bio 207 lec 11
What is biological sex?
Biological sex refers to an organism’s sexual phenotype, such as anatomical and physiological traits that distinguish males from females.
Males produce small gametes (sperm)
Females produce large gametes (eggs)
What is the difference between sex and gender?
Sex = biological traits (gametes, chromosomes)
Gender = identity or social role, shaped by cultural and behavioral factors
What is sex determination?
Sex determination refers to the biological mechanisms that establish an individual’s sexual phenotype. These vary widely and can be:
Chromosomal
Genic
Environmental
What does monoecious mean?
Monoecious (meaning “one house”) refers to organisms that have both male and female reproductive organs in a single individual.
🧪 Example: Many plants, some worms
🧬 Can produce both eggs and sperm.
What is hermaphroditism?
Hermaphroditism is a condition where an individual organism has both male and female reproductive structures. It can be:
Simultaneous (both at once)
Sequential (sex changes over time)
🧠 Note: “Monoecious” is often used for plants, while “hermaphrodite” is used for animals.
What does dioecious mean?
Dioecious (meaning “two houses”) refers to species where individuals are either male or female, but not both.
🧪 Example: Humans, most animals, some plants
Can a species be both monoecious and dioecious?
No — monoecious and dioecious describe opposite reproductive strategies:
Monoecious = one individual, both sexes
Dioecious = two individuals, one sex each
what are autosomes
non-sex chromosomes (same
number/type in male and
female)
What is the XX-XY sex determination system?
A chromosomal system where:
Females = XX (homogametic)
Males = XY (heterogametic)
Found in humans, most mammals, and some plants/reptiles.
What are pseudoautosomal regions?
Small homologous regions on the X and Y chromosomes that allow them to pair and segregate during meiosis. These regions can also undergo recombination.
primary on the top and secondary at the bottom of the chromsomes
How does the duckbill platypus deviate from the standard XX-XY sex determination system?
Unlike most mammals, the duckbill platypus has a complex system with 10 sex chromosomes:
Females: 5 pairs of X chromosomes (XX XX XX XX XX)
Males: 5 X and 5 Y chromosomes (XY XY XY XY XY)
During meiosis, the 10 sex chromosomes form a chain-like structure to ensure proper segregation — a very rare phenomenon in mammals.
Is the Y chromosome actually Y-shaped?
No — the Y chromosome is not physically Y-shaped. The human Y chromosome is acrocentric, meaning it has one very short arm and one longer arm. It looks more like a stick with a small knob than a Y.
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X
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What happens to sex chromosomes during meiosis I?
In meiosis I, homologous sex chromosomes (X and Y in males, X and X in females) pair up, align at the metaphase plate, and segregate into separate cells — just like autosomes.
What happens to sex chromosomes during meiosis II?
In meiosis II, the sister chromatids of each sex chromosome separate and are distributed into different gametes — just like in mitosis.
In an XY male, after meiosis II:
→ One gamete gets an X, the other gets a Y
What is the expected offspring sex ratio when crossing a male and female from the XX-XY system?
A 1:1 ratio — 50% male (XY) and 50% female (XX).
What is the XX-XO system?
simple chromosomal system (e.g., grasshoppers):
Females = XX
Males = X (only one sex chromosome) “O” means absence of the second sex chromosome.
what ratio of progeny phenotypes does XX-XO sytem generate
Similar to XX-XY system, XX-XO system generates 1:1 ratios of sex phenotypes
What is the ZZ-ZW system?
System found in bearded dragons
(right) and birds, butterflies, some
reptiles, fish, amphibians
Females = ZW (heterogametic)
Males = ZZ (homogametic)
presecene of sex chromosomes determine sex in these systems. However, to be more accurate, sex in these systems is
determined by
Not just the chromosomes, but specific genes on sex chromosomes and them wokring in conjunction with autosomal genes.
What is genic sex determination?
In organisms without distinct sex chromosomes, sex is determined by specific genes at one or more loci (e.g., in plants, fungi, some fish).
What is environmental sex determination?
When external factors, like temperature or chemical signals, determine sex.
Example:
Turtles,crocodiles, alligators and a few bird (effected during embryonic development)
turtles: low temps → males, high temps → females
alligators opposite.
Mollusks (Crepidula): sequential hermaphrodites — change sex based on position/social cue
sequential hermaphroditism?
each individual can be both male and female, but not at the same time.
like the mollusks, intially molusks that attach to free substrate are female then males are attracted to chemicals released by the female and lay on top of them gradually becoming female
Juvenile settles on stack → becomes male
Mates with the female below
Eventually becomes female
New juvenile males settle on her
→ This is a social, layered reproductive system.
can sex chromosomes be overrided
Other animals like bearded dragons use sex chromosomes to
determine male and female, but temperature can override this
How is sex determined in Drosophila (fruit flies)?
