Principles of inheritance and variation | ISC Class 12 Biology Notes

This note covers heredity and variation, Mendel’s experiments and laws, genetic crosses, incomplete dominance, co-dominance, blood groups, polygenic inheritance, pleiotropy, chromosomes, linkage, crossing over, sex determination, sex-linked inheritance, pedigree analysis, mutations and human genetic disorders.

What do heredity, variation and Mendel’s experiments explain?

How are inherited characters described?

Heredity is the transmission of characters from parents to offspring through inheritance. Variation is the degree to which offspring differ from their parents. Genetics studies inheritance and variation. A character is an inherited feature, such as stem height; a trait is its form, such as tallness or dwarfness.

Chromosomes carry genes in DNA, or deoxyribonucleic acid, the genetic material. A gene is a unit of inheritance carrying information for expression of a trait. Alleles are slightly different forms of the same gene. Mendel called the inherited units factors, proposing that they pass through reproductive cells, called gametes, without blending. Sperm are male gametes and eggs are female gametes.

Genotype means an organism’s genetic constitution for the character considered. Phenotype means its expressed or observable character. A homozygote has identical alleles of a gene; a heterozygote has different alleles. Under complete dominance, the dominant allele is expressed in the heterozygote while the recessive allele is masked.

Why was Mendel’s approach successful?

Gregor Mendel conducted hybridisation experiments on garden peas during 1856 to 1863. Hybridisation is crossing parents with different inherited traits. His true-breeding lines maintained stable traits over several generations of self-pollination, transfer of pollen to the receptive female surface of the same flower or another flower on that plant.

He selected 14 true-breeding varieties representing seven pairs of contrasting traits. Clear alternatives made offspring easier to classify. Controlled crosses, large samples, mathematical analysis and verification across successive generations helped him distinguish consistent inheritance patterns from isolated observations.

Axial flowers occur along the stem at leaf axils, where leaves join the stem; terminal flowers occur at its end. Inflated pods are full, while constricted pods narrow between seeds.

The biological importance of Mendelism, the principles established through Mendel’s work, is that inheritance can be explained through discrete units and their transmission. It explains both resemblance and the reappearance of a trait absent from the first hybrid generation, and supports predictions about crosses.

How does a monohybrid cross establish dominance and segregation?

How should symbols and generations be read?

A monohybrid cross follows one character with contrasting traits. For pea height, let T represent the dominant tall allele and t the recessive dwarf allele. Thus TT is homozygous tall, Tt is heterozygous tall and tt is homozygous dwarf.

The symbol × means a cross. P denotes the parental generation; F₁ is the first filial, or offspring, generation; F₂ is the second filial generation produced here by self-pollination of F₁.

1. Cross a true-breeding tall parent, TT, with a true-breeding dwarf parent, tt.
2. The TT parent contributes T gametes and the tt parent contributes t gametes. Fertilisation, the union of gametes, produces Tt offspring.
3. All F₁ plants are tall because T is dominant over t. The dwarf allele remains present in their genotype.
4. Self-pollinate the Tt plants. Their gametes contain T or t in equal proportions because the two alleles separate.
5. Combine these gametes to obtain an expected F₂ genotypic ratio of 1 TT : 2 Tt : 1 tt and phenotypic ratio of 3 tall : 1 dwarf.

A Punnett square is a grid showing possible gamete combinations and offspring genotypes. Put one parent’s gametes across the top and the other’s down the side. Each internal cell represents fertilisation and contains one allele from each parent.

What do the two laws state?

The law of dominance states that characters are controlled by paired, discrete factors and that, in a dissimilar pair showing dominance, one factor masks the other. It explains why a Tt plant resembles the TT parent.

The law of segregation states that the two alleles separate during gamete formation, so each gamete receives one. The recovery of dwarf offspring shows that the recessive allele was neither destroyed nor blended. TT and tt produce one allele type each; Tt produces two in equal proportions.

Tall TT and dwarf tt plants lead through labelled gametes to tall Tt offspring. Selfing the Tt generation leads to a second square, with the phenotypic ratio 3 : 1 and genotypic ratio 1 : 2 : 1 printed below.

See Fig. 4.4 in your NCERT textbook

How do test crosses, back crosses and dihybrid crosses differ?

