* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Lecture 3: Chromosomes and sex determination
Survey
Document related concepts
Epigenetics of human development wikipedia , lookup
Hybrid (biology) wikipedia , lookup
Genomic imprinting wikipedia , lookup
Sexual dimorphism wikipedia , lookup
Artificial gene synthesis wikipedia , lookup
Gene expression programming wikipedia , lookup
Transgenerational epigenetic inheritance wikipedia , lookup
Genome (book) wikipedia , lookup
Microevolution wikipedia , lookup
Dominance (genetics) wikipedia , lookup
Designer baby wikipedia , lookup
Skewed X-inactivation wikipedia , lookup
Y chromosome wikipedia , lookup
Quantitative trait locus wikipedia , lookup
Neocentromere wikipedia , lookup
Transcript
Lecture 3: Chromosomes and sex determination 1. 2. 3. 4. 5. Sex chromosomes Sex linkage Chromosome nondisjunction Sex determination Sex-linked traits in humans Red-eyed and white-eyed Drosophila The parallel behavior of Mendel’s factors (=genes) and chromosomes in meiosis Sutton and Bovery in 1902 were the first to come up with this idea. It laid the foundation for the chromosome theory of heredity. ¾ existence alleles in pairs (A, a) is due diploidy (2n) ¾ equal segregation is due to disjoining of homologs in Anaphase I ¾ independent assortment is due to alternative methaphase alignments of two pairs of homolologs ¾ constant number of chromosomes in individuals ¾ different species have different but constant numbers ¼ AB ¾ phenotypic difference between sexes correlate with inheritance of certain chromosomes (sex chromosomes) ¼ ab Fruit fly Drosophila Thomas Hunt Morgan 1866-1945 Columbia Univ. 1910-1927 Nobel Prize in 1933 Caltech 1927-1942 II, III, IV: autosomes I: sex chromosomes Human sex chromosomes Inheritance pattern of sex chromosomes (X and Y) Females: homogametic sex (only X) Males: heterogametic sex (½ X and ½ Y) Notice: equal ratio between the sexes is a consequence of the Law of Equal Segregation Inheritance pattern of sex chromosomes (X and Y) X-chromosomes: from mothers to sons and daughters; from fathers to daughters Inheritance pattern of sex chromosomes (X and Y) from father X-chromosomes: from mothers go to sons and daughters; from fathers to daughters Strictly paternal mode of inheritance of Y chromosome to son Inheritance of X-linked traits (eye color) F1 Thomas Morgan around 1909 all red-eyed Inheritance of X-linked traits (eye color) F2 F1 all red-eyed 3 red (2 ♀:1♂) : 1 white ♂ Inheritance of X-linked traits (eye color) Let’s call the mutant allele of the gene for eye color w (for white mutation). Then the wild type allele is w+ Morgan hypothesized the following: 1.The gene for eye color resides on X chromosome. 2.Females with red eyes are homozygous w+ / w+. 3.Males with white eyes are hemizygous w / Y, because Y chromosome does not carry anyF2 allele of this gene. F1 4.Wild type allele w+ is dominant, mutant allele w is recessive, then heterozygous females w+ / w have red eyes. all red-eyed Then he could explain the results of the cross 3 red (2 ♀:1♂) : 1 white ♂ Inheritance of X-linked traits (eye color) F2 3 red (2 ♀:1♂) : 1 white ♂ Inheritance of X-linked traits (eye color) F2 3 red (2 ♀:1♂) : 1 white ♂ Inheritance of X-linked traits (eye color) Inheritance of X-linked traits (eye color) – a reciprocal cross Reciprocal cross produced a different result! In general, such results indicate sex linkage for a gene in question Morgan’s work indicated that the inheritance of a particular gene (w, eye color) correlates with the inheritance of a particular chromosome (X chromosome). This suggests that the w gene resides on the X chromosome. This correlation was a strong evidence (but yet the final proof) in support of the chromosome theory of heredity. "Non-disjunction as proof of the chromosome theory of heredity" Calvin Bridges 1889-1938 "Non-disjunction as proof of the chromosome theory of heredity" The same cross as Morgan’s reciprocal cross: Most of the progeny were as expected. But rarely (1 case out of 2,000) the cross produced “exceptional” progeny: white-eyed females and red-eyed males expecting the same results: Meiotic non-disjunction of the X-chromosome "Non-disjunction as proof of the chromosome theory of heredity" The same cross as Morgan’s reciprocal cross: Most of the progeny were as expected. But rarely (1 case out of 2,000) the cross produced “exceptional” progeny: white-eyed females and red-eyed males Explanation of the “exceptional” progeny: ♀ expecting the same results: ♂ Humans: ♀XX: No Y chromosomes ♂XY: One Y chromosome X0 - females (sterile) XXY - males Different types of chromosomal sex determination ¾ In humans, Y chromosome actively (dominantly) defines maleness (SRY region) ¾ In Drosophila, Y chromosome does not define maleness, but is required for male fertility ♀XX: Two X chromosomes per 3 pairs of autosomes ♂XY: One X chromosome per 3 pairs of autosomes X0 - males (sterile), XXY – females "Non-disjunction as proof of the chromosome theory of heredity" Explanation of the “exceptional” progeny: ♀ ♂ Aberrant number of chromosomes in ‘exceptional’ progeny was verified by direct microscopic examination Inheritance of X-linked recessive traits in humans: hemophilia in European royalty Queen Victoria was heterozygous for the X-linked recessive hemophilia allele. The trait may have arisen as a mutation in one of her parents’ germ cells or in her own germ line Inheritance of X-linked recessive traits in humans Examples: ¾ hemophilia - inability of blood to clot, caused by defective Factor VIII, there are also other types; ¾ Duchenne’s muscular dystrophy - fatal, death by early adulthood; ¾ red-green color blindness; ¾ testicular feminization syndrome (androgen insensitivity) – the individuals are XY but phenotypically develop as females (although sterile) ¾ 1,500 genes total Inheritance of autosomal recessive traits in humans Both sexes are affected The trait may not be seen for many generations (heterozygotes) Inbreeding is often involved in revealing the trait (to produce homozygotes) Inheritance of autosomal dominant traits in humans Both sexes are affected with equal ratio Both sexes transmit the trait The trait is seen in every generation Inheritance of X-linked dominant traits in humans: faulty tooth enamel and hypertrichosis Both sexes are affected May look similar to autosomal dominant but: Males pass the trait only to daughters Females pass the traits to daughters and to sons Inheritance of Y-linked traits in humans = SRY (Sexdetermining Region of Y)