Q.A, B, D are three independently assorting genes with their recessive alleles a, b, d, respectively. A cross was made between individuals of Aa bb DD genotype with aa bb dd. Find out the type of genotypes of the offspring produced.
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Mendelian Genetics Basics
Imagine you have a box of coloured beads — red and white. If you pick one bead from the box, you get either red or white. Now imagine that the colour of your eyes, or the shape of your earlobe, is decided by something like that: a tiny "packet" inside your cells that comes in two versions, and you inherit one from each parent. That is the core idea of Mendelian genetics.
The everyday intuition
You have probably noticed that children often look like their parents — same hair colour, same dimples, same height. But they are never exact copies. Why? Because each parent contributes half of the "instructions" for building a child. Those instructions come in pairs, one from mother and one from father. Sometimes one instruction overrides the other; sometimes they blend. Gregor Mendel, a 19th-century monk, figured out the rules by watching pea plants — tall vs short, yellow vs green seeds — and counting what appeared in the next generation.
The precise meaning
Mendelian genetics is the study of how traits are passed from parents to offspring through genes. A gene is a unit of heredity — a stretch of DNA that codes for a specific characteristic, like flower colour. Each gene comes in different versions called alleles. For every gene, you inherit two alleles: one from your mother, one from your father.
If the two alleles are identical, you are homozygous for that trait. If they are different, you are heterozygous. In a heterozygous pair, one allele may be dominant — it shows up in the appearance — and the other recessive — it stays hidden unless both alleles are recessive.
Mendel's key insight was that traits are not blended like paint. Instead, alleles remain separate and are passed on intact. A recessive allele can skip a generation and reappear later, unchanged.
Why it matters
Mendelian genetics is the foundation of modern biology. It explains:
- Why some diseases run in families (like cystic fibrosis or sickle-cell anaemia)
- How plant and animal breeders create new varieties
- Why you might have your grandmother's eyes but not your mother's
The NCERT textbook states that Mendel's work established the laws of inheritance — the Law of Dominance, the Law of Segregation, and the Law of Independent Assortment. These laws describe how alleles separate during the formation of eggs and sperm, and how different genes are inherited independently of one another.
Key terms at a glance
- Gene: a unit of heredity on a chromosome
- Allele: a variant form of a gene
- Dominant: the allele that expresses itself even when paired with a different allele
- Recessive: the allele that expresses itself only when paired with an identical recessive allele
- Homozygous: having two identical alleles for a gene
- Heterozygous: having two different alleles for a gene …
When two parents are crossed, each passes one allele per gene to the offspring. Here the first parent is Aa bb DD and the second is aa bb dd.
For gene A: The first parent can give either A or a (heterozygous), while the second parent can only give a (homozygous recessive). The offspring will therefore be either Aa or aa in a 1:1 ratio.
For gene B: Both parents are bb (homozygous recessive), so every offspring must receive b from each parent. All offspring will be bb.
For gene D: The first parent is DD (homozygous dominant) and can only give D, while the second parent is dd and can only give d. Every offspring will be Dd. …
Crossing Aa bb DD x aa bb dd yields only two genotypes — Aa bb Dd and aa bb Dd — in a 1:1 ratio. The B locus stays bb and the D locus is uniformly Dd; only the A locus segregates.
Gametes each parent can form
- Parent 1 (Aa bb DD): heterozygous at A (gives A or a), homozygous at B (only b) and D (only D) -> gametes A b D and a b D.
- Parent 2 (aa bb dd): homozygous recessive at every locus -> a single gamete a b d.
Combining the gametes, locus by locus
Because the genes assort independently, each locus can be analysed on its own:
- A locus: Aa x aa -> Aa or aa (1 : 1)
- B locus: bb x bb -> bb in every offspring
- D locus: DD x dd -> Dd in every offspring …
Use the forked-line (branch) method instead of a combined table: draw the A locus outcome first (Aa or aa, each 1/2), then branch each of those into the B-locus outcome (always bb), then branch each again into the D-locus outcome (always Dd). Multiplying probabilities along each completed branch reproduc …
- GSEB Higher Secondary Certificate (HSC) Examination 2026Set ANNUAL1 markMCQQ.Which process is used when we want to know the Genotype of an unknown flower?(a) Incomplete Dominance(b) Dihybrid Experiments(c) Monohybrid Experiments(d) Test cross
›Reveal solutionSolution
A test cross - mating an individual of unknown genotype with a homozygous recessive individual - reveals the unknown genotype from the phenotypic ratio of offspring.
