Variation across generations & inherited traits
A banana grove grown from cuttings looks like one plant repeated, while a crowd at a bus stop looks like many different people. Both are copies of earlier generations; only the mixing differs.
Heredity is the passing of traits from parents to offspring. Each generation gets the common body design plus small changes (variations) from its parents, and adds new ones. Variations add up over generations, and the environment selects those that help survival.
Equation / formula : Inherited design + inherited variation + new variation → accumulates → environment selects
In the figure : Each generation adds to what it inherited; the environment decides which variants last.
Rule 1 : Why variation is small or large — In asexual reproduction only small DNA copying errors make differences, so offspring look alike. In sexual reproduction DNA of two parents is mixed each generation, so the differences are much bigger.
- Easy : A field of potato plants grown from tubers looks uniform; a group of cousins does not.
- Medium : Only changes in the DNA of germ cells are inherited; a scar or a tan is not.
- For exams : Trait A in 3 percent and trait B in 80 percent of an asexual population: B is older, because an early trait has had more generations to spread.
Rule 2 : Variation and survival — Not every variant suits every environment. If the surroundings change, individuals with a helpful variant survive and pass it on, so it becomes common. A species with no variation can be wiped out by one change, such as a new disease.
- Easy : Heat-tolerant bacteria survive a heat wave while others die.
- Medium : Variation helps the species even if a particular individual is harmed.
- For exams : Selection of variants by the environment is the basis of evolution.
A trait is a visible feature that can have different forms, such as free or attached earlobes.
Thinking a muscle built in a gym or a tan will be inherited. Only germ-cell DNA changes are passed on.
Inherit + new change → accumulate → environment selects. Memory clue : "ICE": Inherit, Change, Environment selects.
Spot the difference
| Point | Asexual | Sexual |
|---|---|---|
| Source of variation | copying errors | mixing of two parents' DNA |
| Variation | small | large |
Practice questions
- 1What is heredity?
- 2Why do four bacteria from one parent differ slightly?
- 3Why is there more variation in sexual reproduction?
- 4Which variations are inherited?
- 5Trait X is in 4 percent and trait Y in 75 percent of an asexual population. Which is older?
- 6How does variation help a species survive?
- 7Why can one disease destroy a field of identical plants?
- 8In a class of 40, 10 have attached earlobes. What percentage have free earlobes?
Show answers (try them yourself first)
- 1Passing of traits from parents to offspring.
- 2DNA copying is not perfectly accurate.
- 3DNA of two parents recombines in every generation.
- 4Changes in the DNA of germ cells.
- 5Y, because an early trait has more time to spread.
- 6Some variants may survive when conditions change.
- 7All share the same weakness; none is resistant.
- 875 percent (30 of 40).
Mendel's experiments: dominant & recessive traits
If a tall and a short plant are crossed and all children are tall, where did the "short" go? It was hidden, and it can return in the next generation.
Mendel crossed pea plants with contrasting traits and counted the progeny in each generation. Each trait is controlled by two copies of a gene (one from each parent). The trait that shows in F1 is dominant; the hidden one is recessive and shows only when both copies are recessive.
Equation / formula : TT x tt → F1: all Tt (tall) → F2: 1 TT : 2 Tt : 1 tt = 3 tall : 1 short
In the figure : A capital letter is the dominant form; the small letter is the recessive form.
Mendel's experiments: dominant & recessive traits
In the app you can move this figure with your finger.
Open in the appRule 1 : Reading the F1 and F2 results — F1 shows only one parental trait (no blending), so that trait is dominant. When F1 plants self-pollinate, the hidden trait returns in about one quarter of F2. A single T makes the plant tall, so TT and Tt are tall; only tt is short.
- Easy : In 800 F2 plants, about 600 are tall and 200 are short.
- Medium : If 450 tall and 150 short plants appear, the ratio is 3 : 1, so tall is dominant.
- For exams : A tall plant crossed with a short plant gives half tall, half short: the tall parent must be Tt.
Rule 2 : Hidden traits and testing purity — A pure tall plant TT gives only T gametes; Tt gives half T and half t. To check whether an F2 tall plant is TT or Tt, let it self-pollinate: only tall progeny means TT; some short progeny means Tt. Always count in each generation, as Mendel did.
- Easy : TT x tt gives all Tt, which look tall.
- Medium : Rr x rr (round x wrinkled) gives half round and half wrinkled seeds.
- For exams : Blood group A man x O woman, daughter O: not enough to say which trait is dominant, since the daughter could have got O from both parents either way.
Dominant does not mean "more common" or "better"; it only means the trait shows with one copy.
Saying F1 plants are "medium" because both traits mix. Traits do not blend; one is expressed.
F1: only dominant shows. F2: 3 dominant : 1 recessive (1 : 2 : 1 in genes). Memory clue : "CAPITAL = Commands": a capital letter commands (dominant), the small letter obeys.
