True-breeding line is one that has undergone continuous self-pollination and shows stable trait inheritance.
Monohybrid cross is a cross involving only one pair of contrasting characters.
Phenotype refers to the observable physical appearance (e.g., Tall), while Genotype is the genetic constitution (e.g., TT or Tt).
Dominant factor expresses itself in both homozygous and heterozygous conditions.
Recessive factor expresses itself only in homozygous conditions.
Law of Dominance states that in a dissimilar pair of factors, one dominates the other.
Law of Segregation states that alleles do not show blending and both characters are recovered in the $F_2$ generation.
Punnett Square is a graphical representation used to calculate the probability of all possible genotypes of offspring.
Test Cross involves crossing an organism showing a dominant phenotype with a recessive parent to determine its genotype.
Post-Mendelian Genetics
Incomplete Dominance occurs when the $F_1$ phenotype does not resemble either parent and is intermediate (e.g., Antirrhinum / Snapdragon).
In Snapdragon, crossing Red (RR) and White (rr) flowers produces Pink (Rr) offspring.
Co-dominance occurs when both alleles express themselves equally in the $F_1$ generation (e.g., ABO blood grouping).
ABO blood groups are controlled by the Gene I, which has three alleles: $I^A$, $I^B$, and $i$.
Multiple Allelism is when more than two alleles govern the same character (can only be studied in a population).
Pleiotropy is a phenomenon where a single gene can exhibit multiple phenotypic expressions (e.g., Phenylketonuria or starch grain size in peas).
Polygenic Inheritance involves characters controlled by three or more genes, where the effect of each allele is additive (e.g., human skin color).
Chromosomal Theory and Linkage
Chromosomal Theory of Inheritance was proposed by Sutton and Boveri.
It states that the behavior of chromosomes is parallel to the behavior of genes.
Thomas Hunt Morgan provided experimental verification of the chromosomal theory using Drosophila melanogaster (fruit flies).
Linkage is the physical association of genes on a chromosome.
Recombination describes the generation of non-parental gene combinations.
Alfred Sturtevant used the frequency of recombination to measure the distance between genes and “map” their position on chromosomes.
Tight linkage leads to very low recombination; loose linkage leads to higher recombination.
Sex Determination
Henking identified a specific nuclear structure called the X-body (later known as the X-chromosome).
Male Heterogamety is seen in humans and Drosophila (XY) and grasshoppers (XO).
Female Heterogamety is seen in birds, where females are ZW and males are ZZ.
In humans, the presence of the Y-chromosome determines the male sex.
Haplodiploid sex determination occurs in honey bees: males (drones) are haploid ($n=16$), females are diploid ($2n=32$).
Male honey bees develop by parthenogenesis and do not have a father (but have a grandfather).
Mutation and Genetic Disorders
Mutation is a phenomenon which results in alteration of DNA sequences and consequently changes the genotype and phenotype.
Point Mutation involves a change in a single base pair of DNA (e.g., Sickle-cell anemia).
Pedigree Analysis is the study of the inheritance of traits in several generations of a family.
Mendelian Disorders are mainly determined by alteration or mutation in a single gene.
Haemophilia is a sex-linked recessive disorder where a single protein involved in blood clotting is affected.
Sickle-cell anemia is an autosomal recessive disorder caused by the substitution of Glutamic acid by Valine at the sixth position of the beta-globin chain.
Phenylketonuria is an inborn error of metabolism where the individual lacks an enzyme that converts phenylalanine into tyrosine.
Thalassemia is an autosomal linked recessive blood disease involving the reduced synthesis of globin chains.
Chromosomal Disorders are caused by the absence, excess, or abnormal arrangement of one or more chromosomes.
Aneuploidy is the gain or loss of a chromosome due to failure of chromatid segregation (e.g., Down’s syndrome).
Polyploidy is an increase in a whole set of chromosomes, often seen in plants.
Down’s Syndrome is caused by the Trisomy of chromosome 21.
Klinefelter’s Syndrome is caused by the presence of an additional X-chromosome (genotype XXY) in males.
Individuals with Klinefelter’s have overall masculine development but express feminine traits like Gynaecomastia.
Turner’s Syndrome is caused by the absence of one of the X-chromosomes (genotype XO) in females.
Females with Turner’s syndrome are sterile as ovaries are rudimentary and lack secondary sexual characters.
