ACARA v9 CONTENT DESCRIPTION “explain the role of meiosis and mitosis and the function of chromosomes, DNA and genes in heredity and predict patterns of Mendelian inheritance”
Builds on the idea that living things are made of cells and that offspring resemble their parents. Here we look at how cells divide to pass on chromosomes, what those chromosomes carry, and how to predict which traits appear in the next generation.
Two kinds of cell division
Body cells divide by mitosis. One cell copies its chromosomes and splits into two cells that are identical to the parent, each with the full set of chromosomes. This is how the body grows and repairs itself. Gametes, the sex cells such as sperm and egg, are made by a different division called meiosis. Meiosis halves the chromosome number, so each gamete carries one copy of each chromosome rather than two. When a sperm and an egg join, the full number is restored, with half the chromosomes coming from each parent.
Mitosis versus meiosis
Mitosis keeps the chromosome number; meiosis halves it. Pick a division and step through it to watch the count.
Body cell, full set. Mitosis copies a body cell into two cells that are identical to the parent, each with the full set of chromosomes. This builds and repairs the body.
What chromosomes, DNA and genes do
The instructions a cell inherits are stored as DNA, a long molecule shaped like a twisted ladder. A short section of DNA that holds one set of instructions, such as the instruction for a single trait, is called a gene. Many genes are packed tightly into each chromosome, and the chromosomes are stored inside the nucleus of the cell. So the order runs from small to large: DNA carries genes, genes sit on chromosomes, and chromosomes sit inside the cell.
From DNA up to the cell
Genetic information is nested. Zoom out from DNA to a gene to a chromosome to the whole cell.
DNA: a long molecule, a twisted ladder, that carries the coded instructions. DNA carries genes, genes are packed into chromosomes, and chromosomes are stored inside the cell.
Passing alleles to the next generation
For many genes a cell carries two copies, one inherited from each parent. The different versions of a gene are called alleles. When gametes form, each one receives only one allele of each gene. A Punnett square crosses the two alleles a mother can pass with the two a father can pass, listing the four equally likely allele pairings an offspring can inherit. Reading the grid predicts the chance of each genotype.
Build a Punnett square
Each parent passes one allele to each offspring. Set both parents and read the four offspring boxes.
The mother passes A or a, the father passes A or a. The grid lists the four equally likely allele pairings the offspring can inherit. A capital A is the dominant allele and a lowercase a is the recessive one.
Dominant and recessive alleles
Alleles can be dominant or recessive. A dominant allele shows its trait whenever it is present, so a plant with the genotype AA or Aa displays the same dominant trait. A recessive trait appears only when both alleles are recessive, written aa, because there is no dominant allele to mask it. This is why two parents that both look the same can have offspring that look different: each may carry a hidden recessive allele.
Dominant or recessive: which trait shows
One dominant allele is enough to show the dominant trait. The recessive trait needs two recessive alleles. Cycle the genotype.
With at least one dominant allele, AA or Aa, the dominant trait shows. The dominant allele masks the recessive one, so AA and Aa look the same on the outside.
Predicting trait ratios
Because each offspring box in a Punnett square is equally likely, the grid predicts a probability. Crossing two Aa parents gives four boxes: three show the dominant trait and one shows the recessive trait, a ratio of three to one. Mendel found this pattern by counting large numbers of pea plants. The ratio is a prediction of probability, so a small family may not match it exactly, but over many offspring the counts settle close to the predicted ratio.
Trait probability in the offspring
Count the four offspring boxes by trait to get a ratio. Set both parents and watch the bars and ratio change.
Crossing two Aa parents gives three offspring boxes that show the dominant trait for every one that shows the recessive trait, the classic three to one ratio Mendel predicted.
Why this matters
Seeing heredity as cell division plus alleles lets us explain why offspring resemble their parents and predict how traits pass on, rather than just observing that they do. The same few ideas, copied chromosomes, halved gametes, dominant and recessive alleles, and a Punnett square, connect what happens inside a cell to the traits we can count in the next generation.
Quick self-check
1. How does meiosis differ from mitosis in terms of the chromosome number?
2. Which statement correctly describes how genetic information is organised?
3. In a Punnett square, what does each box represent?
4. A pea plant has the genotype Aa, where A is dominant. Which trait does it show?
5. Crossing two Aa parents predicts what ratio of dominant to recessive traits in the offspring?