Genetics & Heredity

🧬 Virginia Biology SOL

A self-paced lesson aligned to Virginia Biology SOL BIO.5 — exploring how traits are inherited through genes, alleles, and the process of meiosis.

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SOL BIO.5 Focus: The student will investigate and understand that there are common mechanisms for inheritance — including allele combinations, meiosis, and genetic variation. Note: DNA structure and protein synthesis are covered in BIO.2.

📋 What You Will Learn

        How Mendel's pea plant experiments revealed the laws of inheritance
How to use Punnett squares to predict genotype and phenotype ratios
Patterns beyond simple dominance: codominance, incomplete dominance, sex-linked traits
How to read and interpret pedigree charts across generations
How meiosis produces genetically unique gametes through crossing over and independent assortment

      

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5 Modules

Self-paced content

⚗️

Virtual Lab

Interactive Punnett squares

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SOL Quiz

10 SOL-style questions

📖 Vocabulary

Key Terms

Master these terms — they appear on the Virginia Biology SOL and are essential for understanding genetics and heredity.

Gene

A segment of DNA that codes for a specific trait; the basic unit of heredity passed from parent to offspring.

Foundations

Allele

A variant form of a gene. Each organism has two alleles for each gene — one from each parent.

Foundations

Dominant

An allele that is expressed in the phenotype whenever it is present, masking the recessive allele.

Foundations

Recessive

An allele only expressed in the phenotype when two copies are present (homozygous recessive).

Foundations

Genotype

The genetic makeup of an organism, written as allele pairs (e.g., TT, Tt, tt).

Mendelian

Phenotype

The observable physical expression of the genotype (e.g., tall, short, round seed).

Mendelian

Homozygous

Having two identical alleles for a gene (e.g., TT or tt). Also called true-breeding or purebred.

Mendelian

Heterozygous

Having two different alleles for a gene (e.g., Tt). The dominant allele is expressed.

Mendelian

Codominance

Both alleles are fully expressed simultaneously in the phenotype (e.g., AB blood type, roan cattle).

Beyond Mendel

Incomplete Dominance

Neither allele is fully dominant; heterozygotes show a blended intermediate phenotype (e.g., pink flowers from red × white).

Beyond Mendel

Sex-Linked Trait

A trait controlled by a gene located on a sex chromosome (X or Y). X-linked recessive traits appear more in males.

Beyond Mendel

Carrier

A heterozygous individual who has but does not show a recessive trait, but can pass it to offspring.

Beyond Mendel

Pedigree

A chart that traces the inheritance of a trait through multiple generations of a family.

Pedigrees

Meiosis

Cell division that produces 4 haploid (n) gametes from one diploid (2n) cell, with genetic variation.

Meiosis

Crossing Over

Exchange of segments between homologous chromosomes during prophase I of meiosis, creating new allele combinations.

Meiosis

Gamete

A haploid reproductive cell (sperm or egg) produced by meiosis. Contains one allele for each gene.

Meiosis

🌱 Module 1

Mendel's Laws of Inheritance

How did a monk with pea plants change science forever? Gregor Mendel's careful experiments revealed the mathematical laws that govern how traits are inherited.

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SOL BIO.5c: The variety of traits in an organism are the result of the expression of various combinations of alleles.

01 / 05

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Mendel's Pea Plants

Gregor Mendel (1860s) studied 7 traits in pea plants over 8 years, analyzing over 29,000 plants. He chose peas because they reproduced quickly, had distinct traits, and could be self- or cross-pollinated under controlled conditions.

02 / 05

⚖️

Law of Segregation

Each organism has two alleles for each trait. These alleles separate (segregate) during gamete formation so each gamete carries only ONE allele. Offspring receive one allele from each parent — restoring the pair.

03 / 05

🔀

Law of Independent Assortment

Genes for DIFFERENT traits are inherited independently of one another (when on different chromosomes). The allele you inherit for height does not affect which allele you inherit for seed color.

04 / 05

🔢

The 3:1 Ratio

When two heterozygous parents (Tt × Tt) cross, the F2 offspring show a 3:1 phenotype ratio — 3 dominant : 1 recessive. Mendel observed this in thousands of crosses and recognized it as a mathematical law of nature.

05 / 05

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Dominant vs. Recessive

A dominant allele (capital letter, e.g. T) is expressed whenever present. A recessive allele (lowercase, e.g. t) is only expressed when two copies are present (tt). One dominant allele is enough to show the dominant phenotype.

