How XX and XY Chromosomes Determine Your Sex — The Genetics of Sex and Chromosomes Explained

Whether a baby develops as biologically male or female is decided at the moment of fertilization by a single chromosome delivered by the sperm. That one chromosome, the Y, switches on a tiny gene that sets in motion an entire cascade of hormones, reshaping a default female body plan into a male one. The system is elegant, powerful, and — like all biology — occasionally surprising, producing outcomes that defy the simple "XX is female, XY is male" shorthand.

From the 46 chromosomes packed into nearly every cell of your body, to the master-switch SRY gene discovered in 1990, to the reason an extra chromosome 21 causes Down syndrome while an extra X is usually harmless, the genetics of sex and chromosomes is one of the most revealing chapters in human biology. It explains not only how boys and girls are made, but why certain conditions appear, why they sometimes run in families, and how modern medicine can detect them before birth.

source/credit: pexels@EdwardJenner

The 46 Chromosomes — Your Complete Set

Nearly every cell in your body carries your entire genetic instruction manual, organized into 46 chromosomes arranged in 23 pairs. You inherit one chromosome of each pair from your mother (through the egg) and one from your father (through the sperm), so half of your chromosomes are maternal and half are paternal.

22 Pairs of Autosomes, 1 Pair of Sex Chromosomes

The first 22 pairs are called autosomes — they are numbered 1 through 22, roughly in order of decreasing size, and they are identical in males and females. They carry the vast majority of your roughly 20,000 genes and govern almost every aspect of your body, from eye color to height to disease risk.

The 23rd pair is different: these are the sex chromosomes, and they come in two forms, called X and Y. This single pair is what determines biological sex. Females typically have two X chromosomes (XX), while males typically have one X and one Y chromosome (XY). Because the Y chromosome is far smaller than the X, the total amount of DNA differs slightly between the sexes, but both have the standard 46 chromosomes per cell.

Why You Have Pairs, Not Singles

Having two copies of every chromosome is a safety feature. If a gene on one chromosome is damaged or carries a harmful mutation, the matching gene on the paired chromosome can often compensate. This redundancy is the genetic reason why recessive conditions (discussed in our inheritance article) can stay hidden in carriers. The only chromosome humans do not always have in a matched pair is the Y: males have a single Y paired with a single X, with no second copy to back it up.

How Biological Sex Is Determined — The XX and XY System

Sex determination in humans follows the XY system, the mechanism used by nearly all mammals. The sex of a baby is set entirely by which sex chromosome the sperm contributes at fertilization.

The Egg Always Carries an X

Because females are XX, every egg cell they produce receives one X chromosome. There is no other option — eggs are always X-bearing. Males, however, are XY, so when they produce sperm, roughly half carry an X and half carry a Y. This 50/50 split is why, across large populations, the number of boys and girls born is almost exactly equal (in fact, very slightly more boys are conceived, since the Y-bearing sperm is lighter and swims slightly faster).

Fertilization Decides Everything

At the moment of fertilization, the sperm's sex chromosome joins the egg's X:

• X sperm + X egg = XX = female
• Y sperm + X egg = XY = male

This means that biologically, the father's sperm determines the sex of the child. The mother can only ever contribute an X; it is the sperm that brings either the X or the Y. Despite centuries of folk theories about diet, timing, or position influencing a baby's sex, none of these has any scientific basis — it is a genuine coin flip, decided by which of the hundreds of millions of sperm reaches the egg first.

The SRY Gene — The Master Switch on the Y Chromosome

The Y chromosome's power to trigger male development comes down to a single gene. Without it, even an XY embryo develops as female. With it, even an XX embryo can develop as male.

One Gene That Changes Everything

That gene is SRY — the Sex-determining Region of the Y chromosome. It was identified in 1990 by a team led by Andrew Sinclair, who pinpointed it as the long-sought testis-determining factor (TDF). SRY sits on the short arm of the Y chromosome and encodes a protein that acts as a transcription factor — a molecular switch that turns other genes on or off.

SRY is necessary and sufficient to launch male development. Remarkable experiments in mice confirmed this: when researchers inserted the Sry gene into XX (genetically female) embryos, the animals developed as males. The presence or absence of this single gene flips the entire developmental pathway.

