Block 3 · Heredity · MCAS Reporting Category 2

Complex inheritance

You already know simple dominance from earlier work. This block extends that to three patterns that don't follow the simple rule: codominance (both visible at once), incomplete dominance (alleles blend), and sex-linked traits (carried on the X chromosome).

What you need to know cold

An alleleA version of a gene. The gene for eye color has alleles for blue, brown, and other colors. is a version of a gene. You inherit two — one from each parent.
Your genotypeThe two alleles a person has for a trait, written like Bb, BB, or bb. is the alleles you have (BB, Bb, bb). Your phenotypeThe trait you can see — like brown eyes, blue eyes, pink flowers, or type-A blood. is the trait you see (brown eyes, pink flower).
Simple dominance: Bb shows the dominant trait. The recessive b is hidden.
codominanceInheritance where BOTH alleles are visible at once, not blended. Example: a roan cow has red AND white hairs visible.: both alleles visible AT THE SAME TIME. Roan cattle = red AND white hairs.
incomplete-dominanceInheritance where the two alleles blend into a new in-between trait. Example: red flower × white flower → pink flowers.: alleles BLEND. Red snapdragon × white snapdragon = pink offspring.
sex-linkedA trait carried on the X chromosome. Recessive sex-linked traits show up more in males because males have only one X.: trait on the X chromosome. Recessive sex-linked traits are MORE common in males because males only have one X (color blindness, hemophilia).
A Bb × Bb punnett-squareA grid that shows all the possible offspring genotypes when two parents are crossed. always gives 1 : 2 : 1 genotypes and 3 : 1 phenotypes.
On a pedigreeA family-tree diagram showing which family members have a trait. Squares = males, circles = females, filled-in = has the trait.: square = male, circle = female, filled-in = has the trait.

The Big Rule for this block

Look at the heterozygote. The way it looks tells you the pattern.

Simple dominance hides the recessive. Codominance shows both. Incomplete dominance blends them. Sex-linked changes the male-vs-female math.

Key vocabulary in 8 languages

Words from this block. Use the row in your home language to help your memory. Many of these words are similar across languages because they come from Greek and Latin roots.

English	Español	Português	Français	Italiano	Kreyòl	Tiếng Việt	العربية
allele	alelo	alelo	allèle	allele	alèl	alen	أليل(alīl)
dominant	dominante	dominante	dominant	dominante	dominan	trội	سائد(sāʾid)
recessive	recesivo	recessivo	récessif	recessivo	resesif	lặn	متنحٍّ(mutanaḥḥin)
genotype	genotipo	genótipo	génotype	genotipo	jenotip	kiểu gen	نمط جيني(namaṭ jīnī)
phenotype	fenotipo	fenótipo	phénotype	fenotipo	fenotip	kiểu hình	نمط ظاهري(namaṭ ẓāhirī)
codominance	codominancia	codominância	codominance	codominanza	kodominans	đồng trội	سيادة مشتركة(siyāda mushtaraka)
incomplete dominance	dominancia incompleta	dominância incompleta	dominance incomplète	dominanza incompleta	dominans enkonplè	trội không hoàn toàn	سيادة ناقصة(siyāda nāqiṣa)
sex-linked	ligado al sexo	ligado ao sexo	lié au sexe	legato al sesso	lye ak sèks	liên kết giới tính	مرتبط بالجنس(murtabiṭ bil-jins)
Punnett square	cuadro de Punnett	quadro de Punnett	échiquier de Punnett	quadrato di Punnett	kare Punnett	lưới Punnett	مربع بانيت(murabbaʿ bānit)
pedigree	árbol genealógico	árvore genealógica	arbre généalogique	albero genealogico	ab jenealojik	phả hệ	شجرة العائلة الوراثية(shajarat al-ʿāʾila al-wirāthiyya)

The allele row uses the verified translation from the Quick Reference vocabulary. The other 9 rows are new for Block 3 and have NOT yet been independently verified by GPT-5 / Gemini per Ms Brandolini's verification cycle — they rely on cognate consistency (Romance languages) and standard scientific-vocabulary equivalents (Vietnamese, Arabic, Haitian Kreyòl). If a term feels unfamiliar to a native speaker, please tell Ms Brandolini.

The full picture

Inheritance patterns beyond simple dominance

What this reading is about

In Block 1 you learned that dnaThe molecule that carries the genetic instructions for life. Shaped like a twisted ladder. is the molecule that holds genetic information. In Block 2 you learned how cells use that DNA to build proteins. Now we ask: how do traits actually pass from parents to children?

