Sex linkage | sex linkage in drosophila | T.H Morgan experiment | class 12 скачать в хорошем качестве

Sex linkage | sex linkage in drosophila | T.H Morgan experiment | class 12

Aslam o alikum I am Hassam ur Rahman and I am teaching Fsc biology since 2014 .I am always trying to improve myself and provide best lecture to students.i am taking content for the lecture from authentic and relevant sources but human errors are possible . you are requested to please highlight the mistakes.My lectures are equally reliable for Fsc and mdcat students #visiblescience #mdcatbiology #alevelbiology #neetbiology #fscbiology My personal channel, I am sure it will change your life and very beneficial for you https://youtube.com/@hassamurRahman?f... My Facebook page link https://www.facebook.com/hassam.urahm... This lecture is about SEX LINKAGE Sex Linkage in Drosophila Thomas Hunt Morgan (1910) provided experimental evidence in support of chromosomal theory of heredity through discovery of sex linkage in fruitly Drosophila. Morgan raised cultures of Drosophila lies to study diferent traits, such as colour of the eye. Normal fruit lies, the wild type, have bright red eyes. One of his coworkers Calvin Bridges, observed an unusual white eye mutant male ly. (Fig. 22.25). Morgan mated this white eyed male with a wild type red eyed female. All 1237 ofspring of this cross had red eyes. Morgan concluded that red eye is a dominant trait (Fig 22- 26a). Morgan allowed males and females of F( generation to mate and produce F2 generation. He counted 2459 red-eyed females, 1,011 red-eyed males and 782 white eye males among F2 (Fig. 22.26b). The proportion of 3470 red eyed to 782 white eyed lies did not perfectly it into Mendelian 3 : 1 ratio. The number of recessive phenotype individuals was too small. There was another pecularity in this result. All the white-eyed lies were only males. There was no white eye female in F2 generation. The inheritance of eye colour some how seemed to be related to the ‘sex’ of the ofspring. Morgan proposed that: (i) The gene for eye colour is located on X chromosome, (ii) the alleles for eye colour are present only on X chromosome. There is no corresponding allele for this trait on Y chromosome. Thus even a single recessive allele on X chromosome can express itself in males because Y chromosome is empty for that gene. Males are hemizygous as they carry just one allele on their only X chromosome. Females have two X’chromosomes, each carrying an allele of the trait. Females can be homozygous or heterozygous. Symbol “w” represents the recessive allele for white eye, and “w+” designates its wild type allele for red eye. The genotypes of the parents of Pi cross were: Xw+ Xw+ for red eye female, and Xw Y for the white eye male. Morgan’s hypothesis explained clearly why all the white eyed lies in F2 generation were only males. Step 3: Test cross : Morgan wanted to test his hypothesis (Fig. 22-26c). He crossed the P| white eyed male (XWY) with one of its own daughters, the red eyed heterozygous female from F| generation. This test cross produced 129 red-eyed females, 132 red- eyed males, 88 white-eyed females and 86 white eyed males. White-eyed lies were less viable than red-eyed lies. Half the female ofspring in fact had red eyes and half had white. Similarly half the males had red eyes and half had white. Step 4: Reciprocal cross as a conirmatory test: Appearance of white eyed female provided an opportunity for a further conirmatory test. Morgan mated a white eyed female with a red-eyed male (Fig. 22.26d). All female ofspring had red eyes, and all male ofspring had white eyes. Then these Fi red eyed females and white eyed males were mated to produce F2. Half of the F2 females had red eyes, half had white. Similarly half of the F2 males had red eyes and half had white. This Fi x Fi cross was exactly like step 3 test cross. A trait whose gene is present on X chromosome is called X - linked trait. X - linked traits are commonly referred as sex-linked traits. A gene present only on X chromosome, having no counterpart on Y chromosome, is called X - linked gene. Sex-linked inheritance follows a very speciic pattern. As a son inherits his X chromosome only from his mother, and a daughter gets an X chromosome from each parent, an X linked trait passes in a crisscross fashion from maternal grandfather (Pi) through his daughter (Fi) to the grandson (F2). It never passes direct from father to son because a son inherits only Y chromosome from father. Morgan’s discovery of sex-linked inheritance was a great contribution to the understanding of genes and chromosome. In 1933, T. H. Morgan was awarded a Nobel Prize for his contributions to genetics. .