The Default Pathway Is Female

A key insight of developmental biology is that the default human body plan is female. For the first six weeks of embryonic life, the reproductive system develops identically in XX and XY embryos, with a pair of undifferentiated "bipotential" gonads that could become either ovaries or testes. Around week 6 to 7, SRY switches on in XY embryos and redirects those gonads to become testes. In the absence of SRY — as in XX embryos — the gonads follow the default route and become ovaries.

When SRY Goes Wrong — XX Males and XY Females

Because everything hinges on SRY, errors involving this gene produce striking exceptions to the XX/XY rule:

• XX males — Sometimes, during sperm production, the SRY gene breaks off the Y chromosome and attaches to an X chromosome. An XX embryo that inherits this SRY-bearing X will develop as male, even without a Y chromosome. This condition, called de la Chapelle syndrome, is rare (about 1 in 20,000) and usually causes infertility because other Y genes needed for sperm production are absent.
• XY females (Swyer syndrome) — If the SRY gene is present but mutated and nonfunctional, an XY embryo's gonads never become testes. The embryo follows the default female pathway, developing female reproductive structures. Individuals with Swyer syndrome are raised as girls and have a female appearance, though they typically need hormone treatment and have underdeveloped (streak) gonads that must often be removed because of cancer risk.

These cases powerfully demonstrate that biological sex is driven by genes and the developmental cascade they trigger, not by chromosomes alone.

From SRY to Anatomy — The Hormonal Cascade

SRY is just the first domino. Once it switches on, it sets off a chain reaction of genes and hormones that build male anatomy step by step.

Testes, Sertoli Cells, and Two Key Hormones

After SRY activates, the developing gonads become testes containing two crucial cell types: Sertoli cells and Leydig cells. Sertoli cells produce anti-Müllerian hormone (AMH), which causes the Müllerian ducts — the precursors of the female uterus and fallopian tubes — to wither away. Leydig cells produce testosterone, which drives the Wolffian ducts to develop into male internal reproductive structures such as the epididymis and vas deferens.

Testosterone is also converted into a more potent hormone, dihydrotestosterone (DHT), which directs the development of male external genitalia. The entire process is a precisely timed relay: switch on the right gene at the right moment, and a testis forms; let the cascade run, and testosterone sculpts the male body. Block or disrupt any step, and development can take a different course — which is the basis of several differences of sex development (DSD), formerly called intersex conditions.

Female Development — Active, Not Passive

It is a common misconception that female development is simply the "do-nothing" default. In reality, once the gonads become ovaries (in the absence of SRY), female development is driven by its own set of active genes and signals. The point of the "default" label is only that no Y-chromosome trigger is required for the female pathway to proceed — but it is still a complex, genetically directed process, not a passive fallback.

X-Inactivation — Why Two X's Are Not Double Trouble

If females have two X chromosomes and males have only one, does that mean females produce twice as much of every X-linked gene's protein? They do not — thanks to an ingenious mechanism called X-inactivation.

The Barr Body and the Lyon Hypothesis

Early in female embryonic development, one of the two X chromosomes in each cell is permanently switched off — condensed into a tiny, inactive clump of DNA visible under a microscope as the Barr body (or sex chromatin). This process, explained by geneticist Mary Lyon in 1961 (the "Lyon hypothesis"), equalizes the "dosage" of X-linked genes between males (one X) and females (two X's, but only one active).

Crucially, X-inactivation is random in each cell: some cells switch off the maternal X, others the paternal X. Once a cell makes its choice, all of that cell's descendants keep the same X inactive. This means every female is a genetic mosaic — a patchwork of cells, some expressing her mother's X-linked genes and some her father's.

The Calico Cat and the Mosaic Female

The most vivid illustration is the calico cat. The gene for coat color sits on the X chromosome, and the orange and black patches of a calico directly map which X was inactivated in the skin cells that grew each patch. Because a male cat has only one X, true calico males are extremely rare (and usually XXY). The same mosaic principle, less visibly, operates in every human female's body.

Why Extra X Chromosomes Are Tolerated

X-inactivation is also the reason people can survive with extra X chromosomes — for example, XXY (Klinefelter) or XXX (Triple X) — far better than they can survive with extra autosomes. The extra X's are simply inactivated into additional Barr bodies. The rule is elegant: the number of Barr bodies always equals the number of X chromosomes minus one. A normal female (XX) has one Barr body; a normal male (XY) has none; a person with XXY has one; a person with XXX has two.