If a plant shows the dominant phenotype, its genotype could be either homozygous dominant or heterozygous - the phenotype alone cannot distinguish these. To resolve this, Mendel devised the test cross: crossing the unknown plant with a homozygous recessive plant. If all offspring show the dominant phenotype, the unknown parent was homozygous dominant; if offspring show a 1:1 ratio of dominant:rec …
- GUJCET 2024Set 101 markMCQQ.On which chromosome of each parent gene controlling β-thalassemia is located? (A) 11th (B) 21st (C) 16th (D) 14th
›Reveal solutionSolution
β-globin gene (β-thalassaemia) → chromosome 11.
β-thalassaemia results from mutation of the β-globin gene, which is located on chromosome 11. (α-thalassaemia involve …
- GSEB Higher Secondary Certificate (HSC) Examination 2024Set ANNUAL1 markMCQQ.What is represented by following cross? Tt x tt(a) Test cross(b) Dihybrid cross(c) Co-dominance(d) Incomplete dominance
›Reveal solutionSolution
A test cross crosses an individual of unknown genotype (but known dominant phenotype) with a homozygous recessive individual, to reveal whether the unknown parent is homozygous or heterozygous from the ratio of offspring phenotypes.
In Tt x tt, the tt parent can only contribute a recessive 't' gamete, so the offspring phenotypes directly reveal which gametes the Tt parent produced: half the offspring will be Tt (showing the dominant phenotype) and half tt (showing the recessive phenotype), a 1:1 ratio that confirms the tested parent is indeed heterozygous (Tt) rather than homozygous dominant (which would have given all dominant-phenotype offspring …
- GSEB Higher Secondary Certificate (HSC) Examination 2023Set ANNUAL1 markMCQQ.Which one following is not a mendelian disorders?(a) Sickle - Cell anaemia(b) Ascariasis(c) Phenyl Ketonuria(d) Cystic Fibrosis
›Reveal solutionSolution
Sickle-cell anaemia, phenylketonuria and cystic fibrosis are single-gene (Mendelian) disorders; Ascariasis is a parasitic worm infection, not genetic.
Mendelian disorders are caused by alteration/mutation in a single gene and follow Mendelian inheritance - e.g. sickle-cell anaemia, phenylketonuria, cystic fibrosis, thalassemia, haemophilia, colour blindness.
…
- GUJCET 2022Set 171 markMCQQ.Absence of teeth, bifid tongue and mental retardness are observed in __________. (A) Down's syndrome (B) Albinism (C) Klinefelter's syndrome (D) Oral-facial-digital syndrome
›Reveal solutionSolution
This triad is diagnostic of Oral-facial-digital syndrome.
Concept. Oral-facial-digital (OFD) syndrome affects the mouth, face and digits — features include a bifid/lobed tongue, missing teeth, cleft palate and mental retardation. Down's (trisomy 21), albinism (pi …
- GUJCET 2021Set 151 markMCQQ.Linked genes HBA1 and HBA2 are located on which pair of chromosomes? (A) 11 (B) 14 (C) 22 (D) 16
›Reveal solutionSolution
HBA1 and HBA2 (alpha-globin genes) lie on chromosome 16.
Concept: The two alpha-globin genes, HBA1 and HBA2, are closely linked on the …
- GSEB Higher Secondary Certificate (HSC) Examination 2020Set ANNUAL1 markMCQQ.Expression of only one of the parental characters in the F1 and expression of both in the F2, can be explained by(a) Punnett square(b) Law of segregation(c) Law of Dominance(d) Multiple alleles
›Reveal solutionSolution
Mendel's Law of Dominance explains why only one parental trait appears in the F1 generation (the dominant trait masks the recessive one) while both traits reappear in a 3:1 ratio in the F2 generation.
When Mendel crossed true-breeding tall and dwarf pea plants, all F1 plants were tall — only one of the two parental traits (tallness) was expressed, while the other (dwarfness) was suppressed but not lost, since it reappeared in the F2 generation in a 3:1 (tall:dwarf) ratio. This is explained by the Law of Dominance: of a pair of contrasting factors (alleles), one (dominant) expresses itself in the heterozygous state while the other (recessive) is suppressed but remains present and can reappea …
- GSEB Higher Secondary Certificate (HSC) Examination 2018Set ANNUAL1 markMCQQ.If father Dr. Hansraj blood group is 'A' and mother Komalben blood group is 'B'? What will be the blood group of their son Gajendra?(a) A or B only(b) AB only(c) A or B or AB only(d) O or A or B or AB
›Reveal solutionSolution
If the A parent is IA i and the B parent is IB i, their children can be of any of the four ABO groups.
ABO alleles: IA and IB are codominant, i is recessive.
- Father (A) can be IA IA or IA i.
- Mother (B) can be IB IB or IB i.
Take the heterozygous case IA i x IB i: …
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