The Father Decides Whether a Child Is a Boy or a Girl
Follow how variation builds up across generations, read Mendel's F1 and F2 results for a single trait, work a Punnett square for two independent traits, and see why the sex of a child is decided by the father's sperm.
How can a trait disappear in one generation and come back in the next?
Cross a tall pea plant with a short one and every plant in the next generation is tall. The short trait has apparently vanished.
Now let that generation breed among itself. Short plants reappear — about one in every four.
A trait that reappears cannot have been destroyed. It must have been carried, unexpressed, by plants that looked tall. And that single observation forces a conclusion about how inheritance works:
- Each plant carries two copies of the information for height, one from each parent
- A plant can carry one copy for tall and one for short and still look tall — so one copy can mask the other
- When that plant makes gametes, each gamete gets only one of its two copies
Those three statements are the whole basis of heredity, and they were worked out by breeding pea plants and counting the offspring — long before anyone could see a chromosome.
The copy that shows when both are present is called dominant; the one that is masked is recessive. The recessive trait is not weaker or rarer — it is simply the one that does not appear when its partner is present.
This page covers the CBSE Class 10 Science chapter on heredity: how variation accumulates, dominant and recessive traits with Mendel's results, the Punnett square for two traits, and sex determination in human beings.
Now let that generation breed among itself. Short plants reappear — about one in every four.
A trait that reappears cannot have been destroyed. It must have been carried, unexpressed, by plants that looked tall. And that single observation forces a conclusion about how inheritance works:
- Each plant carries two copies of the information for height, one from each parent
- A plant can carry one copy for tall and one for short and still look tall — so one copy can mask the other
- When that plant makes gametes, each gamete gets only one of its two copies
Those three statements are the whole basis of heredity, and they were worked out by breeding pea plants and counting the offspring — long before anyone could see a chromosome.
The copy that shows when both are present is called dominant; the one that is masked is recessive. The recessive trait is not weaker or rarer — it is simply the one that does not appear when its partner is present.
This page covers the CBSE Class 10 Science chapter on heredity: how variation accumulates, dominant and recessive traits with Mendel's results, the Punnett square for two traits, and sex determination in human beings.
How does variation accumulate over generations?
Each generation inherits the variations already present and adds a few new ones of its own, so the differences build up steadily.
Start with one organism reproducing asexually. Its offspring are near-copies, differing only by copying errors. Their offspring differ from them by fresh copying errors — but they also still carry the earlier differences. So the differences accumulate rather than resetting each generation.
Sexual reproduction speeds this up enormously. Each offspring is a new combination of two parents' DNA, so the variation is not only inherited and added to, it is also reshuffled every generation. Far more different individuals therefore exist in a sexually reproducing population than in an asexual one of the same size.
But not every variation survives, and that is the second half of the story. Consider a population of beetles living on green bushes:
- Most are red, and crows eat them easily against the green leaves
- A few green beetles appear by variation
- Crows find the green ones much harder to see, so more of them survive to reproduce
- After many generations, most of the population is green
Nothing turned red beetles green. The green variation already existed; the crows simply removed more of the red ones. Selection sorts variation that is already there — the same point the reproduction chapter made about bacteria in warming water, now with a visible example.
The environment decides which variations count. If the bushes were red-leaved, the advantage would be reversed, and if there were no crows the colour would not matter at all. A variation is not good or bad in itself; it is good or bad in a particular situation.
And one case shows how little the organism controls. Suppose a disease kills most of the beetles regardless of colour. The few survivors are whichever happened to be elsewhere, not whichever were best suited. Variation can be lost by accident as well as by selection, which is why a small population is more fragile than a large one however well adapted it is.
The link to the rest of the chapter. If traits are inherited in copies that can be masked and reshuffled, then the pattern of inheritance should be predictable by counting. That is exactly what the pea-plant experiments did, and the next two sections are the counting.
Start with one organism reproducing asexually. Its offspring are near-copies, differing only by copying errors. Their offspring differ from them by fresh copying errors — but they also still carry the earlier differences. So the differences accumulate rather than resetting each generation.
