PCB 3063 Lecture 7 sex determination and chromosomal changes

What is sex determination?

The process by which an organism differentiates into one of two distinguishable sexes.

What are autosomes?

Chromosomes that are not directly involved in determining the sex of an organism.

What are sex chromosomes?

Chromosomes that directly determine or help determine the sex of an organism.

What is primary sexual differentiation?

The development and formation of the primary sexual organs, specifically the gonads.

What is secondary sexual differentiation?

The formation of visible external traits indicative of sex, such as genitalia or facial hair.

What does unisexual mean?

An individual having only male or only female reproductive organs.

What does bisexual or hermaphroditic mean?

An individual containing both male and female reproductive organs.

In Protenor insects, how many chromosomes do female somatic cells contain?

14 chromosomes, including two X chromosomes.

In Protenor insects, how many chromosomes do male somatic cells contain?

13 chromosomes, including only one X chromosome.

What mode of sex determination is represented by Protenor insects?

The XX/XO mode of sex determination.

In Lygaeus turicus, how many total chromosomes do males and females have?

14 chromosomes (12 autosomes plus 2 sex chromosomes).

What sex chromosomes do male Lygaeus turicus insects possess?

One X chromosome and one smaller Y chromosome.

What mode of sex determination is found in Lygaeus turicus?

The XX/XY mode of sex determination.

What defines the heterogametic sex?

The sex that produces two distinct types of gametes regarding sex chromosomes.

What defines the homogametic sex?

The sex that produces gametes with uniform sex chromosome content.

In the ZZ/ZW system, which sex is heterogametic?

The female (ZW).

Which organism groups commonly utilize the ZZ/ZW sex determination system?

Moths, butterflies, most birds, some fish, reptiles, and amphibians.

What autosomal gene on chromosome 17 is involved in male gonad development?

SOX9, a transcription factor gene.

How is sex determined in haplodiploid insects like bees and wasps?

By the number of chromosome sets (haploid males, diploid females).

In haplodiploidy, how do males develop?

Males develop from unfertilized, haploid eggs.

In haplodiploidy, how do females develop?

Females develop from fertilized, diploid eggs.

What is the average genetic relatedness between full sisters in haplodiploid systems?

75% genetic relatedness.

What is genic sex determination?

Sex determination controlled by autosomal genes without distinct sex chromosome differences.

What environmental factor determines sex in many reptile embryos?

Incubation temperature during embryonic development.

What role does the human Y chromosome play in sex determination?

It actively directs male development, regardless of the number of X chromosomes.

What phenotype is associated with human XXY individuals?

Male traits with developmental problems and sterility.

What phenotype is associated with human XO individuals?

Female traits with developmental issues and sterility.

Approximately how many genes are located on the human Y chromosome?

As many as 50 genes.

What are pseudoautosomal regions (PARs)?

Homologous regions at chromosome ends that allow X and Y pairing during meiosis.

Why are PAR genes unique compared to other sex-linked genes?

They are present in two copies (diploid) and show autosomal inheritance patterns.

During which meiotic stage do PARs facilitate X and Y pairing?

Prophase I.

What does NRY stand for on the Y chromosome?

Nonrecombining region of the Y.

What is the characteristic of heterochromatin on the Y chromosome?

It remains hypercondensed during interphase and lacks functionally transcribed genes.

What is euchromatin on the Y chromosome?

Active chromatin containing genes that are actively transcribed.

Where is the SRY gene located?

On the short (p) arm of the Y chromosome, near the pseudoautosomal region.

What protein is encoded by the SRY gene?

Testis-determining factor (TDF).

What is the primary molecular function of testis-determining factor (TDF)?

It binds to DNA and induces a bend in the DNA structure.

How can an XX individual develop phenotypically as a human male?

By acquiring the SRY gene via abnormal translocation onto an X chromosome.

How can an XY individual develop phenotypically as a human female?

By carrying a deletion or mutation of the SRY gene on the Y chromosome.

What experiment proved SRY is sufficient for maleness in mammals?

Transfecting the SRY gene into XX mouse embryos caused them to develop as male.

What failure causes abnormal numbers of sex chromosomes during gametogenesis?

Nondisjunction during meiotic divisions.

What is the chromosomal makeup of Klinefelter syndrome?

47,XXY (or additional X/Y variations such as XXXY).

List two physical characteristics of Klinefelter syndrome.

Tall stature with long limbs, and underdeveloped testes resulting in sterility.

How can some feminine traits in Klinefelter syndrome be reduced?

Through regular therapeutic injections of testosterone.

What is the chromosomal karyotype of Turner syndrome?

45,XO.

What happens to most embryos with a 45,XO karyotype?

Most die prior to birth during embryonic development.

List two physical features of Turner syndrome.

Short stature and a webbed neck, along with undeveloped ovaries.

What is Triplo-X syndrome?

A condition in females characterized by three or more X chromosomes (47,XXX).

How does increasing the number of X chromosomes affect Triplo-X phenotype?

Additional X chromosomes generally increase the severity of mental impairment and symptoms.

What physical trait is consistently associated with 47,XYY males?

Above-average height (typically over 6 feet tall).

Does an extra Y chromosome cause increased aggressive or violent behavior?