Exceptions — Genes That Escape Inactivation

X-inactivation is not perfect. A small percentage of genes near the tip of the X chromosome escape silencing and remain active on both copies. This is why individuals with extra X chromosomes are not completely unaffected — the small number of "double-dosed" genes accounts for the subtle features of conditions like Klinefelter syndrome.

Nondisjunction — When Chromosomes Fail to Separate

Most chromosomal disorders arise from a single kind of accident: nondisjunction, the failure of chromosomes to separate properly when a cell divides. This error can occur during meiosis (the cell division that makes eggs and sperm) or, less often, during the early mitotic divisions of the embryo.

How Nondisjunction Produces Aneuploidy

Normally, when a cell divides, each daughter cell receives exactly one copy of every chromosome. In nondisjunction, a chromosome pair fails to separate, so one daughter cell gets an extra chromosome (24) and the other gets none (22). If an egg or sperm carrying an abnormal number of chromosomes takes part in fertilization, the resulting embryo has a chromosomal imbalance called aneuploidy. A baby can end up with 45 chromosomes (monosomy) or 47 (trisomy) instead of the usual 46.

The Maternal Age Effect

One of the most well-established facts in genetics is that the risk of certain chromosomal disorders rises sharply with the mother's age. This is because a woman's eggs are formed before she is even born, and they remain arrested in an early stage of meiosis for decades. The longer they wait, the greater the chance that a chromosome pair will fail to separate properly when ovulation finally occurs. For Down syndrome, the risk is about 1 in 1,500 at age 20, about 1 in 1,000 at age 30, about 1 in 350 at age 35, and about 1 in 100 at age 40. Paternal age has a smaller effect.

How Common Is It?

Chromosomal errors are far more common than most people realize. It is estimated that at least 1 in every 25 sperm or eggs carries an abnormal chromosome number. Most of these errors are lethal early in development and end in very early miscarriage — often before a woman even knows she was pregnant. In fact, chromosomal abnormalities are the single most common cause of miscarriage, present in roughly half of all first-trimester losses. The few aneuploidies compatible with life are the ones we recognize as named syndromes.

Mosaicism — When Only Some Cells Are Affected

If nondisjunction happens not in the parents' gametes but during one of the embryo's own early cell divisions, the result is mosaicism: the individual has a mixture of normal and abnormal cells. The effects are often milder than full trisomy because some cells are unaffected. Mosaic Down syndrome, for instance, can produce a wider and often milder range of features than full trisomy 21. Importantly, mosaicism is not linked to maternal age, because the error occurs after fertilization.

Autosomal Chromosomal Disorders

Because autosomes carry far more genes than the sex chromosomes, and because there is no inactivation mechanism to buffer them, an extra or missing autosome has severe consequences. In fact, the only autosomal aneuploidies that survive to birth involve the smallest chromosomes — and even then, the effects are significant.

Down Syndrome (Trisomy 21)

Down syndrome is the most common chromosomal disorder among live births, occurring in about 1 in 700 babies. It was first described in 1866 by the British physician John Langdon Down, but its chromosomal cause — an extra copy of chromosome 21 — was not discovered until 1959, by the French geneticist Jérôme Lejeune. About 95% of cases are caused by free trisomy 21 (nondisjunction), about 4% by a Robertsonian translocation (in which chromosome 21 material attaches to another chromosome), and about 1% by mosaicism. In roughly 90–95% of nondisjunction cases, the extra chromosome comes from the mother.

Down syndrome is associated with characteristic physical features, varying degrees of intellectual disability, and a higher risk of certain heart defects and other medical conditions — but also, increasingly, with longer, healthier lives. With modern medical care, early intervention, and educational support, many people with Down syndrome live into their 60s and beyond, and lead rich, meaningful lives.

Edwards Syndrome (Trisomy 18)

Edwards syndrome, caused by an extra chromosome 18, is far rarer (about 1 in 6,000 births) and much more severe. It is associated with major heart defects, severe growth restriction, and characteristic features such as clenched fists with overlapping fingers and "rocker-bottom" feet. Tragically, about 90% of affected babies do not survive their first year, though a small number live longer with intensive care.