You already know the basic answer from earlier work: parents pass alleleA version of a gene. The gene for eye color has alleles for blue, brown, and other colors.s to their children, and the child's genotypeThe two alleles a person has for a trait, written like Bb, BB, or bb. determines their phenotypeThe trait you can see — like brown eyes, blue eyes, pink flowers, or type-A blood.. You've used punnett-squareA grid that shows all the possible offspring genotypes when two parents are crossed.s for simple dominance, and you've read pedigreeA family-tree diagram showing which family members have a trait. Squares = males, circles = females, filled-in = has the trait.s. This reading extends that work to three patterns that don't follow simple dominance:

codominanceInheritance where BOTH alleles are visible at once, not blended. Example: a roan cow has red AND white hairs visible. — both alleles fully visible at once
incomplete-dominanceInheritance where the two alleles blend into a new in-between trait. Example: red flower × white flower → pink flowers. — alleles blend into a new in-between trait
sex-linkedA trait carried on the X chromosome. Recessive sex-linked traits show up more in males because males have only one X. — trait carried on the X chromosome

The biggest skill for MCAS is recognizing which pattern a question is showing you. The end of this reading has a quick decision guide.

Quick review: alleles and simple dominance

Every geneA section of DNA that holds the instructions to build one protein. is a section of DNA that holds instructions for one trait. Most genes have more than one version. These versions are called alleleA version of a gene. The gene for eye color has alleles for blue, brown, and other colors.s.

You inherit two alleles for each gene — one from your mother and one from your father. The two alleles together make up your genotypeThe two alleles a person has for a trait, written like Bb, BB, or bb.. The trait you actually show is your phenotypeThe trait you can see — like brown eyes, blue eyes, pink flowers, or type-A blood..

If both alleles are the same, you are homozygousBoth alleles for a trait are the same — either BB or bb. "Homo-" means same. (BB or bb). If the two alleles are different, you are heterozygousThe two alleles for a trait are different — like Bb. "Hetero-" means different. (Bb).

In simple dominance, one allele masks the other. We write the dominantAn allele that hides the other when both are present. Written with a capital letter, like B. allele with a capital letter (B) and the recessiveAn allele that's hidden when paired with a dominant one. Written with a lowercase letter, like b. Only shows when both alleles are recessive. allele with a lowercase letter (b). The genotype Bb shows the dominant trait — the recessive b is hidden.

The classic Punnett square for two heterozygous parents (Bb × Bb) gives:

1 BB : 2 Bb : 1 bb (genotype ratio, 1:2:1)
3 dominant : 1 recessive (phenotype ratio, 3:1)

This is the result you should know cold. It comes up over and over.

Pattern 1: Codominance — both at once

codominanceInheritance where BOTH alleles are visible at once, not blended. Example: a roan cow has red AND white hairs visible. is a pattern where both alleles are fully visible at the same time. They don't blend, and one doesn't mask the other. The "co-" means together.

The classic example is roan cattle. A red bull (genotype R^RR^R) crossed with a white cow (R^WR^W) gives all roan offspring (R^RR^W). A roan animal has red AND white hairs side by side — both colors visible at the same time, not blended.

Another example is the human ABO blood type system. The I^A and I^B alleles are codominant. A person with one of each has type AB blood. Their red blood cells carry both A markers and B markers at the same time. (The third allele, i, is recessive to both.)

How to recognize codominance on MCAS:

The question describes both traits being visible at the same time.
The offspring shows both parent traits — not a blend, not just one.
The notation often uses two capital letters (R^R and R^W, or I^A and I^B) to show neither is recessive.

Pattern 2: Incomplete dominance — blended

incomplete-dominanceInheritance where the two alleles blend into a new in-between trait. Example: red flower × white flower → pink flowers. is a pattern where the two alleles blend together into a new in-between phenotype. Neither allele wins — they mix.

The classic example is snapdragon flower color. A red snapdragon (RR) crossed with a white snapdragon (WW) gives all pink offspring (RW). The pink is a true blend — not partly red and partly white, but a middle color.

Another example is wavy hair. A curly-haired parent (CC) and a straight-haired parent (SS) can have wavy-haired children (CS) — wavy is the blend.

Pattern 3: Sex-linked traits

A sex-linkedA trait carried on the X chromosome. Recessive sex-linked traits show up more in males because males have only one X. trait is carried on a sex chromosome — almost always the X chromosome.

Females have two X chromosomes (XX). Males have one X and one Y (XY). The Y is much smaller than the X and doesn't carry most of the genes that are on the X. So males have only one copy of every X-linked gene.

Why does this matter for inheritance? Because it changes the math:

For an X-linked recessive trait, a female needs the recessive allele on both her X chromosomes (X^bX^b) to show it.
A male needs the recessive allele on his one X (X^bY) to show it.

That's why X-linked recessive traits are far more common in males than in females. About 1 in 12 males is red-green color-blind, but only about 1 in 200 females is.

A female with one recessive X-linked allele (X^BX^b) is a carrierA heterozygote for a recessive trait. Doesn't show the trait but can pass it to their children.. She doesn't have the trait — her normal X^B masks it — but she can pass X^b to half her children.