How can a dominant phenotype conceal two genotypes?

A tall pea plant may be TT or Tt. A test cross crosses an individual showing the dominant phenotype with a homozygous recessive individual to investigate the first individual’s genotype. The recessive parent contributes t, so differences among offspring reveal the other parent’s contributions.

A back cross crosses a hybrid with either parental type. For the hybrid Tt from TT × tt, crossing with TT or tt is a back cross. Crossing with tt also serves as a test cross. The cross with TT does not test for the recessive allele through offspring appearance.

For Tt × tt, the test-cross square is:

Note: Punnett-square ratios are probabilities. They do not require every small group of offspring to contain exactly the predicted proportions. A recessive offspring establishes that the tested dominant parent contributed a recessive allele.

How does independent assortment produce four gametes?

A dihybrid cross follows two characters. For pea seeds, use R for round and r for wrinkled shape, and Y for yellow and y for green colour. These symbols apply to this seed cross. Round and yellow are dominant over wrinkled and green respectively.

1. Cross RRYY, round yellow, with rryy, wrinkled green. Their gametes are RY and ry.
2. The F₁ genotype is RrYy, with round yellow seeds.
3. With independent assortment, segregation of R and r is independent of segregation of Y and y.
4. The hybrid therefore produces RY, Ry, rY and ry gametes, each with an expected frequency of 25 per cent.
5. Place these four gamete types on both sides of a Punnett square. The 16 combinations yield 9 round yellow : 3 wrinkled yellow : 3 round green : 1 wrinkled green.

The law of independent assortment describes this independent segregation of two allele pairs. This is a phenotypic ratio. It applies to the stated dominance and independent-assortment conditions; linked genes can depart from it.

How do incomplete dominance, co-dominance and multiple alleles work?

Why are snapdragon hybrids pink?

Incomplete dominance occurs when the heterozygote has a phenotype intermediate between the two homozygotes. In snapdragon, Antirrhinum, use R for the red-flower allele and r for the white-flower allele. These flower-colour symbols are separate from the pea seed-shape symbols used earlier.

Crossing red RR with white rr produces pink Rr offspring. Selfing Rr gives 1 RR red : 2 Rr pink : 1 rr white. Both genotypic and phenotypic ratios are 1 : 2 : 1 because each genotype is distinguishable in appearance.

The intermediate phenotype does not mean permanent blending of alleles. Red and white reappear in the second generation. Alleles still segregate; the difference from complete dominance lies in expression in the heterozygote.

Red RR and white rr flowers produce pink Rr flowers. The lower Punnett square contains red, pink and white flowers, with both ratios labelled 1 : 2 : 1. A photograph of flowering snapdragons appears beneath the cross.

See Fig. 4.6 in your NCERT textbook

How are human ABO blood groups inherited?

Co-dominance means that both alleles express in a heterozygote. The ABO blood-group gene, denoted I, has three alleles in the population: Iᴬ and Iᴮ produce slightly different surface sugars on red blood cells, while i produces neither of these sugars.

Iᴬ and Iᴮ are each completely dominant over i, but are co-dominant with each other. A person with IᴬIᴮ has both sugar types and blood group AB. A person with ii has blood group O. The letters A, B, AB and O name the blood-group phenotypes.

Multiple allelism means that more than two alleles of one gene exist in a population. A diploid individual, having two chromosome sets, carries only two alleles at this gene. The ABO system has six distinct genotypes and four phenotypes; the repeated AB row shows the reversed parental contributions.

How do polygenic inheritance and pleiotropy differ?

How can several genes contribute to one character?

Polygenic inheritance involves several genes contributing to a character. Traits with a continuous range of forms are generally controlled by three or more genes. Their expression also takes environmental influences into account. Human height and skin colour illustrate such variation.

In an elementary skin-colour model, assume three genes with allele pairs A/a, B/b and C/c. The capital-letter alleles A, B and C contribute towards darker skin; the corresponding small-letter alleles a, b and c towards lighter skin. The effects are additive, meaning their contributions accumulate.

Within this model, AABBCC gives the darkest and aabbcc the lightest skin colour. A genotype with three dominant and three recessive alleles gives an intermediate colour. This is a simplified three-gene model, not a claim that actual human skin colour has no environmental influence.