Patau Syndrome (Trisomy 13)

Patau syndrome, from an extra chromosome 13 (about 1 in 16,000 births), is the most severe of the three common autosomal trisomies. It involves serious brain and facial malformations, heart defects, and extra fingers or toes (polydactyly). Like Edwards syndrome, survival beyond the first year is rare.

Why No Monosomies Survive

With a single exception, losing an entire autosome is lethal. Having only one copy of a large, gene-rich chromosome leaves the body with too little genetic information to develop. The only monosomy compatible with life involves the X chromosome — Turner syndrome (45,X) — and even then, the vast majority of 45,X conceptions miscarry. This asymmetry tells us that the body can tolerate extra genetic material better than it can tolerate a total loss.

Sex Chromosome Disorders — The Aneuploidies

Errors in the number of sex chromosomes are, on the whole, far better tolerated than autosomal errors — thanks largely to X-inactivation and the small number of genes on the Y chromosome. These conditions are surprisingly common and often go undiagnosed for years.

Turner Syndrome (45,X) — Monosomy X

In Turner syndrome, a female has only one X chromosome instead of two. It occurs in about 1 in 2,500 live female births — though this understates how often it arises, because an estimated 95–98% of 45,X conceptions end in miscarriage. (Remarkably, it is one of the most common chromosome abnormalities found in early pregnancy loss.) Features typically include short stature, a webbed neck, heart and kidney abnormalities, and ovarian dysgenesis — the ovaries do not develop properly, leading to lack of puberty and infertility without treatment. Growth hormone therapy and estrogen replacement can greatly improve outcomes. Notably, there is no Barr body, because there is only one X to begin with.

Klinefelter Syndrome (47,XXY)

Klinefelter syndrome is the most common sex chromosome aneuploidy, affecting about 1 in 500 to 1 in 1,000 males. Affected individuals are male (they have a Y chromosome) but carry one or more extra X chromosomes. The condition is often not diagnosed until adolescence or adulthood, when features such as tall stature, small firm testes, low testosterone, gynecomastia (enlarged breast tissue), and infertility become apparent. Because one X is inactivated into a Barr body, the effects are relatively mild, and testosterone replacement therapy can help manage many symptoms.

Triple X Syndrome (47,XXX) and XYY Syndrome (47,XYY)

Two other sex chromosome aneuploidies are often so mild they may go unnoticed entirely:

• Triple X (47,XXX) — females with an extra X chromosome; about 1 in 1,000 females. Many have no symptoms, though some have mild learning difficulties or are taller than average. There are two Barr bodies.
• XYY syndrome (47,XYY) — males with an extra Y chromosome; about 1 in 1,000 males. Again, most are asymptomatic, though they tend to be taller. Contrary to an old and debunked myth, XYY males are not predisposed to violence or criminality.

The overarching lesson: the sex chromosomes are remarkably forgiving of numerical errors, which is why sex chromosome aneuploidies are far more common — and far less devastating — than autosomal ones.

The Shrinking Y Chromosome

The Y chromosome is unusual in several respects — and its evolutionary story is one of the most fascinating in genetics.

Small and Shrinking

The Y chromosome is by far the smallest human chromosome, containing only about 57 million base pairs and roughly 70 protein-coding genes — a tiny fraction of the X's ~800 genes. Over hundreds of millions of years, the Y has been steadily losing genes; it is estimated to have shed around 97% of the genes it once shared with its evolutionary partner, the X. This decline happened because the Y does not recombine (exchange DNA) with a partner over most of its length, so damaged genes cannot be repaired and are gradually lost.

Not Disappearing

Headlines sometimes claim "the Y chromosome is disappearing" and that males are doomed. This is misleading. While the Y has shrunk dramatically over evolutionary time, the genes that remain on it — including SRY and the genes essential for sperm production — are highly conserved and protected. Studies show that the human Y chromosome has been essentially stable for the last 25 million years. The Y is small, but it is not vanishing anytime soon.

The Y-Chromosome Adam

Because the Y chromosome passes essentially unchanged from father to son (with only rare mutations), it can be used to trace paternal lineages deep into the past — the male counterpart to Mitochondrial Eve (traced through mitochondrial DNA, which passes only from mother to child). Geneticists have identified a "Y-chromosomal Adam," the most recent common male ancestor of all living men, who lived in Africa roughly 200,000 to 300,000 years ago. Like Mitochondrial Eve, he was not the only man alive — simply the one whose Y lineage survived unbroken to the present day.

