Inheritance: Genes, Alleles, and Monohybrid Crosses
Inheritance Unlocked: Crack the Genetic Code 🧬
Introduction
1. Introduction
Okay, let's talk inheritance. Ever wonder why you've got your mom's eyes or your dad's height, but you're still 100% your own person? That's genetics doing its thing. Every feature you've got traces back to a molecule called DNA, tucked away inside almost every cell in your body, and this chapter is basically the instruction manual for how that information gets passed down and mixed up between generations. 🧬
We're going to break down the vocabulary you actually need (gene, allele, genotype, phenotype, the whole squad), and then the one diagram that shows up on basically every genetics question, the genetic diagram with a Punnett square. Once you've got the diagram on lock, working out ratios and probabilities is honestly just following a recipe. By the end of this you'll be reading a family tree like it's a group chat drama, working out who's a carrier and who isn't. Let's get into it. 🔬
We're going to break down the vocabulary you actually need (gene, allele, genotype, phenotype, the whole squad), and then the one diagram that shows up on basically every genetics question, the genetic diagram with a Punnett square. Once you've got the diagram on lock, working out ratios and probabilities is honestly just following a recipe. By the end of this you'll be reading a family tree like it's a group chat drama, working out who's a carrier and who isn't. Let's get into it. 🔬
2. Chromosomes, Genes and DNA
Inside the nucleus of almost every cell, your genetic instructions are packed into chromosomes, and each chromosome is basically one very long molecule of DNA. Here's the size order, smallest to biggest concept: a gene is a length of DNA that codes for one specific protein, DNA is the molecule genes are made of, and a chromosome is made of DNA. So: chromosomes are made of DNA, and DNA contains genes.
Think of it like a giant cookbook (the nucleus) full of recipe books (chromosomes), where each recipe book is made of paper (DNA), and each individual recipe (gene) tells the cell how to build one specific dish (a protein).
Think of it like a giant cookbook (the nucleus) full of recipe books (chromosomes), where each recipe book is made of paper (DNA), and each individual recipe (gene) tells the cell how to build one specific dish (a protein).
Key idea🔑 Key idea: Chromosomes are made of DNA. DNA contains genes. A gene codes for one protein.
Worked example
Worked Example: What Does a Gene Actually Do?
Worked Example: The Protein Factory 🏭
A gene is a length of DNA. What does a gene code for?
- 1Go back to the definition: a gene is a length of DNA that codes for a protein.
- 2So whatever the question is really asking, "what does a gene code for", the answer is always the same word.
- 3The answer is a protein. Not "a characteristic", not "an allele", the exact word examiners want is protein.
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3. Alleles
An allele is an alternative form of a gene. Same gene, different version. Like how there's one gene for pea height, but a "tall" allele and a "short" allele of it. You (and every organism) only ever carry two alleles of any one gene at a time, one from each parent, even if more versions exist out there in the wider population.
Quick way to keep gene and allele straight: the gene is the topic ("height"), the allele is the specific answer ("tall" or "short").
Quick way to keep gene and allele straight: the gene is the topic ("height"), the allele is the specific answer ("tall" or "short").
Key idea🔑 Key idea: Allele = an alternative form of a gene. One gene, (at least) two possible alleles.
Worked example
Worked Example: Gene or Allele?
Worked Example: Spot the Difference 🔍
A gene controls whether a pea plant is tall or short. One student says a gene and an allele are exactly the same thing. Another says an allele is one of the alternative versions a gene can take. Who's right?
- 1The gene is the length of DNA that controls the feature, here, plant height.
- 2"Tall" and "short" aren't two different genes, they're two different versions of the one height gene.
- 3So the second student is right: an allele is an alternative form of a gene. There's one gene, but (at least) two alleles of it.
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4. Sex Determination
Biological sex in humans comes down to one special pair of chromosomes. Females are XX. Males are XY. Every egg a woman makes carries an X (that's all she's got to give). Every sperm a man makes carries either an X or a Y, roughly 50/50. So it's actually the sperm that decides the sex of the baby: X-sperm + X-egg = XX = girl, Y-sperm + X-egg = XY = boy.
Because there's roughly equal numbers of X-carrying and Y-carrying sperm swimming around, and either one has an equal shot at fertilising the egg, the expected ratio of girls to boys born is close to 1:1.
Because there's roughly equal numbers of X-carrying and Y-carrying sperm swimming around, and either one has an equal shot at fertilising the egg, the expected ratio of girls to boys born is close to 1:1.
