Sex Chromosomes

Summary

This transcript explores the role of sex chromosomes (X and Y) in determining sex and influencing inheritance patterns. It covers the chromosomal differences between males and females, with males having one X and one Y chromosome, while females have two X chromosomes. The presence of the Y chromosome, specifically the SRY gene, determines male sex. The transcript also explains dosage compensation in females, where one X chromosome is randomly inactivated to balance gene expression between males and females. This process leads to chimerism in female mammals, with observable effects like the calico cat's patchwork fur.

Takeaways

• 😀 The sex chromosomes (X and Y) play a crucial role in determining the sex of an individual in mammals.
• 😀 Humans have 46 chromosomes in total, with 44 being autosomes and 2 being sex chromosomes (XX for females and XY for males).
• 😀 The X chromosome is much larger and contains hundreds of genes, while the Y chromosome is smaller and contains very few genes.
• 😀 Despite their differences, the X and Y chromosomes can pair up in meiosis due to small regions of homology at both ends of the chromosomes.
• 😀 Sex determination in mammals is primarily governed by the presence or absence of the Y chromosome.
• 😀 Individuals with Turner syndrome (45 XO) are phenotypically female, while individuals with Klinefelter syndrome (47 XXY) are phenotypically male, indicating the importance of the Y chromosome for male sex determination.
• 😀 The SRY gene located on the Y chromosome is the critical determinant for male sex, initiating a cascade of gene expressions that lead to male characteristics.
• 😀 Female mammals have two copies of every X-linked gene, while males have only one. This disparity affects gene expression and protein production.
• 😀 Dosage compensation ensures that females, who have two X chromosomes, do not produce double the amount of protein from X-linked genes compared to males.
• 😀 In mammals, one X chromosome in each female cell is randomly inactivated early in development, leading to a balance in gene expression between males and females.
• 😀 This random inactivation of X chromosomes in female mammals leads to the phenomenon of chimerism, where different cells express different alleles of X-linked genes, as seen in calico cats.

Q & A

What are the two basic types of chromosomes in animal cells?

The two basic types of chromosomes in animal cells are autosomes, which are non-sex chromosomes, and sex chromosomes, which determine the sex of an individual.

How do sex chromosomes differ in size and gene content?

The X chromosome is much larger than the Y chromosome and contains hundreds of genes. The Y chromosome is smaller and has very few genes associated with it, with most genes on the Y chromosome not being present on the X chromosome.

How do the X and Y chromosomes pair up during meiosis despite their differences?

Although the X and Y chromosomes are quite different, they have small regions of homology at both ends and a small region in between. These regions are enough to allow the X and Y chromosomes to pair up and act as homologs during meiosis, forming a tetrad in meiosis I.

What determines the sex of an individual based on their sex chromosomes?

The presence of a Y chromosome determines male sex, while the absence of a Y chromosome determines female sex. If an individual has two X chromosomes, they are female (XX), and if they have one X and one Y chromosome, they are male (XY).

What is non-disjunction, and how does it affect sex chromosomes?

Non-disjunction is a process where homologous chromosomes fail to separate properly during meiosis, leading to gametes that either have two sex chromosomes or none. This can result in individuals with extra or missing sex chromosomes, such as 45,XO or 47,XXY, causing disorders like Turner syndrome and Klinefelter syndrome.

How do abnormalities in the sex chromosomes provide evidence for the role of the Y chromosome in sex determination?

Studies of individuals with sex chromosome abnormalities, such as 45,XO (Turner syndrome) and 47,XXY (Klinefelter syndrome), show that the presence of a Y chromosome leads to male phenotypes, even when there are extra or missing X chromosomes. This supports the idea that the Y chromosome, particularly the SRY region, is key in determining sex.

What is the SRY gene, and why is it important in sex determination?

The SRY gene, located on the Y chromosome, is crucial for male sex determination. It encodes a transcription factor that triggers a cascade of gene expressions leading to the development of male-specific characteristics. The presence of the SRY gene determines the male phenotype.

How does dosage compensation balance the gene expression between males and females?

In mammals, females have two X chromosomes, which could lead to double the gene dosage of X-linked genes. To compensate, one of the X chromosomes in each female cell is randomly inactivated, ensuring that only one X chromosome is active, similar to the single X chromosome in males, balancing the gene dosage.

What is the process of X chromosome inactivation, and what is its outcome?

X chromosome inactivation occurs early in female embryonic development when one of the X chromosomes in each cell is randomly converted into heterochromatin, rendering it inactive. This ensures that females, like males, express genes from only one X chromosome, balancing gene dosage across sexes.

How does X-linked gene expression result in chimerism in female mammals?

Female mammals can be considered genetic chimeras for X-linked genes because, in different cells, one of the X chromosomes is inactivated. In heterozygous females, some cells will express one allele of an X-linked gene, while other cells will express a different allele, creating a mosaic pattern of gene expression.