Pedigrees, sex-linked traits, incomplete dominance, codominance, multiple alleles, polygenic traits, chromosomal disorders, genetic testing, and the ethics of genetic knowledge
In Chapter 14, you learned the rules of Mendelian inheritance: alleles, dominance, Punnett squares, and the probability of traits passing from parent to offspring. Mendel worked with pea plants, where traits were discrete and predictable. Human inheritance follows the same underlying principles — but it is considerably more complex, and considerably more interesting.
Human genetics deals with traits controlled by many genes, traits located on the sex chromosomes, traits in which alleles blend or both appear simultaneously, and traits shaped jointly by genes and environment. Understanding how inheritance works in people requires extending the Mendelian model into new patterns without abandoning its foundational logic.
Throughout this chapter, remember that genetics describes probabilities, not destinies. Having a gene variant associated with a condition does not guarantee that condition will develop. Genes, environment, development, and chance all interact. The goal of studying human genetics is to understand biological inheritance honestly — not to reduce people to their genotypes.
You already know how to set up Punnett squares, calculate genotype and phenotype ratios, and apply the rules of dominant and recessive inheritance. Chapter 15 builds directly on that foundation by showing what happens when simple dominance does not apply, when genes are located on sex chromosomes rather than autosomes, and when multiple genes work together to produce a single trait. Every tool from Chapter 14 still applies — but you will now see when additional tools are needed.
When Mendel studied pea plants, he deliberately chose traits that behaved cleanly: tall or short, smooth or wrinkled, yellow or green. Each trait was controlled by a single gene with two alleles, one of which was completely dominant. The mathematics worked beautifully because Mendel selected the simplest possible cases.
Human genetics is rarely so tidy. Consider skin color: it is not controlled by one gene with two alleles. It is controlled by many genes, each contributing somewhat to the final phenotype, and the result is a continuous range of variation rather than a small set of discrete categories. Consider height: it depends on dozens of genes, plus nutrition, health during childhood, and many other environmental inputs. Even traits that follow simpler inheritance patterns — certain genetic disorders, for example — may be more nuanced than the pea plant model suggests.
This chapter focuses on the inheritance patterns most commonly encountered in high school biology and most directly relevant to understanding human health and diversity. Each extends Mendelian logic in a specific way — without replacing it.
Throughout this chapter, genetic conditions are described accurately but without reducing people to their diagnoses. People with genetic conditions live full lives. Chromosomal differences, inherited conditions, and genetic variation are part of human biological diversity — not simply lists of defects to memorize.
A pedigree is a diagram that represents the inheritance of a trait across generations in a specific family. Pedigrees are the primary tool biologists and genetic counselors use to trace inherited conditions through families and to predict the probability that future offspring will inherit a trait.
| Symbol | Meaning |
|---|---|
| Square | Male individual |
| Circle | Female individual |
| Filled shape | Individual who expresses the trait (affected) |
| Half-filled shape | Carrier individual (in some pedigrees) |
| Horizontal line between shapes | Mating pair |
| Vertical line downward | Line of descent to offspring |
| Roman numerals (I, II, III…) | Generation numbers |
| Arabic numerals (1, 2, 3…) | Individual numbers within a generation |
To analyze a pedigree, begin by identifying which individuals are affected and which are unaffected. Then work through these questions systematically.
Dominant or recessive? If two unaffected parents have an affected child, the trait is almost certainly recessive — both parents must be carriers. If the trait appears in every generation without skipping, dominant inheritance is likely.
Sex-linked? If the trait appears much more often in males than females, X-linked recessive is likely. If only males are affected and only females transmit it, X-linked recessive is strongly suggested.
Probable genotypes? Affected individuals with a recessive trait must be homozygous recessive. Unaffected parents of affected children must be carriers (heterozygous).
I-1 genotype: Aa I-2 genotype: Aa II-1 genotype: aa
Pedigrees show probabilities, not certainties. Small family sizes mean expected ratios may not appear. Not every individual may be available for study. Pedigree conclusions are probabilistic — always describe results in terms of "most likely" or "probability," not as guaranteed outcomes.
Human cells typically contain 46 chromosomes arranged in 23 pairs. Twenty-two of these pairs are autosomes — chromosomes that carry the same types of genes in both males and females. The 23rd pair consists of the sex chromosomes. Females typically have two X chromosomes (XX). Males typically have one X chromosome and one Y chromosome (XY).