How Chromosomes Are Tested and Screened

Understanding chromosomes has given medicine powerful tools to detect abnormalities — both before and after birth.

Karyotyping — Photographing the Chromosomes

A karyotype is a laboratory photograph of a person's chromosomes, arranged in pairs and ordered by size. To produce it, technicians grow cells (often from blood), stop them during cell division when chromosomes are most visible, stain them, and photograph them under a microscope. A karyotype can reveal missing, extra, or rearranged chromosomes and remains a foundational test in genetics. Modern higher-resolution techniques, such as chromosomal microarray, can detect much smaller deletions and duplications that a standard karyotype would miss.

Prenatal Screening — NIPT

Today, the most common way to screen for chromosomal conditions before birth is non-invasive prenatal testing (NIPT). During pregnancy, small fragments of the baby's DNA circulate in the mother's bloodstream. From a simple blood draw (usually after 10 weeks), NIPT analyzes this cell-free DNA to estimate the risk of conditions such as Down, Edwards, and Patau syndromes, and can also reveal the baby's sex by detecting the presence or absence of Y-chromosome DNA. NIPT is a screening test — it flags risk but does not diagnose — so a positive result is usually followed by a diagnostic test.

Diagnostic Testing — CVS and Amniocentesis

For a definitive diagnosis, two procedures are used: chorionic villus sampling (CVS), which takes a small sample of placental tissue (usually at 10–13 weeks), and amniocentesis, which samples amniotic fluid (usually at 15–20 weeks). Both can perform a full karyotype or microarray on fetal cells. Because they are invasive and carry a small miscarriage risk, they are generally offered when screening suggests elevated risk, when there is a family history, or when an ultrasound reveals concerning findings.

Newborn Screening and Childhood Diagnosis

Some sex chromosome conditions — such as Turner or Klinefelter syndrome — may first be suspected in childhood due to short stature or delayed puberty, and confirmed with a karyotype. Early diagnosis allows timely treatment (such as growth hormone or hormone replacement) and appropriate support for any learning differences.

When to Consider Genetic Counseling

A genetic counselor is a specialist who helps individuals and couples understand their risk of chromosomal and genetic conditions, interpret test results, and make informed decisions. Consider genetic counseling if any of the following apply:

• You are pregnant or planning a pregnancy, especially if you will be 35 or older at delivery, since maternal age raises the risk of chromosomal conditions.
• You have received an abnormal NIPT or screening result and want to understand what it means and what your options are.
• You have a family history of a chromosomal disorder, repeated miscarriages, or a child born with a chromosomal condition.
• You or your child have features that suggest a possible chromosomal or sex-development condition and you are considering testing.
• You have had fertility problems, as some sex chromosome conditions (such as Klinefelter or Turner syndrome) can underlie infertility.

Genetic counselors provide clear, non-directive guidance — they explain the science and your options so you can make the decision that is right for you and your family.

FAQ

What determines whether a baby is a boy or a girl?

Sex is determined at fertilization by the sex chromosome carried by the sperm. All eggs carry an X chromosome. Sperm carry either an X or a Y. If an X-bearing sperm fertilizes the egg, the baby is XX and develops as female; if a Y-bearing sperm fertilizes the egg, the baby is XY and develops as male. Because the mother always contributes an X, the father's sperm determines the sex — it is, in effect, a 50/50 genetic coin flip. Folk methods of influencing a baby's sex (diet, timing, position) have no scientific support.

Does having a Y chromosome automatically make someone male?

In the vast majority of cases, yes — the Y chromosome's SRY gene triggers male development. But there are important exceptions. If SRY is mutated and nonfunctional, an XY embryo can develop as female (a condition called Swyer syndrome). Conversely, if SRY translocates onto an X chromosome, an XX embryo can develop as male (de la Chapelle syndrome). These cases show that it is the gene and the developmental cascade it triggers — not the chromosome itself — that directs sex development. Biological sex is primarily a genetic and hormonal process, and it is more nuanced than a simple XX-versus-XY rule.

What is Down syndrome, and what causes it?