How can one gene influence several characters?

Pleiotropy occurs when one gene affects several phenotypic features. In most cases, the underlying mechanism involves metabolic pathways, the connected chemical reactions in cells, which contribute to different phenotypes. Thus one altered gene can have several observable consequences.

In phenylketonuria, abbreviated PKU, a mutation affects the gene for phenylalanine hydroxylase. This enzyme, a biological catalyst, helps convert the amino acid phenylalanine into tyrosine. Amino acids are building units of proteins. The condition affects intellectual development and reduces hair and skin pigmentation, or colouring.

Starch synthesis in pea seeds supplies another example. Let B denote the allele associated with effective starch synthesis and b the allele associated with less efficient synthesis. BB seeds have large starch grains and become round; bb seeds have smaller starch grains and become wrinkled.

Bb seeds are round but contain intermediate-sized starch grains. Thus B seems dominant when seed shape is considered, while starch-grain size shows incomplete dominance. Dominance depends on the phenotype examined, as well as the gene product. Polygenic inheritance is many genes affecting one character; pleiotropy is one gene affecting several features.

How do chromosomes, linkage and crossing over explain inheritance?

What is the chromosomal theory?

Homologous chromosomes are corresponding chromosomes of a pair, carrying the same genes at corresponding positions. Each position is a locus; the alleles at corresponding loci may differ.

Meiosis is division that reduces chromosome number for formation of haploid cells, which have one chromosome set. Walter Sutton and Theodore Boveri related chromosome behaviour to Mendel’s factors. Both genes and chromosomes occur in pairs, and their separation explains why a gamete receives one member of each pair.

The chromosomal theory of inheritance places genes on chromosomes and connects chromosome segregation with allele segregation. Independent behaviour of different chromosome pairs explains independent assortment. Thomas Hunt Morgan and his colleagues supplied experimental support using the fruit fly Drosophila melanogaster.

Why do linked genes depart from independent assortment?

Linkage is the physical association of genes on the same chromosome. Recombination produces gene combinations different from parental combinations. Morgan found that linked genes produced more parental than non-parental combinations, with tighter linkage associated with lower recombination.

Crossing over exchanges genetic material between non-sister chromatids of homologous chromosomes. A chromatid is one of the two copies of a duplicated chromosome; non-sister chromatids belong to different homologues. Crossing over creates new combinations and therefore contributes to variation.

1. During zygotene, a stage of prophase I, homologous chromosomes pair. Prophase I is the first preparatory stage of meiosis; the pairing is called synapsis.
2. During the following stage, pachytene, the four chromatids of the paired chromosomes become distinct.
3. Crossing over occurs between non-sister chromatids at recombination sites. The enzyme recombinase participates in this exchange.
4. During diplotene, the following stage, homologues tend to separate but remain connected at crossover sites called chiasmata.

Linkage preserves parental combinations; crossing over can separate linked alleles into new combinations. Alfred Sturtevant used recombination frequency to estimate distances between genes and construct genetic maps, diagrams of relative gene positions along a chromosome.

How is sex determined in humans, birds, grasshoppers and honey bees?

What do the chromosome symbols mean?

Autosomes are chromosomes other than sex chromosomes. Sex chromosomes participate in chromosomal sex determination. X and Y name the sex chromosomes used in the human system; Z and W name those in the bird system. The 0 in X0 means absence of a second sex chromosome.

Homogametic means producing one kind of gamete with respect to sex chromosomes. Heterogametic means producing two kinds. Human females have XX and males XY. Humans have 23 chromosome pairs: 22 pairs of autosomes and one pair of sex chromosomes.

In humans, eggs carry X; half the sperm carry X and half carry Y. Fertilisation by X-bearing sperm produces XX, while fertilisation by Y-bearing sperm produces XY. Each pregnancy has a 50 per cent probability of either chromosomal outcome in this model.

The sperm’s chromosome contribution determines the child’s chromosomal sex. Blaming the mother for the birth of a daughter has no basis in this inheritance mechanism. The equal probabilities do not mean that every family must contain equal numbers of sons and daughters.

How is honey-bee inheritance different?

