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Define and explain the genetic significance of "XX" chromosomes in organisms, particularly in relati...
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Sex determination in organisms is a fundamental genetic process that dictates whether an individual will develop as male or female. In many species, including humans, sex determination is governed by a specific set of chromosomes known as the sex chromosomes. In these species, females typically have two of the same kind of sex chromosome, denoted as "XX," while males have two distinct sex chromosomes, denoted as "XY."
Sex determination in organisms is a fundamental genetic process that dictates whether an individual will develop as male or female. In many species, including humans, sex determination is governed by a specific set of chromosomes known as the sex chromosomes. In these species, females typically have two of the same kind of sex chromosome, denoted as "XX," while males have two distinct sex chromosomes, denoted as "XY."
The "XX" chromosome configuration plays a crucial role in sex determination and inheritance due to the following reasons:
1. Sex Determination: The presence of two X chromosomes typically leads to the development of female sexual characteristics. In humans and other mammals, the sex of an individual is determined at fertilization. If the sperm cell contributes an X chromosome, the resulting zygote will have two X chromosomes (XX) and will develop into a female. If the sperm cell contributes a Y chromosome, the zygote will have one X and one Y chromosome (XY) and will develop into a male.
2. Dosage Compensation: Because females have two X chromosomes and males have only one, there needs to be a mechanism to balance the expression of genes on the X chromosome so that neither sex has a dosage advantage. In mammals, this is achieved through a process called X-chromosome inactivation, where one of the X chromosomes in females is largely inactivated, ensuring that the cells of both males and females have a similar level of gene expression from their sex chromosomes.
3. Genetic Inheritance: The X chromosome carries many genes that are not found on the Y chromosome. This means that X-linked traits are inherited differently than autosomal traits (traits not linked to sex chromosomes). For example, if a gene for a particular trait is located on the X chromosome, a female (XX) will have two copies of this gene, while a male (XY) will have only one. This has implications for the inheritance of X-linked disorders. For instance, if a female carries a recessive X-linked disorder on one of her X chromosomes, she may not express the disorder because she has a second, normal copy of the gene on her other X chromosome. However, a male with a single mutated gene on his X chromosome will express the disorder because he lacks a second X chromosome with a normal copy of the gene.
4. Recombination and Variation: During meiosis, the process that creates gametes (eggs and sperm), homologous chromosomes pair up and can exchange genetic material through a process called recombination. Because females have two X chromosomes, they can undergo recombination, which increases genetic diversity. Males, on the other hand, cannot have recombination between the X and Y chromosomes because these chromosomes are not homologous and share very little genetic material in common.
In summary, the "XX" chromosome configuration is significant in genetic terms because it determines female sex development, ensures dosage compensation, influences patterns of inheritance, particularly for X-linked traits, and contributes to genetic variation through recombination. Understanding the role of the "XX" chromosomes enhances our comprehension of genetic diseases, inheritance patterns, and the fundamental biology of sex determination.
The "XX" chromosome configuration plays a crucial role in sex determination and inheritance due to the following reasons:
1. Sex Determination: The presence of two X chromosomes typically leads to the development of female sexual characteristics. In humans and other mammals, the sex of an individual is determined at fertilization. If the sperm cell contributes an X chromosome, the resulting zygote will have two X chromosomes (XX) and will develop into a female. If the sperm cell contributes a Y chromosome, the zygote will have one X and one Y chromosome (XY) and will develop into a male.
2. Dosage Compensation: Because females have two X chromosomes and males have only one, there needs to be a mechanism to balance the expression of genes on the X chromosome so that neither sex has a dosage advantage. In mammals, this is achieved through a process called X-chromosome inactivation, where one of the X chromosomes in females is largely inactivated, ensuring that the cells of both males and females have a similar level of gene expression from their sex chromosomes.
3. Genetic Inheritance: The X chromosome carries many genes that are not found on the Y chromosome. This means that X-linked traits are inherited differently than autosomal traits (traits not linked to sex chromosomes). For example, if a gene for a particular trait is located on the X chromosome, a female (XX) will have two copies of this gene, while a male (XY) will have only one. This has implications for the inheritance of X-linked disorders. For instance, if a female carries a recessive X-linked disorder on one of her X chromosomes, she may not express the disorder because she has a second, normal copy of the gene on her other X chromosome. However, a male with a single mutated gene on his X chromosome will express the disorder because he lacks a second X chromosome with a normal copy of the gene.
4. Recombination and Variation: During meiosis, the process that creates gametes (eggs and sperm), homologous chromosomes pair up and can exchange genetic material through a process called recombination. Because females have two X chromosomes, they can undergo recombination, which increases genetic diversity. Males, on the other hand, cannot have recombination between the X and Y chromosomes because these chromosomes are not homologous and share very little genetic material in common.
In summary, the "XX" chromosome configuration is significant in genetic terms because it determines female sex development, ensures dosage compensation, influences patterns of inheritance, particularly for X-linked traits, and contributes to genetic variation through recombination. Understanding the role of the "XX" chromosomes enhances our comprehension of genetic diseases, inheritance patterns, and the fundamental biology of sex determination.
Chromosomal Theory and Genetic Linkage a
Modern Understandings of Inheritance
Chromosomal Theory and Genetic Linkage b
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Hi. Until now we've been
studying Mendel's theories of evolution and we're now going to look
more deeply into modern understandings of
inheritance and particularly how chromosomal theory
and genetic linkage that Mendel understood to some degree at least come together.