By the X:A ratio:
X:A = 1.0 → Female
X:A = 0.5 → Male
X = number of X chromosomes; A = haploid sets of autosomes
Values between 0.5–1.0 = intersex or abnormal development
Who discovered how sex is determined in fruit flies, and when?
Calvin Bridges, in the 1920s, studied Drosophila melanogaster (fruit flies) and proposed that sex is determined by the X:A ratio, not just the presence of a Y chromosome.
what did bridges propose
sex determined by balance between
1. female-determining genes on X
2. male-determining genes on the autosomes
normal flies have what ratio of autsomes and x chromosome(s)
2 haploid sets of autosomes and 2X for females and 1X for males, so
Normal male X:A = 1/2 = .5
Normal female X:A = 2/2 =
What is the primary genetic determinant of sex in fruit flies?
Genes on the X chromosome are the primary sex determinants in fruit flies (Drosophila melanogaster).
Do autosomal genes directly determine sex in fruit flies?
No — autosomal genes do not directly determine sex, but they influence developmental timing and regulate X-linked gene expression.
How do autosome numbers influence sex determination in flies if not directly?
The number of autosomes affects the X:A ratio, which controls the activation or repression of sex-determining genes on the X chromosome. (developmental timing and X
linked gene expression.)
What determines sex in humans?
The presence of the SRY gene (Sex-determining Region Y) on the Y chromosome:
If SRY is present → male
If SRY is absent → female
what happened when xx mic were given the SRY gene
Genetic engineering in Mice placed
just the SRY gene in XX mice and
these mice where anatomically
male
What is the role of the SRY gene?
It initiates testis development. Even placing SRY in an XX mouse can make it develop as a male.
Why is the X chromosome essential for human development?
Because it contains genes required by both sexes — at least one X chromosome is needed for survival and development.
Where are fertility genes located in humans?
On both the X and Y chromosomes — both sexes need specific fertility genes from their respective sex chromosomes.
What is the effect of having extra X chromosomes?
More than two X chromosomes (e.g., XXX, XXY) can cause:
Physical abnormalities
Intellectual disabilities
Impaired sexual development
🧠 Examples:
Turner Syndrome (XO)
Klinefelter Syndrome (XXY)
What is Turner syndrome?
XO genotype — female with only one X chromosome.
Short stature, underdeveloped secondary sex characteristics
Normal intelligence
1 in 3000 female births
What is Klinefelter syndrome?
XXY, XXXY, etc. — male with extra X chromosomes
Sterile, tall, small testes, reduced facial/pubic hair
Normal intelligence
1 in 1000 male births
What is Androgen-Insensitivity Syndrome (AIS)?
AIS is a condition where individuals with XY chromosomes and functional SRY gene develop female external characteristics because their body can't respond to testosterone.
What causes AIS?
A mutation in the androgen receptor gene, which is located on the X chromosome. This receptor is required to respond to testosterone.
What are the physical traits of a person with AIS?
Externally female appearance
Vagina present, but no uterus, oviducts, or ovaries
Undescended testes in the abdomen
No menstruation
Why does someone with AIS appear female despite having XY chromosomes?
Because their cells can’t detect testosterone due to a faulty androgen receptor, so male traits do not develop, and female-like external features form instead.
How is AIS inherited?
The defective androgen receptor gene is X-linked and inherited from the mother.
is there alot of homology between x and y
Only small homology
between X and Y
(Pseudoauotosomal
regions)
What are sex-linked genes?
Genes located on the sex chromosomes.
X-linked genes are found on the X chromosome
Y-linked traits exist, but are rare and mostly male-specific
Does recombination happen in females with XX?
Yes.
Recombination occurs normally between the two homologous X chromosomes during meiosis in females, just like autosomes.
Does recombination occur in males with XY?
Yes, but only in small regions.
Recombination happens between the X and Y chromosomes only in the pseudoautosomal regions (PARs) — small areas of shared homology at the ends of the chromosomes.
What does hemizygous mean?
Having only one allele of a gene in a diploid organism.
Males are hemizygous for X-linked genes (they have only one X chromosome).
What is a reciprocal cross, and why does it matter?
A reciprocal cross switches the sex of parents with specific phenotypes to test for sex-linkage.
In autosomal inheritance, results are the same.
In X-linked inheritance, results differ significantly.
What did Thomas Hunt Morgan discover with fruit flies?
He found that white eye color was an X-linked recessive trait in flies — led to the discovery of sex-linked inheritance.
What did Morgan observe in the F1 generation when he crossed a white-eyed male with a red-eyed female?
All F1 flies had red eyes, regardless of sex.
🧠 This showed red was dominant over white.