Haplodiploidy determines sex through the number of chromosome sets. A fertilised honey-bee egg develops into a female, either a queen or worker. An unfertilised egg develops into a male drone by parthenogenesis, development without fertilisation.

Haploid drones produce sperm by mitosis, division retaining chromosome number. A drone has no father and cannot have sons, but has a grandfather and can have grandsons. In birds, by contrast, the egg’s Z or W contribution distinguishes the two chromosomal sexes.

How does sex-linked inheritance operate in flies and humans?

What did Morgan’s fly crosses show?

Sex-linked inheritance concerns genes on sex chromosomes. In Morgan’s Drosophila crosses, genes for yellow body and white eyes were on the X chromosome. Yellow-bodied, white-eyed females were crossed with brown-bodied, red-eyed males; their first-generation offspring were intercrossed.

The second generation departed markedly from 9 : 3 : 3 : 1 because these genes were linked. The yellow-body and white-eye genes showed only 1.3 per cent recombination, whereas white-eye and miniature-wing genes showed 37.2 per cent. Miniature wings are the reduced-wing trait used in the second cross.

Cross A follows y, the yellow-body gene, and w, the white-eye gene. Cross B follows w and m, the miniature-wing gene. A superscript + marks the dominant wild-type allele. The diagram labels parental and recombinant types as 98.7% and 1.3% in A, and 62.8% and 37.2% in B.

See Fig. 4.11 in your NCERT textbook

Wild type means the usual reference form, here brown body, red eyes and normal wings. The higher parental proportion in Cross A demonstrates stronger linkage.

Why are X-linked recessive disorders more frequent in males?

In X-linked recessive inheritance, a male’s single X can express a recessive disorder allele. A heterozygous female can be an unaffected carrier, meaning she carries and can transmit the recessive allele. Red-green colour blindness affects discrimination between red and green through a defect involving the eye’s colour-sensitive cone cells.

Haemophilia affects a protein in the blood-clotting sequence and causes prolonged bleeding. A carrier mother may transmit the allele to sons. An affected female is extremely rare; her mother must be at least a carrier and her father affected.

For a haemophilia cross, let Xᴴ carry the usual allele and Xʰ the haemophilia allele; Y is the male sex chromosome. An XᴴXʰ mother and XᴴY father can produce XᴴXᴴ unaffected daughters, XᴴXʰ carrier daughters, XᴴY unaffected sons and XʰY affected sons in equal expected proportions.

Among sons, the affected probability is one-half; among all children it is one-quarter. A father passes his X to daughters and Y to sons, so he does not pass an X-linked allele directly to a son.

How can a pedigree be constructed and interpreted?

What do pedigree symbols represent?

A pedigree is a family tree recording a trait through generations. Pedigree analysis uses family history to trace inheritance because controlled experimental crosses cannot be performed in humans. A chart records relationships and affected status; it does not automatically reveal every individual’s genotype.

A square denotes a male, a circle a female and a diamond unspecified sex. Filled symbols identify affected individuals. Horizontal lines join parents; a double line marks mating between relatives. Children appear below their parents, ordered by birth from left to right.

Reference: NCERT Class 12 Figure 4.13, pedigree-symbol panel

1. Choose the trait being tracked and state what a filled symbol means before drawing the family.
2. Draw a square or circle for each person whose sex is known, and connect the parental pair horizontally.
3. Connect parents to their children below, placing siblings in birth order from left to right.
4. Fill symbols for affected people. Record known genotypes separately and distinguish them from inferred genotypes.
5. Compare transmission between generations, considering whether the pattern is autosomal or sex-linked and dominant or recessive.

How are genotypes inferred?

An autosomal trait is governed by a gene on an autosome. In a simple autosomal recessive model, two unaffected parents with an affected child must each carry the recessive allele.

For sickle-cell inheritance, Hbᴬ denotes the usual haemoglobin allele and Hbˢ the sickle-cell allele. Haemoglobin is the oxygen-carrying protein in red blood cells. HbᴬHbˢ individuals are apparently unaffected carriers, whereas HbˢHbˢ individuals have sickle-cell anaemia.