Key idea🔑 Key idea: Egg always gives X. Sperm gives X or Y. The sperm decides the baby's sex, giving roughly a 1:1 girl:boy ratio.
Worked example
Worked Example: Who Decides the Baby's Sex?
Worked Example: X Marks the Spot 📍
A sperm carrying a Y chromosome fertilises an egg. What sex will the baby be?
- 1Every egg carries an X chromosome, no exceptions.
- 2This particular sperm is carrying a Y chromosome.
- 3Combine them: X (from egg) + Y (from sperm) = XY, which develops as a boy.
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5. Inheritance: Passing It On
Inheritance is the transmission of genetic information from generation to generation. That's the exact phrase examiners want if you're asked to define it. In practice, it means every offspring gets a mixture of alleles from both parents, which is why you resemble your parents but aren't a carbon copy of either one.
This one definition is basically the umbrella for the rest of the chapter: everything from here on, genotype, phenotype, dominant, recessive, is really just the toolkit for predicting which alleles get passed on and what the offspring end up looking like.
This one definition is basically the umbrella for the rest of the chapter: everything from here on, genotype, phenotype, dominant, recessive, is really just the toolkit for predicting which alleles get passed on and what the offspring end up looking like.
Key idea🔑 Key idea: Inheritance = the transmission of genetic information from generation to generation.
Worked example
Worked Example: Defining Inheritance
Worked Example: Say It Exactly 🎯
What is meant by the term inheritance?
- 1This is a straight-up definition question, examiners want the exact phrasing.
- 2Inheritance is about genetic information moving between generations.
- 3The answer: inheritance is the transmission of genetic information from generation to generation.
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6. Genotype and Phenotype
Two words you need cold: genotype is the alleles an organism has, written as letters, like `Tt`. Phenotype is what the organism actually looks like, its observable characteristics, like "tall".
Here's the plot twist: `TT` and `Tt` can both give the exact same phenotype (tall), because `T` is dominant (more on that below). So just by looking at a tall plant, you genuinely can't always tell whether it's `TT` or `Tt`, phenotype alone doesn't always give away the genotype.
Here's the plot twist: `TT` and `Tt` can both give the exact same phenotype (tall), because `T` is dominant (more on that below). So just by looking at a tall plant, you genuinely can't always tell whether it's `TT` or `Tt`, phenotype alone doesn't always give away the genotype.
Key idea🔑 Key formula: Genotype = the letters. Phenotype = the appearance. Genotype question → answer in letters. Phenotype question → answer in words.
Worked example
Worked Example: Reading a Genotype
Worked Example: Bb Decoded 🕵️
An animal has the genotype `Bb`, where `B` (black spots) is dominant to `b` (yellow spots). What is its phenotype?
- 1`Bb` means the animal has one `B` allele and one `b` allele.
- 2`B` is dominant, so it's expressed even with just one copy present.
- 3The phenotype is black spots, because the dominant `B` allele wins.
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7. Homozygous, Heterozygous and Pure-Breeding
Homozygous = two identical alleles, like `TT` or `tt`. Heterozygous = two different alleles, like `Tt`. This matters for breeding: cross two homozygous individuals with the same genotype and they're pure-breeding, meaning every single offspring matches them, generation after generation. A heterozygous individual is never pure-breeding, because it can pass on either one of its two different alleles, so its offspring can turn out differently from it.
Classic example: a heterozygous purple-flowered plant (`Pp`) can't guarantee purple offspring, some seeds will inherit `p` from both sides and come out white.
Classic example: a heterozygous purple-flowered plant (`Pp`) can't guarantee purple offspring, some seeds will inherit `p` from both sides and come out white.
Key idea🔑 Key idea: Homozygous = same letters twice. Heterozygous = different letters. Only same-genotype homozygous parents are pure-breeding.
Worked example
Worked Example: Pure-Breeding or Not?
Worked Example: The Gardener's Gamble 🌱
A gardener wants every seed from a plant to grow purple flowers. The plant is `Pp`, where `P` (purple) is dominant to `p` (white). Can the gardener rely on this?
- 1`Pp` has two different alleles, so it's heterozygous.
- 2Heterozygous individuals are never pure-breeding, they can pass on either allele.
- 3So no, the gardener can't rely on it, some seeds will inherit `p` from both parents and grow up white, not purple.
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8. Dominant and Recessive Alleles
A dominant allele shows up in the phenotype whenever it's present, even with just one copy (heterozygous). A recessive allele only shows up when there are two copies (homozygous), it gets masked by a dominant allele if one's around. Capital letter = dominant, lower-case = recessive, same letter for both, like `H` (short hair) and `h` (long hair). So `Hh` looks short-haired; only `hh` looks long-haired.