What happened in the F2 generation?
The white-eyed trait reappeared, but only in males.
All females had red eyes
About 50% of males had white eyes
What did Morgan conclude from his F2 results?
He concluded that the white-eye mutation was on the X chromosome, and that males only need one copy of the mutant gene to express it — proving the concept of X-linked inheritance.
What unexpected result did Morgan see when crossing red-eyed females with white-eyed males?
Occasionally, he observed white-eyed females and red-eyed males, even though this shouldn’t happen if X-linked inheritance is followed perfectly.
Example: 3 white-eyed females out of 1240 offspring.
Why was this unexpected?
Because with homozygous red-eyed females (X⁺X⁺) and white-eyed males (XʷY), all:
F1 females should be red-eyed (X⁺Xʷ)
F1 males should be red-eyed (X⁺Y)
So seeing white-eyed females didn't fit the predicted pattern.
What did Calvin Bridges discover?
Nondisjunction — the failure of X chromosomes to separate during meiosis — explained exceptions in eye color inheritance and provided the first evidence linking genes to chromosomes.
so what were the chromsomes of these whtie eyed female flies
Bridges hypothesized that the exceptional white-eyed females
had two X chromosomes and one Y (XwXwY)
What is nondisjunction?
A meiotic error where chromosomes fail to separate, causing gametes to have too many or too few chromosomes.
How did Bridges use nondisjunction to explain Morgan’s unexpected results?
Bridges proposed that in ~10% of meioses, the X chromosomes failed to separate in the female parent. This caused:
Some eggs to receive two X chromosomes
Others to receive none (O)
What happens to the progeny when nondisjunction occurs?
Zygotes that are XʷXʷX⁺ (extra X) or YY (no X) are non-viable — they die
The viable but unexpected phenotypes:
XʷXʷY → white-eyed female (unexpected)
X⁺Y → red-eyed male (unexpected in that cross)
So about 5% of the total offspring had these unexpected phenotypes.
Why was this important?
It was the first experimental proof that genes are located on chromosomes, linking Mendelian inheritance to chromosome behavior in meiosis.
🧠 Key Concept: Bridges showed that gene inheritance could be explained by physical errors in chromosome segregation.
Is red-green color blindness X-linked?
Yes — it is a recessive trait on the X chromosome.
Why is red-green color blindness more common in males?
Males have only one X, so they express the trait if they inherit one copy of the mutant allele.
Females need two mutant X alleles to be affected.
What happens when a color-blind female is crossed with a normal male?
All sons → color-blind (inherit mutant X from mom)
All daughters → normal (but carriers)
How can X-linked traits show multi-generational effects?
X-linked traits often pass in non-Mendelian patterns, especially because males have only one X chromosome (hemizygous).
What are two key multi-generational patterns of X-linked inheritance?
1. Trait can alternate sexes across generations:
A carrier mother (X⁺Xʷ) may have an affected son (XʷY)
That son cannot pass it to his son, but his daughter may become a carrier, who could then have another affected son.
1. Trait can skip a generation:
An affected father (XʷY) passes the mutant X to all his daughters (they are carriers)
But none of his sons inherit the X, so they’re unaffected
His grandsons (via daughters) may then be affected.
Do all genes on the X chromosome show sex-linked inheritance?
No. Some genes are located in the pseudoautosomal regions (PARs) — areas shared by both X and Y chromosomes. Genes in PARs are inherited just like autosomal genes, because both sexes have two copies — one on the X and one on the Y.
Does the presence or absence of the Y chromosome affect sex in fruit flies?
No. In Drosophila, sex is not determined by the presence of a Y.
XO (no Y) = male
XXY = female
✅ The Y chromosome is not needed for male sex determination, but is required for male fertility.
In ZZ-ZW sex determination, which sex is heterogametic?
Females (ZW) are heterogametic; males (ZZ) are homogametic.
🧠 This is the reverse of the XX-XY system.
How are Z-linked traits inherited?
Z-linked traits are inherited similarly to X-linked traits, but the pattern is reversed:
Females are hemizygous (only one Z), so they express Z-linked mutations if inherited from father.
Males can be homozygous or heterozygous.
What is an example of a Z-linked trait?
The Cameo trait in peacocks (dull brown feathers) is a Z-linked recessive mutation.
What are holandric traits?
Traits determined by genes located only on the Y chromosome.
✅ Only males can have them
✅ Passed from father to all sons
Are Y-linked traits common in humans?
No — the human Y chromosome has few functional genes (~350), and many of them relate to male sexual development or fertility. y, 2/3 of Y consist of short DNA sequences repeated many times and no active genes
What are palindromic sequences on the Y chromosome, and why are they important?