Sexual reproduction speeds this up enormously. Each offspring is a new combination of two parents' DNA, so the variation is not only inherited and added to, it is also reshuffled every generation. Far more different individuals therefore exist in a sexually reproducing population than in an asexual one of the same size.
But not every variation survives, and that is the second half of the story. Consider a population of beetles living on green bushes:
- Most are red, and crows eat them easily against the green leaves
- A few green beetles appear by variation
- Crows find the green ones much harder to see, so more of them survive to reproduce
- After many generations, most of the population is green
Nothing turned red beetles green. The green variation already existed; the crows simply removed more of the red ones. Selection sorts variation that is already there — the same point the reproduction chapter made about bacteria in warming water, now with a visible example.
The environment decides which variations count. If the bushes were red-leaved, the advantage would be reversed, and if there were no crows the colour would not matter at all. A variation is not good or bad in itself; it is good or bad in a particular situation.
And one case shows how little the organism controls. Suppose a disease kills most of the beetles regardless of colour. The few survivors are whichever happened to be elsewhere, not whichever were best suited. Variation can be lost by accident as well as by selection, which is why a small population is more fragile than a large one however well adapted it is.
The link to the rest of the chapter. If traits are inherited in copies that can be masked and reshuffled, then the pattern of inheritance should be predictable by counting. That is exactly what the pea-plant experiments did, and the next two sections are the counting.
What do Mendel's F1 and F2 results actually show?
That each trait is controlled by two copies, that one can be dominant over the other, and that the masked copy is passed on unchanged.
The monohybrid cross — one trait at a time. Take a pure tall pea plant and a pure short one, and write the copies each carries:
- The tall plant carries TT
- The short plant carries tt
The first generation, called F1. Each parent contributes one copy, so every offspring is Tt — and every one of them is tall.
So tall is dominant and short is recessive. The short copy is present in every F1 plant and shows in none of them.
The second generation, called F2. Allow the F1 plants to self-pollinate. Each Tt parent can give a T or a t, so four combinations are possible in equal numbers:
- TT — tall
- Tt — tall
- Tt — tall
- tt — short
Three things follow immediately.
- The short trait was not lost in F1 — it reappears in a quarter of F2, so it must have been carried silently
- The two copies separate when gametes are made, since a Tt plant produces both T and t gametes
- Appearance does not reveal the copies. A tall F2 plant may be TT or Tt, and you cannot tell by looking
That last point needs two words to describe it. The phenotype is what you can see — tall or short. The genotype is the pair of copies carried — TT, Tt or tt. Two plants with the same phenotype can have different genotypes, and a question asking for one will not accept the other.
The ratios are the answer to a specific question. Phenotypic ratio means count what you can see: . Genotypic ratio means count the combinations: . Giving one when the other was asked for is the commonest error in the whole chapter, and reading the question word by word prevents it.
How to tell a TT from a Tt without a microscope. Cross it with a short plant. TT crossed with tt gives all tall offspring; Tt crossed with tt gives half tall and half short. The appearance of any short offspring proves the tall parent was carrying a recessive copy — a genuine test, and the reason pure-breeding lines have to be established before an experiment like this can be run at all.
The monohybrid cross — one trait at a time. Take a pure tall pea plant and a pure short one, and write the copies each carries:
- The tall plant carries TT
- The short plant carries tt
The first generation, called F1. Each parent contributes one copy, so every offspring is Tt — and every one of them is tall.
So tall is dominant and short is recessive. The short copy is present in every F1 plant and shows in none of them.
The second generation, called F2. Allow the F1 plants to self-pollinate. Each Tt parent can give a T or a t, so four combinations are possible in equal numbers:
- TT — tall
- Tt — tall
- Tt — tall
- tt — short
Three things follow immediately.
