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This is a laboratory. Not sophisticated as expected, only some milk bottles with fruit flies
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but one of the finest laboratories in the history of science. It is fondly called as Morgan's
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Fly Room, the birthplace of modern genetics at Columbia University, where Morgan and his
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students worked on fruit flies to reveal the mysteries behind genetic inheritance and linkage
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In the last video we discussed about the relationship between independent assortment
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linkage and crossing hour in this video let us understand how Morgan carried out his epic
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experiment in stepwise manner within 5 to 10 minutes if you are new to this channel please consider
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subscribing our channel Morgan was working on drosophila melanogaster or fruit flies he found out
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that majority of the fruit flies are red-eyed and he called it as normal or wild type to his surprise
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he caught a white-eyed mutant. He called it as a mutant as it is a rare one
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and this famous white-eyed fly changed the course of genetics. Let's begin with his conclusion to understand better
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Marken's experiment concluded that white eye color is a sex-linked recessive trait
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The gene for eye color is physically located on X chromosome. How he reached this conclusion
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Let's begin with the first cross. He crossed red-eyed female with white-eyed male
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So red-eyed female, this is the white-eyed male, the famous white-eyed mutant fly
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So as expected, all had red eyes. Both female and male had red eyes
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To understand better, let us write down the genotype. X plus is a red-eyed trait or wild type
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X-W is the white-eyed trait, W indicates the white-eyed trait. W indicates the white-tide rate. So this is homocycus x plus x plus the wild mutant redfly
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and this is white-tide x-w-y as this is a male. It is having a Y chromosome only one X chromosome
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that is with this Y trait. Now let us draw the poinette square. It's better to draw this
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pinnard square along with me to understand this concept better. So these are the female gametes
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x plus and x plus and these are the male gametes x w and y so the offspring are X plus XW then X plus Y X plus XW and X plus Y
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As expected, all had red eyes. And Morgan suspected that this white-eyed trait is a recessive trade
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just like mental proposed. So it is unexpressed in the F1 generation, just like a recessive treat
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So he want to confirm this whether this white-eyed trade is a recessive trade
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So he did the second cross. Here, red-eyed female that is formed in the F-1
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is mated with red-eyed male that is formed in the F-1. Or the flies, the males and the females that is formed in the F-1
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the red-eyed flies is mated with one another. And this would be the genotype X plus X-W as this is a hybrid or heterocygus
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front red dyed female and this will be for red diet it will be x plus y as expected he got the
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3-st-1 ratio as mendel suggested so let us work out this x plus and x w are the female gametes
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male gametes are x plus and y so the offspring are x plus x then x plus y x plus xxx then x plus y xx or x plus x
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and xW and xWY. He got 3 to 1 ratio where 3 flies are red and 1 fly is white
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But he had an important observation. Only male flies had white-colored eye
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Even though 3 to 1 ratio is there as per the Grigar-Mendals ratio
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but here only the male flies had this white eyes. So this is a deviation from the normal pattern
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So this is the white-type male fly and this is the genotype X-W-Y
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He suggested that white-tide rate is carried on X chromosome. That is why it is present only in males
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But there is one more problem. In all these offspring, there are no white-eyed females
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So this white-tie is lethal to females. He wanted to confirm that To confirm that white eye is not lethal to females he did the next cross He used red female of F1 which is heterozygous and white male the famous
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white-eyed mutant. This is hetto-cycus X plus X-W, then XW-Y. Then as this is hetozycous
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if this gene is lethal, there won't be any female white fly. Then he carried out
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the cross as expected he got one is to one is to one is to one ratio as per the
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mandolin ratio where there are red-eyed and white-eyed females red-eyed and white-tied
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males so this was the cross the gametes were x plus and xw here it will be xw and y
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and the offspring were x plus xw then x plus y x plus y xxx
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xW and xWY. So he got this white-eyed female confirming that this white eye color is not lethal to females
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thus ruling out that possibility. Now he had a white-eyed female. He confirmed that white eye is not lethal to females
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Then he finally did the most crucial cross, the confirmatory reciprocal cross
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He used the white-tide female that is formed in that F-2. The genotype will be X-W-X-W
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As this white-tide rate is a recessive rate, it should be in homocycous condition to express in females
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Then he used the red-eyed male, that is X plus Y. Now, he know that white-eye color is a russive trait
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He has already confirmed that. Then he predicted that white-tied female would be homocycous resistive
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If white eye color is a rasive trait, then it should be homocycus in order to express in a female as it is a russive trait
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Then males inherit only X chromosome from the females. So in this course, all males will be white-eyed as this X chromosome, XW, will be inherited by the males or from the mother
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He predicted that all male offspring, resulting from this growth between this white and red will be white whereas all the female would be red as this X chromosome will be inherited from the male as this X plus is a dominant trait it will mask
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this X-W trait or white-eyed trait let us see what happened his predictions was true
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he got all females as red-eyed and all males white-tide. Let us see the gross. So this is xW-X-W homocycus, white-tide female
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As white-tide is a receptive rate, it should be homocycus. Then red-dide rate, that is X plus Y
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Then the offsprings are X plus X-W, that is red. Whereas X-W-Y, that is white-tide male
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then X-W, X plus X-W, then again red-dyed female. And X-T-W. W Y white Tide male so this xW is present in females but it is masked by the dominant trait that is X plus
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whereas this males becomes white-tide as it receives the X chromosome from the females or from the mother
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so males has only one X chromosome so there is no recessive or dominant whatever is there
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that will be expressed. So the XW that is received from the mother is expressed. That is why all
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males becomes white. He confirmed for the first time that the gene for white eye color is
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physically located on the X chromosome. He was the first one to provide experimental evidence
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for chromosome theory and linkage. Genes are present on chromosomes. He is the first one to point out
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that there are linked trades or certain genes are inherited together as against independent
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assortment of Grigermandel. He found out that there are some genes that are inherited together
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or that are linked. For his work with trosophila, Morgan was awarded the Nobel Prize in 1933
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Hope this video is beneficial. You understood this epic experiment. Share your thoughts. We have given a video
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on relationship between linkage, independent assortment and crossing hour as an introductory video
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You can watch that also. Take care. Stay blessed. Thank you so much for your support
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