GCSE AQA Biology: Notes on Reproduction and Inheritance

Sexual and Asexual Reproduction

Ever wondered why you look a bit like both your parents but not exactly like either of them? That's sexual reproduction at work!

In sexual reproduction, two organisms combine their genetic information to create offspring different from both parents. Parents produce gametes (like egg and sperm cells) through a process called meiosis. In humans, each gamete contains 23 chromosomes—half the normal amount. When fertilisation occurs, these gametes fuse, creating a cell with the complete set of chromosomes, which is why you inherit features from both parents.

Plants can reproduce sexually too. Flowering plants have egg cells, and they use pollen as their version of sperm to fertilise those eggs.

Asexual reproduction is quite different—there's only one parent involved! Bacteria, some plants, and certain animals reproduce this way. The offspring are genetically identical to the parent (they're clones) because the cells divide through a process called mitosis, where an ordinary cell simply divides into two identical cells.

Did you know? When organisms reproduce asexually, the offspring are perfect genetic copies of the parent—nature's own cloning process!

Meiosis and Reproduction Methods

Meiosis is like the special behind-the-scenes magic that makes sexual reproduction possible!

During meiosis, cells in reproductive organs divide to form gametes (sex cells). This process halves the chromosome number, changing cells from diploid (full set) to haploid (half set). Without this reduction, the chromosome number would double with each generation!

The meiosis process is fascinating. First, genetic information duplicates, then chromosomes line up in pairs. After being pulled apart in the first division, a second division occurs—resulting in four haploid cells. When gametes join during fertilisation, the normal chromosome number is restored. The fertilised cell then replicates many times using mitosis to produce an embryo.

Both reproduction types have advantages. Sexual reproduction creates variation, increasing survival chances through natural selection. Asexual reproduction requires just one parent, uses less energy, happens faster, and can produce many identical offspring quickly.

Cool fact: Some clever organisms can use both methods depending on circumstances! Malarial parasites reproduce asexually in humans but sexually in mosquitoes, while many plants can produce seeds sexually and also reproduce asexually through runners or bulb division.

DNA and the Genome

DNA is the instruction manual for everything your body does and how it's built!

DNA (Deoxyribonucleic acid) is the genetic material found in the nucleus of your cells. It's a polymer made of two strands twisted in a double helix, containing all the instructions for your body's growth and development. DNA is packaged into structures called chromosomes.

The complete set of genetic material in an organism is called its genome. Scientists have mapped the entire human genome, which is incredibly useful in medicine for identifying disease-linked genes and developing treatments.

DNA structure is fascinatingly simple yet powerful. It's made of repeating units called nucleotides, each containing a sugar, phosphate group, and one of four bases: Adenine (A), Cytosine (C), Thymine (T), or Guanine (G). These bases pair up in a specific way—A always with T, and C always with G—called complementary base pairing.

A sequence of three bases codes for a particular amino acid. The order of bases controls which amino acids join together to form specific proteins that determine your characteristics.

Fascinating! Scientists can track ancient human migration patterns by studying tiny differences in the genomes of different populations around the world.

Protein Synthesis

Your body is constantly making proteins using the instructions in your DNA—it's like a molecular factory!

Proteins are made in the cytoplasm on structures called ribosomes. Since DNA is too large to leave the nucleus, your cells use a messenger molecule called mRNA (messenger RNA) to carry the instructions to the ribosomes.

The process works like this: First, the base code of a gene is copied (transcribed) onto mRNA. This mRNA then travels out of the nucleus to attach to a ribosome. The ribosome reads the code in groups of three bases, while carrier molecules bring specific amino acids to be joined together in the correct order. Once the amino acid chain is assembled, it's released and folds into a unique shape to form the final protein.

Proteins have numerous important jobs in your body. They can act as enzymes to speed up chemical reactions, function as hormones carrying messages around your body, or serve as structural proteins that give physical strength to move around.

Remember: Even small changes in DNA can affect the proteins produced. A change in DNA structure alters the order of bases, potentially changing the amino acids in the protein and affecting how it functions in your body.

Mutations and DNA Changes

Mutations are like typos in your genetic code that can sometimes change how your body works!

