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General Genetics Ayesha M. Khan Spring 2013. Sex Determination in Drosophila  The fruit fly Drosophila melanogaster, has eight chromosomes: three pairs.

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Presentation on theme: "General Genetics Ayesha M. Khan Spring 2013. Sex Determination in Drosophila  The fruit fly Drosophila melanogaster, has eight chromosomes: three pairs."— Presentation transcript:

1 General Genetics Ayesha M. Khan Spring 2013

2 Sex Determination in Drosophila  The fruit fly Drosophila melanogaster, has eight chromosomes: three pairs of autosomes and one pair of sex chromosomes  Presence of the Y chromosome does not determine maleness in Drosophila  Each fly’s sex is determined by a balance between genes on the autosomes and genes on the X chromosome. This type of sex determination is called the genic balance system.  The X chromosome contains genes with female producing effects, whereas the autosomes contain genes with male- producing effects.  Fly’s sex is determined by the X:A ratio, the number of X chromosomes divided by the number of haploid sets of autosomal chromosomes.

3 Sex Determination in Drosophila An X:A ratio of 1.0 produces a female fly; an X:A ratio of 0.5 produces a male.  X:A ratio less than 0.5: a male phenotype is produced, but the fly is weak and sterile—metamales.  X:A ratio between 1.0 and 0.50: intersex fly, with a mixture of male and female characteristics.  X:A ratio > than 1.0 : Female phenotype is produced, but these flies (called metafemales) have serious developmental problems and many never emerge from the pupal case.

4 Chromosome complements and sexual phenotypes in Drosophila

5 Sex Determination in Humans XX-XY sex determination Presence of a gene on the Y chromosome determines maleness

6 Turner syndrome: XO; 1/3000 female births – Immature secondary sex characteristics – Normal intelligence – Sterile Klinefelter syndrome: XXY, or XXXY, or XXXXY, or XXYY; 1/1000 male births – Immature secondary sex characteristics – Most have normal intelligence – Sterile Poly-X females: 1/1000 female births – Normally regular secondary sex characteristics – Fertile – Mental retardation slightly higher

7 The role of sex chromosomes The X chromosome contains genetic information essential for both sexes; at least one copy of an X chromosome is required for human development. The male-determining gene is located on the Y chromosome. A single copy of this chromosome, even in the presence of several X chromosomes, produces a male phenotype.

8 The role of sex chromosomes (contd) The absence of the Y chromosome results in a female phenotype. Genes affecting fertility are located on the X and Y chromosomes. A female usually needs at least two copies of the X chromosome to be fertile. Additional copies of the X chromosome may upset normal development in both males and females, producing physical and mental problems that increase as the number of extra X chromosomes increases.

9 The male-determining gene in humans David Page (1987)  Analyzed the chromosomes of sex-reversed XX men, rare individuals who look like men but have two X chromosomes instead of one X chromosome and one Y chromosome.  Page discovered that sex-reversed males carried genes from a 140-kilobase region on the short arm of the Y chromosome.  Presumably, this region had been transferred to the X chromosome during a translocation.  Subsequent experiments narrowed down this region and found that one gene, the sex- determining region of the Y, or SRY was the master regulator of sex determination. The presence of just this region from the Y chromosome is thus sufficient to cause male development.

10 The SRY Gene How the Y chromosome determines sex: The SRY gene, located on the Y chromosome, is the primary determinant of sexual development. – That is, if a developing embryo has a functional SRY gene in its cells, it will develop as a male. And, if there is no functional SRY, the embryo develops as female. Although the SRY gene is usually on the Y chromosome, it occasionally gets transferred to the X. – this leads to 46,XX males Also, sometimes the SRY gene is inactivated by mutation. – Leading to 46,XY females (Swyer syndrome) – it is also possible to have a partially inactive SRY gene

11 Androgen-insensitivity syndrome -Females; X and Y chromosome -Caused by the defective androgen receptor; cells are insensitive to testosterone, and female characteristics develop. -The gene for the androgen receptor is located on the X chromosome; so persons with this condition always inherit it from their mothers.

12 Sex-linked characteristics Sex-Linked Characteristics Are Determined by Genes on the Sex Chromosomes Genes on the X chromosome determine X- linked characteristics; those on the Y chromosome determine Y-linked characteristics.

13 Thomas Morgan ( ) The first person to explain sex- linked inheritance was the American biologist Thomas Hunt Morgan X-Linked White Eyes in Drosophila In both humans and fruit flies (Drosophila melanogaster) females have two X chromosomes, while males have X and Y

14 Sex Linkage Morgan (1910) found a mutant white-eyed male fly, and used it in a series of experiments that showed a gene for eye color located on the X chromosome. a.First, he crossed the white-eyed male with a wild-type (red-eyed) female. All F 1 flies had red eyes. Therefore, the white-eyed trait is recessive. b.Next, F 1 were interbred. They produced an F 2 with: i. 3,470 red-eyed flies. ii. 782 white-eyed flies. c. All of the F 2 white-eyed flies were male.

15 X-linked inheritance of white eyes in Drosophila: Red-eyed female  white-eyed male

16 X-linked inheritance of white eyes in Drosophila: The F 1 flies are interbred to produce the F 2 =>This finding was clearly not the expected result for a simple recessive trait, which should appear in ¼ of both male and female F 2 offspring.

17 What happened when white eyed males and red eyed females from second generation were crossed? =>Equal number of offspring with each eye color

18 Morgan’s hypothesis was that this eye color gene is located on the X chromosome.  Males therefore cannot be either homozygous or heterozygous but are said to be hemizygous for X-linked loci. -Females may be homozygous or heterozygous. The wild-type female in the original cross was w + /w + (homozygous for red eyes). -Females only show the white eyes trait if they inherit mutant genes on both X chromosomes. “Sex-linked inheritance”

19 Reciprocal cross: Homozygous white-eyed female  red-eyed ( wild-type) male

20 Reciprocal cross: The F 1 flies are interbred to produce the F 2

21  Morgan’s discovery of X-linked inheritance showed that when results of reciprocal crosses are different, and ratios differ between progeny of different sexes, the gene involved is likely to be X-linked (sex- linked).  This was strong evidence that genes are located on chromosomes.  Morgan received the 1933 Nobel Prize for Physiology or Medicine for this work.

22 Non-Disjunction of X Chromosomes 1.Morgan’s work showed that crossing a white-eyed female (w/w) with a red-eyed male (w + /Y) produces an F 1 of white-eyed males (w/Y) and red-eyed females (w + /w). His student, Bridges, found that about 1 in 2,000 of the offspring was an exception, either a white-eyed female or red-eyed male. 2.Bridges’ hypothesis was that chromatids failed to separate normally during anaphase of meiosis I or II, resulting in non- disjunction. 3.Non-disjunction can involve either autosomes or sex chromosomes. For the eye-color trait, X chromosome non- disjunction was the relevant event. Non-disjunction in an individual with a normal set of chromosomes is called primary non-disjunction.

23 Nondisjunction in meiosis involving the X chromosome


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