Crossing two HbᴬHbˢ carriers gives 1 HbᴬHbᴬ : 2 HbᴬHbˢ : 1 HbˢHbˢ as the expected ratio. The same segregation principle explains how a recessive condition can appear after an unaffected parental generation.

Dominant traits can pass from an affected parent to offspring, while X-linked patterns depend on which parent supplies the X chromosome. Interpret a small pedigree cautiously: an observed pattern may be compatible with more than one inheritance model. Use the stated family information and inheritance assumptions together.

What types of mutation generate genetic variation?

How do spontaneous and induced mutations differ?

A mutation is an alteration in genetic material. Changes in DNA can change genotype and phenotype; mutation and recombination both contribute to variation. A spontaneous mutation arises without an identified external mutagenic treatment, whereas an induced mutation follows exposure to a mutagen.

A mutagen is a physical or chemical factor that induces mutation. Ultraviolet radiation, abbreviated UV, is one example. A gene mutation changes the DNA sequence within a gene; a point mutation changes a single base pair, two corresponding bases on the paired DNA strands.

What are transitions, transversions and frameshifts?

A nucleotide, the repeating unit of DNA, contains a sugar, phosphate and nitrogenous base. Adenine and guanine are bases in the purine group; cytosine and thymine belong to the pyrimidine group. A base substitution replaces one base with another.

A codon is a three-base unit specifying an amino acid or a stop signal in protein synthesis. The reading frame is the grouping of a sequence into these triplets. Inserting or deleting bases in a coding sequence in numbers not divisible by three shifts this grouping.

Such a frameshift mutation changes the subsequent codon grouping. Addition or removal of three bases, or multiples of three, leaves the subsequent reading frame unchanged, although amino acids may be added or removed. Therefore not every insertion or deletion is a frameshift.

Sickle-cell anaemia illustrates a point mutation affecting the beta chain of haemoglobin. Larger losses, gains or rearrangements of chromosome segments are chromosomal aberrations. These are commonly observed in cancer cells. A single-base change and a whole-chromosome change operate at different structural scales.

How do sickle-cell anaemia, phenylketonuria and thalassaemia arise?

What distinguishes a Mendelian disorder?

Mendelian disorders are mainly determined by alteration of a single gene and follow recognisable inheritance patterns. They may be dominant or recessive and autosomal or sex-linked. Sickle-cell anaemia, PKU and thalassaemia are autosomal recessive examples; haemophilia and red-green colour blindness illustrate X-linked recessive inheritance.

In sickle-cell anaemia, glutamic acid is replaced by valine at the sixth position of the beta-globin chain, one protein chain of haemoglobin. Under low oxygen tension, meaning low oxygen availability, the altered haemoglobin polymerises, joining into larger assemblies that distort red blood cells.

The cells change from biconcave discs, with depressions on both faces, to elongated sickle-like forms. The low-oxygen condition matters: the statement concerns the behaviour of altered haemoglobin under that condition. HbᴬHbˢ carriers are apparently unaffected; HbˢHbˢ individuals show the disease phenotype.

How does an enzyme defect have multiple effects?

In PKU, the enzyme needed to convert phenylalanine to tyrosine is lacking. Phenylalanine accumulates and is converted into phenylpyruvic acid and other derivatives, related chemical products. Their accumulation in the brain affects intellectual development; the derivatives are also excreted in urine.

This metabolic defect connects the single-gene inheritance pattern with pleiotropy. The condition includes effects on intellectual development and reduced hair and skin pigmentation.

Why is thalassaemia called a quantitative defect?

Thalassaemia reduces synthesis of alpha-globin or beta-globin chains. Alpha and beta name the two globin-chain types. Reduced chain production causes abnormal haemoglobin formation and anaemia, reduced blood oxygen-carrying capacity associated with insufficient functional haemoglobin.

Alpha-thalassaemia involves HBA1 and HBA2, the two closely linked alpha-globin genes on chromosome 16 of each parent. Mutation or deletion of one or more of the four copies reduces alpha-globin production. The more copies affected, the less alpha-globin produced.

Beta-thalassaemia involves HBB, the beta-globin gene on chromosome 11 of each parent, with mutation affecting one or both copies. Thalassaemia is a quantitative problem of producing too few globin molecules; sickle-cell anaemia is a qualitative problem of an incorrectly functioning globin.

