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State Board · Class 12 · Biology

Lesson 3: Inheritance and Variation

7 concepts · 56 practice questions with answers · Free to read

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Mendel and the monohybrid cross

Link to Std 9–10

Std 11 covered cell division (meiosis); here we use it to study Mendel's laws, linkage, sex determination and genetic disorders.

From daily life

Children look like their parents yet differ from siblings — genes follow rules.

Simple definition

Heredity = transmission of traits across generations. A monohybrid cross involves pure parents differing in one pair of contrasting traits.

Monohybrid crossMendel and the monohybrid crossTT Talltt Dwarf×F1 : Tt TallF2 : 1 TT : 2 Tt : 1 tt3 Tall : 1 DwarfTTTtTtttPunnett square (Tt × Tt)Mendel: 1822, Moravia; garden pea; 7 pairs of traitsSuccess: large samples, careful records, ratios, clear contrastsPhenotype = visible; genotype = genetic make-upHomozygous (TT/tt) pure; heterozygous (Tt) hybridPhenotypic 3:1; genotypic 1:2:1Allele = alternative form of a gene; dominant / recessive1900: rediscovered by de Vries, Correns, Tschermak

In the figure : Mendel, Tt × Tt and the Punnett square.

Rule 1 : Terminology and why Mendel succeeded — Character = feature (height); trait = its form (tall/dwarf); gene = segment of DNA; alleles = alternative forms of a gene (at the same locus); dominant = expressed in F1; recessive = masked in F1. Phenotype = what is seen (tall), genotype = genetic make-up (TT/Tt). Homozygous = identical alleles (pure; one type of gamete); heterozygous = different alleles (hybrid; two types of gametes). Mendel (1822, Moravia) succeeded with the pea, 7 clear trait pairs, large samples, careful records and ratios. He proposed ‘factors’ (today's genes) occur in pairs and do not blend.

  • Easy : Phenotype = seen; genotype = written.
  • Medium : TT, tt = homozygous; Tt = heterozygous.
  • Exam-type (CET) : 7 pairs = 7 traits.

Rule 2 : Monohybrid cross: F1 and F2 — Pure tall (TT) × pure dwarf (tt) → gametes T and t → F1 all Tt (tall). Selfing F1 gives F2: Punnett square TT : Tt : Tt : tt. Phenotypic ratio 3 tall : 1 dwarf; genotypic ratio 1 : 2 : 1. The dwarf trait was not lost in F1, only masked; it returned in F2 = factors do not blend. A Punnett square is a probability table of fertilizations. A pure line is a line from a single pure parent by self-fertilization.

  • Easy : F1 = 100% tall.
  • Medium : F2: 3:1 and 1:2:1.
  • Exam-type (CET) : 1 TT + 2 Tt = 3 tall.
Remember

3:1 visible, 1:2:1 genetic: P = 3:1, G = 1:2:1.

Common mistake and CET trap

‘The recessive trait is lost in F1’ is wrong. Trap: ‘Tt is homozygous’ is wrong (heterozygous).

Shortcut

Memory clue : "Homo = same alleles; hetero = hybrid".

Spot the difference

PointPhenotypeGenotype
MeaningVisibleGenetic make-up
ExampleTallTT / Tt
F2 ratio3 : 11 : 2 : 1

Practice questions

Easy
  1. 1What is an allele?
  2. 2Heterozygous means?
  3. 3F1 phenotype of TT × tt?
Medium
  1. 4F2 phenotypic ratio?
  2. 5F2 genotypic ratio?
  3. 6Use of Punnett square?
Challenging
  1. 7What is a pure line?
  2. 8Plant used by Mendel?
Show answers (try them yourself first)
  1. 1Alternative form of a gene.
  2. 2Different alleles (Tt).
  3. 3All tall.
  4. 43 : 1.
  5. 51 : 2 : 1.
  6. 6Shows probable offspring.
  7. 7True-breeding line.
  8. 8Garden pea (Pisum sativum).