- The short trait was not lost in F1 — it reappears in a quarter of F2, so it must have been carried silently
- The two copies separate when gametes are made, since a Tt plant produces both T and t gametes
- Appearance does not reveal the copies. A tall F2 plant may be TT or Tt, and you cannot tell by looking
That last point needs two words to describe it. The phenotype is what you can see — tall or short. The genotype is the pair of copies carried — TT, Tt or tt. Two plants with the same phenotype can have different genotypes, and a question asking for one will not accept the other.
The ratios are the answer to a specific question. Phenotypic ratio means count what you can see: . Genotypic ratio means count the combinations: . Giving one when the other was asked for is the commonest error in the whole chapter, and reading the question word by word prevents it.
How to tell a TT from a Tt without a microscope. Cross it with a short plant. TT crossed with tt gives all tall offspring; Tt crossed with tt gives half tall and half short. The appearance of any short offspring proves the tall parent was carrying a recessive copy — a genuine test, and the reason pure-breeding lines have to be established before an experiment like this can be run at all.
How do you work a Punnett square for two traits at once?
List the gametes each parent can make, combine every gamete of one with every gamete of the other, and count the results.
The dihybrid cross — two traits together. Take pea plants differing in both seed shape and seed colour:
- One parent is round and yellow, carrying RRYY
- The other is wrinkled and green, carrying rryy
F1. Every offspring gets one copy of each from each parent, so all are RrYy — and all are round and yellow. So round is dominant over wrinkled, and yellow over green.
F2. Each RrYy plant can make four kinds of gamete, because the two traits are assorted independently:
Combining four kinds of gamete from one parent with four from the other gives boxes in the Punnett square. Counting the appearances:
- 9 round and yellow
- 3 round and green
- 3 wrinkled and yellow
- 1 wrinkled and green
The important result is in the middle two groups. Round-and-green and wrinkled-and-yellow plants are new combinations — neither parent looked like that. So the two traits were inherited independently of each other, and shape did not travel with colour.
Check the totals a second way. Count shape alone: round appears times and wrinkled times, which is . Count colour alone: yellow appears and green , again . Each trait on its own still gives the monohybrid ratio, which is exactly what independent inheritance means — and it is the quickest way to check a square you have just filled in.
Worked example — a simpler cross. What offspring does Tt crossed with tt give?
The Tt parent makes T and t gametes; the tt parent makes only t. So the combinations are Tt, Tt, tt, tt:
**A ratio is the signature of a cross with a recessive parent, and it is the test cross described in the previous section.
The systematic method, which is what the marks are for.
- Write both parents' genotypes
- List the gametes each can make — one copy of each trait per gamete
- Draw the square and fill every box
- Read off the phenotypes, then count
- State whether the ratio asked for is phenotypic or genotypic
The error to avoid. A gamete carries one copy of each gene, never two. Writing RY and rY as gametes is right; writing RrY is not, because that gamete would carry two shape copies. Count the letters in each gamete — one per trait — and the square cannot go wrong.**
The dihybrid cross — two traits together. Take pea plants differing in both seed shape and seed colour:
- One parent is round and yellow, carrying RRYY
- The other is wrinkled and green, carrying rryy
F1. Every offspring gets one copy of each from each parent, so all are RrYy — and all are round and yellow. So round is dominant over wrinkled, and yellow over green.
F2. Each RrYy plant can make four kinds of gamete, because the two traits are assorted independently:
Combining four kinds of gamete from one parent with four from the other gives boxes in the Punnett square. Counting the appearances:
- 9 round and yellow
- 3 round and green
- 3 wrinkled and yellow
- 1 wrinkled and green
The important result is in the middle two groups. Round-and-green and wrinkled-and-yellow plants are new combinations — neither parent looked like that. So the two traits were inherited independently of each other, and shape did not travel with colour.
Check the totals a second way. Count shape alone: round appears times and wrinkled times, which is . Count colour alone: yellow appears and green , again . Each trait on its own still gives the monohybrid ratio, which is exactly what independent inheritance means — and it is the quickest way to check a square you have just filled in.
Worked example — a simpler cross. What offspring does Tt crossed with tt give?
The Tt parent makes T and t gametes; the tt parent makes only t. So the combinations are Tt, Tt, tt, tt:
**A ratio is the signature of a cross with a recessive parent, and it is the test cross described in the previous section.
