Sexual Development and Sex Chromosomes

Sexual Development

• Maleness or femaleness is determined at conception based on sex chromosomes: XX for female and XY for male.
• Gender differences become apparent around the 9th week of prenatal development.
• During the 5th week, embryos develop two unspecialized gonads near two sets of ducts that allow developmental options.
• Mullerian ducts develop into female sexual structures, while Wolffian ducts develop into male sexual structures.
• Activation of the SRY gene on the Y chromosome prompts hormone release,steering development along the male route.
• In the absence of SRY gene activation, a female develops.
• Mutation of the Wnt4 gene, essential for female development, causes high levels of male sex hormones.

Sex Chromosomes

• Heterogametic sex refers to the sex with two different sex chromosomes (males = XY).
• Homogametic sex signifies having two of the same sex chromosome (female = XX).
• The X chromosome has more genes, making it larger than the Y chromosome.

Phenotype Formation

• Transcription factors control the expression of other genes.
• The SRY transcription factor stimulates male development by signaling the indifferent gonads to destroy potential female structures while stimulating development of male structures.
• Prenatal sexual development is a multistep process where mutations can cause Hermaphroditism, Intersex, or Pseudohermaphroditism.
• Hermaphroditism is when an individual has both male and female sexual structures.
• Intersex is when internal and external structures are inconsistent.
• Pseudohermaphroditism is the presence of both structures but at different life stages

Sexual Orientation

• Homosexuality is complex, reflecting input from genes and the environment.
• Genetic influence is shown through strong feelings as young children, as seen in twin studies.

Sex Ratio

• Sex ratio is the proportion of males to females in a population calculated as the number of males divided by the number of females multiplied by 1,000.
• Primary sex ratio is the sex ratio at conception.
• Secondary sex ratio is the sex ratio at birth.

Traits Inherited on Sex Chromosomes

• Y-linked genes are located on the Y chromosome, are rare, and are passed from male to male.
• X-linked genes are located on the X chromosome, where a disproportionate number of mutant X-linked genes cause illness are expressed in males with only one copy.
• In females, X-linked traits are passed like autosomal traits.

X-linked Recessive Inheritance

• X-linked recessive inheritance is expressed in females only if the causative allele is present in two copies.
• It mostly passes from an unaffected heterozygous mother to an affected son, which may be healthy enough to transmit it to offspring.
• Examples include colorblindness, and hemophilia B.

X-linked Dominant Inheritance

• X-linked dominant conditions and traits are rare.
• A female who inherits the gene has the illness, but a male who inherits it is usually more severely affected.
• Examples include Rett syndrome, and incontinentia pigmenti.

Solving Problems of X-linked Inheritance

• Mendel’s first law (segregation) applies to genes on the X chromosome.
• The same logic is used to solve problems of X-linked inheritance as to trace traits transmitted on autosomes, considering the X and Y chromosomes in Punnett squares.

Steps:

• Look at the pattern of inheritance. Different frequencies of affected males and females in each generation may suggest X linkage.
• Draw the pedigree.
• List all genotypes and phenotypes and their probabilities.
• Assign genotypes and phenotypes to the parents considering clues in the phenotypes of relatives.
• Determine how alleles separate into gametes for the genes of interest on the X and Y chromosomes.
• Unite the gametes in a Punnett square.
• Determine the phenotypic and genotypic ratios for the F1 generation.
• To predict further generations, use the F1 genotypes and repeat applicable steps.

Sex-Limited and Sex-Influenced Traits

• Sex-limited traits affect a structure or function of the body that is present in only males or only females.
• Sex-influenced traits occur when an allele is dominant in one sex but recessive in the other.
• The difference in expression can be caused by hormonal differences between the sexes.

X Inactivation

• X-inactivation is a mechanism that balances the apparent inequality in the expression of genes on the X chromosome.
• The gene XIST in the X inactivation center part of the X chromosome controls X inactivation.
• XIST encodes an RNA that binds to a specific site on the same (inactivated) X chromosome to silence it.
• Once an X chromosome is inactivated, all its daughter cells have the same X chromosome shut off.
• X inactivation can alter the phenotype but not the genotype and can be used to check the sex of an individual.

Effect of X Inactivation on the Phenotype

• For homozygous X-linked genotypes, X inactivation has no effect.
• For heterozygotes, X inactivation leads to the expression of one allele or the other.
• Manifesting heterozygotes are carriers who express the phenotype.
• X inactivation can be used to identify carriers of some X-linked diseases, though in reality, females may not be equivalent in gene expression because they have 2 cell populations, while males only have 1.

Parent-of-Origin Effects

• Genomic imprinting is when methyl groups cover a gene or several linked genes and prevent them from being accessed to synthesize protein.
• Imprinting is an epigenetic alteration, in which a layer of meaning is stamped upon a gene without changing its DNA sequence.
• It has implications for understanding early human development and can explain incomplete penetrance.
• Incomplete penetrance is when an individual is known to have inherited a genotype associated with a particular phenotype but has no signs of the trait.

Imprinting Diseases in Humans

• The effects of genomic imprinting are noticeable only when an individual has one copy of a normally imprinted allele, and the other, active allele is inactivated or deleted.
• Imprinting diseases can be dramatic, such as Prader-Willi syndrome and Angelman syndrome.

Different Timetables in Sperm and Oocyte Formation

• Differences in the timetables of sperm and oocyte formation explain some parent-of-origin effects.
• Examples include Huntington disease, and Noonan syndrome.