Dihybrid cross, Mendel's laws and test cross

From daily life

Tracking two traits makes new pairings, like picking a shirt and a cap.

Simple definition

A dihybrid cross involves two pairs of contrasting traits (RRYY × rryy). A test cross = F1 × homozygous recessive, used to find an unknown genotype.

Dihybrid crossDihybrid cross: 9 : 3 : 3 : 1RRYY (round, yellow) × rryyF1 : RrYy (all round-yellow)F1 gametes: RY, Ry, rY, ryF2: 16 combinations9R_Y_ round3R_yy round3rrY_ wrinkled1rryy wrinkled4 phenotypes; 9 genotypes; 16 combinationsNew combinations: round-green and wrinkled-yellowTest cross: F1 × homozygous recessive (Tt × tt) → 1:1Dihybrid test cross (RrYy × rryy) → 1:1:1:1Back cross: F1 × either parent; test cross is a back crossMendel's 3 laws: dominance, segregation, independent assortmentSegregation = pure gametes; assortment rests on dihybrid cross

In the figure : Dihybrid 9:3:3:1 and test cross.

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Dihybrid cross, Mendel's laws and test cross

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Rule 1 : Dihybrid cross and 9:3:3:1 — Round-yellow (RRYY) × wrinkled-green (rryy) → gametes RY and ry → F1 RrYy, all round-yellow. F1 makes 4 gamete types: RY, Ry, rY, ry. 4 × 4 = 16 combinations: 9 round-yellow : 3 round-green : 3 wrinkled-yellow : 1 wrinkled-green; 4 phenotypes, 9 genotypes. The new combinations (round-green, wrinkled-yellow) show the two trait pairs do not depend on each other.

  • Easy : 9 + 3 + 3 + 1 = 16.
  • Medium : 4 phenotypes; 9 genotypes.
  • Exam-type (CET) : Gametes = 2 × 2 = 4.

Rule 2 : Mendel's 3 laws; test cross and back cross — 1. Dominance: of two contrasting traits from pure parents, the one seen in F1 is dominant. 2. Segregation (purity of gametes): alleles separate at gamete formation; each gamete is pure (one allele). 3. Independent assortment: in a dihybrid, the two pairs are distributed independently (9:3:3:1). Test cross: F1 (or a dominant-looking plant) × tt; all dominant offspring = pure (TT); 1:1 = hybrid (Tt). Back cross: F1 × either parent; a test cross is a type of back cross. Mendel's laws are not universal.

  • Easy : Test cross: × tt (recessive).
  • Medium : Segregation = purity.
  • Exam-type (CET) : Test cross ⊂ back cross.
Remember

D-S-I: Dominance, Segregation, Independent assortment. Test cross = ‘× recessive’.

Common mistake and CET trap

‘Every back cross is a test cross’ is wrong (the reverse holds). Trap: dihybrid ratio 3:1 is wrong.

Shortcut

Memory clue : "Test = recessive tester (tt)".

Spot the difference

CrossF2 / result
Monohybrid3 : 1
Dihybrid9 : 3 : 3 : 1
Test cross Tt × tt1 : 1

Practice questions

Easy
  1. 1Dihybrid F2 ratio?
  2. 2Gametes of RrYy?
  3. 3Combinations in F2?
Medium
  1. 4How many genotypes?
  2. 5Law of segregation states?
  3. 6Aim of test cross?
Challenging
  1. 7Phenotype ratio of Tt × tt?
  2. 8Independent assortment is based on?
Show answers (try them yourself first)
  1. 19:3:3:1.
  2. 2RY, Ry, rY, ry.
  3. 316.
  4. 49.
  5. 5Gametes are pure.
  6. 6Identify genotype.
  7. 71 : 1.
  8. 8Dihybrid cross.

Deviations from Mendelism (Neo-Mendelism)

From daily life

A pink flower looks like red and white blended, yet the genes are not mixed.