Big exam pattern to memorise: if two normal-looking parents have a child with a condition, that condition has to be recessive, and both parents must secretly be carriers (`Dd`), because a dominant allele could never hide like that.
Big exam pattern to memorise: if two normal-looking parents have a child with a condition, that condition has to be recessive, and both parents must secretly be carriers (`Dd`), because a dominant allele could never hide like that.
Key idea🔑 Key idea: Dominant shows with one copy. Recessive needs two copies. Two unaffected parents + an affected child = recessive condition, both parents are carriers.
Worked example
Worked Example: Spotting the Carrier Parents
Worked Example: The Hidden Allele 🫥
A recessive allele `d` causes a condition, `D` (dominant) gives no condition. Two healthy parents have a child with the condition. What must the parents' genotypes be?
- 1The child has the condition, and it's caused by a recessive allele, so the child must be `dd`.
- 2Both parents look healthy, so neither can be `dd` themselves.
- 3But each parent had to pass on one `d`, so both parents must be `Dd`, healthy carriers.
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9. Genetic Diagrams and Punnett Squares
Every genetic diagram follows the exact same recipe: 1. parental phenotypes, 2. parental genotypes, 3. gametes each parent can make (circle them), 4. a Punnett square combining the gametes, 5. offspring genotype ratio, 6. offspring phenotype ratio. Nail this order and you can answer almost any monohybrid cross question.
Two crosses come up again and again. Cross two heterozygotes (`Tt × Tt`) and you always get a 3:1 phenotype ratio. Cross a heterozygote with a homozygous recessive (`Tt × tt`) and you always get a 1:1 ratio. And if you're given real offspring numbers instead of a clean ratio (like "93 tall, 28 short"), simplify: divide the bigger number by the smaller, and see which textbook ratio it's closest to.
Two crosses come up again and again. Cross two heterozygotes (`Tt × Tt`) and you always get a 3:1 phenotype ratio. Cross a heterozygote with a homozygous recessive (`Tt × tt`) and you always get a 1:1 ratio. And if you're given real offspring numbers instead of a clean ratio (like "93 tall, 28 short"), simplify: divide the bigger number by the smaller, and see which textbook ratio it's closest to.
Key idea🔑 Key formula: Tt × Tt → 3:1 phenotype ratio. Tt × tt → 1:1 phenotype ratio. Probability = (boxes with that phenotype) ÷ (total boxes).
Worked example
Worked Example: Cracking a Cystic Fibrosis Cross
Worked Example: The Carrier Couple 👨👩👧
Cystic fibrosis is caused by a recessive allele `f`, dominant allele `F` gives no cystic fibrosis. Two parents who don't have cystic fibrosis are both `Ff`. What's the probability their first child has cystic fibrosis?
- 1Set up the cross: `Ff × Ff`. Each parent can make gametes `F` or `f`.
- 2Fill in the Punnett square: `1 FF : 2 Ff : 1 ff`.
- 3Only the `ff` box has the condition, that's 1 box out of 4, so the probability is , which is 25%.
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10. Pedigree Diagrams
A pedigree diagram is a family tree that tracks a characteristic across generations, using a symbol key: square = male, circle = female, shaded = affected, unshaded = unaffected. A horizontal line joins a couple; vertical lines connect down to their kids.
Good news: you already have all the genetics you need to read one. Two unaffected parents with an affected child always means the condition is recessive and both parents are carriers, exact same logic as any other cross. A condition popping up in every generation, always with an affected parent involved, points instead towards a dominant allele.
Good news: you already have all the genetics you need to read one. Two unaffected parents with an affected child always means the condition is recessive and both parents are carriers, exact same logic as any other cross. A condition popping up in every generation, always with an affected parent involved, points instead towards a dominant allele.
Key idea🔑 Key idea: Two unaffected parents + an affected child on a pedigree = recessive condition, both parents are carriers.
Worked example
Worked Example: Solving a Family Pedigree
Worked Example: Whodunnit, Genetically 🕵️♀️
A pedigree shows two unaffected parents with one child affected by albinism (`a` recessive, `A` dominant). What does this tell you about the parents' genotypes?
- 1Both parents are unshaded (unaffected), but their child is shaded (affected).
- 2A dominant allele could never "hide" in two unaffected parents like this, only a recessive allele can.
- 3So the affected child must be `aa`, meaning both parents must be `Aa` carriers, even though neither one shows albinism themselves.
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