The passing of traits from parents to their offspring is called heredity. For example, if a child has dimples like one of their parents, that is an inherited trait.

• They grow quickly

• They produce many offspring

• Their traits are easy to see (such as purple or white flowers)

• He could control which plants pollinated each other

By studying these plants, Mendel discovered that traits are controlled by genes, which are passed from parents to offspring.

1. First, Mendel crossed a plant that always produced purple flowers with a plant that always produced white flowers.

2. All of the offspring had purple flowers, none were white.

This showed Mendel that the purple trait was dominant over the white trait.

Next, Mendel pollinated those purple offspring with each other.

• In the next generation, most plants had purple flowers, but some had white flowers.

Mendel determined how this happened:

• Each plant had two genes for flower color

• The dominant purple gene can hide the recessive white gene

• The white flowers only appeared when a plant received two recessive white genes

So:

• Dominant trait: shows up if at least one dominant gene is present

• Recessive trait: only shows up when there are two recessive genes

How we see the genes in the offspring's physcial appearance is called the phenotype.

The genotype determine phenotype.

Imagine a gene for dimples:

• The dominant gene = dimples

• The recessive gene = no dimples

Let's use D to represent dimples and d to represent no dimples. An offspring could inherit:

• DD → it has dimples

• Dd → it has dimples (dominant trait shows, recessive trait hides)

• dd → it does not have dimples

Even if both parents have dimples, they can still have a child without dimples if they both carry the recessive gene, just like the white flowers reappeared in Mendel’s plants.

The genes that get passed on determine the what traits can be seen in offspring.

Homozygous = Two of the Same Genes

• When both genes for a trait are the same, the organism is homozygous.

• There are two types:

  • Homozygous dominant → has two dominant genes

  • Homozygous recessive → has two recessive genes

In humans:

• DD → dimples, homozygous dominant

• dd → no dimples, homozygous recessive

In Mendel’s pea plants:

• A plant with two purple genes was homozygous dominant

• A plant with two white genes was homozygous recessive

Heterozygous = Two Different Genes

• When the two genes are different, the organism is heterozygous.

  • Heterozygous genes always contain one dominant gene and one recessive gene

In humans:

• Dd → has dimples, heterozygous

  • Even though there is one gene for “no dimples,” the dominant gene for dimples hides the recessive gene.

In Mendel’s experiment:

• The first generation of purple-flowered plants were heterozygous

  • They had one purple gene and one white gene

  • They looked purple because the purple gene is dominant

Being homozygous or heterozygous affects:

• What trait you see (phenotype)

• What genes you can pass on to your offspring (genotype)

Two parents who both show a dominant trait (like dimples) can still have a child without the trait if they are heterozygous, because they can pass on their recessive genes.

This is exactly why the white flowers reappeared in Gregor Mendel’s pea plant experiment.

In humans, we know:

• Traits are controlled by genes

• Genes sections of DNA located on chromosomes

• We have 46 chromosomes in most body cells

• We inherit 23 chromosomes from egg and 23 chromosomes from sperm

This means we also receive two genes for each trait (one from each parent) just like Mendel’s pea plants.