Chapter 4

Non-Disjunction

Error in cell division where sister chromatid fail to separate.

F1

The first filial generation (first offspring)

Sex Determination Mechanisms

-Chromosomal Systems: Looking at the chromosomes to determine if the organism is male or female o Example: in mammals, XX means female and XY means male. This is different than insects or birds who use XX and XO, or ZW ZZ to determine male or female. -Environmental Systems: The same genotype of an organism can result in a different phenotype based on the different environments that each are in. o Example: Different temperature windows in some reptiles can determine if the offspring is male or female (22-27 degrees Celsius is the window between male and female determination) -Genic Systems: No sex chromosomes, so a set of loci determines sex. No obvious differences in male and female chromosomes. o Example: plants, fungi, and protozoans, loci on "sex" chromosomes determines sex, not the differences in the chromosomes themselves.

List and discuss the three dosage compensation mechanisms

-Hyperactivation: Activation is heightened to match that of the female organism o Example: Drosophila generate 2x the amount of gene product for the X chromosome in males when compared to females (this makes them equal, since they only have one X while females have 2). -Inactivation: X chromosome is inactivated to match that of males (happens at random in each cell, can inactivate either maternal or paternal X chromosome) o Example: Female mammals such as Calico cats use inactivation to inactivate certain X chromosomes, which can lead to a mosaic, as they have a mix of both paternal and maternal information. -Down Regulation: X chromosomes in females is halved so that together they equal the same amount as one. o Example: Female nematodes will reduce the activity on both of their X chromosomes so that they are equal to that of the males one x chromosome.

List and briefly discuss the three chromosomal abnormality (non-disjunct) disorders discussed in lecture.

-Turners Syndrome: Occurs when an individual is XO, meaning that they only have one sex chromosome. -Klinefelter Syndrome: Occurs when an individual has multiple sex chromosomes with at least one Y (XXY, XXXY, XXYY, etc.) -Poly-X Females: Occurs when a female organism has more than the regular 2 X chromosomes.

X-Linked

A gene carried on the X chromosome (If a male inherits an X-linked recessive trait from his mother, he expresses that trait because the Y from his father has no counteracting gene. Females are more likely to be carriers of X-linked traits but are less likely to express them).

What is a reciprocal cross experiment and what does it achieve?

A reciprocal cross experiment would be a cross in which the male and female parent phenotypes are reversed (for example, a colorblind male with a normal seeing female). This leads to a ¼ to ¼ to ¼ to ¼ ratio (much different than the Mendelian inheritance which was 3:1).

What is dosage compensation as it relates to genetics?

Dosage compensation is where organisms regulate and equalize X linked gene product regardless of chromosome dosage. This means that although we would expect females to have twice the gene expression of the X chromosomes (since they are XX), dosage compensation for mammals creates an inactivation of one of the X chromosomes to equal each other out.

Genetic Mosaic

An individual with somatic regions that are genetically different from each other, since the random deactivation of one of the X chromosomes occurs (2 or more sets of genotypes active within an individual organism).

Haplodiploidy System

Haploid set - male Diploid set - female Bees, wasps, and ants

Pseudoautosomal Regions

Regions at the ends of the sex chromosomes (X and Y) that are homologous, and allow the different sex chromosomes to pair and segregate during meiosis (allows the X to pair with the Y to create XY males).

Heterogametic

Sex chromosomes are of different types (XY)

Homogametic

Sex chromosomes are of one type (XX)

Describe Sex Determination in Humans

Sex determination in humans follows the chromosomal system, where we observe X and Y genes to determine if an organism is male or female. In both sexes, the organism obtains the first X chromosome. The differences between the sexes arise from the second chromosome. If the second chromosome is another X, the organism is female. If the second chromosome is Y, the organism is a male. - If we have XX, we will not have an SRY gene (due to the absence of the Y chromosome) and TDF will not be produced. In the absence of TDF, ovaries will be generated, and from there so will female sex characteristics. - If we have XY, the SRY gene will signal the TDF and with its presence testes will be generated, and from there male sex characteristics.

Sex Linked Traits

Sex-linked traits are traits that are inherited, based on the fact that they reside on the sex chromosomes. If the trait resides on the X chromosome, it would be considered an X- linked trait. If the trait resides on the Y chromosome, it is a Y-linked trait.

Y-Linked

The pattern of inheritance that results from genes located only on the Y chromosome.

Testicular Feminization

The presence of the SRY gene along with a Tfm mutation, where an XY male is left with female secondary sex characteristics. -Testicular feminization is when an XY male has the SRY gene, but there is a Tfm mutation that does not allow for the testosterone receptor. This means that there is no testosterone receptor complex even though there is testosterone being produced, no signal, and thus female secondary sex characteristics are produced rather than male, even though they have XY.

F2

The second filial generation; offspring resulting from the crossing of individuals of the F1 generation