Basic Genetics and Inheritance

Overview

• Topic: Genetics — basic concepts, Mendel's laws, chromosomes, genes, alleles, inheritance.
• Purpose: Explain heredity, variation, sex determination, Mendelian ratios, applications like genetic engineering and counseling.
• Context: Lecture aimed at ICSE students with examples and practice prompts.

Key Concepts

• Genetics: Study of heredity — transmission of features from parents to offspring.
• Heredity: Passing of characters (features) from parents to offspring; includes similarities and differences.
• Variation: Small differences among individuals of the same species; responsible for unique appearances.

Chromosomes And Karyotype

• Humans have 23 pairs (46) chromosomes arranged in a karyotype.
• Chromosomes occur in pairs: one from mother, one from father.
• Homologous chromosomes: pairs similar in shape and size.
• Autosomes: chromosome pairs 1–22; determine body features (height, eye color).
• Allosomes (sex chromosomes): 23rd pair; determine sex (XX = female, XY = male).
• Chromosome number is constant within a species (e.g., humans always 46).

Genes, Alleles, Genotype, Phenotype

• Gene: Segment of DNA with a specific nucleotide sequence that determines a character.
• Alleles: Alternative forms of a gene occupying same locus on homologous chromosomes.
• Dominant allele: Expressed in offspring; denoted by capital letter.
• Recessive allele: Masked in presence of dominant allele; denoted by lowercase letter.
• Homozygous: Both alleles same (e.g., TT or tt).
• Heterozygous: Alleles different (e.g., Tt).
• Genotype: Genetic constitution (set of genes) of an organism.
• Phenotype: Observable expression of a trait in the organism.

Mendel And Pea Experiments

• Father of Genetics: Gregor Mendel; used garden pea (Pisum sativum).
• Pea chosen because many distinct varieties exist, true-breeding forms available, and it self-pollinates; easy to cross-pollinate artificially.

Mendel’s Three Laws (Important)

1. Law Of Dominance

   • In a pair of contrasting characters, one allele (dominant) expresses, the other (recessive) is masked.
   • Remember definition for exam.

2. Law Of Segregation

   • Two members of a factor pair separate during formation of gametes (during meiosis).
   • Alleles segregate into different gametes; fertilization randomly fuses gametes to form zygote.

3. Law Of Independent Assortment

   • When two pairs of characters are considered, distribution of alleles of one character is independent of the other.
   • Applies to dihybrid crosses (two characters).

Mendelian Ratios And Punnett Square

• Monohybrid cross (single character):
  • F2 phenotypic ratio: 3 : 1 (dominant : recessive)
  • F2 genotypic ratio: 1 : 2 : 1 (homozygous dominant : heterozygous : homozygous recessive)
• Dihybrid cross (two characters):
  • F2 phenotypic ratio: 9 : 3 : 3 : 1 (for two independently assorting traits)
• Punnett square used to predict genotypes and phenotypes from parental gametes.

Sex-Linked Traits

• Sex-linked (X-linked) inheritance: trait governed by allele on X chromosome.
• Many X-linked disorders are more common in males (XY) because males have single X.
• Examples:
  • Color blindness: difficulty distinguishing colors (red/green); male-oriented.
  • Hemophilia: “bleeder” disease; blood clotting impaired; mainly affects males.

Mutation And Causes

• Mutation: Sudden heritable change in one or more genes; alters genetic material (DNA).
• Mutations can change phenotype and be passed to future generations if in germ cells.
• Examples / effects:
  • Sickle cell anemia: mutation causes sickle-shaped RBCs; leads to blockages.
  • Radiation (e.g., atomic bomb in Hiroshima, Nagasaki) caused lasting genetic defects in descendants.

Applications Of Genetics

• Genetic counseling: advice before marriage or conception to assess genetic disease risk (e.g., thalassemia, hemophilia, sickle cell).
• Genetic engineering: modifying genetic constitution; creation of organisms (e.g., Dolly the sheep).
• Breeding and agriculture: develop new varieties, hybrid fruits, specialized breeds; captive breeding programs.
• Medical benefits: organ transplantation across species, predicting offspring traits.

Definitions And Quick Facts

• Karyotype: Arrangement of chromosomes by shape and size.
• Homologous: Similar shape and size chromosomes from each parent.
• Heterologous (heterologous sex chromosomes): X and Y are dissimilar.
• Gametes: Sex cells (sperm and egg); carry single set of chromosomes.
• Zygote: Fertilized egg formed by fusion of egg and sperm.

Important Points To Memorize

• Mendel is Father of Genetics (Gregor Mendel).
• Human somatic cells: 46 chromosomes (23 pairs).
• Autosomes: pairs 1–22; Allosomes: pair 23 (sex chromosomes).
• XX = female, XY = male; sex determined by paternal contribution (X or Y).
• Monohybrid F2 ratio: phenotypic 3:1, genotypic 1:2:1.
• Dihybrid F2 phenotypic ratio: 9:3:3:1.
• Dominant alleles shown by capital letters; recessive by lowercase.
• Recessive traits express only in homozygous recessive genotype.
• Key X-linked disorders: color blindness, hemophilia.

Practice/Next Steps

• Use Punnett squares to practice monohybrid and dihybrid crosses.
• Solve problems determining genotype vs phenotype from crosses.
• Practice pedigree analysis for sex-linked traits.
• Revise definitions and memorize Mendel’s three laws and key ratios.
• Seek genetic counseling examples and real-life case studies for application questions.