Down syndrome is caused by an extra copy of chromosome 21 (trisomy 21), so that affected individuals have 47 chromosomes instead of 46. About 95% of cases result from nondisjunction — a failure of chromosome 21 to separate properly during the formation of an egg or sperm — and the risk rises significantly with the mother's age. A smaller percentage result from a translocation (inherited chromosome 21 material attached to another chromosome) or mosaicism (only some cells affected). Discovered by Jérôme Lejeune in 1959, Down syndrome is the most common chromosomal disorder among live births, affecting about 1 in 700 babies, and is associated with characteristic features, intellectual disability of varying degree, and a higher risk of certain medical conditions — though modern care has greatly extended life expectancy and quality of life.

Why can someone survive with an extra sex chromosome but not an extra autosome?

The key is X-inactivation. When a person has extra X chromosomes (such as in XXY or XXX), the extras are switched off into inactive Barr bodies, so the body is largely protected from a dangerous overdose of X-linked genes. And because the Y chromosome carries very few genes, an extra Y (as in XYY) has little effect. Autosomes, by contrast, carry hundreds or thousands of essential genes and have no inactivation mechanism to buffer an extra copy — so an extra autosome floods cells with too much gene product, which is usually lethal. This is why only the smallest autosomal trisomies (21, 18, 13) ever survive to birth, while extra sex chromosomes are relatively common and often mild.

Does the mother's age really affect the risk of chromosomal disorders?

Yes. This is one of the best-established relationships in genetics. A woman's eggs are formed before birth and remain paused in an early stage of cell division for decades; the longer they wait, the greater the chance a chromosome will fail to separate correctly (nondisjunction) at ovulation. The risk of Down syndrome rises from about 1 in 1,500 at age 20 to about 1 in 100 at age 40. The risks for Edwards and Patau syndromes and other aneuploidies rise similarly. This maternal-age effect is why prenatal screening is especially recommended for pregnant individuals aged 35 and older. Paternal age contributes as well, but to a much smaller degree.

Is the Y chromosome really disappearing?

It has shrunk dramatically over evolutionary time, but it is not disappearing in humans. The Y lost most of its ancestral genes over hundreds of millions of years because it does not recombine over most of its length, so damaged genes cannot be repaired. However, the genes that remain — including SRY and those essential for sperm production — are tightly conserved and protected. Research shows the human Y chromosome has been essentially stable for the last 25 million years. So while the Y is small and has a dramatic evolutionary history, men are not in danger of losing it.

References

• Sinclair AH et al: A gene from the human sex-determining region encodes a protein with homology to a conserved DNA-binding motif — identification of the SRY gene (Nature, 1990).
• MedlinePlus Genetics: SRY gene, Swyer syndrome, Klinefelter syndrome, and Turner syndrome — reference summaries on sex determination and sex chromosome conditions (National Library of Medicine, updated 2024).
• Lyon MF: Gene action in the X-chromosome of the mouse (Mus musculus L.) — the Lyon hypothesis of X-inactivation (Nature, 1961; updated reviews 2022).
• Nussbaum RL, McInnes RR, and Willard HF: Thompson & Thompson Genetics in Medicine — chromosomal basis of inheritance, aneuploidy, nondisjunction, and sex chromosome disorders (updated editions, 2022).
• Lejeune J, Gautier M, and Turpin R: Étude des chromosomes somatiques de neuf enfants mongoliens — discovery that Down syndrome is caused by trisomy 21 (1959).
• Hassold T and Hunt P: To err (meiotically) is human: the genesis of human aneuploidy — the maternal age effect and nondisjunction (Nature Reviews Genetics, updated 2023).
• Graves JAM: Evolution of vertebrate sex chromosomes and the origin of the mammalian Y — the shrinking Y chromosome and its conservation (Nature Reviews Genetics, updated 2022).
• American College of Obstetricians and Gynecologists (ACOG): Noninvasive prenatal testing (NIPT), chorionic villus sampling, and amniocentesis — clinical guidance (updated 2024).
• National Down Syndrome Society (NDSS) and global registries: Down syndrome incidence, features, life expectancy, and care (updated 2024).
• National Society of Genetic Counselors: Genetic counseling for chromosomal conditions, prenatal screening, and differences of sex development (updated 2024).

This article is for educational purposes only and is not a substitute for professional medical or genetic advice, diagnosis, or treatment. If you have questions about chromosomes, prenatal testing, or a chromosomal or sex-development condition, please consult a qualified healthcare provider or a certified genetic counselor.