The Y chromosome is small and carries few genes beyond those involved in male development. The X chromosome is much larger and carries many genes unrelated to sex determination — genes for color vision, blood clotting proteins, muscle proteins, and more. Because males carry only one X chromosome, genes on the X are inherited differently in males and females. This difference is the basis of sex-linked (or X-linked) traits.
| Genotype | Sex | Description |
|---|---|---|
X^A X^A | Female | Homozygous dominant — unaffected, not a carrier |
X^A X^a | Female | Heterozygous — carrier (unaffected but can pass allele) |
X^a X^a | Female | Homozygous recessive — affected |
X^A Y | Male | Hemizygous dominant — unaffected |
X^a Y | Male | Hemizygous recessive — affected |
Males have only one X chromosome. Whatever allele is present on that X will be expressed, because there is no second X to provide a dominant allele. Males are hemizygous for X-linked genes — they carry only one copy. This is why X-linked recessive conditions are far more common in males: a male needs only one recessive allele to be affected. A female needs two.
A female who carries one X-linked recessive allele and one dominant allele (X^A X^a) is a carrier. She does not express the condition but can pass the recessive allele to her children. Each son of a carrier female has a 50% probability of inheriting the recessive X and being affected. Each daughter has a 50% probability of being a carrier.
| X^A | X^a | |
|---|---|---|
| X^A | X^A X^A | X^A X^a |
| Y | X^A Y | X^a Y |
X^A X^A (25%) — unaffected female, not a carrier
X^A X^a (25%) — carrier female, unaffected
X^A Y (25%) — unaffected male
X^a Y (25%) — affected male
Color vision deficiency (commonly called "color blindness") affects approximately 8% of males and under 1% of females in populations of European ancestry. The most common form reduces the ability to distinguish red from green. The gene responsible is on the X chromosome, which explains the striking difference in prevalence between males and females.
Hemophilia A results from a recessive allele on the X chromosome that produces a non-functional clotting protein. Affected individuals can experience serious bleeding from even minor injuries. Hemophilia A affected several members of European royal families in the 19th and 20th centuries — historical pedigrees that helped establish the principles of X-linked inheritance in humans.
In Mendelian genetics, dominant alleles completely mask recessive alleles. A heterozygote (Aa) looks identical to the homozygous dominant (AA). But this complete dominance relationship is not universal. In incomplete dominance, neither allele is completely dominant over the other. Heterozygotes show a phenotype that is intermediate between the two homozygous phenotypes — a blend of both parental types.
| R^R | R^W | |
|---|---|---|
| R^R | R^R R^R (Red) | R^R R^W (Pink) |
| R^W | R^R R^W (Pink) | R^W R^W (White) |
Phenotype ratio: 1 Red : 2 Pink : 1 WhiteIncomplete dominance often results from gene dosage: one copy of a gene produces a reduced amount of a product (such as a pigment). Two copies produce full color; one copy produces less pigment and a lighter, intermediate color. The alleles are not blending — they are producing different quantities of product.
Incomplete dominance in humans: Familial hypercholesterolemia, a condition affecting cholesterol processing in the blood, shows a pattern resembling incomplete dominance. Individuals with two non-functional alleles have very high cholesterol and develop severe cardiovascular disease early in life. Those with one functional and one non-functional allele have intermediate cholesterol levels and moderate risk. Those with two functional alleles have normal cholesterol processing.
Codominance is different from incomplete dominance. In codominance, both alleles are expressed simultaneously in the heterozygote — not blended together, but both fully visible as distinct products. The heterozygote does not show an intermediate; it shows both parental phenotypes at once.
| Genotype | Blood Type | Antigens on Red Blood Cells |
|---|---|---|
I^A I^A or I^A i | Type A | A antigen only |
I^B I^B or I^B i | Type B | B antigen only |
I^A I^B | Type AB | Both A and B antigens — codominance |
ii | Type O | Neither A nor B antigen |
A person with genotype I^A I^B has type AB blood — they produce both A and B antigens on their red blood cells simultaneously. This is codominance: both alleles are fully and independently expressed. There is no blending. The I^A allele produces A antigen; the I^B allele produces B antigen; both are present in the heterozygote.
Blood type matters clinically because the immune system produces antibodies against antigens not present on a person's own red blood cells. Receiving blood with the wrong antigens can trigger a dangerous immune response. Understanding blood type inheritance is directly relevant to transfusions, organ transplants, and prenatal care.
"Codominance and incomplete dominance are the same thing." They are not. In incomplete dominance, the heterozygote shows a blended intermediate phenotype (pink from red and white). In codominance, both phenotypes are fully expressed simultaneously — type AB blood shows both A and B antigens, not some intermediate "AB antigen." The distinction is whether alleles blend or coexist distinctly.