Simple definition

Neo-Mendelism = exceptions found after Mendel: incomplete dominance, co-dominance, multiple alleles, pleiotropy, polygenic inheritance.

Neo-MendelismDeviations from Mendel (Neo-Mendelism)IncompleteCo-dominanceMultiple allelesRR × rr → Rr (pink)RR × WW → RW (roan)A, B, O (IA, IB, i)F2 1:2:1 (3 phenotypes)Both shown: cattlemore than 2 allelesMirabilis jalapaPleiotropy: one gene, many traits (sickle-cell, HbA/HbS)Carrier × carrier → 1:2:1; HbS HbS dies → survivors 2:1Polygenic: one trait, many genes (skin colour)Epistasis, supplementary, complementary = intergenicIntragenic: between alleles of one gene (incomplete, co-dominance)Drosophila wings: vg+, vg-ni, vg-no, vg-st, vgMendel: 1 trait = 1 gene = 2 alleles; complete dominanceIn incomplete/co-dominance genotypic = phenotypic ratio

In the figure : Deviations from Mendel.

Rule 1 : Incomplete dominance, co-dominance, multiple alleles — Incomplete dominance: both alleles partly expressed; intermediate phenotype. Mirabilis jalapa: red (RR) × white (rr) → F1 pink (Rr); F2 1 red : 2 pink : 1 white. Co-dominance: both alleles fully expressed; in cattle red × white → F1 roan (RW) (red + white hairs); F2 1:2:1. In both, genotypic = phenotypic ratio. Multiple alleles: more than 2 alleles at one locus (from mutation); e.g. human A, B, O blood groups, Drosophila wings (vg).

  • Easy : Incomplete = blend-like; co-dominance = both.
  • Medium : Both give 1 : 2 : 1.
  • Exam-type (CET) : A, B, O = multiple alleles.

Rule 2 : Pleiotropy and polygenic inheritance — Pleiotropy: one gene, several unrelated traits. Sickle-cell anaemia: HbA (normal, dominant) and HbS; the carrier (HbA HbS) shows mild anaemia (RBCs sickle only at low O₂); HbS HbS dies. Carrier × carrier → 1 normal : 2 carrier : 1 anaemic (dies) ⇒ survivors 2 : 1, hence 1:2 instead of 3:1. Polygenic: one trait, many genes (human skin colour) — epistasis, additive. Intragenic interactions (alleles of one gene) and intergenic (different genes: pleiotropy, polygenes, epistasis, supplementary, complementary).

  • Easy : Pleiotropy = one gene, many traits.
  • Medium : Polygenic = many genes, one trait.
  • Exam-type (CET) : HbS HbS = lethal.
Remember

Pleio = plenty of traits; poly = plenty of genes.

Common mistake and CET trap

‘Genes blend in incomplete dominance’ is wrong. Trap: ‘co-dominance gives an intermediate colour’ is wrong (both appear).

Shortcut

Memory clue : "Incomplete = in between; co-dominance = both".

Spot the difference

TypeF1Example
IncompletePinkMirabilis
Co-dominanceRoanCattle
Multiple alleles> 2 allelesA, B, O
PleiotropyOne gene, many traitsSickle-cell

Practice questions

Easy
  1. 1Mirabilis F1 colour?
  2. 2F2 ratio in incomplete dominance?
  3. 3Example of co-dominance?
Medium
  1. 4Example of multiple alleles?
  2. 5What is pleiotropy?
  3. 6Survivor ratio in sickle-cell carrier cross?
Challenging
  1. 7Example of polygenic trait?
  2. 8What is Neo-Mendelism?
Show answers (try them yourself first)
  1. 1Pink.
  2. 21 : 2 : 1.
  3. 3Roan cattle.
  4. 4ABO blood groups.
  5. 5One gene, many traits.
  6. 62 : 1.
  7. 7Skin colour.
  8. 8Post-Mendelian deviations.

Chromosomal theory, chromosomes and ploidy

From daily life

Genes are lines of text and chromosomes are the book that holds them.