The systematic method, which is what the marks are for.
- Write both parents' genotypes
- List the gametes each can make — one copy of each trait per gamete
- Draw the square and fill every box
- Read off the phenotypes, then count
- State whether the ratio asked for is phenotypic or genotypic
The error to avoid. A gamete carries one copy of each gene, never two. Writing RY and rY as gametes is right; writing RrY is not, because that gamete would carry two shape copies. Count the letters in each gamete — one per trait — and the square cannot go wrong.**
Why is the sex of a child decided by the father?
Because the mother can only contribute an X chromosome, while the father contributes either an X or a Y.
The chromosome arrangement. Human beings have 23 pairs of chromosomes. Twenty-two pairs are matched, and one pair is the sex chromosomes:
- A female has two of the same kind: XX
- A male has two different ones: XY
What each parent can pass on. A gamete carries one chromosome from each pair, so:
- Every egg from the mother carries an X — she has nothing else to give
- A sperm from the father carries either an X or a Y, in roughly equal numbers
The four possible combinations, from a Punnett square of XX crossed with XY:
- X from mother with X from father — XX, a girl
- X from mother with X from father — XX, a girl
- X from mother with Y from father — XY, a boy
- X from mother with Y from father — XY, a boy
So the sex of the child is decided entirely by which kind of sperm happens to fertilise the egg, and the mother's contribution is the same whatever the outcome. She cannot influence it, because she has no Y chromosome to give.
That has a consequence far outside the laboratory. A widespread belief blames the mother when a couple has daughters and no sons. The biology says plainly that this is impossible, because the deciding chromosome comes from the father — and the roughly equal numbers of X-carrying and Y-carrying sperm mean that neither parent can choose. Understanding one Punnett square is enough to see that the belief is simply wrong.
Why the ratio is about equal in the population. Since half the sperm carry X and half carry Y, boys and girls are born in roughly equal numbers wherever the natural process is left alone. A large departure from that balance in any region is therefore evidence of interference, which is why prenatal sex determination for that purpose is prohibited by law in India.
One boundary case to be precise about. The ratio is a probability, not a rule about individual families. A couple with four daughters has not beaten the odds any more than four heads in a row beats a coin — each conception is independent, and the previous children do not influence the next. That is the same probability logic as any Punnett square, and it is why the ratios in this chapter are always described as expected rather than guaranteed.
The chromosome arrangement. Human beings have 23 pairs of chromosomes. Twenty-two pairs are matched, and one pair is the sex chromosomes:
- A female has two of the same kind: XX
- A male has two different ones: XY
What each parent can pass on. A gamete carries one chromosome from each pair, so:
- Every egg from the mother carries an X — she has nothing else to give
- A sperm from the father carries either an X or a Y, in roughly equal numbers
The four possible combinations, from a Punnett square of XX crossed with XY:
- X from mother with X from father — XX, a girl
- X from mother with X from father — XX, a girl
- X from mother with Y from father — XY, a boy
- X from mother with Y from father — XY, a boy
So the sex of the child is decided entirely by which kind of sperm happens to fertilise the egg, and the mother's contribution is the same whatever the outcome. She cannot influence it, because she has no Y chromosome to give.
That has a consequence far outside the laboratory. A widespread belief blames the mother when a couple has daughters and no sons. The biology says plainly that this is impossible, because the deciding chromosome comes from the father — and the roughly equal numbers of X-carrying and Y-carrying sperm mean that neither parent can choose. Understanding one Punnett square is enough to see that the belief is simply wrong.
Why the ratio is about equal in the population. Since half the sperm carry X and half carry Y, boys and girls are born in roughly equal numbers wherever the natural process is left alone. A large departure from that balance in any region is therefore evidence of interference, which is why prenatal sex determination for that purpose is prohibited by law in India.
One boundary case to be precise about. The ratio is a probability, not a rule about individual families. A couple with four daughters has not beaten the odds any more than four heads in a row beats a coin — each conception is independent, and the previous children do not influence the next. That is the same probability logic as any Punnett square, and it is why the ratios in this chapter are always described as expected rather than guaranteed.
