Biology119 views·Updated Jun 13, 2026·7 pages

Basics of Genetics: Understanding Heredity

Katy Hodge@theonlykatinthehat

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Genetics is all about how traits get passed from parents...

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9TH GRADE
BIOLOGY Unit 6

Table of Contents

Video 6.1 Intro to Genetics
Gregor Mendel
"the father of Genetics"
Why Pea Plants?
3 Principles

Introduction to Genetics

Gregor Mendel, nicknamed the "father of Genetics," was an Austrian monk who conducted groundbreaking experiments with pea plants in the 1850s. He combined his knowledge of botany and math to collect tons of data that revealed how traits pass from one generation to the next.

Why pea plants? They were perfect for Mendel's experiments because they grow quickly, have many obvious traits (like purple or white flowers), and their pollination can be controlled. Mendel could either allow them to self-pollinate (when a plant's own pollen fertilizes its eggs) or perform cross-pollination by manually moving pollen between plants.

Through his meticulous experiments, Mendel established three fundamental principles of inheritance: the Principle of Dominance, the Principle of Segregation, and the Principle of Independent Assortment. These principles form the foundation of what we now call Mendelian inheritance.

Quick Fact: When Mendel crossed plants with different traits, he called the offspring "hybrids" because they were a mix of the two parent plants. These hybrids formed what scientists call the F1 generation.

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Key Genetics Vocabulary

When we talk about genetics, we need to understand some important terms. A gene is a section of DNA on a chromosome that codes for a specific protein, which creates a trait like eye color. An allele is a specific version of that gene - for example, one version might make blue eye pigment while another makes brown.

Every organism inherits two alleles for each gene—one from mom and one from dad. When both alleles are the same (like BB or bb), we call this homozygous. When the alleles are different (like Bb), we call this heterozygous or hybrid.

Your genotype refers to the actual alleles you have (BB, Bb, or bb), while your phenotype is the physical trait that's expressed - what you can actually observe, like brown or blue eyes. Phenotypes can include anything observable, from eye color to blood type to certain behaviors.

Remember This: Dominant alleles (written as capital letters like B) are always expressed when present, while recessive alleles (written as lowercase letters like b) only show up when no dominant allele is present.

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Mendel's Principles & Probability

Mendel's first principle, the Principle of Dominance, states that some alleles are dominant and mask recessive alleles. His second principle, the Principle of Segregation, explains that during reproduction, the two alleles for each trait separate randomly into different gametes (egg or sperm cells) during meiosis.

Probability plays a huge role in genetics. Just like there's a 1/2 chance of getting heads when flipping a coin, there are predictable odds for inheriting specific alleles. Mendel used this concept to predict the outcomes of his pea plant crosses.

When Mendel crossed purple-flowered plants with white-flowered plants (the P generation), all the offspring (F1 generation) had purple flowers! The white color seemed to disappear. But when he let these F1 plants self-pollinate, the F2 generation showed a ratio of about 3/4 purple to 1/4 white flowers.

Try This: A cross between two organisms for a single trait is called a monohybrid cross. You can predict the results using a Punnett Square, a tool developed by scientist Reginald Punnett based on Mendel's work.

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Beyond Simple Inheritance Patterns

Not all traits follow Mendel's simple dominant/recessive pattern. In incomplete dominance, neither allele is completely dominant, resulting in a blended phenotype. For example, when a red snapdragon is crossed with a white one, the offspring are pink!

With codominance, both alleles are fully expressed in the heterozygous individual. Blood type AB is a perfect example - both the A and B alleles are expressed equally, not blended.

Many genes have multiple alleles in a population, not just two. Human blood type is determined by three alleles (A, B, and O), creating four possible blood types (A, B, AB, and O).

Cool Connection: Your biological sex is determined by special chromosomes called sex chromosomes. Females typically have XX chromosomes, while males have XY. The other 22 pairs of chromosomes (called autosomes) are the same in both sexes.

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Sex-Linked Traits

Sex-linked traits follow a unique inheritance pattern because they're located on sex chromosomes. The X chromosome carries many genes unrelated to determining sex, while the Y chromosome mainly contains genes for male development.

Since males have only one X chromosome (XY), they'll show sex-linked recessive traits more often than females, who have two X chromosomes (XX). This explains why conditions like color blindness and hemophilia affect males more frequently than females.

For example, hemophilia is an X-linked recessive disorder where people bleed excessively when injured because they lack proteins necessary for blood clotting. Red-green color blindness is another X-linked recessive trait caused by a malfunction in the eye's light-sensitive cells.

Think About This: If a woman with normal vision (but carrying a recessive color blindness allele) has children with a color-blind man, what are the chances their sons will be color-blind? Their daughters? Try solving this with a Punnett square!

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Biology119 views·Updated Jun 13, 2026·7 pages

Basics of Genetics: Understanding Heredity

Katy Hodge@theonlykatinthehat

Add to Google sources

Genetics is all about how traits get passed from parents to children. It's like solving a puzzle to understand why you have blue eyes or why your friend is taller than you. In this unit, we'll explore how Gregor Mendel,...

of 7

Introduction to Genetics

Quick Fact: When Mendel crossed plants with different traits, he called the offspring "hybrids" because they were a mix of the two parent plants. These hybrids formed what scientists call the F1 generation.

of 7

Key Genetics Vocabulary

Remember This: Dominant alleles (written as capital letters like B) are always expressed when present, while recessive alleles (written as lowercase letters like b) only show up when no dominant allele is present.

of 7

Mendel's Principles & Probability

Try This: A cross between two organisms for a single trait is called a monohybrid cross. You can predict the results using a Punnett Square, a tool developed by scientist Reginald Punnett based on Mendel's work.

of 7

Beyond Simple Inheritance Patterns

With codominance, both alleles are fully expressed in the heterozygous individual. Blood type AB is a perfect example - both the A and B alleles are expressed equally, not blended.

Many genes have multiple alleles in a population, not just two. Human blood type is determined by three alleles (A, B, and O), creating four possible blood types (A, B, AB, and O).

Cool Connection: Your biological sex is determined by special chromosomes called sex chromosomes. Females typically have XX chromosomes, while males have XY. The other 22 pairs of chromosomes (called autosomes) are the same in both sexes.

of 7

Sex-Linked Traits

Think About This: If a woman with normal vision (but carrying a recessive color blindness allele) has children with a color-blind man, what are the chances their sons will be color-blind? Their daughters? Try solving this with a Punnett square!

of 7