Simple definition

The chromosomal theory (Sutton–Boveri, 1903): genes lie on chromosomes. Ploidy = number of chromosome sets.

Chromosomes and ploidyChromosomal theory, chromosome, ploidychromatidsCentromere4 types (centromere)Metacentric : VSubmetacentric : LAcrocentric : jTelocentric : iPloidyEuploidy: n, 2n, 3n, 4nAneuploidy: 2n−1 monosomy2n+1 trisomy2n−2 nullisomy2n+2 tetrasomySutton and Boveri (1903): genes lie on chromosomesParallel behaviour: pairs, segregate and assort in meiosisChromosome = DNA + histone + non-histone proteinsNamed by Waldeyer (1888); clearest at metaphaseKinetochore: spindle attaches; secondary constriction → nucleolusTelomeres = ends; human 2n = 46 (23 pairs)Euploid = whole sets; aneuploid = gain/loss of some

In the figure : Chromosome, centromere types and ploidy.

Rule 1 : Chromosomal theory — Mendel's work (1866) was ignored until 1900, when de Vries, Correns and Tschermak rediscovered it. Sutton and Boveri (1903): genes and chromosomes behave in parallel — chromosomes occur in pairs; in meiosis homologues pair, segregate and assort independently; each gamete gets one of each pair. Gametes link parents to offspring; fertilization restores the diploid number. So chromosomes carry hereditary material.

  • Easy : Genes and chromosomes both occur in pairs.
  • Medium : Gamete = one of each pair.
  • Exam-type (CET) : 1866: published; 1900: rediscovered.

Rule 2 : Chromosome structure and ploidy — Chromosome (Waldeyer, 1888) = DNA + histone + non-histone proteins; clearest at metaphase; 2 chromatids joined at the centromere (primary constriction; kinetochore = spindle attachment). Secondary constriction: nucleolus; ends = telomeres. Shapes (centromere position): metacentric (V), submetacentric (L), acrocentric (j), telocentric (i). Euploidy: monoploid (n), diploid (2n), triploid (3n), tetraploid (4n). Aneuploidy: number is not a whole multiple of the set — monosomy (2n−1), nullisomy (2n−2), trisomy (2n+1), tetrasomy (2n+2).

  • Easy : Kinetochore = spindle attachment.
  • Medium : Euploid = whole sets.
  • Exam-type (CET) : Trisomy = 2n + 1.
Remember

M-S-A-T: V, L, j, i. Mono = −1, tri = +1.

Common mistake and CET trap

‘A chromosome is only DNA’ is wrong (proteins too). Trap: ‘trisomy = 2n−1’ is wrong.

Shortcut

Memory clue : "Acrocentric = ‘j’, telocentric = ‘i’ (centromere at the end)".

Spot the difference

TypeShape
MetacentricV (middle)
SubmetacentricL
Acrocentricj
Telocentrici (terminal)

Practice questions

Easy
  1. 1Who proposed the chromosomal theory?
  2. 2Who coined ‘chromosome’?
  3. 3Chromosomes are made of?
Medium
  1. 4Role of kinetochore?
  2. 5Which is V-shaped?
  3. 6What is a telomere?
Challenging
  1. 7Example of euploidy?
  2. 8Monosomy = ?
Show answers (try them yourself first)
  1. 1Sutton and Boveri.
  2. 2Waldeyer (1888).
  3. 3DNA + proteins.
  4. 4Spindle attachment.
  5. 5Metacentric.
  6. 6End of chromosome.
  7. 73n (triploid).
  8. 82n − 1.

Linkage, crossing over and sex chromosomes

From daily life

Beads on one string move together; they separate only if the string breaks.

Simple definition

Linkage = genes on one chromosome inherited together. Crossing over = exchange of segments between non-sister chromatids of homologous chromosomes.