Exam tip
What layout keeps a genetics answer complete?
Write the genotypes, list the gametes, draw the square, then answer the question that was actually asked. Genetics questions are marked on the working as much as the ratio.
- Use capital letters for dominant and the same letter in small for recessive — T and t, R and r. Using two different letters for one trait is a serious error
- Write both parents' genotypes before anything else, and state which trait is dominant
- List the gametes separately. A gamete has one copy of each gene — RY, Ry, rY, ry for RrYy
- Draw the Punnett square and fill every box. An answer with a stated ratio and no square earns much less
- **Read the question for phenotypic or genotypic.** Monohybrid F2 is phenotypic and genotypic
- Check a dihybrid square by counting each trait alone — both should come to
- For sex determination, say that the mother can only give X, and that the father gives X or Y
- **Call the ratio expected, since it is a probability over many offspring
The distinction to state carefully. Phenotype is the appearance; genotype** is the pair of copies. A tall F2 plant may be TT or Tt, so tall does not identify the genotype, and only a test cross with a recessive parent can. A question asking how you would find out whether a tall plant is TT or Tt is asking for that cross, and the answer is that any short offspring proves it was Tt.
- Use capital letters for dominant and the same letter in small for recessive — T and t, R and r. Using two different letters for one trait is a serious error
- Write both parents' genotypes before anything else, and state which trait is dominant
- List the gametes separately. A gamete has one copy of each gene — RY, Ry, rY, ry for RrYy
- Draw the Punnett square and fill every box. An answer with a stated ratio and no square earns much less
- **Read the question for phenotypic or genotypic.** Monohybrid F2 is phenotypic and genotypic
- Check a dihybrid square by counting each trait alone — both should come to
- For sex determination, say that the mother can only give X, and that the father gives X or Y
- **Call the ratio expected, since it is a probability over many offspring
The distinction to state carefully. Phenotype is the appearance; genotype** is the pair of copies. A tall F2 plant may be TT or Tt, so tall does not identify the genotype, and only a test cross with a recessive parent can. A question asking how you would find out whether a tall plant is TT or Tt is asking for that cross, and the answer is that any short offspring proves it was Tt.
Did you know
How can a trait skip a generation entirely?
Two parents who both show a trait can have a child who does not, and a trait absent in both parents can appear in their child. Neither is a contradiction — both follow directly from one Punnett square.
Take two plants, both tall, but both carrying a hidden recessive copy — both Tt. Their offspring:
- TT — tall
- Tt — tall
- Tt — tall
- tt — short
A quarter of the offspring are short, from two tall parents. The short copy was present in both parents all along, masked in each of them, and it only shows when a child receives it from both.
So the trait did not appear out of nowhere and it did not skip a generation in the sense of disappearing. It was carried, silently, by individuals who did not show it — and the word for such an individual is a carrier.
That is why family resemblances are so unpredictable. A feature may be visible in a grandparent, absent in a parent and visible again in a child, simply because the parent was a carrier. Nothing was lost and nothing was created; the copies were reshuffled.
And it explains why the recessive trait is not rare. Short pea plants are not unusual in a population, because a great many tall-looking plants are carrying the short copy without showing it. A recessive trait can be common in the copies and uncommon in appearance, and the two counts are quite different numbers.
One useful consequence for the exam. When a question says both parents show the dominant trait but one child shows the recessive, you can immediately write both parents as heterozygous — Tt and Tt — because that is the only combination that permits it. The appearance of a recessive child is a proof about the parents' genotypes, and questions are built on exactly that deduction.
The general point is worth keeping. What you inherit is the copies, not the appearance, and appearance is only the visible consequence of the pair you happen to have. Heredity is about information being passed on, some of it hidden — which is the same idea the chapter opened with.
Take two plants, both tall, but both carrying a hidden recessive copy — both Tt. Their offspring:
- TT — tall
- Tt — tall
- Tt — tall
- tt — short
A quarter of the offspring are short, from two tall parents. The short copy was present in both parents all along, masked in each of them, and it only shows when a child receives it from both.