Pedigrees: reading the family tree

A pedigreeA family-tree diagram showing which family members have a trait. Squares = males, circles = females, filled-in = has the trait. shows how a trait passes through generations. The symbols:

Square = male
Circle = female
Filled-in shape = person has the trait
Empty shape = person does not have the trait
Horizontal line between a male and a female = partners
Vertical line down = their children

Generations are labeled with Roman numerals (I, II, III) on the left.

The skill on MCAS is to look at the pattern of filled vs empty shapes and figure out how the trait is inherited:

Trait skips a generation? (Grandparents have it, parents don't, grandkids do.) Probably recessive.
Every affected person has at least one affected parent? Probably dominant.
Far more common in males than females? Possibly sex-linked recessive.
An affected father has all affected daughters and no affected sons? Almost certainly X-linked dominant.

How to spot the pattern (the decision guide)

When MCAS shows you a question about a non-simple inheritance pattern, walk this path:

Are both traits visible at the same time in the heterozygote? → codominance (red AND white hairs).
Do the two traits blend into a new color or trait? → incomplete dominance (red + white = pink).
Is the trait much more common in males, or do affected fathers pass it to all daughters but no sons? → sex-linked.
None of the above, just dominant masking recessive? → simple dominance.

The Big Rule for this block: look at the heterozygote. The way the heterozygote looks tells you which pattern is in play. Simple dominance hides the recessive. Codominance shows both. Incomplete dominance blends them. Sex-linked involves the X chromosome and changes the male-vs-female ratio.

Why this matters

Most real-world inheritance is more complex than the simple BB/Bb/bb model. Many traits — eye color, height, skin color, blood type, intelligence, risk of disease — depend on many genes working together (called polygenic inheritance) and on environment. The patterns in this reading are stepping stones toward understanding that complexity.

For MCAS, knowing the three "non-simple" patterns and being able to recognize each from a description, a Punnett square result, or a pedigree is the goal. The questions are pattern-recognition questions: which inheritance pattern is this?

Diagram: a heterozygous cross (Bb × Bb)

The most common Punnett square on the test. Both parents are heterozygous. The four boxes show every possible offspring genotype. Three boxes (BB, Bb, Bb) show the dominant phenotype. One box (bb) shows the recessive phenotype. Genotype ratio 1 : 2 : 1; phenotype ratio 3 : 1.

Punnett square: heterozygous cross (Bb × Bb)

Diagram: codominance vs incomplete dominance

Both panels show the same parental cross — a homozygous red parent crossed with a homozygous white parent. The offspring is what changes. On the left, codominance: the offspring shows BOTH colors at once (red and white side by side). On the right, incomplete dominance: the colors blend into pink. The visual contrast IS the lesson.

Codominance vs incomplete dominance

Pictures to recognize on the test

The picture shows…	The answer is…
An animal with red AND white hairs visible side by side (a roan cow).	Codominance. Both alleles visible at once.
A red parent flower × a white parent flower → all pink offspring.	Incomplete dominance. The alleles blend.
A 2 × 2 Punnett square with B and b across the top and B and b down the side.	A Bb × Bb cross. Genotypes 1 : 2 : 1, phenotypes 3 : 1.
A pedigree with a filled-in square in generation I, no affected children in generation II, and filled-in shapes again in generation III.	A recessive trait. (It "skipped" generation II — generation II must be carriers.)
A pedigree where most affected individuals are male.	Possibly sex-linked recessive. (Color blindness, hemophilia.)
A pedigree where an affected father has affected daughters and unaffected sons.	X-linked dominant (or possibly autosomal dominant — look at the broader family).

Pattern rules

If the question says…	Pick…
"Both traits visible at the same time, NOT blended."	Codominance. (Orange + black cat hairs visible side by side. AB blood with both A and B markers.)
"Two traits blend into a new color or trait."	Incomplete dominance. (Red + white snapdragon → pink.)
"Trait more common in males than in females."	Sex-linked recessive. (Males only have one X, so one recessive allele is enough.)
"Punnett square — 1 Bb × 1 Bb."	3 : 1 ratio of dominant to recessive phenotypes.
"Pedigree with a filled-in shape."	That person has the trait.
"What is the genotype of a heterozygote?"	Bb — one dominant, one recessive allele.
"Where do new alleles come from?"	Mutations. (From Block 1 / Topic 7.)
"A color-blind father has children. Which children get the color-blindness allele from him?"	All his daughters (who become carriers); none of his sons (who get his Y, not his X).

Where to practice

Practice the Block 3 — Complex inheritance test on Pear Assessment. You can retake it as many times as you want — the questions and answer choices shuffle each time, so every attempt feels a little different. Try it without looking at this page first. If you get stuck, come back, look up the pattern, then try again.