The ABO blood type system also illustrates multiple alleles: the existence of more than two alleles for a single gene within a population. The ABO gene has three common alleles — I^A, I^B, and i — though any one individual can carry at most two of them (one on each homologous chromosome).
Multiple alleles arise through mutation over time. As populations accumulate different mutations at the same genetic locus, multiple functional variants can exist and persist. The existence of multiple alleles increases phenotypic diversity in populations beyond what two alleles alone can produce. This population-level diversity is a source of variation that natural selection can act upon — a concept you will revisit in Chapter 17.
| I^A | i | |
|---|---|---|
| I^B | I^A I^B (AB) | I^B i (B) |
| i | I^A i (A) | ii (O) |
Type AB: 25% Type B: 25% Type A: 25% Type O: 25%Most of the traits that visibly vary among people — skin color, height, eye color, facial features — are polygenic traits: traits controlled by two or more genes acting together. In polygenic inheritance, each contributing gene may have multiple alleles, each allele contributes a small amount to the phenotype, and the total phenotype results from the cumulative contribution of all contributing alleles. Environmental factors often interact with genotype to produce the final phenotype.
The result is not a few discrete categories but a continuous distribution of phenotypes across a range. If you measure the heights of 1,000 people, you will not see two groups — "short" and "tall." You will see a bell-shaped curve with most people near the average and fewer individuals at the extremes.
Imagine a trait controlled by just 3 genes, each with two alleles (capital = more pigment, lowercase = less). The number of "contributing" alleles a person inherits ranges from 0 to 6. Very few people will inherit all 0 or all 6 (the extremes). Most will inherit a mix, producing an intermediate phenotype. As the number of contributing genes increases, the distribution becomes smoother — until it approaches the continuous bell-shaped curve we observe for traits like height and skin color.
Environment and polygenic traits: Height is a classic example. Tall parents tend to have tall children (genetic component), but children who experience severe malnutrition during growth will not reach the heights their genotype would otherwise support. The phenotype is a product of genotype and environment together. This means knowing someone's genotype does not tell you their phenotype for polygenic traits — it can only describe a range of likely outcomes, not a fixed result.
It is important to understand that skin color is a biological trait shaped by natural selection in response to UV radiation at different latitudes. It carries no information about intelligence, character, or health. The continuous variation in skin color helps explain why it does not sort neatly into a small number of categories — and why attempts to do so have no biological basis.
During meiosis, homologous chromosomes are supposed to separate into different cells. Sometimes this separation fails — a process called nondisjunction. Nondisjunction results in gametes with an abnormal number of chromosomes. If a gamete with an extra chromosome (n + 1) is fertilized by a normal gamete (n), the resulting zygote has three copies of one chromosome instead of two — a condition called trisomy. If a gamete missing a chromosome (n − 1) is fertilized, the zygote has only one copy — a condition called monosomy.
Most trisomies and monosomies are incompatible with development. Fetuses with most chromosomal trisomies do not survive to birth. A few specific conditions are survivable and result in recognizable patterns of features.
Gamete with 2 copies of chr. 21 + normal gamete (1 copy)
→ Zygote with 3 copies of chr. 21 = Trisomy 21Down syndrome results from trisomy 21 — three copies of chromosome 21 instead of two. It is the most common chromosomal disorder in live births. People with Down syndrome have a characteristic set of physical features, including differences in facial structure and lower muscle tone at birth, and often some degree of intellectual disability that varies widely among individuals. They are also at increased risk for certain heart defects, thyroid conditions, and early-onset Alzheimer's disease.
People with Down syndrome can live full lives, attend school, form relationships, participate in their communities, and hold employment. Intellectual abilities vary considerably — Down syndrome does not produce a single fixed level of cognitive function. The probability of trisomy 21 increases with the age of the biological mother at conception, because older eggs have a higher rate of nondisjunction during meiosis.
Errors in the number of sex chromosomes are more common than autosomal trisomies and are generally more compatible with survival.
| Condition | Chromosome Pattern | Features |
|---|---|---|
| Klinefelter syndrome | 47, XXY (male) | Reduced fertility; reduced testosterone; sometimes learning differences; often undiagnosed until puberty or later |
| Turner syndrome | 45, X (female) | Short stature; reduced fertility or infertility; some heart abnormalities; typical cognitive function (some spatial reasoning differences) |
| Triple X syndrome | 47, XXX (female) | Usually typical development and fertility; often undiagnosed |
| XYY syndrome | 47, XYY (male) | Usually typical development; no significant health concerns |
Chromosomal disorders can also result from structural changes rather than changes in chromosome number. A deletion occurs when a segment of a chromosome is lost. A duplication occurs when a segment is copied twice. An inversion occurs when a segment is reversed within the chromosome. A translocation occurs when a segment moves from one chromosome to a non-homologous chromosome. Structural errors may disrupt gene function by removing genes, duplicating them, or placing them in new chromosomal environments. Their effects depend on which genes are involved and how they are disrupted.