No, scientific evidence disproved the hypothesis linking 47,XYY to aggression.

What is the primary sex ratio?

The ratio of male to female embryos conceived in a population.

What is the approximate value of the human primary sex ratio?

Approximately 1.40 (140 male conceptions per 100 female conceptions).

What is the secondary sex ratio?

The ratio of male to female births in a population.

What is the approximate human secondary sex ratio?

Approximately 1.05 (105 male births per 100 female births).

Why is the secondary sex ratio lower than the primary sex ratio in humans?

Male fetuses experience higher mortality rates during pregnancy.

What hypothesis explains why Y-bearing sperm might fertilize eggs more often?

Y-bearing sperm have less mass than X-bearing sperm and may be more motile.

What is dosage compensation?

A mechanism equalizing expression of X-linked genes between males and females.

At what embryonic stage does random X-inactivation occur in human females?

At approximately the 8-cell stage of development.

What is a Barr body?

A condensed, inactivated X chromosome visible against the nuclear membrane during interphase.

How many Barr bodies are present in somatic cells of a 47,XXY male?

One Barr body.

How many Barr bodies are found in a cell from a 45,XO female?

Zero Barr bodies.

How many Barr bodies are present in a 47,XXX female?

Two Barr bodies.

What region on the X chromosome is essential for initiating X-inactivation?

The X-inactivation center (XIC).

What is Xist?

A long non-coding RNA produced from XIC that coats the inactivated X chromosome.

Is the Xist RNA transcript ever translated into a protein?

No, Xist functions directly as a non-coding RNA molecule.

From which X chromosome is Xist transcribed?

Exclusively from the X chromosome destined to be inactivated.

What epigenetic modifications are associated with Xist coating?

Inhibition of histone acetylation and induction of DNA/histone methylation.

What is aneuploidy?

A condition where an organism gains or loses individual chromosomes but not entire sets.

What is monosomy?

The loss of a single chromosome from a diploid genome (2n - 1).

What is trisomy?

The gain of a single extra chromosome in a diploid genome (2n + 1).

What is polyploidy?

A condition in which an organism possesses three or more complete sets of chromosomes.

What chromosomal deletion causes Cri-du-chat syndrome?

A deletion of part of the short arm (p arm) of chromosome 5.

What key anatomical malformation leads to the characteristic cry in Cri-du-chat?

Malformation of the larynx and glottis.

What chromosomal alteration causes Prader-Willi syndrome?

A partial deletion on the long arm of chromosome 15.

Why are autosomal trisomies in animals rare among live births?

Most autosomal trisomies disrupt gene dosage severely and are embryonic lethal.

What autosomal trisomy is commonly known as Down syndrome?

Trisomy 21.

What is the primary origin of the extra chromosome in Down syndrome cases?

Nondisjunction during maternal meiosis I (95% of cases).

What maternal factor strongly correlates with the frequency of Down syndrome?

Advanced maternal age.

What is Patau syndrome?

Trisomy 13, causing severe organ system defects and early infant mortality.

What is Edward syndrome?

Trisomy 18, causing severe organ defects with most surviving infants being female.

What is autopolyploidy?

Polyploidy arising from the duplication of chromosome sets from the same species.

What is allopolyploidy?

Polyploidy resulting from the hybridization and combination of chromosome sets from different species.

State one mechanism that produces an autotriploid (3n) organism.

Fertilization of an unreduced diploid gamete (2n) by a normal haploid gamete (n).

How can an egg fertilized by two sperm affect chromosome number?

It results in dispermy, yielding a triploid (3n) zygote.

How do autotetraploids (4n) originate in early embryos?

From a failure or stoppage of cell division during early embryonic mitosis.

Why are most autopolyploid plants sterile?

Unequal chromosome segregation during meiotic synapsis leads to unbalanced gametes.

Name two common commercial seedless fruits that are triploid.

Seedless watermelons and commercial bananas.

What is an allotetraploid?

An allopolyploid containing two complete diploid chromosome sets from two different species (AABB).

Why are initial interspecific hybrid offspring (AB) typically sterile?

Chromosomes A and B are non-homologous and cannot pair during prophase I.

How do sterile interspecific hybrids (AB) become fertile allotetraploids (AABB)?

Through spontaneous chromosomal doubling in early embryonic cell divisions.

Why are allotetraploid plants typically fertile?

Each chromosome has an identical homologous partner for proper meiotic pairing.

What usually initiates structural changes in chromosomes?

Chromosomal breaks caused spontaneously or induced by radiation and chemical mutagens.

What property do broken chromosome ends possess that allows rejoining?

They have sticky, un-capped ends that non-specifically ligate with other broken ends.

What is a terminal deletion?

A single break near the end of a chromosome that removes the terminal fragment.

What is an intercalary deletion?

A deletion resulting from two internal breaks followed by loss of the middle segment.

Which fragment is retained by the cell when a chromosome breaks into pieces?

The fragment containing the centromere.

How do homologous chromosomes synapse during meiosis if one has a deletion?

The normal homolog forms a deletion loop (compensation loop) to align genes.

What is a gene duplication?

A structural mutation where a chromosomal region or gene is present in multiple copies.

What event during meiotic prophase I causes gene duplications and deletions?

Unequal crossing over between misaligned homologous chromosomes.