How do chromosome-number changes cause human disorders?

How do aneuploidy and polyploidy differ?

Chromosomal disorders arise from absence, excess or abnormal arrangement of chromosomes. Failure of proper chromosome or chromatid separation during division is called nondisjunction. It can produce aneuploidy, gain or loss of individual chromosomes rather than a complete set.

Trisomy means an extra copy of a particular chromosome; monosomy means one chromosome is missing from a pair. In contrast, polyploidy is an increase in whole chromosome sets. Failure of cytokinesis, division of the cell’s cytoplasm, after nuclear division can cause polyploidy, which is often seen in plants.

A typical human body cell has 46 chromosomes. A karyotype describes chromosome number and composition. In the notation 47, XXY, the number gives the total chromosome count and the letters identify the sex chromosomes.

How should the disorders be distinguished?

Down’s syndrome involves an autosome, chromosome 21. Its features include delayed physical, psychomotor and intellectual development. Psychomotor development concerns the coordination of mental activity and movement. The cause is a chromosome-copy change, not a mutation restricted to the haemoglobin gene.

In Klinefelter’s syndrome, gynaecomastia means development of breasts. In Turner’s syndrome, rudimentary ovaries are poorly developed ovaries. Secondary sexual characters are bodily features developing with sexual maturation beyond the reproductive organs themselves. Sterility means inability to produce offspring.

Keep the genetic scale clear: a Mendelian disorder primarily concerns one gene, aneuploidy concerns individual chromosome numbers, and polyploidy concerns whole sets. These distinctions connect the observed condition to the kind of inherited or cellular change that produced it.

Glossary

• Allele — One of the alternative forms of a gene governing a particular inherited character.
• Genotype — The genetic constitution of an organism for the gene or genes being considered.
• Phenotype — The expressed character of an organism, such as tallness or dwarfness in pea plants.
• Homozygote — An individual possessing two identical alleles at the gene being considered.
• Heterozygote — An individual possessing two different alleles at the gene being considered.
• Segregation — Separation of an allele pair during gamete formation, with one allele entering each gamete.
• Test cross — A cross with a homozygous recessive individual to investigate a dominant individual’s genotype.
• Co-dominance — Expression of both alleles in a heterozygote, as in the human AB blood group.
• Polygenic inheritance — Inheritance in which several genes contribute to a character, with environmental influences also involved.
• Pleiotropy — The influence of a single gene on more than one phenotypic feature.
• Linkage — Physical association of genes on the same chromosome, favouring transmission of parental combinations.
• Crossing over — Exchange of genetic material between non-sister chromatids of homologous chromosomes during meiosis.
• Pedigree — A family chart that records the inheritance of a trait across successive generations.
• Mutation — An alteration of genetic material that can change an organism’s genotype and phenotype.
• Aneuploidy — Gain or loss of individual chromosomes following failure of proper separation during cell division.

Common errors and misconceptions

• Misconception: A dominant phenotype proves homozygosity. Correct: A tall pea plant can be TT or Tt; a test cross helps investigate which genotype is present.
• Misconception: Recessive alleles disappear in hybrids. Correct: They remain in the genotype and can reappear in offspring after segregation.
• Misconception: Pink snapdragons prove permanent blending. Correct: Red and white flowers reappear after selfing because their alleles remain distinct.
• Misconception: Multiple alleles mean three ABO alleles in each person. Correct: Three occur in the population, but a diploid individual carries two.
• Misconception: Every dihybrid cross gives 9 : 3 : 3 : 1. Correct: That expectation requires the specified dominance and independent-assortment conditions; linkage can change it.
• Misconception: Half the sons affected means half of all children affected. Correct: Sons and all children are different groups; a carrier-mother haemophilia cross illustrates the distinction.
• Misconception: Every DNA insertion causes a frameshift. Correct: Insertion of three bases or a multiple of three leaves the subsequent coding frame unchanged.
• Misconception: Thalassaemia and sickle-cell anaemia are the same globin defect. Correct: Thalassaemia reduces globin production; sickle-cell anaemia changes globin quality.