▶

▶ AMOEBA SISTERS

Mendelian Genetics — Amoeba Sisters

YouTube · Recommended

▶

▶ CRASH COURSE

Heredity: Crash Course Biology

YouTube · Crash Course

🔑 Key Concepts to Remember

        P generation = original parents; F1 = first generation offspring; F2 = second generation
True-breeding (homozygous) plants always produce offspring with the same trait
The dominant phenotype appears in 3 out of every 4 F2 offspring (on average)
Mendel's laws are the foundation of modern genetics

      

🔲 Module 2

Punnett Squares

Punnett squares are the essential tool for predicting the probability of offspring genotypes and phenotypes. Master these and you can solve any genetics problem.

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SOL BIO.5c: Use Punnett squares to predict genotype and phenotype ratios for mono- and dihybrid crosses.

01 / 04

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How to Set Up a Punnett Square

Write one parent's alleles across the TOP and the other parent's alleles down the LEFT side. Each box inside the grid is filled by combining the allele from its column header with the allele from its row header.

02 / 04

📊

Monohybrid Cross Ratios

Tt × Tt gives: TT, Tt, Tt, tt → Genotype ratio 1:2:1 (1 TT : 2 Tt : 1 tt) → Phenotype ratio 3:1 (3 tall : 1 short). The 3:1 ratio is the hallmark of a monohybrid cross between two heterozygotes.

03 / 04

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The Test Cross

A test cross (Tt × tt) reveals whether an unknown dominant phenotype individual is TT or Tt. If any recessive offspring appear, the parent must be Tt (heterozygous). Result is always 1:1 (50% tall : 50% short).

04 / 04

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Dihybrid Cross — 9:3:3:1

A dihybrid cross (TtRr × TtRr) tracks TWO independently assorting genes. It uses a 4×4 Punnett square with 16 boxes. The expected phenotype ratio is 9:3:3:1 — a hallmark of two independently assorting gene pairs.

💡 Use the Punnett Square Lab (in the Practice section) to click through interactive grids for all cross types — monohybrid, test cross, P generation cross, and dihybrid!

📐 Steps to Solve Any Punnett Square

1

Identify the cross

Determine the genotypes of both parents (e.g., Tt × Tt). Identify which allele is dominant and which is recessive.

2

Set up the grid

Write parent 1 alleles across the top. Write parent 2 alleles down the left side. Draw a 2×2 grid (monohybrid) or 4×4 grid (dihybrid).

3

Fill in each box

Combine the column letter + row letter to fill each cell. Always write the dominant allele first (Tt not tT).

4

Calculate ratios

Count genotypes (TT:Tt:tt) and phenotypes (dominant:recessive) and express as ratios or percentages.

🔬 Module 3

Beyond Mendelian Genetics

Not every trait follows simple dominant/recessive patterns. Real genetics is more complex — and more interesting.

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SOL BIO.5c: The variety of traits in an organism are the result of the expression of various combinations of alleles — including codominance, incomplete dominance, multiple alleles, and sex linkage.

01 / 05

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Codominance

Both alleles are FULLY expressed simultaneously — neither masks the other. Example: ABO Blood Type. A person with genotype I^AI^B expresses BOTH A and B antigens → Blood type AB. Roan cattle (red + white hairs both present).

02 / 05

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Incomplete Dominance

Neither allele is fully dominant — heterozygotes show an INTERMEDIATE blended phenotype. Example: Red snapdragon (RR) × White (WW) = Pink (RW). The F2 cross of pink × pink gives 1 red : 2 pink : 1 white.

03 / 05

♂♀

Sex-Linked Traits

Genes on the X chromosome show sex-linked inheritance. X-linked recessive traits (color blindness, hemophilia) appear more in males (XY) because males have only ONE X chromosome — there's no second X to mask the recessive allele.

04 / 05

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Polygenic Traits

Some traits are controlled by MULTIPLE genes, producing a continuous range of phenotypes rather than discrete categories. Examples: human skin color, height, eye color. When graphed, polygenic traits show a bell curve distribution.

05 / 05

🔢

Multiple Alleles

Some genes have MORE than two alleles in the population (though each person still only has two). Example: ABO blood type has three alleles — I^A, I^B, and i — producing four possible blood types: A, B, AB, and O.

