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Questions and Answers
What term describes an organism that has two of the same type of sex chromosomes?
In temperature-dependent sex determination, which factor is typically crucial for determining the biological sex of offspring?
How does climate change potentially affect the sex ratios of certain reptiles like turtles?
What is the primary difference between X-linked and Y-linked inheritance?
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Which of the following is a characteristic of mammals regarding sex determination?
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What characterizes genic sex determination?
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How does temperature affect sex determination in reptiles like turtles?
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What is the concept of sequential hermaphroditism?
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In mammals, which sex is typically heterogametic?
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Which of the following statements accurately describes temperature-based sex determination in alligators?
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Which factor influences the sex ratios in certain species as global temperatures rise?
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What term describes a chromosome configuration where an organism has two different sex chromosomes?
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What mechanism must occur to ensure proper gene dosage for certain genes on the X chromosome?
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Which reproductive strategy describes organisms that do not have distinguishable sex chromosomes?
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How do environmental factors influence sex determination in slipper limpets?
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What happens to ZZ individuals if they are incubated at high temperatures?
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At what temperature do male crocodiles typically develop?
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How does global warming impact sex determination in species that rely on temperature for sex determination?
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Which of the following is essential for male mammalian sex determination?
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What is the consequence of losing key genes from the Y chromosome in an XY individual?
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What initiates the development of testes in mammals?
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Which of the following statements about the pseudoautosomal region (PAR) is true?
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What is a hallmark of X-linked inheritance as demonstrated by Morgan's fruit fly experiments?
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What was one of the main goals of Thomas Hunt Morgan’s experiments?
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In the absence of the SRY gene, which reproductive organs are most likely to develop?
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What does hemizygous mean in the context of male inheritance patterns?
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What kind of inheritance pattern is primarily associated with the X chromosome?
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Study Notes
Terminology
- Sex Chromosomes: Chromosomes that determine biological sex and influence secondary sexual characteristics.
- Homogametic: Individuals with gametes containing the same sex chromosomes.
- Heterogametic: Individuals with gametes containing different sex chromosomes.
- Hemizygous: Containing only one copy of a chromosome that is typically observed in a pair.
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Haploinsufficiency: Loss of one copy of a gene results in insufficient product, leading to reduced function.
- Genes exclusive to the Y or X in humans are NOT haploinsufficient as proper gene dosage is 1.
- Nondisjunction: Failure of chromosomes to separate properly during meiosis, resulting in an abnormal number of chromosomes in gametes.
Sex Chromosomes and Sex Determination
- Sex determination can vary across organisms.
- Difference in biological sex is established by key genes.
- Sex chromosomes do not always involve two distinct chromosome types.
- Humans: XX (female) vs XY (male)
- Grasshoppers: XX (female) vs XO (male) - males have fewer chromosomes.
- In organisms with unequal chromosome numbers, meiosis results in gametes with unequal chromosome counts.
Heterogametic Sex
- The sex with two different sex chromosomes.
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ZZ vs ZW: In many species, the female is the heterogametic sex (ZW).
- Found in birds, snakes, butterflies, amphibians, and some fish.
XY Heredity (Humans)
- Males are heterogametic (XY).
- X and Y can pair during meiosis due to structural similarity (regions of homology).
- Pseudoautosomal regions.
- Crossing-over occurs in these regions.
- These homologous regions contain genes that are NOT inactivated in women, ensuring proper gene dosage.
Sex Determination without Sex Chromosomes
- Biological sex can be determined by genes or the environment, even in the absence of sex chromosomes.
- Genic Determination: Genes at specific loci determine sex.
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Environmental Sex Determination: Environment partially or completely determines sex.
- Example: Slipper limpet (Crepidula fornicata) - sequential hermaphroditism.
- Example: Reptiles including turtles, crocodiles, and alligators.
- Temperature-based sex determination.
- Turtles: warmer temperatures = females, colder temperatures = males
- Alligators: colder temperatures = females, warmer temperatures = males
Global Warming and Sex Determination
- Increasing global temperatures are affecting male:female ratios in temperature-dependent sex determination, potentially impacting reproductive potential.
Mammalian Biological Sex Determination
- Genes on the Y chromosome are crucial for testes development.
- Loss of these genes in an XY individual leads to female phenotypic development.
- Presence of a small portion of the Y chromosome in XX individuals can cause male phenotypic development, indicating a small region of the chromosome is vital for sex determination.
Human Biological Sex Determination
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SRY gene on the Y chromosome triggers testes development.
- Transcription factor activated at around 6 weeks of development.
- Before SRY activation, gonads are undifferentiated.
- Testes secrete testosterone (promotes male characteristics) and Mullerian Inhibitory Substance (MIS) (suppresses female reproductive ducts).
- In the absence of SRY, ovaries develop (estrogen and no MIS).
- Improper gene activity can hinder development and lead to sterility.
The X and Y Chromosomes
- Contain genes not associated with sexual development.
- Inheritance of these genes does not follow Mendel's laws.
- Males in XY inheritance are hemizygous for genes on the X chromosome.
Pseudoautosomal Regions
- Located on the distal ends of the short and long arms of X and Y chromosomes.
- These regions contain highly similar DNA sequences (PAR1 & PAR2).
- Crossing-over occurs in these regions during male meiosis.
- All genes within PAR1 are NOT inactivated in women, ensuring proper gene dosage.
- High recombination frequency in PAR1.
Sex-Linked Inheritance
- Inheritance patterns for genes on sex chromosomes differ from those on autosomes.
- Most sex-linked inheritance is X-linked, due to the X chromosome having more genes.
- Results in differential expression of traits between males and females.
Morgan’s Fruit Fly Experiments
- Demonstrated X-linked inheritance and differential expression of traits between males and females.
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Parental Generations:
- Red-eyed female x White-eyed male: All offspring red-eyed (no difference between males and females).
- White-eyed female x Red-eyed male: All females red-eyed, all males white-eyed.
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F1 Crosses:
- Heterozygous female x Red-eyed male: All females red-eyed, half the males white-eyed.
- Heterozygous female x White-eyed male: 50% red-eyed males and females, 50% white-eyed males and females.
- The discrepancy between males and females in phenotype presentation is a hallmark of X-linked inheritance.
Morgan’s Fruit Fly Experiments Summary
- Sex-linked inheritance based on mutations observed in males only.
- Gene linkage based on the inheritance of genes as a single unit.
- Chromosome mapping based on recombination frequencies between linked genes.
X-Linked Inheritance in Humans
- Can cause various genetic disorders, including hemophilia and color blindness.
Nondisjunction
- Can lead to sex chromosome aneuploidy (abnormal number of sex chromosomes).
- Impact on genotype and phenotype depends on the specific chromosomes involved and the dosage of genes.
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Description
Test your knowledge on sex chromosomes and their role in biological sex determination. This quiz covers key terms such as homogametic, heterogametic, and nondisjunction. Explore the complexities of chromosome behavior during meiosis and how it affects genetic functions.