A mutation is a random change in the sequence of DNA bases or a chromosome. Most mutations don't actually change a protein's function—they either don't alter the protein at all or change it so slightly that it still works normally. However, some mutations can lead to significant changes in the proteins they code for.

One type of mutation is an insertion, where a new base gets inserted into the DNA sequence when it shouldn't be there. This changes how the bases are read—like inserting an extra letter in a word—shifting everything that comes after it. Because DNA is read in groups of three bases, an insertion can affect multiple amino acids at once.

These changes in DNA structure alter what protein is synthesised because the order of bases would be different. Since proteins carry out most functions in your body, changes to proteins can sometimes lead to differences in how your body works or develops.

Think about it: Mutations might sound scary, but they're actually the driving force behind evolution! Without occasional mutations creating genetic diversity, species couldn't adapt to changing environments.

Genetic Inheritance

The reason you might have your mum's eyes and your dad's hair comes down to genetic inheritance!

Understanding genetic inheritance requires knowing some key terms: Gametes are sex cells like sperm and eggs. Chromosomes carry genetic information in the form of genes. Alleles are different versions of genes, and they can be either dominant (always expressed) or recessive (only expressed if two copies are present).

When describing genetics, we use terms like homozygous (having two identical alleles of a gene), heterozygous (having two different alleles), genotype (the combination of alleles), and phenotype (the observable characteristics).

We inherit two alleles of each gene—one from our mother and one from our father. Some characteristics are controlled by single genes (monohybrid inheritance), while most characteristics result from multiple genes interacting (polygenic inheritance).

To predict how characteristics might be inherited, scientists use Punnett squares. These diagrams show possible combinations of alleles in offspring. In these squares, dominant alleles are shown with capital letters, and recessive alleles with lowercase letters.

Fascinating fact: While we often think of traits as simply dominant or recessive, many characteristics like height, skin colour and intelligence are influenced by multiple genes working together, plus environmental factors!

Inherited Disorders

Some medical conditions run in families because they're passed down through our genes.

Cystic fibrosis is an inherited disorder affecting cell membranes, causing the body to produce sticky mucus that blocks air passages and the pancreas. It's caused by a recessive allele (usually shown as 'f'). About 1 in 25 people carry this allele, but carriers don't have the disorder. If both parents are carriers, there's a 1 in 4 (25%) chance their child will have cystic fibrosis.

Using a Punnett square helps visualise this inheritance pattern. If both parents are carriers (Ff), they can produce gametes with either F or f. When these combine, possible offspring genotypes are: FF (unaffected), Ff (carrier), Ff (carrier), and ff (has cystic fibrosis).

If only one parent is a carrier, there's a 0% chance of producing a child with cystic fibrosis, though some children may be carriers.

The Punnett squares shown in your notes illustrate how these probabilities work. With both parents as carriers, one square in four shows the ff combination that causes the disorder—hence the 25% probability.

Think about this: Understanding inheritance patterns helps families make informed decisions about having children when there's a known genetic disorder in the family.

More Inherited Disorders and Sex Determination

Some genetic conditions are much more likely to be passed on than others, depending on whether they're dominant or recessive.

Polydactyly is a genetic disorder where a child is born with extra fingers or toes. Unlike cystic fibrosis, it's caused by a dominant allele (usually shown as 'D'). This means it can be inherited even if only one parent carries the defective allele. The parent with the allele will have the condition themselves, and if one parent has the allele, there's a 50% chance of passing it to each child.

Modern medicine offers options like embryo screening, where a cell can be taken from an embryo and its genes analysed to detect genetic disorders. This allows the option to end a pregnancy or not use an IVF embryo with disorders. This raises social, ethical, and economic concerns.

Gene therapy is another approach, where normal alleles are inserted into chromosomes of individuals with defective ones. While it can alleviate suffering, it's not always successful and some consider it unnatural.

The inheritance of biological sex is determined by chromosomes. Females have two X chromosomes (XX), while males have one X and one Y (XY). Using a Punnett square, we can see that mothers always contribute an X chromosome, while fathers can contribute either X (resulting in a girl) or Y (resulting in a boy).

Did you know? The Y chromosome is much smaller than the X chromosome and contains fewer genes, mainly ones that determine male characteristics!