▶

▶ AMOEBA SISTERS

Incomplete Dominance, Codominance & Multiple Alleles

YouTube · Amoeba Sisters

▶

▶ AMOEBA SISTERS

Sex-Linked Traits — Amoeba Sisters

YouTube · Amoeba Sisters

Pattern	Heterozygote Looks Like	Example
Simple Dominance	Dominant parent	Tall pea plants
Incomplete Dominance	Blend of both	Pink snapdragon
Codominance	Both expressed fully	AB blood type

🧾 Module 4

Pedigrees & Probability

Pedigree charts let scientists and doctors trace how traits move through families — and predict who might be a carrier or at risk.

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SOL BIO.5c: Genetic information can be used to predict trait inheritance across generations using pedigree analysis and probability.

01 / 04

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Reading a Pedigree

Circles = females. Squares = males. Shaded = affected (shows the trait). Horizontal line = mating pair. Vertical lines connect to offspring below. Roman numerals label each generation (I, II, III).

02 / 04

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Determining Inheritance Pattern

If two unaffected parents have an affected child → trait is RECESSIVE. If every affected individual has an affected parent → likely DOMINANT. If it appears more in males → may be X-linked. Use these clues together to identify the pattern.

03 / 04

👤

Identifying Carriers

A CARRIER is heterozygous (Cc) — unaffected but carries one recessive allele and can pass it to children. If two unaffected parents have an affected child (cc), BOTH parents must be carriers (Cc). This is shown on pedigrees as unshaded with a dot.

04 / 04

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Probability in Genetics

Probability predicts the CHANCE of inheriting a genotype. Carrier × Carrier (Cc × Cc): 25% CC, 50% Cc, 25% cc. Each pregnancy is an INDEPENDENT event — even if three children are unaffected, the 4th still has a 25% chance of being affected.

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▶ AMOEBA SISTERS

Pedigrees — Amoeba Sisters

YouTube · Amoeba Sisters

▶

▶ TUTORIAL

How to Read a Pedigree Chart

YouTube · Genetics Tutorial

📋 Pedigree Symbols Quick Reference

        ⭕ Unaffected female  |  ◯● Affected female
□ Unaffected male  |  ■ Affected male
◯—□ Mating pair (horizontal line)
◯⦿ or □⬜ with dot = Carrier (heterozygous, unaffected)
Vertical line down from mating pair = offspring
Horizontal bracket connecting siblings

      

🔀 Module 5

Meiosis & Genetic Variation

Meiosis is the cellular process that explains WHY offspring are never exact copies of their parents — and how Mendel's laws actually work at the chromosome level.

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SOL BIO.5d: Meiosis has a role in genetic variation between generations through independent assortment and crossing over.

01 / 05

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What Is Meiosis?

Meiosis is a specialized cell division that produces 4 haploid (n) gametes from one diploid (2n) cell. In humans: 46 chromosomes (2n) → 4 cells each with 23 chromosomes (n). These gametes fuse at fertilization to restore 2n.

02 / 05

✂️

Crossing Over (Prophase I)

During Prophase I, homologous chromosomes pair up and exchange segments in a process called crossing over (recombination). This shuffles alleles between chromosomes, creating NEW combinations that neither parent had — a major source of genetic variation.

03 / 05

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Independent Assortment (Metaphase I)

At Metaphase I, homologous chromosome pairs line up randomly at the cell's equator. Either chromosome of each pair can face either pole — completely by chance. In humans, this produces 2²³ = over 8 million possible chromosome combinations per gamete.

04 / 05

⚡

Meiosis vs. Mitosis

Mitosis: 1 division → 2 identical diploid cells (for growth/repair). Meiosis: 2 divisions → 4 unique haploid cells (for reproduction). Meiosis includes crossing over and independent assortment; mitosis does not. Results are genetically different; mitosis results are identical.

05 / 05

🔗

Connection to Mendel's Laws

Mendel's Law of Segregation = alleles separating into different gametes during Meiosis I. Mendel's Law of Independent Assortment = the random alignment of chromosome pairs at Metaphase I. Meiosis is the physical basis for all of Mendel's mathematical observations.

