Difficulty: Intermediate | Prerequisites: Basic Mendelian genetics (Ch. 14), understanding of meiosis and mitosis
This material connects Mendel's abstract "factors" to real, physical structures: chromosomes. You are learning that genes sit at specific positions on chromosomes, and that chromosomal behaviour during meiosis is what drives segregation and independent assortment. The chapter also introduces the first major exception to simple Mendelian ratios: sex-linked inheritance, where a gene's location on the X chromosome changes who expresses it and how. If you are comfortable with meiosis and Punnett squares, you are ready for this.
Genes occupy specific loci on chromosomes, and chromosomes, not the genes themselves, physically segregate and independently assort during meiosis. Genes on the X chromosome follow a distinctive inheritance pattern in which males, having only one X, express every X-linked allele they carry, recessive or not. Female mammals compensate for their double dose of X-linked genes by randomly inactivating one X chromosome per cell early in development.
Chromosome theory of inheritance
Mendelian genes have specific loci (positions) along chromosomes, and it is the chromosomes that undergo segregation and independent assortment during meiosis. Developed by Walter Sutton and Theodor Boveri.
In simple terms, this means the "factors" Mendel described are physically carried on chromosomes, so chromosomal behaviour explains inheritance patterns.
Locus (plural: loci)
The specific position of a gene on a chromosome.
Think of it as the gene's address on a particular chromosome.
Wild type
The phenotype for a character most commonly observed in natural populations. In Drosophila notation, the wild-type allele is marked with a superscript + (e.g. w+ for red eyes).
In simple terms, this means the "normal" version of a trait you would see in nature.
Mutant phenotype
A trait that is an alternative to the wild type, arising from a change (mutation) in the wild-type allele.
Sex-linked gene
A gene located on either sex chromosome (X or Y).
X-linked gene
A gene located on the X chromosome. Humans have roughly 1,100 X-linked genes. Most code for characters unrelated to sex.
Y-linked gene
A gene located on the Y chromosome. Humans have about 78 Y-linked genes coding for around 25 proteins, many involved in testicular function.
Hemizygous
The condition of having only one copy of a gene at a given locus. Males are hemizygous for X-linked genes because they carry only one X chromosome.
In simple terms, this means "homozygous" and "heterozygous" do not apply to X-linked genes in males, because there is no second allele to compare.
SRY (sex-determining region of Y)
A gene on the Y chromosome required for the development of testes. Without SRY, gonads develop as ovaries, even in an XY embryo.
WNT4
A gene on chromosome 1 (an autosome) that encodes a protein promoting ovary development. An XY embryo with extra copies of WNT4 can develop rudimentary female gonads.
Barr body
The compact, inactive X chromosome visible along the inside of the nuclear envelope in female mammalian cells. Most genes on the Barr body are not expressed.
Think of it as the "switched-off" X that balances gene dosage between males and females.
XIST (X-inactive specific transcript)
A gene that becomes active only on the X chromosome destined to become the Barr body. Its RNA product coats that chromosome and initiates inactivation.
Dosage compensation
The mechanism ensuring males and females have the same effective dose of X-linked gene products, achieved by X inactivation in females.
Thomas Hunt Morgan provided the first evidence linking a specific gene to a specific chromosome in the early 1900s.
He chose Drosophila melanogaster (fruit flies) because they breed rapidly (a new generation every two weeks), have only 4 pairs of chromosomes, and are easy to maintain.
Morgan discovered a single white-eyed male fly among the usual red-eyed (wild-type) population.
The white-eye cross
He crossed the white-eyed male with a red-eyed female. All F1 offspring had red eyes.
When F1 flies were crossed with each other, the F2 showed a 3:1 ratio of red to white eyes, but the white-eye trait appeared only in males.
All F2 females had red eyes.
Half the F2 males had red eyes, half had white eyes.
Morgan concluded that the eye-colour gene is on the X chromosome, with no corresponding allele on the Y.
Why it works this way
Males have one X, so a single recessive allele (w) on that X is expressed, with no wild-type allele to mask it.
Females need two copies of the recessive allele to show white eyes. In this cross, every F2 female inherited at least one w+ allele from her F1 father, so no F2 female could be white-eyed.
This was direct evidence that a specific gene sits on a specific chromosome.
Mammals use the X-Y system: females are XX, males are XY.
The Y chromosome is much smaller than the X. Only short segments at either end of the Y are homologous with the X, allowing the two to pair during meiosis.
Each egg carries one X. Sperm carry either an X or a Y, so the sperm determines the sex of the offspring.
Anatomical sex differentiation begins around 2 months of embryonic development.
The SRY gene on the Y triggers testis development. Without it, gonads become ovaries.
Sex-linked genes: X-linked vs Y-linked
X-linked genes: roughly 1,100 in humans, many unrelated to sex.
Y-linked genes: about 78 genes coding for around 25 proteins, many expressed only in the testis.
Because Y-linked genes are so few, very few disorders pass from father to son via the Y.
Fathers pass X-linked alleles to all daughters but to no sons (sons get the Y).
Mothers can pass X-linked alleles to both sons and daughters.
A female expresses a recessive X-linked trait only if she is homozygous for that allele.
A male expresses any X-linked allele he inherits, because he is hemizygous.
