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Hi friends, in this video let us
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understand the genic balance theory
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proposed by Calvin Bridges regarding the
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sex determination in Drosophila.
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Let's begin with the background for
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better understanding. While working in
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Drosophila, Calvin Bridges found out
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that the genotype XX is the genotype and
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the phenotype is female in Drosophila.
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Whereas XY is male just like humans. But
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later studies found out that the Y
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chromosome does not determine sex. Or in
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other words, XXY, even though presence
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of Y is there, it is female. Y has no
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role in determining the sex. Whereas XO,
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without a Y chromosome,
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a single X will make the phenotype male.
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So, if you remove the Y chromosome, the
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fly is still male. If you add a Y
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chromosome to the female, that is XXY,
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the fly is still female. That means Y
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chromosome does not determine sex.
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And this finding actually lead to
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further research that lead to the
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discovery of genic balance theory. It
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was proposed by Calvin Bridges in 1921.
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And this was a summary of the findings.
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Autosomes are responsible for male-ness,
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whereas female-ness is determined by the
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X chromosome. He observed that Y
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chromosome in Drosophila does not
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determine sex. It is only required for
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male fertility. Sex is determined by the
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balance between female determining
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factors on the X chromosome and male
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determining factors on the autosomes.
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And he formulated a ratio, a fundamental
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ratio, which is called as sex index
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ratio. That is X by A ratio, where
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that is equal to number of X chromosomes
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by sets of autosomes. Let us understand
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this theory with the classical
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experimental example of Calvin Bridges.
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His experiment was on Drosophila.
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He used polyploid flies and categorized
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the results based on specific ratios.
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And he used this ratio, that is number
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of X chromosomes by sets of autosomes.
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First, let us see the female. In
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females, there are two X chromosomes.
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Therefore, number of X chromosome is
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two. Whereas sets of autosomes, two
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sets. It's diploid, it's diploid,
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therefore two sets of autosomes. Two by
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two, one. If the ratio is one, it will
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that is XY, there is only one X
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chromosome and two sets of autosomes,
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that is one by two. That is 0.5. If the
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ratio is 0.5, the phenotype is normal
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male. Then he further studied and found
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out many ratios in between. Let us see.
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There are three X chromosomes.
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That is three by two, two or sets of
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Then three by two, it will be 1.5. If
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the ratio is greater than one, it will
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be super female. And this is sterile
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also. If there are three sets of
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there is only one X chromosome and three
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sets of autosomes, one by three. Then
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the ratio will be 0.33. If the ratio is
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less than 0.5, it will be super male and
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that is sterile also.
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And finally, he found out that
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if there are two X chromosome and three
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sets of autosome, then it will be two by
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That the ratio between 0.5 and 0.1
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will be intersex. That shows mixed
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traits of both males and females. And
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this is the summary of genic balance
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theory. And what is the reason for this
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manifestation? Later studies found out
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that there is a gene which is called as
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which acts as a master switch. It acts
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as a biological calculator that
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interprets the XA ratio and decides
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whether the fly will be male or female.
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In Drosophila, this SXL gene must be on
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for females, whereas it should be off
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for males. Otherwise,
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this will be lethal to both females and
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males. That's why it is called as sex
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lethal genes. In the later video, we'll
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be discussing how SXL gene
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determines the sex of in Drosophila.
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Now, finally, the role of Y is
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fertility, not sex determination. Y
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chromosome contains genes that is
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essential for sperm development and
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motility. That is in XY male, whereas in
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XO male, Y chromosome is absent.
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Therefore, it leads to defective sperm
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without motility that lead to sterility.
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This Y chromosome has genes that codes
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for fertility factors
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that codes for dynein motors that makes
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sperm tails move. So, without Y, the fly
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is male, but it is sterile.
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Hope you are clear. Take care. Stay
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blessed. Thank you so much. You are with