▶

▶ AMOEBA SISTERS

Meiosis — Amoeba Sisters

YouTube · Amoeba Sisters

▶

▶ CRASH COURSE

Meiosis: Crash Course Biology

YouTube · Crash Course

Feature	Mitosis	Meiosis
Purpose	Growth / repair	Sexual reproduction
Divisions	1	2
Cells produced	2	4
Chromosome #	Diploid (2n)	Haploid (n)
Genetic result	Identical to parent	Unique — genetically varied
Crossing over?	No	Yes — Prophase I

⚗️ Virtual Lab

Interactive Punnett Square Lab

Click each cell to reveal the offspring genotype. Switch between crosses using the tabs. Use "Reveal All" when ready to check your work.

LAB 1 Monohybrid Crosses

Click each cell in the grid to reveal the offspring genotype and phenotype. Try to predict each cell BEFORE clicking!

Monohybrid Cross: Tt × Tt — Heterozygous tall × Heterozygous tall. T = tall (dominant), t = short (recessive).

T

t

T

tapTTTall

tapTtTall

t

tapTtTall

tapttShort

Expected Ratios

Genotype: 1 TT : 2 Tt : 1 tt Phenotype: 3 Tall : 1 Short

Test Cross: Tt × tt — Heterozygous tall × Homozygous short. Used to determine whether an unknown dominant individual is TT or Tt.

T

t

tapTtTall

tapttShort

t

tapTtTall

tapttShort

Expected Ratios

Genotype: 2 Tt : 2 tt Phenotype: 1 Tall : 1 Short (1:1)

P Generation Cross: TT × tt — Homozygous tall × Homozygous short. This is Mendel's original cross — all F1 offspring are heterozygous (Tt) and tall.

T

t

tapTtTall

t

tapTtTall

Expected Ratios

100% Tt — all heterozygous 100% Tall phenotype (F1 generation)

LAB 2 Beyond Mendel Crosses

Practice incomplete dominance and carrier crosses — common on the Virginia Biology SOL.

Incomplete Dominance — P Cross: RR × WW — Red snapdragon × White snapdragon. RW = Pink (blended intermediate phenotype).

R

W

tapRWPink

W

tapRWPink

Result

100% RW = Pink — all F1 are pink

Incomplete Dominance — F2 Cross: RW × RW — Two pink snapdragons crossed. What happens in the F2 generation?

R

W

R

tapRRRed

tapRWPink

W

tapRWPink

tapWWWhite

F2 Ratio

1 RR — Red 2 RW — Pink 1 WW — White

Carrier × Carrier: Cc × Cc — Both parents unaffected carriers for cystic fibrosis. C = unaffected (dominant), c = cystic fibrosis (recessive).

C

c

C

tapCCUnaffected

tapCcCarrier

c

tapCcCarrier

tapccAffected

Result

25% CC — Unaffected, non-carrier 50% Cc — Unaffected carrier 25% cc — Has cystic fibrosis

Carrier × Affected: Cc × cc — One parent is a carrier; the other is affected. What are the odds of an affected child?

C

c

tapCcCarrier

tapccAffected

c

tapCcCarrier

tapccAffected

Result

50% Cc — Unaffected carrier 50% cc — Affected

LAB 3 Dihybrid Cross

The dihybrid cross uses a 4×4 grid (16 boxes) to track two independently assorting traits. Expected phenotype ratio: 9:3:3:1.

Dihybrid Cross: TtRr × TtRr — T = tall, t = short; R = round seed, r = wrinkled seed. Both genes on different chromosomes — independently assorting.

TR

Tr

tR

tr

TR

tapTTRRTall, Round

tapTTRrTall, Round

tapTtRRTall, Round

tapTtRrTall, Round

Tr

tapTTRrTall, Round

tapTTrrTall, Wrinkled

tapTtRrTall, Round

tapTtrrTall, Wrinkled

tR

tapTtRRTall, Round

tapTtRrTall, Round

tapttRRShort, Round

tapttRrShort, Round

tr

tapTtRrTall, Round

tapTtrrTall, Wrinkled

tapttRrShort, Round

tapttrrShort, Wrinkled

Expected Phenotype Ratio — 9:3:3:1

9 — Tall, Round 3 — Tall, Wrinkled 3 — Short, Round 1 — Short, Wrinkled

📝 SOL Quiz

Genetics & Heredity SOL Quiz

10 SOL-style questions covering all five modules. Select your answer — feedback appears immediately.

📝

Virginia Biology SOL — BIO.5

10 questions · Genetics & Heredity · Select an answer to see feedback

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