8 massive palindromic sequences on Y. These are repeated DNA segments that can recombine within the Y chromosome (intrachromosomal recombination), helping prevent genetic decay from lack of recombination with X.
What is dosage compensation?
A mechanism that equalizes gene expression (protein produced) between sexes despite differences in sex chromosome number (e.g., XX vs XY).(with respect to autosomal genes and X-linked genes in females)
Why is dosage compensation necessary?
Without it, females (XX) would produce twice as much X-linked protein as males (XY), disrupting cell function. (males would have insuffcient gene dosage)
How does dosage compensation work in fruit flies?
In males, X-linked genes are upregulated (doubled) to match expression from the two Xs in females.
How does dosage compensation work in placental mammals (e.g., humans)?
In females, one X chromosome is randomly inactivated in each cell. This is known as X-inactivation.
who saw barr bodies first
In 1949, Murray Barr noticed
dark “bodies” inside the nuclei
of only female cells of cats.
What is a Barr body?
A Barr body is an inactivated X chromosome seen in female cells as a dark-staining body in the nucleus.
Who proposed the idea of X-inactivation?
Mary Lyon in 1961 — called the Lyon hypothesis.
Is X-inactivation random?
Yes. In each cell, either the maternal or paternal X is randomly inactivated early in development. (so if heterzygous could be the X with the B or b allele)
🧠 This creates mosaic expression in females.
What is an example of mosaicism caused by X-inactivation?
Calico cats — different coat colors appear depending on which X (carrying which coat color gene) is active in each patch of skin.
What is the result of X-inactivation in female placental mammals?
Females become functionally hemizygous at the cellular level for X-linked genes — meaning each cell only expresses one allele of each X-linked gene.
How is gene expression divided among cells in a heterozygous female?
Roughly 50% of cells express one allele (e.g., from the maternal X), and 50% express the other (from the paternal X), due to random X-inactivation.
🧠 Key term: This leads to mosaicism, where different cells/tissues in the same individual may express different alleles.
📌 Example: Calico cats with patches of fur color depending on which X chromosome is active.
When does random X-inactivation occur in female development?
Early in embryonic development, when the embryo is made of just a few cells.
What happens to the inactivated X chromosome in the daughter cells?
nce an X chromosome is inactivated in a cell, all daughter cells (from mitosis) will keep that same X inactivated.
🧠 Result: Clusters of cells in the body will express only one X allele, leading to visible mosaicism in heterozygous females.
📌 Example: Different fur color patches in calico cats or random expression of X-linked traits in human females.
How many X chromosomes are inactivated in cells with multiple Xs?
All but one X chromosome is inactivated.
🧠 Each additional X forms a Barr body.
How does a cell know how many X chromosomes are present?
The exact mechanism isn't fully understood, but cells can "count" X chromosomes and ensure that only one remains active — the rest are inactivated.
🧠 This is done through the Xist gene, which becomes active on extra X chromosomes to initiate inactivation.
Why do people with extra X chromosomes (e.g., XXX, XXY) still have developmental issues, even with X-inactivation?
Because X-inactivation is not 100% complete — about 15% of X-linked genes escape inactivation and remain active, producing extra protein.
📌 That overexpression disrupts normal development and contributes to physical or cognitive symptoms in conditions like:
Triple-X (XXX)
Klinefelter (XXY)
Poly-X (XXXX, XXXXX)
Dosage compensation limits damage, but cannot fully erase the effects of extra X-linked gene expression.
What is the Xist gene?
Xist stands for X-inactive specific transcript — it's a gene that produces a long noncoding RNA (17,000 nucleotides) involved in X chromosome inactivation in female mammals.
How does Xist RNA lead to X-inactivation and Barr body formation?
Xist gene is activated on the X chromosome to be inactivated.
It produces long Xist RNA, which coats that entire X chromosome.
Xist RNA recruits protein complexes that:
Condense chromatin
Add epigenetic marks (like DNA methylation and histone modification)
This transforms the chromosome into a tightly packed, transcriptionally inactive structure called a Barr body.
How is the active X chromosome protected from being inactivated by Xist?
On the active X chromosome, Xist is repressed by other regulatory genes that block its transcription.
The active X produces repressor proteins or noncoding RNAs (like Tsix, the antisense RNA of Xist).
These repressors prevent Xist RNA from being made or spreading.
Without Xist RNA, the active X remains open and transcriptionally active.
Do all species have dosage compensation?
No — some species (e.g., birds, butterflies, platypus) do not use dosage compensation, and yet function normally.
📌 The mechanism is not universal.
How many Barr bodies are present in different karyotypes?
(inactivated till only 1 x left)
Karyotype Barr Bodies
XX 1
XY 0
XO (Turner) 0
XXY 1
XXX 2
XXXY 2
XXXX 3