Spot the difference
| Point | Dominant trait | Recessive trait |
|---|---|---|
| Shows when | one copy is enough | both copies are recessive |
| Example | T (tall) | t (short) |
Practice questions
- 1Who worked out the laws of inheritance using peas?
- 2What is the F1 generation?
- 3Why were there no medium-height plants in F1?
- 4What is the genotype ratio in F2 of Tt x Tt?
- 5What is the tall : short ratio in F2?
- 6Which F2 tall plants breed pure on self-pollination?
- 7F2 has 1600 plants in a 3 : 1 ratio. How many show the recessive trait?
- 8A tall plant x short plant gives half tall, half short. Find the tall parent's genes.
Show answers (try them yourself first)
- 1Gregor Mendel.
- 2The first generation of progeny from the parental cross.
- 3Traits do not blend; the dominant one is expressed.
- 41 TT : 2 Tt : 1 tt.
- 53 : 1.
- 6TT.
- 7400 (one fourth).
- 8Tt.
Independent inheritance of traits
Choosing a shirt colour and a trouser colour are separate choices; any shirt can go with any trouser. Two traits can be shuffled in the same way.
When two traits are followed together (tall/short and violet/white, say), F2 shows the parental forms and also new combinations. This happens because the traits are inherited independently. The F2 ratio is 9 : 3 : 3 : 1.
Equation / formula : TtVv gives 4 gametes: TV, Tv, tV, tv → 16 boxes → 9 : 3 : 3 : 1
In the figure : Only 10 of 16 look like a parent; 6 of 16 are new combinations.
Rule 1 : Gametes and the 16 boxes — A plant TtVv puts one gene of each pair into each gamete, so it makes four kinds in equal numbers: TV, Tv, tV, tv. Four male x four female kinds give 16 boxes (a Punnett square), with 9 genotypes and 4 looks.
- Easy : In 160 F2 plants, about 90 show both dominant traits and 10 show both recessive.
- Medium : Each trait alone still gives 3 : 1: tall : short = 12 : 4 = 3 : 1.
- For exams : How many of 16 F2 plants are short with round seeds? 3 (genes tt with R_).
Rule 2 : Why traits are independent — Genes are not one long thread: the DNA is in separate pieces called chromosomes, and each pair is sorted into germ cells on its own. If all genes travelled as one unit, a tall plant would always keep its parent's seed shape and no new combinations would appear.
- Easy : A tall plant can have round or wrinkled seeds; the two traits do not decide each other.
- Medium : New combinations add to variation, which helps a species survive change.
- For exams : Cross TtRr x ttrr gives four types in equal numbers: 100 each in 400 progeny.
Tall x short and round x wrinkled together: F1 is all tall and round (both dominant).
Saying the 9 : 3 : 3 : 1 ratio is for one trait. It is for two traits; one trait gives 3 : 1.
Two traits → 4 gametes → 16 boxes → 9 : 3 : 3 : 1; 6 of 16 are new combinations. Memory clue : "9-3-3-1 = both-top, one-new, other-new, both-low".
Spot the difference
| Point | One-trait cross | Two-trait cross |
|---|---|---|
| F2 ratio | 3 : 1 | 9 : 3 : 3 : 1 |
| Gamete types from F1 | 2 (T, t) | 4 (TV, Tv, tV, tv) |
Practice questions
- 1What does a two-trait cross show?
- 2Name the four gametes of TtVv.
- 3How many boxes are in the Punnett square?
- 4Write the F2 ratio of a two-trait cross.
- 5How many of 16 show new combinations?
- 6In 320 F2 plants, how many are expected to show both recessive traits?
- 7Why would no new combinations appear if all genes were linked?
- 8Why are independently inherited traits useful to a species?
Show answers (try them yourself first)
- 1Whether two traits are inherited independently.
- 2TV, Tv, tV, tv.
- 316.
- 49 : 3 : 3 : 1.
- 56 (3 + 3).
- 620 (one sixteenth).
- 7Traits would travel together and only parental forms would appear.
- 8New combinations add variation.
Genes, chromosomes & equal contribution of parents
A recipe card tells a cook how to make a dish; a gene tells a cell how to make a protein. A child gets one card of each pair from each parent.
A gene is a section of DNA that gives the information for one protein. Proteins (enzymes, hormones) produce the trait. DNA is packed in separate pieces called chromosomes. Body cells have pairs of chromosomes (one from each parent); each germ cell has just one of each pair, so fusion restores the full number.
Equation / formula : Gene → protein (enzyme) → hormone / pigment → trait · 23 + 23 = 46 chromosomes
In the figure : A change in a gene changes the protein, and so can change the trait.
Genes, chromosomes & equal contribution of parents
In the app you can move this figure with your finger.
Open in the appRule 1 : How a gene makes a trait — A gene tells the cell how to make a protein, such as an enzyme. If the enzyme is efficient, plenty of hormone or pigment is made; if the gene is altered and the enzyme works poorly, less is made. This is why one working copy (T) can be enough to make a plant tall.
- Easy : If the enzyme for red pigment does not work, the flower is pale.