Exam-style questions with model answers

Q1. Define genotype and phenotype, illustrating both with a tall heterozygous pea plant. T is the tall allele, dominant over the dwarf allele t. [2 marks]

1. Genotype is genetic constitution: the heterozygous plant has genotype Tt, with one allele of each kind.
2. Phenotype is the expressed character: the plant is tall because T is dominant over t.

Q2. In peas, T produces tallness and is dominant over dwarf allele t. A Tt plant is crossed with tt. Assume equal segregation and random fertilisation. State the gametes, expected offspring genotypes and phenotype ratio. [3 marks]

1. The Tt parent produces T and t gametes in equal proportions, while the homozygous recessive tt parent produces only t gametes.
2. Combining the possible gametes gives Tt and tt offspring in equal expected proportions, so the genotypic ratio is 1 Tt : 1 tt.
3. Tt offspring are tall and tt offspring are dwarf. The expected phenotypic ratio is therefore 1 tall : 1 dwarf.

Q3. Snapdragon RR flowers are red, rr flowers white and Rr flowers pink. Self a pink plant, assuming equal segregation and random fertilisation. State its gametes, the offspring genotypes, the phenotype ratio and the inheritance pattern. [4 marks]

1. The pink parent has genotype Rr and produces R and r gametes in equal proportions on self-pollination.
2. The four equally likely combinations are RR, Rr, Rr and rr, giving the genotypic ratio 1 RR : 2 Rr : 1 rr.
3. Using the stated genotype-to-colour relationships, the phenotypic ratio is 1 red : 2 pink : 1 white.
4. This is incomplete dominance: the heterozygote is intermediate, while the reappearance of both parental colours demonstrates segregation without permanent allele blending.

Q4. Human ABO alleles Iᴬ and Iᴮ express A and B surface sugars and are co-dominant; each dominates i, which produces neither sugar. Cross Iᴬi with Iᴮi, assuming equal segregation and random fertilisation. Explain the gametes, offspring and two inheritance concepts illustrated. [5 marks]

1. The Iᴬi parent produces Iᴬ and i gametes in equal proportions. The Iᴮi parent similarly produces Iᴮ and i gametes.
2. Combining these gametes gives IᴬIᴮ, Iᴬi, Iᴮi and ii as the four equally likely offspring genotypes, each with a probability of one-quarter.
3. The corresponding blood groups are AB, A, B and O respectively, so the expected phenotypic ratio is 1 AB : 1 A : 1 B : 1 O.
4. IᴬIᴮ illustrates co-dominance because both surface-sugar types are expressed together in the same individual, rather than producing an intermediate type.
5. The population’s three alleles illustrate multiple allelism. Each individual in this cross still possesses only two alleles, one inherited from each parent.

Q5. Explain chromosomal sex determination in humans in four points, including chromosome constitution, gametes, fertilisation outcomes and probability. [4 marks]

1. Human females have 22 pairs of autosomes plus XX sex chromosomes; males have 22 pairs of autosomes plus XY sex chromosomes.
2. Eggs carry X, whereas half the sperm carry X and half carry Y. The male is therefore heterogametic.
3. Fertilisation by an X-bearing sperm produces XX, a female; fertilisation by a Y-bearing sperm produces XY, a male.
4. Each outcome has a 50 per cent probability per pregnancy. The sperm’s sex chromosome determines the outcome, so blaming the mother is biologically unjustified.

Q6. Haemophilia is X-linked recessive. Xᴴ carries the usual allele, Xʰ carries the haemophilia allele and Y denotes the male sex chromosome. An unaffected carrier mother XᴴXʰ and unaffected father XᴴY produce equally likely gametes with random fertilisation. Describe the outcomes and affected probabilities in four points. [4 marks]

1. The mother produces Xᴴ and Xʰ eggs; the father produces Xᴴ and Y sperm, with equal proportions of each kind.
2. Daughters are XᴴXᴴ or XᴴXʰ in equal expected proportions. Both are unaffected, but XᴴXʰ daughters are carriers.
3. Sons are XᴴY or XʰY in equal expected proportions. XᴴY sons are unaffected and XʰY sons have haemophilia.
4. One-half of sons are expected to be affected, but the affected probability among all children is one-quarter because only one of four equally likely combinations is affected.

