Mendelian Genetics Basics

You'll need to nail the key terms first - they're the building blocks for everything else in genetics. A gene is basically a section of DNA that controls a specific trait, like eye colour. Alleles are different versions of the same gene (think blue eyes vs brown eyes).

Your genotype is the actual genetic code you carry (written as letters like TT or Tt), whilst your phenotype is what you actually look like. Dominant alleles (capital letters) always show up in your appearance, even if you only have one copy. Recessive alleles (lowercase letters) only appear when you have two copies.

Homozygous means having two identical alleles (like TT or tt), whilst heterozygous means having two different ones (like Tt). The generations have special names too: P generation (parents), F1 (first offspring), and F2 (second generation offspring).

Quick Tip: Think of alleles like different flavours of the same ice cream - same product, different varieties!

Mendel's Laws and Monohybrid Crosses

Mendel's first law (segregation) tells us that trait-controlling factors separate during gamete formation, so each parent only passes on one allele for each trait. This happens during meiosis when chromosomes get pulled apart.

A monohybrid cross tracks just one characteristic at a time. You'll use Punnett squares to predict outcomes - they're like genetic calculators! The classic example is crossing two heterozygous parents (Tt × Tt).

Here's your step-by-step process: state parent phenotypes and genotypes, work out possible gametes, draw your Punnett square, fill it in, then state the offspring ratios. The magic ratio you'll see constantly is 3:1 for phenotypes (3 dominant : 1 recessive) and 1:2:1 for genotypes.

Exam Gold: That 3:1 ratio from Tt × Tt crosses appears in loads of questions - memorise it!

Independent Assortment and Dihybrid Crosses

Mendel's second law (independent assortment) shows that different genes get inherited independently - but only if they're on different chromosomes or far apart on the same one. This relates to how chromosomes line up randomly during meiosis.

Dihybrid crosses track two traits simultaneously, like seed shape and colour. The Punnett square becomes much bigger (16 squares instead of 4), but don't panic - the process is the same.

When you cross two parents who are heterozygous for both traits (like RrYy × RrYy), you get the famous 9:3:3:1 ratio. That's 9 with both dominant traits, 3 with each mixed combination, and 1 with both recessive traits.

Sex linkage happens when genes sit on sex chromosomes. Most sex-linked traits are X-linked, which means males are more likely to show recessive traits since they only have one X chromosome.

Memory Trick: For dihybrid crosses, remember "9-3-3-1" - it's the genetic equivalent of a phone number!

Working Through Genetic Problems

Let's tackle a monohybrid cross step by step. Say you're crossing a heterozygous tall plant (Tt) with a dwarf plant (tt). The tall plant produces T and t gametes, whilst the dwarf only produces t gametes.

Your Punnett square gives you 50% Tt (tall) and 50% tt (dwarf) offspring. Notice how this gives a 1:1 ratio instead of 3:1 - that's because one parent is homozygous recessive.

For sex-linked problems, always write alleles as superscripts on the X chromosome (like X^N for normal vision). Remember that the Y chromosome carries no allele for most X-linked traits.

The key is reading questions carefully - are they asking for the probability of any child having a trait, or specifically a son or daughter? Your answer format matters too (percentage, fraction, or ratio).

Pro Tip: Always show your working - even if you get the final answer wrong, you can still pick up marks for correct method!

Dihybrid Cross Example

Guinea pig genetics makes a perfect dihybrid example! With black coat (B) dominant over brown $b$, and short hair (S) dominant over long $s$, let's cross two heterozygous parents (BbSs × BbSs).

Each parent can produce four different gamete types: BS, Bs, bS, and bs. Use the FOIL method (First, Outer, Inner, Last) to work these out systematically.

Your 16-square Punnett square will show the classic 9:3:3:1 phenotypic ratio. Count carefully: 9 black short-haired, 3 black long-haired, 3 brown short-haired, and 1 brown long-haired guinea pig.

This ratio appears whenever you cross two individuals who are heterozygous for both traits. It's one of the most important patterns in genetics, so make sure you can recognise and reproduce it.

Study Hack: Practice drawing 16-square Punnett squares until you can do them quickly - they're exam favourites!

Sex-Linked Inheritance

Sex-linked crosses work differently because the inheritance pattern depends on whether you're male or female. Let's look at colour blindness: a carrier woman $X^N X^n$ married to a normal-vision man $X^N Y$.

Their Punnett square shows four possible outcomes. Daughters can be normal $X^N X^N$ or carriers $X^N X^n$, whilst sons can be normal $X^N Y$ or colour-blind $X^n Y$.

The probability of having a colour-blind son is 25% of all children, but 50% of all sons. Always check what the question is actually asking for!

Males show X-linked recessive traits more often because they only need one copy of the recessive allele. Females need two copies, which is much less likely.

Remember: In sex-linkage, it's all about that X chromosome - males get their only copy from mum!

Exam Success Strategy

Your exam toolkit needs these essential ratios: 3:1 for monohybrid crosses and 9:3:3:1 for dihybrid crosses. But don't just memorise them - understand where they come from so you can work them out from scratch.

Always show your complete working: state your alleles, write parental genotypes, list possible gametes, draw your Punnett square, and calculate ratios. Even if you make an error early on, you can still earn marks for correct method.

Never mix up genotype (the letters like Bb) with phenotype (the description like "black coat"). This is a classic mistake that costs easy marks.

For probability questions, express answers exactly as requested - ratio, percentage, or fraction. Be precise about whether it's probability of any child or specifically a son or daughter having the trait.

Final Tip: Practice past paper questions until genetic crosses become second nature - repetition builds confidence!

Quick Revision Summary

Mendel's laws form the foundation: segregation (alleles separate during meiosis) and independent assortment (different genes inherit independently if on different chromosomes).

Cross types you must know: monohybrid (one trait, gives 3:1 ratio when both parents heterozygous) and dihybrid (two traits, gives 9:3:3:1 ratio when both parents heterozygous for both traits).

Sex-linkage means genes on sex chromosomes, with X-linked recessive traits more common in males since they only have one X chromosome.

Master your terminology: the difference between allele/gene, genotype/phenotype, and homozygous/heterozygous. These distinctions appear in every genetics question, so getting them right is non-negotiable for success.

Revision Gold: Create flashcards for all key terms - you'll need instant recall under exam pressure!