- Medium : Gene (small part of DNA) is smaller than a chromosome, which carries many genes.
- For exams : Both parents contribute equally: each gives one copy of every gene, so a child has two versions of each trait.
Rule 2 : Chromosome numbers stay steady — A human body cell has 23 pairs, 46 chromosomes. A germ cell has only 23 (one of each pair, maternal or paternal by chance). A sperm (23) and an egg (23) fuse to make a zygote with 46. If germ cells had 46, the number would double each generation.
- Easy : Sperm 23 + egg 23 = zygote 46.
- Medium : If a zygote has 30 chromosomes, a sperm of that species has 15.
- For exams : Halving in germ cells (by meiosis) and fusion at fertilisation keep the DNA of the species stable.
Asexually reproducing organisms also follow inheritance rules, because DNA is copied to the offspring.
Saying a germ cell has the full 46 chromosomes. It has 23 (half).
Gene → protein → trait. Chromosomes: 23 + 23 = 46. Memory clue : "GPT": Gene, Protein, Trait.
Spot the difference
| Point | Gene | Chromosome |
|---|---|---|
| What it is | part of DNA for one protein | piece of DNA carrying many genes |
| Number in a human cell | thousands | 46 (23 pairs) |
Practice questions
- 1What is a gene?
- 2How does a gene control a trait?
- 3What is a chromosome?
- 4How many chromosomes are in a human egg and in a zygote?
- 5How do both parents contribute equally?
- 6Why would the number double if germ cells had 46?
- 7Why is one T enough to make a plant tall?
- 8A body cell has 20 chromosomes. How many are in its germ cell?
Show answers (try them yourself first)
- 1A section of DNA that gives information for one protein.
- 2By controlling the proteins (enzymes, hormones) that produce the trait.
- 3A separate piece of DNA carrying many genes.
- 423 and 46.
- 5Each gives one copy of every gene (one chromosome of each pair).
- 6Fusion would add two full sets, giving 92.
- 7One working copy makes enough enzyme and hormone.
- 810.
Sex determination in human beings
A coin toss decides heads or tails with equal chance. The father's sperm is like that toss: it carries either X or Y with equal chance.
In humans, 22 pairs of chromosomes are ordinary and one pair is the sex chromosomes. Women are XX; men are XY (Y is shorter). Every egg carries X. A sperm carries X or Y. So the father's sperm decides the child's sex.
Equation / formula : Egg (X) + sperm X → girl (XX) · Egg (X) + sperm Y → boy (XY) · chance 1 : 1
In the figure : The mother always gives X, so the father decides: X gives a girl, Y gives a boy.
Sex determination in human beings
In the app you can move this figure with your finger.
Open in the appRule 1 : The XX / XY rule — A woman has the matched pair XX, so each egg gets one X. A man has the mismatched pair XY, so half his sperm carry X and half carry Y. A girl gets X from both parents; a boy gets X from his mother and Y from his father.
- Easy : A boy cannot get his Y from his mother, because women have no Y.
- Medium : A couple with three daughters has a 1/2 chance of a boy next time; each birth is independent.
- For exams : In 1000 births, about 500 boys and 500 girls are expected.
Rule 2 : Other ways sex is decided — Sex is not decided the same way in all animals. In some reptiles the temperature at which eggs are kept decides it. In snails an individual can change sex during life, so sex is not fixed by genes. In humans it is mainly genetic: a sperm with X gives a girl, with Y a boy, fixed at fertilisation.
- Easy : Hotter or cooler egg temperature can decide male or female in some lizards.
- Medium : The sex of a human baby is fixed at fertilisation, not at birth.
- For exams : A woman is not responsible for the sex of her child: the father's sperm (X or Y) decides.
Sperm cells carrying X and Y are made in nearly equal numbers.
Blaming the mother for the child's sex. Her eggs all carry X; the father's sperm decides.
Mother XX gives X. Father XY gives X (girl) or Y (boy). Chance 1 : 1. Memory clue : "Dad Decides": Dad's X = girl, Dad's Y = boy.
Spot the difference
| Point | Woman | Man |
|---|---|---|
| Sex chromosomes | XX (matched) | XY (mismatched) |
| Germ cells | all eggs carry X | sperm carry X or Y |
Practice questions
- 1Name the sex chromosomes of a woman and of a man.
- 2Which sex chromosome does every egg carry?
- 3Who decides the sex of the child and how?
- 4What is the chance of a boy in each birth?
- 5A couple has four daughters. What is the chance the fifth child is a boy?
- 6Give an example where temperature decides sex.
- 7How many chromosomes (including sex chromosomes) does a human sperm have?
- 8Why is it wrong to blame a mother for having only daughters?
Show answers (try them yourself first)
- 1Woman XX; man XY.
- 2X.
- 3The father: his sperm carries X (girl) or Y (boy).
- 41/2 (one in two).
- 51/2; each birth is independent.
- 6Some reptiles (egg temperature).
- 723, with either X or Y.
- 8Her eggs always carry X; the sperm decides.