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Lecture 18 - sex determination and sex linked characteristics

Lecture 18 - sex determination and sex linked characteristics

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Unformatted text preview: Lecture 18 June 7, 2011 Chapter 4 Sex Determination and SexLinked Characteristics 1. Sex determination and Human sex chromosomes An introduction 2. X linked gene inheritance 3. Dosage compensation Sex and Sex Reproduction Sex refers to sexual phenotype. Most organisms have only two sexual phenotypes: male and female. Sexual reproduction involves the combination of haploid gametes (sperm and egg) from two sexes to form a diploid embryo (zygote) Sex Determination Sex determination refers to the mechanism or genetic basis for sex establishment 1. Chromosomal sex determination (focus of our course) 2. Genic sex determination (by genes on undifferentiated chromosomes) 3. Environmental sex determination (e.g. marine mollusk) Turtles Warm temp. = females Cool temp. = males Alligators Warm temp. = males Cool temp. = females slipper limpet Human Sex Chromosomes Humans have 23 pairs of chromosomes (discovered in 1956) 22 autosomal chromosomes (same in male and female) 2 sex chromosomes: X and Y Y is 1/3 the size of the X and has only 1/100th as many genes Though differ in shape and size, X and Y can still pair in meiosis X and Y contain at least 20 common genes near the ends of the chromosomes (pseudoautosomal region). Allow for pair and recombination between the X and Y . Human Y Chromosome The Y chromosome is an acrocentric chromosome (NOT Yshaped!) 95% of Y = nonrecombining region of the Y (NRY) or malespecific region of the Y (MSY) Y chromosome has the SRY gene, which determines the male and encodes a protein called the testis-determining factor (TDF) XY female miss functional SRY gene XX male has SRY gene Absence of Y chromosome results in female phenotype (turner syndrome) Human Sex Inheritance Female: homogametic XX Produce two same gametes in terms of sex chromosome Male: heterogametic XY Produce two different gametes in terms of sex chromosome Inheritance of human sex results in equal numbers of male and female offspring. http://www.youtube.com/watch?v=8YMhkweABfQ&feature=related Inheritance of Sex-linked Genes The Y, although behaving like a homolog of X, has much fewer genes If a locus controlling a trait is located on X not Y chromosome, the frequency at which the trait is observed will be different in males and females In Female XX: a gene that is on the X will be two copies In male XY: a gene on the X will be one copy E.g. Color blindness, hemophilia (reduced blood clotting) = x linked recessive abnormality. Different ratio from the Mendelian principles A major extension of Mendelian inheritance principles X-linkage Firstly discovered by Thomas H. Morgan in 1910. He found genes controlling eye color, body color and other traits in Drosophila are located on X chromosomes. The linked genes are inherited together (linkage) according to sex. http://bcs.whfreeman.com/WebPub/Biology/pierce4e/Animations%20%20Podcasts/ch04/0401_X_linked_inheritance.html Wild-type eye color in Drosophila = red Due to the production of two pigments. Morgan found a mutant male with white eyes. Two pigments are missing due to a recessive allele. Crosses between the white-eyed male mutant to the wildtype (red) females results in all (F1) progeny flies with red eyes. Indicates red eye is dominant to white eye. Male and female F1 progeny were then crossed with each other. Results: Females: 100% red eyes. Males: ½ red eyes: ½ white eyes (1:1). Explanation The gene that controls eye color is located on the X chromosome. The Y chromosome does not carry any alleles associated with the eye color gene. Males; therefore, have only one copy of the gene. Inherited from their mother along with their only X chromosome. Explanation In Meiosis Examples of X-linked Inheritance in Humans Males need to inherit only one recessive allele to show an X-linked trait, but females need to inherit two, one from the mother and one from the father. Male has more chance to exhibit the recessive phenotype Color Blindness The genes for visual pigments that are responsible for the perception of red and green are found on the X chromosome. X-linked recessive trait in human F i g : 4-14, K & G Color Blindness Inheritance Sex-linked Gene Inheritance Practice Betty (wife): normal vision, but her mom is color blind. Bill (husband) is color blind color blind? 50% Dosage Compensation Human female XX: 2 copies of X-linked genes Human male XY: 1 copy of X-linked genes So, X-linked gene in females would produce twice as much products as in males? No, because there is a genetic mechanism in females that compensates for X dosage. Inactivation of one X-chromosome in female Dosage Compensation In female somatic cells, one can see an inactivated X chromosome = Barr body (darkly staining chromatin) during interphase Barr body = inactivated X-chromosome Areas (genes) on one of the X chromosomes visible in nucleus of a cell in a female mammals Female cell bar body Male cell, no bar body Number of bar body in human cells with different complements of sex chromosomes Turner Syndrome (1/2000 female births) 45, X: no Barr body person is female, abnormal ovaries Klinefelter Syndrome (1/500 male births) 47,XXY: one Barr body male, but sterile and slight feminization - 47, XXX: two Barr bodies (1/1200 female births) many times they are normal females - 47, XYY: one Barr body normal male (> 6 feet) - 46, XX (normal): one Barr body Dosage Compensation Which X gets inactivated? Is it the same X in every cell? Mary Lyon (1961): Lyon Hypothesis X inactivation is random Happens early in development (within first few weeks) In heterozygous female, 50% cell express one allele and 50% cell express the other allele. Once an X is inactivated, is remains inactivated in all somatic cells that descend from the cell. Neighboring cells have the same X chromosome inactivated Produce a patchy or mosaic pattern for the expression of an X-linked character in heterozygous females. Patchy distribution caused by X inactivation E.g. Calico cat or tortoiseshell gene (of orange or black allele) for coat color on X chromosome orange/black patches in female So, females are mosaics for all heterozygous X-linked alleles Mechanism of X-Inactivation X inactivation yielding a Barr body = lyonization Xist (X-inactivation-specific-transcription) gene On the X chromosome destined to become inactivated, the Xist gene produce an RNA molecule that costs the X chromosome and inactivate the genes on it by altering the chromatin structure On the active X chromosome, the Xist gene is repressed. ...
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