); the order and time of entry of donor markers into the recipient give a map measured in minutes, reflecting the linear oriented transfer from oriT.

What did Benzer's fine-structure analysis of the rII region of phage T4 reveal?

It demonstrated that the gene is divisible: the cistron is the unit of function (defined by complementation), while the recon (recombination) and muton (mutation) are smaller, down to single nucleotides — showing recombination occurs within a gene.

What is pedigree analysis used for?

Pedigree analysis traces the inheritance of a trait across generations using standardized symbols to determine its mode of inheritance (autosomal/X-linked, dominant/recessive) and to assess carrier status and recurrence risk.

List key pedigree features of autosomal recessive vs autosomal dominant inheritance.

Autosomal recessive: trait skips generations, affected from unaffected carrier parents, both sexes equally, often consanguinity. Autosomal dominant: every affected has an affected parent (vertical transmission), both sexes equally, no skipping.

What is the LOD score and the threshold for declaring linkage?

LOD (logarithm of odds) = log10 (likelihood of data assuming linkage at recombination fraction θ / likelihood assuming no linkage, θ=0.5). A LOD score >= +3 (odds 1000:1) indicates significant linkage; <= -2 excludes linkage.

What is a karyotype?

A karyotype is the complete set of chromosomes of a cell/individual arranged in order by size, centromere position, and banding pattern, used to detect numerical and structural chromosomal abnormalities.

Name the chromosomal basis of Down, Turner, and Klinefelter syndromes.

Down syndrome: trisomy 21 (47,XX/XY,+21); Turner syndrome: monosomy X (45,X); Klinefelter syndrome: 47,XXY.

Give one example each of an autosomal recessive, autosomal dominant, and X-linked recessive genetic disorder.

Autosomal recessive: cystic fibrosis / sickle-cell anemia; Autosomal dominant: Huntington's disease / achondroplasia; X-linked recessive: hemophilia A / Duchenne muscular dystrophy.

What is polygenic (quantitative) inheritance?

Polygenic inheritance is the control of a continuously varying (quantitative) trait by many genes (polygenes), each with a small additive effect, together with environmental influence, producing a normal distribution of phenotypes.

Define heritability and distinguish broad-sense from narrow-sense heritability.

Heritability is the proportion of phenotypic variance due to genetic variance. Broad-sense H^2 = VG/VP (all genetic variance); narrow-sense h^2 = VA/VP (only additive genetic variance), which predicts response to selection.

What is the breeder's equation relating selection to response?

R = h^2 x S, where R is the response to selection, S is the selection differential, and h^2 is the narrow-sense heritability. Realized heritability = R/S.

What is QTL mapping?

QTL (Quantitative Trait Locus) mapping locates chromosomal regions controlling quantitative traits by testing statistical association between molecular markers and trait values in a segregating population (e.g., by interval mapping, composite interval mapping).

What are the major types and causes of mutations?

Types: point/gene mutations (substitutions — transitions/transversions, insertions/deletions causing frameshifts) and chromosomal mutations. Causes: spontaneous (replication errors, tautomeric shifts) or induced by mutagens (radiation, chemicals, transposons).

Name common methods for detecting mutations.

Ames test (bacterial reversion for mutagens/carcinogens), CIB and attached-X methods in Drosophila, specific-locus tests in mice, replica plating for selecting bacterial mutants, and modern DNA sequencing/SSCP/DGGE.

Distinguish germinal from somatic mutations.

Germinal (germ-line) mutations occur in gametes/germ cells and are heritable, transmitted to offspring; somatic mutations occur in body cells, are not inherited, and affect only the individual (e.g., a mosaic patch or cancer).

Classify mutant types by effect on function (loss vs gain).

Loss-of-function: amorph (null, complete loss) and hypomorph (partial reduction). Gain-of-function: hypermorph (increased activity), neomorph (novel function), and antimorph (dominant-negative, antagonizes wild-type).

What is insertional mutagenesis?

Insertional mutagenesis is gene disruption caused by integration of a DNA element (transposon, T-DNA, retrovirus) into or near a gene; the insert acts as a known molecular tag enabling cloning of the mutated gene (gene/transposon tagging).

Compare the four main structural chromosomal aberrations.

Deletion: loss of a chromosome segment; Duplication: extra copy of a segment; Inversion: a segment is reversed 180 degrees; Translocation: a segment moves to a non-homologous chromosome (reciprocal or Robertsonian).

Differentiate paracentric from pericentric inversion and their meiotic consequences.

Paracentric inversion excludes the centromere; a crossover within the loop yields a dicentric bridge and an acentric fragment. Pericentric inversion includes the centromere; a crossover within it yields duplication-deletion (unbalanced) recombinant chromatids. Inversions act as crossover suppressors.

What cytological structure forms in a heterozygote for a reciprocal translocation at meiosis, and what is its genetic consequence?

A cross-shaped (cruciform) configuration of four chromosomes forms, segregating in alternate or adjacent patterns; adjacent segregation produces unbalanced gametes, causing semi-sterility.

Define ploidy variations: euploidy vs aneuploidy.

Euploidy = variation in complete chromosome sets (monoploid n, diploid 2n, polyploid 3n/4n etc.); aneuploidy = gain or loss of individual chromosomes (e.g., monosomy 2n-1, trisomy 2n+1, nullisomy 2n-2).

Distinguish autopolyploidy from allopolyploidy and give the genetic implication of polyploidy.

Autopolyploidy: multiple sets from the same species (e.g., autotetraploid); allopolyploidy: sets from two different species after hybridization + doubling (e.g., wheat, Triticale, Raphanobrassica). Odd-numbered polyploids are usually sterile due to abnormal meiotic pairing; polyploidy often increases vigor and is common in plants.

What is homologous recombination?

Homologous recombination is the exchange of nucleotide sequences between two DNA molecules sharing extensive sequence similarity, mediated by strand invasion (RecA/Rad51), Holliday junction formation and resolution; it underlies crossing over and double-strand break repair.

What is the Holliday junction and how is it resolved?

The Holliday junction is a four-stranded cross-shaped DNA intermediate of homologous recombination formed by reciprocal single-strand exchange; branch migration extends it, and resolvases cut it to give either crossover or non-crossover (patch/splice) products.

What is non-homologous recombination, and how does transposition fit in?

Non-homologous recombination joins DNA without sequence homology — e.g., non-homologous end joining (NHEJ) of double-strand breaks and site-specific/illegitimate recombination. Transposition is a form of non-homologous (illegitimate) recombination in which transposable elements move via cut-and-paste (conservative, DNA transposons) or copy-and-paste (replicative/retrotransposons via RNA intermediate).