So the trait did not appear out of nowhere and it did not skip a generation in the sense of disappearing. It was carried, silently, by individuals who did not show it — and the word for such an individual is a carrier.
That is why family resemblances are so unpredictable. A feature may be visible in a grandparent, absent in a parent and visible again in a child, simply because the parent was a carrier. Nothing was lost and nothing was created; the copies were reshuffled.
And it explains why the recessive trait is not rare. Short pea plants are not unusual in a population, because a great many tall-looking plants are carrying the short copy without showing it. A recessive trait can be common in the copies and uncommon in appearance, and the two counts are quite different numbers.
One useful consequence for the exam. When a question says both parents show the dominant trait but one child shows the recessive, you can immediately write both parents as heterozygous — Tt and Tt — because that is the only combination that permits it. The appearance of a recessive child is a proof about the parents' genotypes, and questions are built on exactly that deduction.
The general point is worth keeping. What you inherit is the copies, not the appearance, and appearance is only the visible consequence of the pair you happen to have. Heredity is about information being passed on, some of it hidden — which is the same idea the chapter opened with.
Exam relevance
Why does NEET keep returning to Mendel's ratios?
This is foundation work for one of the largest and most scoring Class 12 Biology chapters, and the ratios are examined directly.
Where the monohybrid and dihybrid crosses lead. Class 12 Principles of Inheritance and Variation rebuilds them as the Law of Dominance, the Law of Segregation and the Law of Independent Assortment, and then adds what happens when those laws break down — incomplete dominance, codominance, multiple alleles and linkage. **The and ratios are the baseline against which every exception is described, so knowing where they come from is more useful than remembering them.
Where the Punnett square leads. In Class 12 it is used for blood groups, for sex-linked inheritance and for pedigree analysis, and NEET sets questions in which you must work a cross and read off a probability. The gamete-listing step is where most marks are lost, and the discipline of one copy per gamete is exactly what carries forward.
Where sex determination leads. Class 12 covers the XX-XY system alongside the XX-XO and ZZ-ZW systems of other organisms, and then sex-linked disorders such as haemophilia and colour blindness, whose inheritance follows from the mother having two X chromosomes and the father one. The Class 10 fact that the father contributes the deciding chromosome is the key to every one of those pedigrees.
Where variation leads. Class 12 Evolution uses the accumulation of variation and its sorting by selection as the core mechanism, with the beetle example replaced by named cases. The distinction between variation arising and variation being selected is examined as an assertion-reason item.
Question types to expect. At this level: state the ratio, work a cross, explain why the father determines sex. In competitive papers: probability questions from a cross, pedigree interpretation, and assertion-reason items on dominance.
The single trap that costs marks. Giving the genotypic ratio when the phenotypic was asked for, or the reverse. and describe the same F2 generation, and the question decides which is wanted. In NEET the wrong one is always among the options.
A second trap. Writing a gamete with two copies of one gene. A gamete carries one copy of each gene, so RrYy makes RY, Ry, rY and ry — four kinds, not two and not one. Getting this wrong changes the number of boxes and therefore every ratio in the answer.
Board versus competitive emphasis. The CBSE paper marks the genotypes, the gametes, the filled square and the stated ratio; a competitive paper marks a probability or a single genotype. The transferable asset is the method** — genotypes, gametes, square, count — because it answers every cross you will ever be given.
Where the monohybrid and dihybrid crosses lead. Class 12 Principles of Inheritance and Variation rebuilds them as the Law of Dominance, the Law of Segregation and the Law of Independent Assortment, and then adds what happens when those laws break down — incomplete dominance, codominance, multiple alleles and linkage. **The and ratios are the baseline against which every exception is described, so knowing where they come from is more useful than remembering them.
Where the Punnett square leads. In Class 12 it is used for blood groups, for sex-linked inheritance and for pedigree analysis, and NEET sets questions in which you must work a cross and read off a probability. The gamete-listing step is where most marks are lost, and the discipline of one copy per gamete is exactly what carries forward.
