Living things: cells, systems, and heredity (grade 8) · lesson

Asexual and sexual reproduction

💡 Asexual offspring are copies of one parent, while sexual offspring get a new mix of genes from two parents.

In asexual reproduction, there is one parent. The offspring gets a copy of that parent's genes. A sea star growing from a broken arm is one example. Strawberry runners are another.

In sexual reproduction, there are two parents. The offspring gets half its genes from each. So it is a new mix, not an exact copy.

Each way has a use. Asexual copies keep a good trait the same. Sexual mixing makes variety.

🧠 Let's work it out

Count the parents.
One parent means a genetic copy.
Two parents means a mix.

What do you think comes next?

asexual reproduction: making offspring from one parent
sexual reproduction: making offspring from two parents

🔗 Khan Academy: Inheritance and variation · Khan Academy ↗

Genes make proteins

💡 Genes are sections of DNA that carry instructions for proteins, and proteins help build traits.

Genes are made of DNA. Most of a cell's DNA is packed into chromosomes in the nucleus.

A gene carries the instructions for making a protein.

Proteins do much of the work in your body. They help build traits, like the color of your eyes.

🧠 Let's work it out

Think of a recipe.
The gene is the recipe.
The protein is the dish it makes.
The trait is what the dish does for the body.

What do you think comes next?

DNA: the material that carries genetic instructions
chromosome: a tightly packed strand of DNA
protein: a molecule that does work in the body and helps build traits

Mutations

💡 A mutation changes a gene, which can change a protein and a trait, and its effect depends on the environment.

A mutation is a change in the DNA of a gene. The cell then makes a protein with a different shape.

That protein may work differently, or not at all. Then the organism may show a different trait.

A mutation can be harmful, helpful, or have no effect. It depends on the change and on where the organism lives.

🧠 Let's work it out

Judge a mutation in its setting.
White fur is easy for a fox to spot on brown dirt.
But on snow, white fur hides a rabbit.

What do you think comes next?

mutation: a change in the DNA of a gene

Dominant and recessive

💡 Each parent passes one allele, and a Punnett square shows the chances of each trait in the offspring.

You get two copies of each gene, one from each parent. Different versions of a gene are called alleles.

A dominant allele (P) shows up even with just one copy. A recessive allele (p) shows only when there are two copies, as in pp.

A Punnett square shows every way the parents' alleles can combine.

🧠 Let's work it out

Cross Pp × Pp.
Put P and p across the top and down the side.
The boxes are PP, Pp, Pp, and pp.
One in four is pp.
So about 1/4 of the offspring show the recessive trait.
Out of 400 seeds, that is about 100.

What do you think comes next?

allele: one version of a gene
dominant: an allele that shows with just one copy
recessive: an allele that shows only with two copies

Spreading seeds

💡 Plants have structures, like sweet fruit, that help spread their seeds to new places.

Young plants that grow right next to their parent must compete for light and water.

Many plants make sweet fruit. Animals eat the fruit, and the seeds pass through them. The seeds land far away, where they have room to grow.

🧠 Let's work it out

Ask what the plant gains.
The plant gives away its fruit, so the fruit is not for the plant to eat.
The payoff is where the seeds end up.

What do you think comes next?

seed dispersal: the spreading of seeds away from the parent plant