Heredity · Class 10 Science: 66 flashcards

The transmission of characters (traits) from parents to their offspring through DNA — the process that makes offspring resemble their parents and keeps species breeding true.

Each generation inherits the variations already present in its parents AND adds new ones (from DNA-copying errors and sexual gene-mixing). The differences therefore pile up generation after generation.

A 19th-century monk who performed the first quantitative inheritance experiments on garden peas — counting offspring types across generations and deducing the laws of inheritance from the ratios, long before genes or DNA were known.

(1) Easy to grow with short generations and many offspring; (2) naturally self-pollinating (pure lines) yet easily hand-crossed; (3) they show clearly contrasting, either/or traits such as tall/short stems and round/wrinkled seeds.

Tall vs short (dwarf) plants; round vs wrinkled seeds; yellow vs green seeds (also flower colour/position and pod traits).

A cross between two plants differing in ONE contrasting trait pair — e.g., pure tall (TT) × pure short (tt) — followed through the F1 and F2 generations.

ALL plants were tall (genotype Tt). The tall factor T is dominant — it expresses even with one copy — while the short factor t, though present, stays masked.

When F1 (Tt) plants self-pollinated, short plants REAPPEARED among tall ones in a ratio of about 3 tall : 1 short.

It proves the shortness factor was not destroyed or blended away in F1 — it was carried, hidden but intact, and expressed again when two copies met (tt). Traits are particulate, inherited as discrete factors in pairs.

Genotype: 1 TT : 2 Tt : 1 tt. Phenotype: 3 tall : 1 short — because TT and Tt are visibly identical (both tall).

The trait version that expresses itself even when only one copy is present — e.g., tallness (T) in peas: both TT and Tt plants are tall.

The version expressed only when BOTH copies are that version — e.g., shortness (t): only tt plants are short; in Tt the trait stays hidden.

Genotype is the genetic makeup — the factor pair (TT, Tt or tt). Phenotype is the observable characteristic — tall or short. Different genotypes (TT and Tt) can share one phenotype (tall).

Exactly ONE of the pair — germ cells receive a single factor for each trait (the halved-DNA principle), so that fertilisation, combining one from each parent, restores the pair.

A grid tool for predicting cross outcomes: each parent's possible gametes are written along the edges, the cells are filled with their combinations, and counting the cells gives the expected genotype and phenotype ratios.

Because Mendelian traits do not blend — the dominant factor T expresses completely, masking t entirely. The clean reappearance of pure short plants in F2 is itself the disproof of blending.

By its offspring: self-pollinate it (or cross with tt). If any short offspring appear, the parent must carry t (i.e., it is Tt); a TT parent gives tall offspring only.

The ratio is a probability — each offspring draws factors independently like coin tosses. Small samples can deviate by chance; the ratio emerges reliably only across large numbers, which is why Mendel counted thousands.

Alleles — alternative versions of a gene. Genes are stretches of DNA located on chromosomes in the cell nucleus.

The pairing rule: each trait's factors come one from the mother's gamete and one from the father's — every pair is an equal, one-from-each inheritance.

Pure lines (TT or tt) produce only their own kind when selfed, so any new pattern after crossing must be caused by the cross itself — establishing clean cause-and-effect for the ratios he counted.

One-third. The tall plants comprise 1 TT and 2 Tt; only the TT third breeds true for tallness when selfed.

A cross between plants differing in TWO contrasting trait pairs — e.g., seed shape (round/wrinkled) and seed colour (yellow/green) — tracked together through F1 and F2.

P: pure round-yellow (RRYY) × pure wrinkled-green (rryy). F1: all RrYy — round-yellow, since round and yellow are dominant.

9 round-yellow : 3 round-green : 3 wrinkled-yellow : 1 wrinkled-green.

Round-green and wrinkled-yellow — combinations present in NEITHER parent. Their appearance proves the shape and colour factor pairs separated and recombined freely: traits are inherited independently.

The principle that each trait's factor pair segregates into gametes independently of other traits' pairs — so an RrYy plant produces all four gamete combinations (RY, Ry, rY, ry) equally.

RY, Ry, rY and ry — in equal proportions, because R/r and Y/y are chosen independently.

Shape: (9+3) round : (3+1) wrinkled = 12:4 = 3:1. Colour: (9+3) yellow : (3+1) green = 12:4 = 3:1. The 9:3:3:1 is the product of two independent 3:1 ratios.

A section of DNA that carries the information for making a specific protein — the functional unit of heredity, occurring in pairs (one copy on each chromosome of a pair).

The gene's information builds a protein — here an enzyme needed to produce growth hormone. An efficient enzyme yields ample hormone → tall plant; an altered gene gives a less efficient enzyme → less hormone → short plant. Gene → protein → performance → phenotype.

In a heterozygote (Tt), the one working gene copy still produces enough functional protein to run the process — the dominant trait shows. The recessive trait appears only in tt, when no working copy (and hence no functional protein) exists.