Q7. In a pedigree, two unaffected parents have a child with sickle-cell anaemia. Assume simple autosomal recessive inheritance, equal allele segregation and random fertilisation. Hbᴬ is the usual allele; Hbˢ causes disease only as HbˢHbˢ. Infer the parental genotypes, show their gametes and give offspring expectations and their meaning. [5 marks]

1. The affected child has genotype HbˢHbˢ, so one Hbˢ allele must have come from each parent under the stated inheritance model.
2. Both parents are unaffected but carry Hbˢ. Each must therefore be heterozygous HbᴬHbˢ, rather than homozygous for either the usual or the disease allele.
3. Each parent produces Hbᴬ and Hbˢ gametes in equal proportions. Random fertilisation combines these into HbᴬHbᴬ, HbᴬHbˢ, HbᴬHbˢ and HbˢHbˢ.
4. The expected genotypic ratio is 1 HbᴬHbᴬ : 2 HbᴬHbˢ : 1 HbˢHbˢ. Thus one-quarter are affected, one-half are carriers and one-quarter lack Hbˢ.
5. These are probabilities for offspring under the model, not a requirement that a family has exactly four children or exactly one affected child in every four births.

Q8. Distinguish Down’s, Klinefelter’s and Turner’s syndromes by stating the chromosomal change and one characteristic effect of each. Give two separate points per syndrome. [6 marks]

1. Down’s syndrome is caused by an additional copy of chromosome 21. This is trisomy of an autosome rather than an alteration restricted to one gene.
2. One characteristic effect of Down’s syndrome is delayed physical, psychomotor and intellectual development.
3. Klinefelter’s syndrome involves an additional X chromosome, giving 47, XXY.
4. An individual with Klinefelter’s syndrome has overall masculine development but shows breast development, termed gynaecomastia, and is sterile.
5. Turner’s syndrome involves absence of one X chromosome, giving 45, X0. Here 0 indicates the missing second sex chromosome.
6. A female with Turner’s syndrome has rudimentary ovaries and is sterile, with reduced development of secondary sexual characters.

Key takeaways

• Mendel’s contrasting pea traits, true-breeding lines, large samples and repeated crosses revealed regular patterns of inheritance.
• Segregation separates allele pairs into gametes; complete dominance explains why two different genotypes can share one phenotype.
• Incomplete dominance changes the heterozygote’s phenotype, while co-dominance permits both alleles to express in the same individual.
• ABO blood groups illustrate three alleles in a population, although each diploid individual carries only two.
• Polygenic inheritance involves several genes contributing to one character; pleiotropy involves one gene influencing several features.
• Linkage favours parental combinations, while crossing over between non-sister chromatids produces recombination and contributes to variation.
• Human males and bird females are heterogametic; honey-bee sex determination depends on the number of chromosome sets.
• Pedigrees trace inheritance through families, but genotype deductions require a stated inheritance model and adequate family information.
• Gene mutations, changes in individual chromosome numbers and changes in whole chromosome sets represent distinct genetic alterations.

Test yourself

Why can tall pea plants have different genotypes?

The dominant tall allele expresses in both homozygous TT and heterozygous Tt plants, so appearance alone does not distinguish them.

How does a test cross differ from selfing a heterozygote?

A test cross uses a homozygous recessive partner, whereas selfing combines gametes from the same heterozygous plant.

Why does pink snapdragon colour not imply permanent allele blending?

Selfing pink heterozygotes produces red and white offspring again, showing that the alleles remain distinct and segregate.

What is the difference between multiple allelism and polygenic inheritance?

Multiple allelism concerns several population-level alternatives of one gene; polygenic inheritance concerns several genes contributing to a character.

Which type of chromatid participates in meiotic crossing over?

Non-sister chromatids of paired homologous chromosomes exchange genetic material during the pachytene stage of prophase I.

Why can an X-linked allele not pass directly from father to son?

A father contributes his Y chromosome to a son and his X chromosome to a daughter.

Why does insertion of three coding bases differ from insertion of one?

Three bases add a complete triplet, leaving the subsequent reading frame unchanged; one base shifts the grouping into codons.

How do aneuploidy and polyploidy differ?

Aneuploidy changes the number of individual chromosomes; polyploidy increases the number of whole chromosome sets.