Linkage and crossing overLinkage and crossing overA Ba bChiasma → recombinationLinkageComplete: Drosophila male XIncomplete: Zea mays (colour, shape)Linkage groups = haploid numberDrosophila 4; pea 7; human 23Linkage: Bateson (plants); Morgan = Drosophila experimentsGenes close → few recombinants (1.3%); far → more (37.2%)Crossing over: non-sister chromatids, pachyteneSteps: synapsis, tetrad, crossing over, terminalizationParental combinations more; recombinants fewerX: metacentric, longer, rich in euchromatinY: acrocentric, heterochromatin-rich = genetically inertHomologous region: crossing over; non-homologous: sex-linked

In the figure : Crossing over and linkage.

Rule 1 : Linkage and Morgan's experiments — Linkage was found in plants by Bateson; Morgan tested it in Drosophila. Complete linkage: no crossing over (Drosophila male X). Incomplete linkage: distant genes, crossing over possible (maize: grain colour–shape). Linkage group = all linked genes of one chromosome; number = haploid number: Drosophila 4, pea 7, human 23. Morgan: close genes (yellow body–white eye) = 1.3% recombination; distant (white–miniature wing) = 37.2%. Close = stronger linkage, fewer recombinants.

  • Easy : Close = fewer recombinants.
  • Medium : Linkage groups = n.
  • Exam-type (CET) : Parental combinations are more.

Rule 2 : Crossing over and sex chromosomes — Crossing over (term by Morgan): at pachytene (prophase I) between non-sister chromatids; steps: synapsis, tetrad, crossing over, terminalization. It increases variation → useful for natural selection. Sex chromosomes (allosomes): X and Y. X: straight, long, metacentric, rich in euchromatin (active). Y: acrocentric, rich in heterochromatin (genetically inert). Both have homologous (crossing over here) and non-homologous (X-linked / Y-linked genes) regions. The X's non-homologous region is longer.

  • Easy : Pachytene = crossing over.
  • Medium : X: metacentric; Y: acrocentric.
  • Exam-type (CET) : Non-homologous = sex-linked.
Remember

Crossing over: S-T-C-T (synapsis, tetrad, crossing, terminalization).

Common mistake and CET trap

‘Crossing over is between sister chromatids’ is wrong (non-sister). Trap: ‘close genes = more recombination’ is wrong.

Shortcut

Memory clue : "Close genes cling together".

Spot the difference

PointXY
ShapeMetacentricAcrocentric
ChromatinMore euchromatinMore heterochromatin
ActivityActiveInert

Practice questions

Easy
  1. 1What is linkage?
  2. 2Example of complete linkage?
  3. 3Linkage groups in humans?
Medium
  1. 4When does crossing over occur?
  2. 5Between which chromatids?
  3. 6What are allosomes?
Challenging
  1. 7Y chromosome shape?
  2. 8What does 1.3% recombination show?
Show answers (try them yourself first)
  1. 1Genes on one chromosome inherited together.
  2. 2Drosophila male X.
  3. 323.
  4. 4Pachytene (prophase I).
  5. 5Non-sister chromatids.
  6. 6Sex chromosomes.
  7. 7Acrocentric.
  8. 8Strong linkage.

Autosomal and sex-linked inheritance

From daily life

Colour blindness is commoner in boys because they have only one X.

Simple definition

Autosomal = inheritance of genes on the 22 pairs of autosomes. Sex-linked = inheritance of genes on non-homologous parts of sex chromosomes (X-linked, Y-linked).

Autosomal and sex-linkedAutosomal and sex-linked inheritanceAutosomalX-linkedY-linked (holandric)Widow's peak; PKUColour blindness, haemophiliaHypertrichosis (father → son)Colour blind: XcY × XCXC → carrier daughters, normal sonsCarrier ♀ × normal ♂ → ¼ normal ♀, ¼ carrier ♀, ¼ normal ♂, ¼ affected ♂X-linked recessive: more in males (no allele on Y = hemizygous)Criss-cross: grandfather → daughter (carrier) → grandsonHaemophilia: lacks factor VIII/IX; ‘royal disease’ (Queen Victoria)A female is affected only with the gene on both XPKU: no phenylalanine hydroxylase → brain damageAutosomal recessive: both sexes equal; may skip generationsWidow's peak: autosomal dominant (WW, Ww)Genotypes: XCY normal; XcY affected; XCXc carrier

In the figure : Autosomal, X-linked and Y-linked inheritance.