At fixed positions on chromosomes, which reside in the nucleus. Each body cell carries two copies of each chromosome — and therefore two copies of each gene.

When gametes form, each germ cell receives just ONE chromosome from every pair — a complete single set carrying one copy of each gene. The factor pair is thus physically separated, one member per gamete.

Fertilisation: the gametes' two single sets combine, rebuilding every chromosome pair — one member from the mother, one from the father — and restoring the species' full chromosome number.

Because each contributes one complete set of chromosomes (equal DNA) through its gamete — every gene pair in the child has one maternal and one paternal copy.

Their genes were located on DIFFERENT chromosomes, and separate chromosome pairs segregate into gametes independently of one another.

No — genes riding the same chromosome tend to be inherited together rather than recombining freely, so their crosses deviate from 9:3:3:1.

Nine distinct genotypes (e.g., RRYY, RrYY, RRYy, RrYy, RRyy, Rryy, rrYY, rrYy, rryy) — collapsing into four phenotypes by dominance.

1/16 — the wrinkled-green class, requiring rryy: an r from each parent AND a y from each parent.

3/16 — round (R_) combined with green (yy): one of the two new-combination classes.

They unite desirable traits from different parents in one organism — e.g., disease resistance from one variety with high yield from another — which is the practical engine of crop and livestock improvement.

The recessive version is typically an ALTERED gene whose protein works poorly or not at all — it isn't an active 'shortness instruction' but a less functional variant, unmasked only when no working copy remains.

Segregation: the two chromosomes of a pair (with their gene copies) separate into different gametes — one factor per gamete. Independent assortment: different chromosome pairs separate independently — so genes on different chromosomes recombine freely.

23 pairs in all — 22 matched pairs (identical in both sexes) plus 1 pair of sex chromosomes (the 23rd), which differs between women (XX) and men (XY).

Females: XX — a matched pair of two X chromosomes. Males: XY — a mismatched pair, the Y being much shorter than the X.

Because its two members differ — the Y chromosome is far shorter than the X and carries different (far fewer) genes, unlike the 22 pairs whose members are alike.

An X — the mother is XX, and since each gamete receives one member of every pair, either choice from XX is still an X. Her contribution to the child's sex is therefore constant.

X-carrying and Y-carrying sperm, in EQUAL numbers (half and half) — because the father's X and Y separate one-per-gamete during sperm formation.

Egg (X) + X-sperm → XX → girl. Egg (X) + Y-sperm → XY → boy. Both are equally probable.

The father's — because the egg always carries X, the deciding variable is whether an X-sperm or a Y-sperm fertilises it. And that is pure chance.

Her genetic contribution to sex is identical for every child — an X. The only variable lies in the father's sperm, and even there the outcome is uncontrollable chance. The blame has no biological basis whatsoever.

Because X- and Y-sperm are produced in equal numbers and have an equal chance of fertilising the egg — each conception is a 50:50 event, which sums to a roughly equal ratio across a population.

Still 50:50 (about 1/2). Each conception is an independent event — probability has no memory, and previous outcomes do not alter the next one's odds.

Some reptiles — notably turtles and crocodiles: the temperature at which fertilised eggs are incubated decides whether offspring develop as male or female.

Some snails — their sex is not fixed at birth and can change during their lifetime, showing that sex determination is not always genetic or permanent.

It acts as a switch: its presence initiates male development; in its absence (XX), development proceeds along the female path. Despite carrying few genes, its effect is decisive.

No — the 22 non-sex pairs (autosomes) are the same in both sexes. Only the 23rd pair differs, which is why it alone governs sex.

It is the one-chromosome-per-pair rule applied to the 23rd pair: XX segregates to all-X eggs, XY segregates to half-X half-Y sperm, and fertilisation re-pairs them — a Punnett square giving 1 XX : 1 XY.

Because the sex is fixed at the instant of fertilisation by which chromosome the winning sperm carried — an event already randomised among equal swimmers. Nothing external reaches into that race or rewrites the chromosome afterwards.

Since biology delivers roughly equal numbers, a large skew signals human interference — such as illegal prenatal sex determination followed by sex-selective abortion — which is why the practice is banned.

The mother's — birds reverse the mammalian system: females carry the mismatched pair (ZW) and males the matched (ZZ), so the egg, not the sperm, carries the deciding chromosome.

Their sex is set by nest incubation temperature; warming beaches push hatchling ratios heavily toward one sex (often female), undermining future breeding — a demographic threat unique to temperature-dependent species.

No — systems vary: birds use ZW (female mismatched), some insects use different arrangements. XY is the human/mammalian version of a broader family of chromosomal strategies.

Half — the X and Y separate into different sperm during gamete formation, producing 50% X-carrying and 50% Y-carrying sperm.

XX — one X inherited from the mother's egg, the other X from the father's X-carrying sperm. (A son's XY has the X from the mother and the Y necessarily from the father.)