What is Topic 6 in DU Admission (Bangladesh)?

Topic 6 is a topic in the Science syllabus of DU Admission (Bangladesh). It carries a weight of 3% in the exam. Our notes cover the key concepts, formulas, and problem-solving approaches you need to master this topic.

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Topic 6

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Topic 6 — Key Facts for DU Admission (Bangladesh) Core concept: Genetics and Heredity — the foundation of biological inheritance High-yield point: Mendel’s Laws, Punnett squares, and human genetic disorders ⚡ Exam tip: One or two genetics problems appear every year in DU admission test

🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Topic 6 — Genetics and Heredity Study Guide Overview: Genetics is one of the most scoring chapters in the biology section Core principles: Mendel’s Laws of Inheritance, chromosome theory, DNA as genetic material Key points: Dominant/recessive alleles, genotype/phenotype, monohybrid and dihybrid crosses Study strategy: Practice Punnett square problems daily until they become automatic

🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Genetics and Heredity — Complete Study Notes

What is Heredity?

Heredity is the process by which offspring inherit traits from their parents. The field of genetics studies how traits are transmitted from one generation to the next. In the DU admission test, genetics questions test your understanding of Mendelian inheritance, chromosomal basis of heredity, and human genetic disorders.

Mendel’s Laws of Inheritance

Gregor Mendel (1822–1884), the father of genetics, performed experiments on garden pea (Pisum sativum) and derived three fundamental laws:

1. Law of Dominance

In a heterozygote, the dominant allele masks the recessive allele. The trait that appears in the F1 generation is the dominant trait.

Example: When pure tall (TT) and pure dwarf (tt) pea plants are crossed, all F1 offspring are tall (Tt). The矮 allele is hidden but still present.

2. Law of Segregation

During gamete formation, paired alleles separate randomly so each gamete receives only one allele from each pair.

Key concept: The 3:1 ratio in F2 generation of a monohybrid cross arises from this segregation.

3. Law of Independent Assortment

Alleles of different genes assort independently during gamete formation. This holds true when genes are on different chromosomes.

Example: Dihybrid cross TtYy × TtYy produces a 9:3:3:1 phenotypic ratio.

Important Genetic Terminology

Term	Definition
Gene	Unit of heredity located on DNA
Allele	Alternate form of a gene
Dominant	Allele that expresses its trait in heterozygote
Recessive	Allele expressed only in homozygous condition
Genotype	Genetic makeup (e.g., Tt, tt)
Phenotype	Physical appearance (e.g., tall, dwarf)
Homozygous	Both alleles same (TT or tt)
Heterozygous	Different alleles (Tt)
Codominance	Both alleles fully expressed (e.g., AB blood group)
Incomplete Dominance	Intermediate phenotype (e.g., Snapdragon flowers)

Punnett Square — Problem Solving

Monohybrid Cross: Tt × Tt

       T       t
  T   TT      Tt
  t   Tt      tt

Offspring: 3 Tall : 1 Dwarf (Phenotypic ratio 3:1) Genotypic ratio: 1 TT : 2 Tt : 1 tt

Dihybrid Cross: TtYy × TtYy

Produces 16 genotype combinations
Phenotypic ratio: 9 Round-Yellow : 3 Round-green : 3 Wrinkled-Yellow : 1 Wrinkled-green

Non-Mendelian Inheritance

Codominance

Both alleles are fully expressed. Classic example: ABO blood group system

Genotype	Blood Group
I^A I^A or I^A i	A
I^B I^B or I^B i	B
I^A I^B	AB
ii	O

Multiple Alleles

More than two alleles exist for a gene. ABO system has three alleles: I^A, I^B, i.

Pleiotropy

One gene affects multiple phenotypic traits. Example: Phenylketonuria (PKU) affects skin, hair, and brain.

Polygenic Inheritance

Multiple genes control one trait. Example: Human skin color, height.

Chromosomal Basis of Inheritance

Genes are located on chromosomes
Humans have 46 chromosomes (23 pairs)
22 pairs are autosomes, 1 pair is sex chromosomes (XX or XY)
Sex-linked genes are carried on X or Y chromosome

Important: X-linked recessive diseases (e.g., Hemophilia, Color blindness) predominantly affect males because they have only one X chromosome.

Human Genetic Disorders

Autosomal Recessive

Cystic fibrosis, Sickle cell anemia, Phenylketonuria
Both parents must be carriers (heterozygous)

Autosomal Dominant

Huntington’s disease, Marfan syndrome
Affected individual needs only one copy

X-Linked Recessive

Hemophilia A, Duchenne muscular dystrophy, Red-green color blindness
Males (XY) are more frequently affected
Carrier females usually unaffected

X-Linked Dominant

Vitamin D-resistant rickets
Both males and females affected, females less severely

Sex Determination

Human sex is determined by X and Y chromosomes:

Female: XX ( homogametic)
Male: XY (heterogametic)
Y chromosome carries the SRY gene, which triggers male development

DNA as Genetic Material

DNA is the primary genetic material in all eukaryotes and most prokaryotes
Double helix structure (Watson and Crick, 1953)
Semi-conservative replication
DNA → RNA → Protein (Central Dogma)
RNA is genetic material in some viruses (e.g., HIV, Influenza)

Quick Reference — Must Remember Facts

Mendel’s law of segregation applies to meiosis
Independent assortment occurs during meiosis I (metaphase I)
Incomplete dominance = intermediate phenotype (no dominance)
Codominance = both alleles expressed (e.g., MN blood group, Roan coat)
Epistasis = one gene masks another gene (9:3:4 ratio in recessive epistasis)
A person with genotype I^A I^B has AB blood group — universal recipient
A person with genotype ii has O blood group — universal donor
Turner syndrome: XO (female, sterile)
Klinefelter syndrome: XXY (male, sterile)
Down syndrome: Trisomy 21 (47, XX+21 or 47, XY+21)

Common DU Admission Questions Pattern

Problem on monohybrid cross (find phenotypic ratio)
Blood group genetics (Rh factor combined with ABO)
Sex-linked inheritance (color blindness in家族)
Difference between gene and allele
Crossing over and recombination

Exam Tips

Always write the gametes along the edges of Punnett square before filling
For X-linked problems, remember males are hemizygous (only one copy of X-linked gene)
Test cross (unknown genotype × homozygous recessive) determines if unknown is homozygous or heterozygous
Remember: Phenotypic ratio ≠ Genotypic ratio in dominance relationships

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