By the end of this section,
you should be able to discuss Sutton's chromosomal theory of inheritance.
You should be able to describe the genetic linkage,
and you should be able to explain the process
of homologous recombination or crossing over.
First of all, let's now look at Mendel's ideas in brief,
just to remind you.
He is considered the father of modern genetics.
He began studying heredity in 1843.
Of course, not knowing anything about DNA,
and not knowing anything at all about chromosomes even.
He just observed traits after crosses as we learned.
But later improved microscopic techniques
allowed people to observe cell division and meiosis.
Particularly, they could stain the cells and see chromosomes.
With each mitotic division,
chromosomes that they saw replicated,
condensed, and migrated to separate cellular poles.
They notice these chromosomes.
They call them chromosomes because they were stained.
Chromo means color.
They could see them colored when they looked at
cells that had been stained in a particular way.
Now, the speculation was that chromosomes might be the key to heredity,
because of the way they moved during meiotic and mitotic divisions.
Several scientists began to examine
Mendel's publications and see if they could put 2 and 2 together.
If they could understand the movement of the chromosomes with Mendel's ideas.
In particular, Theodor Boveri in 1902,
noticed that proper sea urchin embryonic development
does not occur unless all the chromosomes are present.
Yes. If there was a problem in our language today,
during anaphase in which the chromosomes didn't separate properly,
maybe all the chromosomes wouldn't be present, and then,
if that were the case, embryonic development did not occur properly.
Maybe there's a connection between the chromosomes and development.
Walter Sutton in the same year, 1902,
said that the chromosomes separate into daughter cells during meiosis.
Sutton then looked at the chromosomes and
he saw that they separated into daughter cells during meiosis.
This led to the chromosomal theory of inheritance in
which they said that chromosomes are the genetic material.
There was material, there was the genetic material,
something physical that was responsible for the Mendelian inheritance.
Now, during meiosis, homologous chromosomes they saw migrate
as discrete structures that are independent of other chromosome pairs.
They could see this by looking through the microscope,
and the chromosomes sorting from each homologous pair
into pre gametes appears to be random.
That was just by visualization, and of course,
that jive very nicely with Mendel's ideas.
Each parent then synthesizes gametes that contain only half their chromosomal complement,
as we learned earlier.
Even though male and female gametes,
sperm and egg differ in size and morphology.
Yes, the sperm are much smaller and they certainly look different than the egg.
Both of them are haploid.
Yes. Both of them are haploid.
That is, they contain the same number of chromosomes,
half the number that is found in the diploid zygote,
and that's suggested equal genetic contributions from each parent.
Of course, the gametic chromosomes combined during fertilization to produce
offspring that had the same chromosome number as their parents, the 2n.
Yes, as we learned before.
Scientists proposed the chromosomal theory of inheritance,
long before there was any direct evidence that chromosomes carried traits.
All these were indirect,
circumstantial reasoning that made people think that these things they saw,
those chromosomes, in fact, carried traits.
But there were critics,
and the critics said that they saw that individuals had far
more independently segregating traits than they had chromosomes.
People were thinking maybe each chromosome only had 1 trait.
But here people saw that there were many independently segregating traits.
If several traits might be on the same chromosome,
so they should not segregate independently.
But in fact, people did see that they segregated independently.
There were critical thinkers that
were worried about the chromosomal theory of inheritance.
But Thomas Morgan, who worked on Drosophila Melanogaster.
Drosophila are the common fruit fly,
provided experimental evidence to support the chromosomal theory of inheritance.
Let's go back for a moment to Mendel.
Mendel's work suggested that traits are inherited independently of each other.
If we go through across here with
the same homozygous flies and then dihybrid test crosses that we saw before.
Then if we look at 2 different traits, the b trait,
which is something that encodes for the body color of these flies,
and this vg or vestigial wings traits,
that is either the wings could be long like this normal or they could be just very short.
If you do these crosses with homozygous plus
wild-type in both of those traits together with negatives in both traits,
then of course you get an F1 hybrid in which you will have
all the various combination of eggs that will come out of that across.
Now if you mate that,
or each of these together with a bb,
in other words, in vgvg,
that is, these are the black body vestigial wings,
that's not the wild-type male,
then there are actually 4 possibilities of
flies that you'll get out and they're listed over here.
You would expect, of course,
each of them according to Mendel,
to be found in a 1:1:1:1 ratio.
However, when this was done by Morgan,
he did not find that they were in a 1:1:1:1 ratio.
But that is not what he found.
What he found was a 965: 944: 206: 185.
Let's see if that could be explained.
It seems actually from this and Morgan saw that it's not
a discrete segregation like Mendel saw but rather you could have something in the middle.
His suggestion was that alleles,
that is, particular traits could be very close to each other on the same chromosome,
or potentially they could be far apart from each other on the same chromosome,
and that would lead to different kinds of segregation.
Let's pause here and continue with this in the next video.
This video discusses the modern understanding of inheritance, particularly the chromosomal theory of inheritance and genetic linkage. Mendel's theories of evolution are discussed in brief, as well as the improved microscopic techniques that allowed scientists to observe cell division and meiosis. Theodor Boveri and Walter Sutton's observations of chromosomes during meiosis led to the chromosomal theory of inheritance, which states that chromosomes are the genetic material. Thomas Morgan's experiments with Drosophila Melanogaster provided evidence to support the chromosomal theory of inheritance, as well as the concept of genetic linkage, which explains why alleles can be close or far apart on the same chromosome.
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