Live figure

Autosomal and sex-linked inheritance

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Rule 1 : Autosomal inheritance — Humans have 23 pairs: 22 autosomes + 1 sex pair. Autosomal recessive traits affect both sexes equally and may skip generations. Widow's peak (V-shaped hairline) = autosomal dominant (WW, Ww); ww = straight hairline. Phenylketonuria (PKU) = autosomal recessive; without phenylalanine hydroxylase phenylalanine is not converted to tyrosine, accumulates in blood and CSF, impairs brain development, and is excreted in urine.

  • Easy : Autosomal: both sexes equal.
  • Medium : PKU: missing enzyme.
  • Exam-type (CET) : Widow's peak: dominant.

Rule 2 : X-linked and Y-linked inheritance — X-linked recessive: a female needs the gene on both X; with one it is a carrier (looks normal). Males are hemizygous (no allele on Y) → affected, hence commoner in males. Colour blindness (XcY, XcXc; red-green; cones fail) and haemophilia (clotting factor VIII/IX lacking; XhY). Genotypes: XCY, XcY; XCXC, XCXc, XcXc. Colour-blind man (XcY) × normal woman (XCXC) → all daughters carriers, sons normal. Carrier woman × normal man → ¼ normal daughter, ¼ carrier daughter, ¼ normal son, ¼ affected son. Criss-cross: grandfather → daughter → grandson. Y-linked (holandric): father to son (hypertrichosis).

  • Easy : Carrier ♀ looks normal.
  • Medium : Criss-cross = grandfather → grandson.
  • Exam-type (CET) : Y-linked: only in males.
Remember

X-linked: a son's X comes from his mother, Y from his father.

Common mistake and CET trap

‘Haemophilia is Y-linked’ is wrong (X). Trap: ‘a carrier woman is ill’ is wrong.

Shortcut

Memory clue : "Hemizygous = half-zygous (one X)".

Spot the difference

DisorderType
PKUAutosomal recessive
Colour blindnessX-linked recessive
HaemophiliaX-linked recessive
HypertrichosisY-linked

Practice questions

Easy
  1. 1Pairs of autosomes?
  2. 2Cause of PKU?
  3. 3Widow's peak is?
Medium
  1. 4Colours confused in colour blindness?
  2. 5Another name for haemophilia?
  3. 6Daughters of XcY × XCXC?
Challenging
  1. 7What is hemizygous?
  2. 8Where are holandric genes?
Show answers (try them yourself first)
  1. 122.
  2. 2Lack of phenylalanine hydroxylase.
  3. 3Autosomal dominant.
  4. 4Red and green.
  5. 5Bleeder's disease.
  6. 6All carriers.
  7. 7Single X with no allele on Y.
  8. 8On the Y chromosome.

Sex determination and genetic disorders

From daily life

Whether a baby is a boy or a girl depends on the father's sperm.

Simple definition

Sex determination = mechanism that fixes sex. Genetic disorders = Mendelian (gene changes) and chromosomal (number/structure) disorders.

Sex determination and disordersSex determination and genetic disordersHuman XX–XYBirds ZW–ZZHoney bee♂ 44+XY ; ♀ 44+XX♀ ZW (heterogametic)♀ 2n=32; ♂ n=16Father decidesEgg decidesHaplo-diploidMendelian: thalassemia, sickle-cell, haemophilia, PKU (gene)Chromosomal: Down, Turner, Klinefelter (number/structure)Down: trisomy 21; 47 chromosomes; non-disjunctionTurner: 44+XO (♀); short, webbed neck, sterileKlinefelter: 44+XXY (♂); tall, gynaecomastia, sterileThalassemia: α (HBA1/2, chr 16) / β (HBB, chr 11) globinSickle-cell: HbA/HbS; differs from thalassemiaBonellia: environment decides sex; human ratio 1:1Royal jelly → queen; drone = parthenogenesis

In the figure : Sex-determination types and disorders.

