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Unlock This CourseGenetics is like a game of chance, but with rules we can predict! When organisms reproduce, they pass on their genes to their offspring. But which genes get passed on follows patterns we can work out using maths. This is where probability comes in - it helps us predict what traits the babies might have.
Think of it like flipping coins. Each parent has two versions of each gene (called alleles) and they randomly pass one to their child. By understanding these patterns, we can predict things like eye colour, height, or even genetic diseases.
Key Definitions:
Every time reproduction happens, it's like rolling dice. Each parent contributes one allele from their pair and the combination determines what the offspring will be like. If we know what alleles the parents have, we can work out the chances of different outcomes in their children.
A monohybrid cross looks at just one characteristic at a time, like eye colour or plant height. This is the simplest type of genetic cross to understand and calculate.
Punnett squares are like a grid that helps us work out all the possible combinations when two organisms reproduce. They're named after Reginald Punnett, who invented this handy tool.
Put one parent's alleles across the top and the other parent's alleles down the side. Each box shows one possible combination.
Combine the alleles from each row and column. Each box represents one possible offspring genotype.
Count up the different types and work out the ratios and percentages.
Let's say brown eyes (B) are dominant and blue eyes (b) are recessive. If both parents are heterozygous (Bb), what are the chances their child will have blue eyes? Using a Punnett square: Bb × Bb gives us BB, Bb, Bb, bb. That's a 1:2:1 ratio, meaning 25% chance of blue eyes, 50% chance of brown eyes (but carrying the blue gene) and 25% chance of brown eyes (pure brown).
When we do genetic crosses, we often express our results as ratios or percentages. This helps us understand the likelihood of different outcomes.
Different types of crosses give us predictable ratios that appear again and again in genetics.
This classic ratio appears when two heterozygotes cross (Bb × Bb). You get 3 offspring showing the dominant trait for every 1 showing the recessive trait. In percentages, that's 75% dominant and 25% recessive.
This happens in a test cross - when you cross a heterozygote with a homozygous recessive (Bb × bb). Half the offspring show each trait, giving you a 50:50 split.
Sometimes we want to look at two characteristics at once, like both eye colour and hair colour. These are called dihybrid crosses and they're more complex but follow the same basic rules.
When two organisms that are heterozygous for two different traits reproduce, you get a famous ratio called 9:3:3:1. This was first discovered by Gregor Mendel when he studied pea plants.
Mendel crossed pea plants that were heterozygous for both seed colour (yellow/green) and seed shape (round/wrinkled). Yellow (Y) and round (R) were dominant. The cross YyRr × YyRr gave him 9 yellow round : 3 yellow wrinkled : 3 green round : 1 green wrinkled. This 9:3:3:1 ratio appears whenever you cross two double heterozygotes!
Genetics follows the same probability rules as any other chance event. Understanding these rules helps us solve more complex genetic problems.
When you want to know the probability of two independent events both happening, you multiply their individual probabilities together.
If there's a 1/4 chance of blue eyes and a 1/2 chance of curly hair, then the chance of both blue eyes AND curly hair is 1/4 × 1/2 = 1/8 or 12.5%.
When you want to know the probability of one thing OR another happening, you add their probabilities together.
If there's a 1/4 chance of having type A blood and a 1/4 chance of having type B blood, then the chance of having either A OR B blood is 1/4 + 1/4 = 1/2 or 50%.
Understanding genetic probability isn't just for exams - it has real applications in medicine, agriculture and conservation.
Genetic counsellors use these calculations to help families understand the risks of inherited diseases.
Cystic fibrosis is caused by a recessive allele. If both parents are carriers (heterozygous), each child has a 25% chance of having the disease, a 50% chance of being a carrier and a 25% chance of being completely unaffected. This information helps families make informed decisions about family planning.
Farmers and breeders use genetic probability to develop crops and animals with desirable traits.
Plant breeders might cross varieties to combine disease resistance with high yield. By understanding the genetics, they can predict which crosses are most likely to produce the plants they want.
Even experienced students sometimes make errors with genetic probability. Here are the most common mistakes and how to avoid them.
Remember that genotype ratios and phenotype ratios can be different. Always check which one the question is asking for.
Each offspring is independent - if the first child has blue eyes, it doesn't change the odds for the second child.
Use multiplication for 'AND' problems and addition for 'OR' problems. Don't mix them up!