"Trisomy 21 always causes severe intellectual disability." Down syndrome produces a wide range of cognitive and developmental outcomes. Some individuals have significant intellectual disabilities; others have mild difficulties and live largely independent lives. Assuming a chromosomal diagnosis predetermines a specific level of function is not scientifically supported and can cause real harm by leading to reduced expectations.
Advances in molecular biology have made it possible to detect specific alleles, chromosomal abnormalities, and even whole genome sequences from small biological samples. Genetic testing is used in several distinct contexts, each with different goals and implications.
| Type of Testing | Purpose | When Used |
|---|---|---|
| Carrier testing | Determine if a person carries one copy of a recessive allele | Before or during pregnancy when family history is known |
| Prenatal testing | Examine fetal chromosomes or genes before birth | During pregnancy, especially with risk factors |
| Newborn screening | Test infants for treatable metabolic and genetic conditions | Shortly after birth; standard in most countries |
| Diagnostic testing | Confirm or identify cause of symptoms that may have a genetic basis | When symptoms suggest a genetic condition |
| Predictive testing | Test individuals at risk for late-onset conditions before symptoms appear | Adults with family history of hereditary diseases |
Amniocentesis involves withdrawing a small amount of amniotic fluid from the fluid-filled sac surrounding a developing fetus. The fluid contains fetal cells whose chromosomes can be karyotyped or whose DNA can be analyzed. It is typically performed in the second trimester and carries a small risk of miscarriage.
Chorionic villus sampling (CVS) involves taking a small sample of placental tissue, which shares the same genetic material as the fetus. CVS can be performed earlier than amniocentesis but also carries a small procedural risk.
Cell-free fetal DNA testing (noninvasive prenatal testing, or NIPT) analyzes fragments of fetal DNA that circulate in the mother's blood. It carries no procedural risk to the fetus but screens for certain conditions rather than providing a definitive diagnosis.
A karyotype is an organized image of an individual's chromosomes, arranged by size and type. To produce a karyotype, cells are collected, treated to stop division at a stage when chromosomes are most visible, stained, photographed through a microscope, and then the chromosome images are digitally arranged in matching pairs. The pattern of chromosome number, size, and banding allows biologists to identify trisomies, monosomies, and structural rearrangements.
Genetic counseling is a professional service that helps individuals and families understand genetic information and its implications. Genetic counselors are trained in genetics, medicine, and counseling skills. They explain inheritance patterns, calculate probabilities, describe the nature and medical implications of genetic conditions, explain available testing options, and support families in making decisions that align with their own values — without directing or pressuring individuals toward specific choices.
Genetic test results can be complex, probabilistic, and emotionally significant. A trained genetic counselor can explain what a result means (and what it does not mean), describe the range of outcomes associated with a condition, provide access to support resources, and help a family think through their situation without imposing values. This human support cannot be replaced by internet searches alone.
Genetic information is unique among personal information: it is not just information about one individual. It is also information about biological relatives who may not have chosen to share or receive it. A person's genetic test results can reveal information about their parents, siblings, children, and cousins.
This creates difficult questions. Should employers have access to genetic information? Should insurance companies? Should family members be entitled to know about a relative's results if those results are relevant to their own health? Different societies and legal systems have answered these questions differently. In the United States, the Genetic Information Nondiscrimination Act (GINA) prohibits health insurers and employers from discriminating based on genetic information — but this law has limitations and does not cover life insurance or disability insurance.
Genetic determinism is the mistaken idea that genes completely determine traits, behaviors, or life outcomes. This is scientifically inaccurate and socially harmful. For almost all complex traits — including personality, intelligence, disease risk, and behavior — genes interact with environment, development, and chance in ways that make simple prediction impossible. Having a gene variant associated with a condition increases risk, but does not guarantee an outcome.
When genetic determinism is applied to groups rather than individuals, the errors and harms multiply. The history of eugenics — the attempt to improve human populations by controlling reproduction based on assumed genetic traits — is a sobering reminder of how badly genetics can be misapplied. Eugenics policies in the 20th century caused serious harm to many people and were built on flawed genetics, flawed reasoning, and profound disrespect for human dignity. Modern genetics does not support eugenics or deterministic views of human potential.
