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examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: Genetic Variation from Fertilisation
Have you ever wondered why you don't look exactly like your siblings, even though you share the same parents? The answer lies in genetic variation that occurs during sexual reproduction, particularly during fertilisation. This fascinating process ensures that each new individual is genetically unique!
Key Definitions:
Sexual reproduction involves two parents who each contribute genetic information to their offspring. This happens when a male gamete (sperm) fuses with a female gamete (egg) during fertilisation. Unlike asexual reproduction, which produces genetically identical clones, sexual reproduction creates offspring with a unique mix of genes from both parents.
Genetic variation is crucial for species survival! It provides the raw material for evolution by natural selection. Populations with greater genetic diversity are more likely to contain individuals who can survive environmental changes, disease outbreaks, or other challenges. Without variation, a single threat could wipe out an entire species.
Fertilisation is a key moment in creating genetic variation. Let's explore how this process works and why it's so important for diversity.
Before fertilisation can occur, specialised sex cells called gametes must be produced through a process called meiosis. This special type of cell division ensures that each gamete contains half the normal number of chromosomes (haploid). When fertilisation happens, these gametes combine to create a zygote with the full set of chromosomes (diploid).
Human body cells have 46 chromosomes (23 pairs). Through meiosis, gametes are formed with just 23 chromosomes. When sperm and egg fuse during fertilisation, the resulting zygote has the full 46 chromosomes again - half from mum and half from dad!
In flowering plants, pollen (male gametes) fertilises the ovule (female gamete) to form seeds. Each seed contains a unique genetic combination, which is why plants grown from seeds show variation, unlike those grown from cuttings.
Many animals release huge numbers of gametes. For example, female frogs can lay hundreds of eggs and males release millions of sperm. This increases the chances of fertilisation and creates many genetically diverse offspring.
Fertilisation creates variation in two main ways: through the random combination of chromosomes during meiosis and through the random fusion of gametes. Let's look at these processes in more detail.
During meiosis, chromosomes from each parent are randomly shuffled and distributed to gametes. This is like shuffling two decks of cards (one from each parent) and dealing them out in new combinations. With 23 pairs of chromosomes in humans, there are over 8 million possible combinations of chromosomes in each gamete!
During meiosis, sections of chromosomes can swap places in a process called crossing over or recombination. This mixes up the genes on individual chromosomes, creating even more variation. It's like taking parts from two different jigsaw puzzles and creating a new, unique puzzle.
The second major source of variation comes from random fertilisation - which particular sperm fertilises which particular egg is completely random. When you consider the millions of possible sperm and hundreds of eggs, the possible combinations are astronomical!
If we calculate the number of possible genetic combinations from random assortment in meiosis and random fertilisation, the result is mind-boggling. For humans, there are approximately 70 trillion different possible combinations! This is why siblings (except identical twins) look different, even though they have the same parents.
Let's see how genetic variation works with a simple example using just two gene pairs.
Imagine two parents who both have the genotype AaBb (where A and B are dominant alleles and a and b are recessive).
Through meiosis, each parent can produce four types of gametes: AB, Ab, aB and ab.
When these gametes combine randomly during fertilisation, they can produce 16 different possible combinations in their offspring!
Eye colour is determined by multiple genes, but let's simplify and consider just one. The brown eye allele (B) is dominant over the blue eye allele (b). If two parents are both heterozygous (Bb), they can each produce gametes with either B or b. When fertilisation occurs, their children could have any of these combinations: BB (brown eyes), Bb (brown eyes), or bb (blue eyes). This explains how two brown-eyed parents can sometimes have a blue-eyed child!
Genetic variation isn't just interesting - it's essential for life as we know it!
Genetic variation provides the raw material for natural selection. When environments change, individuals with advantageous genetic traits are more likely to survive and reproduce. Over time, this leads to adaptation and evolution. Without genetic variation, species couldn't adapt to changing conditions.
Genetic variation helps populations survive disease outbreaks. If all individuals were genetically identical (like in clones or highly inbred populations), a single disease could potentially wipe out the entire population. Variation means some individuals may have natural resistance.
Farmers and plant breeders rely on genetic variation to develop crops with desirable traits like higher yields, disease resistance, or better taste. Without the variation that comes from sexual reproduction, we wouldn't have the diverse range of food crops we enjoy today.
When species become endangered and their populations shrink, they often lose genetic variation. This is called a genetic bottleneck. The cheetah is a famous example - all modern cheetahs have very low genetic diversity due to a population bottleneck thousands of years ago. This low variation makes them vulnerable to diseases and environmental changes, highlighting why genetic variation is so important for species survival.
Genetic variation from fertilisation is a fundamental concept in biology that explains why offspring from the same parents can look different. This variation arises from:
This genetic diversity is crucial for evolution, adaptation and the survival of species in our ever-changing world. Next time you notice how different you look from your siblings or friends, remember - it's all thanks to the amazing genetic variation created during fertilisation!
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