Mendel's laws, inheritance, sex determination, evolution, and speciation
Heredity: The transmission of traits/characteristics from parents to offspring through generations.
Gene: A unit of inheritance; a segment of DNA that codes for a specific trait.
Trait: A specific characteristic of an organism (e.g., eye colour, height).
Variation: Differences in traits among individuals of same species.
Gregor Johann Mendel (1822â1884), an Austrian monk, is called the "Father of Genetics." He conducted experiments with pea plants (Pisum sativum) for 8 years, studying 7 pairs of contrasting traits. His work was ignored for 35 years and only recognised after his death!
Dominant trait: Trait that appears in Fâ generation when two contrasting traits are crossed.
Recessive trait: Trait that gets masked in Fâ but reappears in Fâ.
Alleles: Two different forms of the same gene (TT, Tt, or tt for height)
Genotype: Genetic composition (TT, Tt, tt)
Phenotype: Observable trait (tall, tall, dwarf)
Homozygous: Both alleles same (TT or tt)
Heterozygous: Alleles different (Tt)
T = Tall (dominant), t = dwarf (recessive)
P generation: TT (tall) Ă tt (dwarf)
Fâ generation: All Tt (tall) â all plants are tall! (recessive hidden)
Fâ Ă Fâ: Tt Ă Tt
Fâ genotypes: TT : Tt : tt = 1 : 2 : 1
Fâ phenotypes: Tall : Dwarf = 3 : 1
This is Mendel's Law of Segregation: Two alleles of a gene separate during gamete formation.
R = Round (dominant), r = wrinkled; Y = Yellow (dominant), y = green
RRYY Ă rryy â Fâ: all RrYy (Round-Yellow)
Fâ Ă Fâ: RrYy Ă RrYy â 4 types of phenotypes
Round-Yellow : Round-Green : Wrinkled-Yellow : Wrinkled-Green = 9 : 3 : 3 : 1
This shows Law of Independent Assortment: genes for different traits are inherited independently.
Humans have 23 pairs of chromosomes. The 23rd pair = sex chromosomes.
Female: XX | Male: XY
Mother always contributes X chromosome to child.
Father contributes either X or Y â determines sex of child.
Probability of boy = 1/2 = 50% | Probability of girl = 1/2 = 50%
Mother (XX) â egg always carries X chromosome
Father (XY) â sperm carries either X or Y (50% each)
X-sperm + egg (X) = XX = Girl
Y-sperm + egg (X) = XY = Boy
So the father's contribution determines whether a child is male or female â not the mother!
Evolution is the gradual change in the heritable characteristics of biological populations over successive generations. Over time, these changes can lead to new species.
⢠All organisms produce more offspring than can survive.
⢠There is variation among organisms of same species.
⢠Organisms with favourable variations are better adapted â they survive and reproduce more ("survival of the fittest").
⢠Favourable traits are passed to next generation â gradually accumulate â new species may form.
Before industrialisation: White moths on light-coloured tree bark were camouflaged â survived. Black moths were visible â eaten by birds.
After industrialisation: Soot darkened tree bark â now black moths survived â white moths got eaten. The population shifted from white to black moths â evolution by natural selection!
Speciation: Formation of new species from existing ones, usually due to geographic isolation + natural selection + genetic drift.
Genetic drift: Random changes in allele frequencies in a small isolated population (may not be advantageous â just random).
Geographic isolation â population split â different environments â different selection pressures â accumulation of different traits â eventually, two populations cannot interbreed â new species!