Heredity and Evolution — Karnataka (SSLC) Class 10 Science Solutions (Free)

Heredity is the transmission of traits from parents to their offspring, carried physically through genes located on chromosomes. Mendel's Law of Segregation: the two alleles (versions of a gene) an organism carries for a trait SEPARATE from each other during the formation of gametes (sperm or egg cells), so each gamete receives only ONE of the two alleles, never both. Why this matters: it explains why an organism with two DIFFERENT alleles for a trait (say, Aa) doesn't simply produce offspring that are a uniform "blend" of both — instead, each individual gamete carries only A or only a, and which allele ends up in a particular offspring depends on which gamete from each parent happens to combine at fertilisation. This is exactly why two children of the same two parents can inherit visibly different combinations of traits from each other, despite both parents contributing from the exact same genetic pool.

A gene is a specific segment of DNA, located at a fixed position on a chromosome, that carries the instructions controlling a particular inherited trait. Every individual carries two alleles (versions) of each gene — one inherited from each parent. Using A to represent the dominant allele and a for the recessive allele: - AA (homozygous dominant): dominant trait is expressed. - Aa (heterozygous): dominant trait is STILL expressed — a dominant allele's effect shows up even when only one copy is present, effectively "masking" the recessive allele's effect. - aa (homozygous recessive): the recessive trait is expressed — this only happens when BOTH inherited alleles are the recessive version, since there is no dominant allele present to mask it. This is exactly why a recessive trait can seemingly "skip" a generation — two Aa parents, both outwardly showing the dominant trait, can still each pass on their hidden recessive allele, producing an aa child who visibly shows the recessive trait that neither parent displayed.

Natural selection is the process by which individuals possessing traits better suited to their environment survive and reproduce more successfully than individuals lacking those traits, passing their advantageous traits on to more offspring. The mechanism, step by step: 1. Within any population, individuals naturally show VARIATION in their traits (due to genetic differences already present). 2. Environmental pressures (limited food, predators, disease, climate) mean not every individual survives to reproduce equally. 3. Individuals whose traits happen to suit the current environment better are more likely to survive AND reproduce, passing those advantageous alleles on to the next generation in greater numbers. 4. Over many generations, this repeated, gradual filtering causes advantageous traits to become more common in the population, while disadvantageous traits become rarer. This gradual shift in the genetic makeup of a population over successive generations, driven by this filtering, IS evolution. A classic illustrative case: the peppered moth in industrial England — as soot darkened tree bark during the Industrial Revolution, darker-coloured moths (better camouflaged against predators on sooty bark) survived and reproduced at higher rates than lighter moths, shifting the population's overall colouring within a few generations.

Cross: Aa × Aa Each Aa parent produces two types of gametes in equal proportion: half carrying A, half carrying a. Using a Punnett square to combine all possible gamete pairings: | | A | a | |-------|------|------| | **A** | AA | Aa | | **a** | Aa | aa | This gives four equally likely outcomes: AA, Aa, Aa, aa — so: - AA (homozygous dominant): 1/4 - Aa (heterozygous): 2/4 - aa (homozygous recessive): 1/4 The probability of a homozygous recessive (aa) child is 1/4, or 25%. Note that 3/4 of the offspring (AA + Aa) will outwardly SHOW the dominant trait, even though only 1/4 are genetically "pure" for it (AA) — this 3:1 ratio of dominant-to-recessive appearance is one of Mendel's most famous original observations.

Variation refers to the naturally occurring DIFFERENCES in traits among individuals of the same species, arising from the different combinations of existing genes each individual inherits (and, to some extent, from environmental influence). Variation is the raw diversity already present within a population at any given time. Mutation is a sudden, random CHANGE in an organism's DNA sequence, creating a genuinely NEW version of a gene that did not exist in that lineage before. How they relate: mutation is one of the fundamental SOURCES that generates new variation in the first place. Without mutation occasionally introducing new alleles, a population's genetic variation would be limited only to reshuffling the alleles already present — mutation is what periodically adds genuinely new genetic material into the pool. Most mutations are neutral or harmful, but the rare beneficial mutation is exactly the kind of new variation that natural selection can act on to drive evolutionary change, which is why mutation and natural selection are often described as working together as the two core engines of evolution.

1. Fossil records: fossils found in progressively younger rock layers show a gradual, ordered sequence of anatomical changes within related lineages over enormous timescales, exactly as evolution by gradual accumulation of change would predict. 2. Comparative anatomy (homologous structures): structures that share the same underlying skeletal arrangement despite serving very different functions in different species — for example, the forelimb bones of a human arm, a bat's wing, and a whale's flipper — strongly suggest descent from a shared ancestor, whose basic limb structure was then modified differently in each lineage. 3. Vestigial organs: structures that serve little or no function in a present-day organism, but closely resemble a fully functional structure in a related species — such as the human appendix or the tiny remnant leg bones found in some snakes — are best explained as evolutionary leftovers from ancestors in which that structure WAS functional. 4. Embryological similarities: the embryos of very different vertebrate species (fish, birds, humans) show striking structural similarities at early stages of development, including features like gill slits that later develop into completely different structures — evidence of a shared evolutionary starting point. 5. Molecular biology: comparing DNA and protein sequences directly between species shows that more CLOSELY related species (as already established by fossil and anatomical evidence) consistently share MORE similar genetic sequences than distantly related ones — an independent line of evidence that converges on the same evolutionary relationships the other methods suggest.

Colour blindness is an X-linked recessive trait, meaning the gene is carried on the X chromosome, and the recessive allele (Xᵇ) must be present on BOTH X chromosomes for a female to be colour blind, but only needs to be present on a male's single X chromosome (since males have only one X, paired with a Y). Cross: mother XᴮXᵇ (carrier) × father XᵇY (colour blind) Mother's gametes: Xᴮ or Xᵇ (equal chance) Father's gametes: Xᵇ or Y (equal chance) Possible offspring combinations: - XᴮXᵇ (daughter, carrier — outwardly normal vision) - XᵇXᵇ (daughter, colour blind) - XᴮY (son, normal vision) - XᵇY (son, colour blind) Result: of the daughters, 50% are carriers (XᴮXᵇ) and 50% are colour blind (XᵇXᵇ) — none have fully normal (non-carrier) vision, since every daughter receives the father's Xᵇ. Of the sons, 50% have normal vision (XᴮY) and 50% are colour blind (XᵇY), depending purely on which of the mother's two X chromosomes they happen to inherit.