When prenatal genetic testing reveals that a fetus has a chromosomal disorder or serious genetic condition, parents face difficult decisions. Different families — based on their values, religious beliefs, medical circumstances, and support systems — may make different decisions. These decisions are personal and should be supported rather than directed by medical or governmental authority. Genetic counselors are trained to provide information and support without imposing values on the families they serve.
Human genetic variation is enormous, and much of it does not cause disease — it is simply variation, the natural result of evolution, mutation, recombination, and population history. Even genetic conditions that reduce typical function in some respects are part of the spectrum of human variation. Many people in communities such as the Deaf community and the Down syndrome community emphasize that their identities and experiences are not simply defined by what they cannot do, and that genetic knowledge should not lead to devaluing the lives of people with genetic differences.
Studying human genetics should deepen respect for biological complexity and human diversity, not reduce people to genotypes. Genetic knowledge gives us insight into biological inheritance — but it does not tell us what a person's life is worth, or what they are capable of.
"Having a gene for a condition means you will develop that condition." For most conditions, having a risk allele increases probability but does not guarantee an outcome. Many conditions require additional genetic or environmental factors to develop. Even for single-gene disorders, the expression of a gene (penetrance and expressivity) can vary significantly among individuals. Genetics describes probabilities and tendencies — not fixed destinies.
These questions do not have single correct answers. Support your reasoning with evidence and acknowledge more than one perspective.
Use this pedigree for questions 1–4: Generation I — unaffected female (I-1) and unaffected male (I-2). Their children: affected female (II-1), unaffected male (II-2), unaffected female (II-3). II-2 marries unaffected female (II-4); they have an affected son (III-1) and unaffected daughter (III-2).
For each scenario, identify the inheritance pattern and explain your reasoning.
Each problem shows a student's reasoning. Identify the error and write the correct answer.
Identify the error. Explain the correct distinction and give one example of each.
What is wrong with this reasoning? Where did the man actually inherit his X-linked allele?
Is this claim correct? Use genotype reasoning to explain whether this parentage is biologically possible.
Explain the scientific error in this statement. How do both genes and environment contribute to a polygenic trait like height?
What is wrong with this conclusion? What other inheritance pattern also causes traits to skip generations?
A child is born with type O blood. The biological father listed on the birth record has type AB blood.
Questions:
A genetic counselor is working with a family in which the mother is 38 years old and a prenatal test reveals that the fetus has trisomy 21.
"X-linked traits always skip a generation." Not always. Whether an X-linked trait appears in a given generation depends on whether carrier females in that generation have sons who inherit the recessive allele. In some pedigrees the trait appears in every generation; in others it appears to skip. The key is carrier status, not a fixed generational pattern.
"Codominance and incomplete dominance are the same thing." They are not. In incomplete dominance, the heterozygote shows a blended intermediate phenotype (pink from red and white parents). In codominance, both phenotypes are fully and separately expressed at once — type AB blood shows both A and B antigens simultaneously, not some intermediate "AB antigen."
"Trisomy 21 always causes severe intellectual disability." Down syndrome produces a wide range of cognitive and developmental outcomes. Some individuals have significant intellectual disabilities; others have mild difficulties and live largely independent lives. Assuming a diagnosis predetermines a specific level of function is not scientifically supported.
"Having a gene for a condition means you will develop that condition." For most conditions, having a risk allele increases probability but does not guarantee an outcome. Environment, other genes, developmental factors, and chance all interact with genotype. Genetics describes probabilities — not fixed destinies.
"Polygenic traits are too complicated to study scientifically." While polygenic traits are more complex than single-gene traits, scientists study them effectively using large population studies, statistical analysis of genetic data, and careful measurement of traits across populations. Complexity makes individual-level prediction difficult, but it does not prevent scientific understanding of how multiple genes contribute.
Chapter 15 extended Mendelian genetics into the complexity of human inheritance — patterns in which simple dominance does not apply, traits encoded on sex chromosomes, multiple alleles, many-gene systems, and chromosomal errors. But throughout this chapter, existing alleles were transmitted, not created. Chapter 16 asks a deeper question: where do new alleles come from in the first place? You will learn that mutations are the original source of all genetic variation, examine the different types of mutations and their effects on organisms, and explore how genetic recombination shuffles existing variation into new combinations. Chapter 16 will then extend into how humans have learned to read, copy, and deliberately edit genetic information — from selective breeding to PCR to CRISPR — and will examine the benefits, risks, and ethical responsibilities that come with the power to alter biological information.