This is why X-linked recessive disorders are far more common in males.
Clinical examples
Duchenne muscular dystrophy: progressive muscle weakening due to absence of dystrophin. Affects about 1 in 3,500 males born in the US.
Haemophilia: absence of one or more clotting-factor proteins. Also X-linked recessive.
In each cell, one X chromosome is randomly inactivated early in embryonic development, condensing into a Barr body along the nuclear envelope.
This achieves dosage compensation: both sexes effectively have one active X per cell.
Which X is inactivated is random and independent in each cell. Once set, all mitotic descendants of that cell inactivate the same X.
Consequence: heterozygous females are mosaics, with patches of cells expressing different X-linked alleles. The calico pattern in cats is a visible example.
The XIST mechanism
A region on each X chromosome contains genes involved in inactivation.
The two regions briefly associate in each cell early in development.
The XIST gene becomes active only on the chromosome that will become the Barr body.
XIST RNA coats that chromosome, triggering inactivation through DNA methylation and histone modification.
In the ovaries, Barr body chromosomes are reactivated in cells that will become eggs.
Students often think "sex-linked" and "X-linked" mean the same thing. They do not. Sex-linked covers genes on either sex chromosome (X or Y). Most sex-linked genes discussed in exams are X-linked, but the distinction matters.
Students often assume that a carrier female will show some symptoms of an X-linked recessive disorder. She will not, because her second X carries the dominant wild-type allele. (She is a mosaic at the cellular level, but the overall phenotype is wild type.)
Students often confuse hemizygous with homozygous. A hemizygous male has one allele, not two identical ones. The term only applies when there is a single copy of the locus.
Students sometimes think X inactivation is selective, shutting down the "worse" X. It is random. Which X is inactivated in each cell has nothing to do with which alleles it carries.
⚠️ Morgan's white-eye cross is a staple exam question. Be able to diagram the cross through F2, showing genotypes on both X chromosomes and the Y, and explain why white eyes appear only in F2 males.
⚠️ Know the inheritance pattern for X-linked recessive traits: fathers pass X-linked alleles to daughters only; affected males inherit the allele from their carrier mothers.
⚠️ Understand why males are more frequently affected by X-linked recessive disorders (hemizygosity).
⚠️ Be able to explain X inactivation, Barr bodies, and mosaicism. The calico cat example is commonly tested.
⚠️ The SRY gene and its role in sex determination is a frequent short-answer topic.
True or False: A male fruit fly can be heterozygous for an X-linked gene. (False. Males are hemizygous for X-linked genes.)
Fill in the blank: The gene on the Y chromosome required for testis development is called ______. (SRY)
True or False: In female mammals, the choice of which X to inactivate is determined by which carries better alleles. (False. It is random.)
Fill in the blank: The inactive X chromosome in a female cell is visible as a ______. (Barr body)
True or False: A colour-blind father will pass the colour-blindness allele to all of his sons. (False. He passes it to all of his daughters, not his sons.)
Q: A white-eyed female Drosophila is crossed with a red-eyed male. What are the expected phenotypes and ratios of the offspring?
A: The female is X^w X^w, the male is X^w+ Y. All daughters will be red-eyed carriers (X^w+ X^w). All sons will be white-eyed (X^w Y). The ratio is 1:1 red-eyed females to white-eyed males.
Q: Why are X-linked recessive disorders more common in males than in females?
A: Males are hemizygous for X-linked genes. They have only one X chromosome, so a single recessive allele is expressed with no dominant allele to mask it. Females would need to be homozygous (receiving the recessive allele from both parents) to express the disorder.
Q: A woman who is a carrier for haemophilia marries a man with normal clotting. What proportion of their sons would you expect to be affected?
A: 50%. The carrier mother is X^H X^h. She has a 50% chance of passing the X^h allele to each son. Sons who receive it will be affected because they have no second X to carry the normal allele.
Q: Explain why calico cats are nearly always female.
A: The orange and black fur-colour alleles are on the X chromosome. A female heterozygous for these alleles undergoes random X inactivation, producing patches of cells expressing either orange or black, creating the calico pattern. Males, with only one X, express whichever allele they carry and cannot be mosaics for this trait (except in rare XXY cases).
Q: What is the role of the XIST gene in dosage compensation?
A: XIST is expressed only on the X chromosome that will become the Barr body. Its RNA product coats that chromosome, triggering DNA methylation and histone modification that silence most of its genes, ensuring both sexes have one functional copy of X-linked genes per cell.
This connects to Mendelian genetics (Ch. 14) because the chromosome theory explains the physical basis for Mendel's laws of segregation and independent assortment. It also connects to meiosis (Ch. 13), since the behaviour of chromosomes during anaphase I and metaphase I is what drives allele separation. X inactivation and the role of DNA methylation connect forward to gene regulation and epigenetics.
chromosome theory of inheritance, sex-linked inheritance, X-linked inheritance, Y-linked genes, Morgan Drosophila experiment, white-eye fruit fly, hemizygous, Barr body, X inactivation, dosage compensation, XIST gene, SRY gene, sex determination, calico cat genetics, Duchenne muscular dystrophy genetics, haemophilia inheritance, wild type vs mutant, carrier female, X-linked recessive disorders, WNT4, sex chromosomes, autosomes vs sex chromosomes