🧬 What is Genetics?
Genetics is a branch of biology that studies how traits are passed from one generation to the next and why there is variation among individuals of the same species[1].
Core Concepts of Genetics:
1. Heredity: Transmission of traits (characters) from parents to offspring.
2. Variation: Differences in traits between individuals of the same species.
Historical Note: The term "Genetics" was coined by William Bateson in 1905[1].
PYQ Corner - Genetics Basics
- Who developed Punnett Square? (CBSE 2007)[2]
- Why do certain genes tend to be inherited together in a cell at the time of cell division? (CBSE 2008)[2]
- What is sex chromosome complement of male bird? (CBSE)[2]
👨🔬 Gregor Mendel - The Father of Genetics
Gregor Johann Mendel, an Austrian monk, laid the foundation of modern genetics through his experiments on pea plants (Pisum sativum) from 1856–1863[1].
His Work:
• Published as "Experiments in Plant Hybridisation" in 1865
• Rediscovered in 1900 by de Vries, Correns, and Tschermak
Why Pea Plants (Pisum sativum)?
Mendel chose pea plants for the following reasons[1]:
- Short life cycle – Fast growth and quick reproduction
- Large number of seeds per plant
- Distinct contrasting traits (e.g., tall vs. dwarf)
- Self-pollinating nature (ensures purity), yet can be cross-pollinated manually
- Availability of many true breeding varieties
PYQ Corner - Mendel's Work
- Mention the advantages of selecting a pea plant for the experiment by Mendel[8]
- Mendel published his work on inheritance of characters in 1865, but it remained unrecognised till 1900. Give three reasons for the delay in accepting his work (CBSE 2014)[2]
Mendel's Seven Characters in Pea Plant
| Character | Dominant Trait | Recessive Trait |
|---|---|---|
| Stem Height | Tall | Dwarf |
| Seed Shape | Round | Wrinkled |
| Seed Color | Yellow | Green |
| Pod Shape | Inflated | Constricted |
| Pod Color | Green | Yellow |
| Flower Position | Axial | Terminal |
| Flower Color | Violet | White |
📚 Basic Genetic Terms
| Term | Definition | Example |
|---|---|---|
| Gene | Unit of inheritance, a segment of DNA that codes for a trait | Gene for plant height |
| Allele | Alternate forms of a gene | T (tall) and t (dwarf) |
| Homozygous | Identical alleles | TT or tt |
| Heterozygous | Different alleles | Tt |
| Genotype | Genetic makeup | TT, Tt, tt |
| Phenotype | Observable trait | Tall or Dwarf |
PYQ Corner - Basic Terms
- Differentiate between Dominance and Recessive; Homozygous and Heterozygous; Monohybrid and Dihybrid[8]
- Define the respective pattern of inheritance where F1 phenotype: (i) does not resemble either of the two parents and is in between the two (ii) resembles only one of the two parents (CBSE 2012)[2]
⚖️ Mendel's Laws of Inheritance
1. Law of Dominance
In heterozygotes, only one trait (dominant) is expressed[1].
Key Points:
• Characters are controlled by discrete units called factors
• Factors occur in pairs
• In a dissimilar pair, one member dominates (dominant) the other (recessive)
2. Law of Segregation
Alleles separate during gamete formation. Each gamete receives only one allele of each gene[1].
3. Law of Independent Assortment
Genes for different traits segregate independently during gamete formation[1].
PYQ Corner - Mendel's Laws
- State the law of independent assortment. Using Punnett square, demonstrate the law of independent assortment in a dihybrid cross involving two heterozygous parents (CBSE 2010)[2]
- Name and state the law that can be derived from this cross and not from a dihybrid cross (CBSE 2012)[2]
🔬 Mendel's Monohybrid Cross
A cross between parents differing in one character (e.g., Tall vs Dwarf)[1].