Rule 1 : Sex determination — Humans: XX–XY. Female = 44 + XX (homogametic, one egg type); male = 44 + XY (heterogametic; 50% sperm X, 50% Y). X sperm + egg → girl; Y sperm → boy; hence a 1 : 1 sex ratio, and the father decides. Birds: ZW–ZZ — female heterogametic (ZW), male ZZ; the egg decides sex. Honey bee: haplo-diploid — fertilized egg (2n = 32) → female (queen/worker; royal jelly makes a queen); unfertilized egg (n = 16) by parthenogenesis → male (drone). Bonellia: environment (where the larva settles) decides sex.

  • Easy : Male: XY (heterogametic).
  • Medium : Birds: female = ZW.
  • Exam-type (CET) : Bee drone = haploid.

Rule 2 : Genetic disorders — Mendelian (gene changes): thalassemia, sickle-cell, colour blindness, haemophilia, PKU. Thalassemia: autosomal recessive; α-globin (HBA1/HBA2, chromosome 16) or β-globin (HBB, chromosome 11) gene deleted/mutated; anaemia, pale-yellow skin; needs transfusions; differs from sickle-cell (a faulty globin). Chromosomal (number/structure): Down syndrome = trisomy 21, 47 chromosomes, non-disjunction (risk rises in mothers over 45); Turner = 44 + XO, female, short, webbed neck, underdeveloped ovaries; Klinefelter = 44 + XXY, male, tall, gynaecomastia, sterile.

  • Easy : Down = 47; Turner = 45; Klinefelter = 47.
  • Medium : Turner = female; Klinefelter = male.
  • Exam-type (CET) : Thalassemia ≠ sickle-cell.
Remember

Turner = tiny (XO, female); Klinefelter = kinda extra (XXY, male).

Common mistake and CET trap

‘The mother decides the child's sex’ is wrong (the father). Trap: ‘Turner = male’ is wrong.

Shortcut

Memory clue : "Down = trisomy 21; Klinefelter = XXY".

Spot the difference

DisorderChromosomesSex
Down47 (trisomy 21)Both
Turner45 (44+XO)Female
Klinefelter47 (44+XXY)Male

Practice questions

Easy
  1. 1Who decides human sex?
  2. 2Who is heterogametic in humans?
  3. 3Female bird genotype?
Medium
  1. 4How does a drone arise?
  2. 5Cause of Down syndrome?
  3. 6Genotype of Turner?
Challenging
  1. 7A Klinefelter feature?
  2. 8What is non-disjunction?
Show answers (try them yourself first)
  1. 1Father (sperm).
  2. 2Male (XY).
  3. 3ZW.
  4. 4From unfertilized egg (n = 16).
  5. 5Trisomy of chromosome 21.
  6. 644 + XO.
  7. 7Gynaecomastia / sterility.
  8. 8Failure of chromosomes to separate.

Quick revision of the whole lesson

ConceptKey fact
Monohybrid3:1 and 1:2:1
Dihybrid9:3:3:1; test cross 1:1
DeviationsIncomplete, co-dominance, pleiotropy
ChromosomesSutton–Boveri; euploid/aneuploid
LinkageClose = fewer recombinants
Sex-linkedColour blindness, haemophilia (X)
Sex determinationXY, ZW, haplo-diploid

🎯 Most important points for the exam

  • Mendel, monohybrid cross.
  • Dihybrid cross and test cross.
  • Incomplete dominance, pleiotropy.
  • Chromosomal theory, ploidy.
  • Linkage, crossing over.
  • X-linked disorders.
  • Sex determination, Down/Turner/Klinefelter.
Study mantra

Understand the concept → draw the figure → memorise the key points → solve questions yourself.