Where sex determination leads. Class 12 covers the XX-XY system alongside the XX-XO and ZZ-ZW systems of other organisms, and then sex-linked disorders such as haemophilia and colour blindness, whose inheritance follows from the mother having two X chromosomes and the father one. The Class 10 fact that the father contributes the deciding chromosome is the key to every one of those pedigrees.
Where variation leads. Class 12 Evolution uses the accumulation of variation and its sorting by selection as the core mechanism, with the beetle example replaced by named cases. The distinction between variation arising and variation being selected is examined as an assertion-reason item.
Question types to expect. At this level: state the ratio, work a cross, explain why the father determines sex. In competitive papers: probability questions from a cross, pedigree interpretation, and assertion-reason items on dominance.
The single trap that costs marks. Giving the genotypic ratio when the phenotypic was asked for, or the reverse. and describe the same F2 generation, and the question decides which is wanted. In NEET the wrong one is always among the options.
A second trap. Writing a gamete with two copies of one gene. A gamete carries one copy of each gene, so RrYy makes RY, Ry, rY and ry — four kinds, not two and not one. Getting this wrong changes the number of boxes and therefore every ratio in the answer.
Board versus competitive emphasis. The CBSE paper marks the genotypes, the gametes, the filled square and the stated ratio; a competitive paper marks a probability or a single genotype. The transferable asset is the method** — genotypes, gametes, square, count — because it answers every cross you will ever be given.
Key takeaways
What should you know about heredity?
Two copies per trait, two ratios, one square and one chromosome pair.
- Variation accumulates because each generation inherits earlier variations and adds new ones; sexual reproduction also reshuffles them
- Selection sorts existing variation — the green beetles were already there before the crows favoured them
- Each trait is controlled by two copies, one from each parent, and a gamete carries one of them
- Dominant shows when present; recessive is masked but passed on unchanged
- Monohybrid F1: TT crossed with tt gives all Tt, all tall. F2: phenotypic and genotypic
- Phenotype is the appearance; genotype is the pair of copies — a tall plant may be TT or Tt
- A test cross with a recessive parent reveals a hidden copy: any recessive offspring proves the parent was heterozygous
- Dihybrid F2: , with new combinations appearing — so the two traits are inherited independently
- Check a dihybrid square by counting each trait alone; both should give
- Humans have 23 pairs of chromosomes, one pair being the sex chromosomes: XX female, XY male
- Every egg carries X; sperm carry X or Y — so the father's sperm decides the sex, in an expected ratio
- Two carriers can have a child showing the recessive trait, which is how a trait appears to skip a generation
The sharpest self-test is the dihybrid square. Write out the four gametes of RrYy, fill all sixteen boxes, and check that the shape alone and the colour alone each come to .
- Variation accumulates because each generation inherits earlier variations and adds new ones; sexual reproduction also reshuffles them
- Selection sorts existing variation — the green beetles were already there before the crows favoured them
- Each trait is controlled by two copies, one from each parent, and a gamete carries one of them
- Dominant shows when present; recessive is masked but passed on unchanged
- Monohybrid F1: TT crossed with tt gives all Tt, all tall. F2: phenotypic and genotypic
- Phenotype is the appearance; genotype is the pair of copies — a tall plant may be TT or Tt
- A test cross with a recessive parent reveals a hidden copy: any recessive offspring proves the parent was heterozygous
- Dihybrid F2: , with new combinations appearing — so the two traits are inherited independently
- Check a dihybrid square by counting each trait alone; both should give
- Humans have 23 pairs of chromosomes, one pair being the sex chromosomes: XX female, XY male
- Every egg carries X; sperm carry X or Y — so the father's sperm decides the sex, in an expected ratio
- Two carriers can have a child showing the recessive trait, which is how a trait appears to skip a generation
The sharpest self-test is the dihybrid square. Write out the four gametes of RrYy, fill all sixteen boxes, and check that the shape alone and the colour alone each come to .