Principles of Inheritance and Variation notes

This chapter explains how characters pass from parents to offspring and why offspring show variation. It begins with Mendel’s pea plant experiments, where dominant and recessive traits revealed the laws of inheritance. It then extends Mendelism to incomplete dominance, codominance, multiple alleles, pleiotropy and polygenic inheritance. The chromosomal theory connects genes with chromosomes, while Morgan’s work explains linkage and crossing over. Sex determination shows how chromosome combinations decide sex in humans, insects, birds and honeybees. The chapter ends with pedigree analysis and genetic disorders such as haemophilia, sickle-cell anaemia, Down syndrome, Turner syndrome and Klinefelter syndrome, all highly important for NEET.

Mendelian genetics explains inheritance using pea plant crosses performed by Gregor Johann Mendel. Mendel selected true-breeding pea varieties with contrasting traits such as tall/dwarf stem, yellow/green seed and round/wrinkled seed. In a monohybrid cross, he followed one character and obtained a 3:1 phenotypic ratio in F2, leading to the laws of dominance and segregation. In a dihybrid cross, two characters were followed together and the F2 ratio was 9:3:3:1, supporting independent assortment. Test crosses reveal unknown genotypes by crossing with a homozygous recessive parent. Punnett squares and probability rules help predict offspring genotypes and phenotypes.

Deviations from Mendelism are inheritance patterns that do not show simple dominant-recessive Mendelian ratios. In incomplete dominance, the heterozygote has an intermediate phenotype, as seen in snapdragon flower colour. In codominance, both alleles express equally, as in the AB blood group. Multiple alleles involve more than two allelic forms in a population, although an individual carries only two. Pleiotropy occurs when one gene influences multiple characters, such as sickle-cell anaemia. Polygenic inheritance occurs when many genes additively control a trait, producing continuous variation such as human skin colour. ABO blood group inheritance combines multiple alleles and codominance, making it a major NEET concept.

The chromosomal theory of inheritance, proposed independently by Sutton and Boveri, states that chromosomes are the physical carriers of genes. Mendel’s factors were abstract, but chromosome behaviour during meiosis provided a physical explanation for segregation and independent assortment. Homologous chromosomes occur in pairs, separate during meiosis I and enter different gametes, just as alleles segregate. Different chromosome pairs orient independently at metaphase I, explaining independent assortment. Morgan’s experiments on Drosophila showed that genes are located on chromosomes in linear order and that genes on the same chromosome may be linked. His work provided experimental evidence for linkage, recombination and chromosome-based inheritance.

Sex determination explains how an organism develops as male or female. In many animals, sex is determined by specific sex chromosomes. In the XX-XY system, seen in humans and Drosophila, females are XX and males are XY; males produce two types of sperm, so the father determines the sex of the child. In the XO system, seen in some insects such as grasshoppers, males have only one X chromosome and no Y chromosome. In birds, the ZW-ZZ system operates, where females are ZW and males are ZZ, so females determine offspring sex. In honeybees, haplodiploidy occurs: fertilised diploid eggs become females, while unfertilised haploid eggs become males.

Linkage is the tendency of genes located on the same chromosome to be inherited together. It reduces the frequency of independent assortment and increases parental combinations. Crossing over is the exchange of genetic material between non-sister chromatids of homologous chromosomes during pachytene of prophase I in meiosis. It produces recombinant chromosomes and new allele combinations. Morgan discovered linkage while studying Drosophila and found that tightly linked genes show fewer recombinants, while genes far apart show more recombination. Recombination frequency is used to estimate distance between genes and construct linkage maps. This topic is central for NEET questions involving parental types, recombinants and gene mapping.

Genetic disorders are diseases caused by changes in genes or chromosomes. Pedigree analysis helps trace inheritance patterns in families because controlled crosses cannot be performed in humans. Mendelian disorders are caused by mutation in a single gene and follow predictable inheritance patterns such as autosomal dominant, autosomal recessive or sex-linked recessive. Important examples include haemophilia, colour blindness, sickle-cell anaemia and thalassaemia. Chromosomal disorders occur due to absence, excess or abnormal arrangement of chromosomes, usually caused by nondisjunction during meiosis. Major NCERT examples are Down syndrome due to trisomy 21, Klinefelter syndrome due to XXY constitution and Turner syndrome due to XO constitution.