Example: Tall (TT) × Dwarf (tt)
F1 Generation: All Tall (Tt) - 100%
F2 Generation:
• Phenotypic Ratio: 3 Tall : 1 Dwarf
• Genotypic Ratio: 1 TT : 2 Tt : 1 tt
PYQ Corner - Monohybrid Cross
- When a tall pea plant was selfed, it produced one-fourth of its progeny as dwarf. Explain with the help of a cross (CBSE 2010)[2]
- Write the possible genotypes Mendel got when he crossed F1 tall pea plants with dwarf pea plants (CBSE 2012)[2]
- Why in a test cross, did Mendel cross a tall pea plant with a dwarf pea plant only? (CBSE 2012)[2]
🔬 Mendel's Dihybrid Cross
A cross between parents differing in two characters[1].
Example: Round Yellow (RRYY) × Wrinkled Green (rryy)
F1 Generation: All Round Yellow (RrYy)
F2 Generation:
• Phenotypic Ratio: 9:3:3:1
• 9 Round Yellow : 3 Round Green : 3 Wrinkled Yellow : 1 Wrinkled Green
PYQ Corner - Dihybrid Cross
- A tall pea plant with yellow seeds (heterozygous for both) is crossed with a dwarf pea plant with green seeds. Using a Punnett square work out the cross to show the phenotypes and genotypes of F1 generation (CBSE 2008)[2]
- In a dihybrid cross, when would the proportion of the parental gene combinations be much higher than non-parental type as experimentally shown by Morgan and his group? (CBSE 2012)[2]
🧪 Test Cross
A cross between an individual with unknown genotype and a homozygous recessive individual[1].
Purpose: To determine the genotype of an individual showing dominant phenotype.
PYQ Corner - Test Cross
- How does a test cross help in identifying the genotype of the organism? Explain (CBSE 2010)[2]
- A test cross is performed between a heterozygous and homozygous recessive individual[8]
🌈 Types of Inheritance Beyond Mendel
1. Incomplete Dominance
Neither allele is completely dominant. F1 hybrid shows intermediate phenotype[1].
Example: 4 o'clock plant (Mirabilis jalapa)
Red (RR) × White (rr) → Pink (Rr)
F2: 1 Red : 2 Pink : 1 White
2. Co-dominance
Both alleles express themselves fully in heterozygous condition[1].
Example: Human Blood Group AB (I^A I^B)
Both A and B antigens are expressed
3. Multiple Alleles
More than two alleles exist for a gene, but an individual has only two[1].
Example: ABO Blood group system
Alleles: I^A, I^B, i
Genotypes: I^A I^A, I^A I^B, I^B I^B, I^A i, I^B i, ii
Phenotypes: A, AB, B, A, B, O
PYQ Corner - Non-Mendelian Inheritance
- Snapdragon shows incomplete dominance for flower colour. Work out a cross and explain the phenomenon. How is this inheritance different from Mendelian pattern of inheritance? (CBSE 2012)[2]
- Explain the phenomenon of multiple allelism and co-dominance taking ABO blood group as an example (CBSE 2012)[2]
- A woman with blood group O married a man with AB group. Show the possible blood groups of the progeny. List the alleles involved in this inheritance (CBSE 2008)[2]
🎨 Polygenic Inheritance
Trait controlled by many genes. Shows continuous variation[1].
Examples:
• Skin colour in humans
• Height in humans
• Weight in humans
Characteristics:
• Quantitative traits
• Bell-shaped distribution
• Environmental influence
PYQ Corner - Polygenic Inheritance
- What is the pattern of inheritance for polygenic trait?[12]
- Given below are two statements about polygenic inheritance. Identify if they are correct[12]
🧬 Chromosomal Theory of Inheritance
Proposed by Sutton and Boveri (1902)[1].
