Genetic Chapter 7 - Sex Determination, Sex Chromosomes

What is Phenotypic Dimorphism?

Males and Females Show Visible Differences

What is Heteromorphic Chromosomes?

Sex Chromosomes Differ in Structure

What are Sex Chromosomes?

Carry Genes That Determine Sex in Many Species

What is Sex Determination Controlled By?

Controlled by Specific Genes, Not the Whole Chromosome

What Gene Triggers Male Development?

The SRY Gene on the Y Chromosome

What is Homogametic Sex?

Produces Like Chromosomes, Like XX For Example, Human Females are Homogametic

What is Heterogametic Sex?

Produces Unlike Chromosomes, Like XY For Example, Humans Males are Heterogametic

When are Females Heterogametic?

In the ZW System, Like Chickens and Frogs

What is Klinefelter Syndrome?

When a Male Has an Extra X (XXY). Leads to Reduced Fertility and Some Feminization

What is Turner Syndrome?

When a Female is Missing an X (XO). Leads to Short Stature and Gonadal Dysgenesis

What Sex Chromosome Syndrome Does This Represent?

Klinefelter Syndrome

What Sex Chromosome Syndrome Does This Represent?

Turner Syndrome

What is XXX Syndrome?

A Female With an Extra X Chromosome. May Be Taller Than Average, Fertile or Infertile, and Sometimes Have Learning Difficulties

What is XYY Syndrome?

A Male With an Extra Y Chromosome. May Be Unusually Tall and May Have Learning or Behavioral Issues

What are Pseudoautosomal Regions (PARs)?

Found at Both Ends of the Y Chromosome and Synapse and Recombine With the X Chromosome During Meiosis. Ensures Proper Pairing of X and Y in Sperm Formation

What are Y Chromosome Basics?

• Contains ~50 Genes (Fewer Than X Chromosome, ~100 Genes)

• Essential For Male Development

What is the Pairing Region (PAR)?

Essential For Proper Segregation of X and Y During Male Gametogenesis

What is the Male-Specific Region of Y (MSY)?

Nonrecombining, Divided Into:

• X-Transposed (15%)

• X-Degenerative (20%)

• Amplicon (30%) → Encodes Testis-Specific Proteins

What is the Sex-Determining Region of Y (SRY)?

Adjacent to PAR on Short Arm of Y. Controls Male Development and Encodes Testis-Determining Factor (TDF)

What is the Testis-Determining Factor (TDF)?

Active at 6-8 Weeks in XY Embryos and Triggers Testes Formation

What is Dosage Compensation?

A Genetic Mechanism That Balances X Chromosome Gene Expression in Males and Females. Prevents Excessive Expression of X-Linked Genes

What are Barr Bodies?

Results of X Chromosome Inactivation in Females. Highly Condensed Inactive X Chromosome Seen as a Dark Spot in the Nucleus

Barr Bodies Follow What Rule?

N-1 Rule, Only One X Stays Active, All Others Form Barr Bodies

1. What is the Relationship Between G6PD and X Chromosome?

2. Why is G6PD Deficiency Less in Females?

3. What Causes Females to be Mosaics for X-linked Traits?

1. G6PD is a X-Linked Gene.

2. Because Females Have a Chance to Inactivate the X Chromosome With the Deficiency.

3. Because X Inactivation is Random, Some Cells Express One X, Some Cells Express the Others.

What is the X Inactivation Center (Xic)?

A DNA Region on the X Chromosome That Controls X-Inactivation

What are the Genes of Xic?

XIST, Tsix, and Xite

What is the XIST Gene?

Produces a Nonconding RNA That Coats and Silences One X Chromosome

What is the Tsix Gene?

Antisense to XIST, Block XIST on the X That Stays Active

What is the XITE Gene?

Helps Tsix Work and Ensures the Correct X is Chosen

What is X-X Transient Pairing?

Required to Initiate X-Inactivation

What Happens in Tsix Deletion?

Prevents Pairing Which Leads to Chaotic X-Inactivation

What Happens in Xic Transgene Addition?

Blocks Pairing Which Prevents X-Inactivation

How is Sex Determined in Drosophila?

Determined By X:A Ratio

Is There Dosage Compensation in Drosophila?

Nope! So No X-Inactivation

What is a Hermaphrodite?

An Individual That Has Both Testes and Ovaries, Can Self-Fertilize

What Organisms Have Hermaphrodites?

C. elegans. AKA Nematode Worms

Reptiles Sex are Temperature-Dependent…

So For Crocodiles and Most Turtles

• Eggs at Low Temperatures = Male

• Eggs at High Temperature = Female