Basic Principles of Inheritance Chapter Notes | Biotechnology for Class 11 - NEET PDF Download

Introduction to Inheritance

• Family members often share common features such as facial features, hair color, and skin color due to genetic inheritance.
• These characteristics are passed from parents to offspring through genetic information, a phenomenon observed in all plants and animals.
• Heredity is the transmission of traits from one generation to the next, where offspring inherit parental characteristics.
• Inherited traits are encoded in genes, which are located on chromosomes.
• Offspring resemble their parents but also exhibit variations due to the combination of genetic material from both parents.
• Gregor Johann Mendel, known as the 'father of genetics,' laid the foundation for understanding heredity through his experiments on pea plants.

Linkage and Crossing Over

• Phenotypic traits, such as flower color or pollen shape in peas, are controlled by pairs of alleles located at specific gene loci on homologous chromosomes.
• Humans have 20,000 to 25,000 protein-coding genes distributed across 23 pairs of chromosomes.
• Genes on the same chromosome are inherited together, a phenomenon called linkage.
• Linked genes form a linkage group, and the traits they control are known as linked characters.
• W. Bateson and R.C. Punnett demonstrated linkage in sweet pea experiments, crossing red-flowered, long-pollen plants with white-flowered, short-pollen plants.
• Their F1 progeny all had red flowers and long pollen, indicating dominance of these traits.
• In F2, parental combinations (red-long, white-short) were more frequent than expected, suggesting linkage.
• Thomas Hunt Morgan’s experiments on Drosophila (1910) confirmed that linked genes are arranged linearly on chromosomes.
• Crossing over, the exchange of chromosomal segments during meiosis, results in non-parental combinations.
• In Morgan’s Drosophila cross between grey-bodied, vestigial-winged and black-bodied, long-winged flies, F1 hybrids were grey-bodied and long-winged.
• A test cross with double recessive flies produced 83% parental and 17% non-parental combinations, confirming crossing over.

Recombination

• Recombination was recognized shortly after the rediscovery of Mendel’s work in 1900.
• Bateson and Punnett’s sweet pea experiments showed that genes do not always assort independently, deviating from the expected 9:3:3:1 dihybrid ratio.
• In their cross, parental combinations (red-long, white-short) exceeded 50%, while recombinants (red-short, white-long) were less than 50%.
• This deviation was attributed to linkage, where genes on the same chromosome are inherited together.
• Recombination occurs due to crossing over, where homologous chromosomes exchange segments during meiosis.
• Morgan’s Drosophila experiments further established linkage and recombination, allowing the creation of chromosome maps based on recombination frequencies.
• One percent recombination between two traits equals one map unit or one centimorgan (cM).
• Cytological evidence from Harriet Creighton and Barbara McClintock’s 1931 maize experiment confirmed crossing over.
• They used morphologically distinct chromosome 9 (one normal, one with a heterochromatic knob and extra segment) to track recombination.
• Two traits—kernel color (colored vs. colorless) and texture (starchy vs. waxy)—were studied, showing that recombinants had exchanged chromosomal segments.

Sex-linked Inheritance

• Sex-linked inheritance involves genes located on sex chromosomes, first described by Thomas H. Morgan in 1910 through Drosophila experiments.
• Morgan observed a white-eyed male in a red-eyed Drosophila culture and crossed it with a red-eyed female.
• All F1 progeny (males and females) were red-eyed, indicating that the white-eye allele (w) is recessive to the red-eye allele (W).
• In F2, red and white-eyed flies appeared in a 3:1 ratio, but all white-eyed flies were male, and all females were red-eyed.
• Morgan concluded that the eye color gene is located on the X chromosome, making it a sex-linked gene.
• Sex-linked inheritance explains traits like hemophilia and color blindness in humans, which are often observed in males due to their single X chromosome.

Extrachromosomal Inheritance

• DNA is present not only in the nucleus but also in mitochondria and plastids, contributing to extrachromosomal or cytoplasmic inheritance.
• During fertilization, sperm contribute primarily nuclear DNA, while the egg provides both nuclear and cytoplasmic DNA (mitochondria and plastids).
• Thus, mitochondrial and plastid DNA are inherited maternally, leading to uniparental or maternal inheritance.
• Traits controlled by mitochondrial or plastid genes do not follow Mendelian inheritance patterns.
• In Four O’clock plants (Mirabilis jalapa), leaf color (green, white, or variegated) is determined by plastid DNA.
• Crosses in Four O’clock plants showed that offspring leaf color matches the maternal phenotype, confirming maternal inheritance.
• Examples include mitochondrial genes coding for cellular respiration enzymes and plastid genes for chlorophyll or pigments.

Polyploidy

• Ploidy refers to the number of chromosome sets in an organism: haploid (1 set), diploid (2 sets), or polyploid (more than 2 sets).
• Polyploids include triploids (3 sets), tetraploids (4 sets), hexaploids (6 sets), octoploids (8 sets), etc.
• Most organisms are diploid, but polyploidy is common in plants (over 30% of plant species) and rare in animals.
• In some insects like bees and ants, males are haploid, and females are diploid.
• Polyploid plants often have larger leaves and cells and are more tolerant to harsh environments compared to diploids.
• Examples of polyploid plants include bananas (diploid or triploid), peanuts (tetraploid), cotton (tetraploid), and wheat (hexaploid).
• Aneuploidy occurs when one or more chromosomes are missing or extra, resulting from irregular meiotic division.

Reverse Genetics

• Forward genetics studies phenotypic variations (macro-variations like size or micro-variations like DNA sequences) to identify regulating genes.
• Reverse genetics starts with a known DNA or protein sequence and investigates its function by inducing variations.
• Advances in DNA sequencing have enabled the identification of all genes in an organism’s genome.
• In reverse genetics, a candidate gene is cloned, reinserted into the genome, and its expression is silenced to observe phenotypic effects.
• This approach is useful for characterizing genes with unknown functions, as many genes do not produce visible phenotypic variations.
• Techniques like RNA interference (RNAi) or targeted gene disruption via homologous recombination are used to alter gene expression.
• RNAi uses small double-stranded RNA molecules to inhibit gene expression or translation.