Key Points:
- Genes are located on chromosomes
- Chromosomes occur in pairs
- During meiosis, homologous chromosomes segregate
- Independent assortment explains Mendel's laws
PYQ Corner - Chromosomal Theory
- Who proposed the chromosomal theory of inheritance? (CBSE)[8]
- Define the chromosomal theory of inheritance[11]
🔗 Linkage & Recombination
Linkage
Genes located close together on the same chromosome are inherited together[1].
• More linkage = less recombination
• Example: Morgan's fruit fly experiments
• Linked genes don't follow independent assortment
Recombination
Exchange of genetic material between homologous chromosomes during crossing-over in meiosis[1].
• Greater distance = more recombination
• Recombination frequency = Map distance
• Maximum recombination frequency = 50%
PYQ Corner - Linkage
- During his studies on genes in Drosophila that were sex-linked, T.H. Morgan found F2 population phenotypic ratios deviated from expected 9:3:3:1. Explain the conclusion he arrived at (CBSE 2010)[2]
- Explain the contribution of Alfred Sturtevant in chromosome mapping (CBSE 2012)[2]
♂♀ Sex Determination
In Humans (XY System):
Male: XY → produces two types of gametes (X and Y)
Female: XX → produces only X gametes
Sex ratio: 1:1 (50% male, 50% female)
Other Systems:
| Species | Female | Male | Type |
|---|---|---|---|
| Birds | ZW | ZZ | ZW system |
| Grasshopper | XX | XO | XO system |
| Honeybee | Diploid | Haploid | Haplodiploidy |
PYQ Corner - Sex Determination
- Explain the sex determination mechanism in humans. How is it different in birds? (CBSE 2010)[2]
- Why are grasshopper and Drosophila said to show male heterogamety? Explain female heterogamety with an example (CBSE 2010)[2]
- Women are often blamed for producing female children. How will you address this issue scientifically? (CBSE 2014)[2]
🏥 Genetic Disorders
A. Mendelian Disorders (Monogenic)
| Disorder | Inheritance | Features |
|---|---|---|
| Haemophilia | X-linked recessive | Delayed blood clotting |
| Colour Blindness | X-linked recessive | Can't differentiate red-green colors |
| Sickle Cell Anaemia | Autosomal recessive | Sickle-shaped RBCs, pain, anaemia |
| Thalassemia | Autosomal recessive | Faulty Hb synthesis |
| Phenylketonuria | Autosomal recessive | Mental retardation, excess phenylalanine |
B. Chromosomal Disorders
| Disorder | Karyotype | Symptoms |
|---|---|---|
| Down's Syndrome | Trisomy 21 (47 chromosomes) | Mental retardation, short height, flat face |
| Turner's Syndrome | XO (45 chromosomes) | Female, sterile, no menstruation |
| Klinefelter's Syndrome | XXY (47 chromosomes) | Male, tall, sterile, feminine features |
PYQ Corner - Genetic Disorders
- Write the genotype of: (i) An individual who is carrier of sickle-cell anaemia gene but apparently unaffected (ii) An individual affected with the disease (CBSE 2010)[2]
- A human being suffering from Down's syndrome shows trisomy of 21st chromosome. Mention the cause of this chromosomal abnormality (CBSE 2010)[2]
- Why is the father never passing on the gene for haemophilia to his sons? Explain (CBSE 2012)[2]
- Explain the causes, inheritance pattern and symptoms of any two mendelian genetic disorders (CBSE 2010)[2]
🔄 Mutation - A Sudden Heritable Change
Types of Mutation:
1. Gene Mutation: Change in DNA sequence within a gene
Example: Sickle Cell Anaemia (GAG → GTG in beta-globin gene)
2. Chromosomal Mutation: Structural or numerical changes in chromosomes
Examples: deletion, duplication, inversion, translocation
PYQ Corner - Mutation
- Name the event during cell division cycle that results in gain or loss of chromosome (CBSE 2011)[2]
- Sickle cell anaemia in humans is a result of point mutation. Explain (CBSE 2011)[2]
📝 Test Your Knowledge - Enhanced PYQ Edition
Question 1 of 25