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Transcript
Asexual and Sexual Reproduction • Genetically identical cells are produced from a single parent cell by mitosis. • Fission or Budding Sexual reproduction occurs when new individual is formed through union of two sex cells (gametes). Produce zygote. Haploid gametes formed by meiosis in sex organs (gonads). Asexual and Sexual Reproduction • Different Approaches to Sex Parthogenesis - Virgin birth - Exclusive Switching Sexual Reproduction - monoecious / dioecious Hermaphroditism - Both Sex organs Evolution of Reproduction • Three Strategies of Internal Fertilization: Oviparity - Eggs are fertilized internally and deposited outside mother’s body to complete development. Ovoviviparity - Fertilized eggs are retained within mother to complete development, but all nourishment gained from yolk sac. Viviparity - Young develop within mother and obtain nourishment directly from mother’s blood. Porifera and Cnidaria • • Sponges reproduce by both asexual and sexual means. Most poriferans that reproduce by sexual means are hermaphroditic and produce eggs and sperm at different times. Cnidarians reproduce by both asexual and sexual means. Arthropoda • reproduction dioecious - many dimorphic oviparous, or ovoviviparous metamorphosis - complete / incomplete some parthenogenic – development from a unfertilized ovum. - Fertilization can be internal- males have an ovipositor Annelida • reproduction monoecious / dioecious trochophore larva asexual - budding Fertilization external Gametes passed into the coelom (body cavity) Evolution of Vertebrate Reproduction • Vertebrate sexual reproduction evolved in the ocean before vertebrates colonized land. Most marine bony fish utilize external fertilization. Most other vertebrates utilize internal fertilization. - Gametes could not be released on dry land without significant mortality. Sexual Reproduction • Sex Determination In some organisms, environmental conditions can determine sex of offspring. - In mammals, sex is determined early in embryonic development. ¾ Embryonic gonads are indifferent. Y chromosome produce males. Animals: Sex Determination Alternation of generations in Gynosperms Alternation of generations in angiosperms Comparison between Animal and Plant life cycles • • • Plants gametes divide so individuals are haploid or diploid Animals individuals are diploid and gametes are haploid Patterns of Inheritance Pattern of inheritance hypotheses heredity – the tendency for traits to be passed from parent to offspring “blending” hypothesis – the genetic material of the parents mixes in a manner similar to blending paint. Lamark’s inheritance of acquired characteristics. example a giraffe's neck Mendel’s inheritance of characteristic traits What were Mendel’s experiments? Mendel performed simple test crosses Parent generation P1 X P2 First Filial generation F1 Second Filial generation F2 In each generation he recorded the phenotype of each individual and the number with each phenotype How did Mendel derive his rules? Used pea plants to control parental crosses Chose only characters that were present or not present. Started parental cross with true-breeders Crossed two true-breeders of different varieties – hybridization cross Dominant phenotype is given by two genotypes: Homozygote and heterozygote: RR and Rr Gene – a discrete unit of heredity Allele – alternate forms of a gene when expressed result in variation within traits Phenotype – the physical trait expressed Genotype – the alleles present Somatic cells have two alleles of each gene but gametes have only one allele Alleles of each gene segregate during gamete formation Principle of allele segregation is Mendel’s first rule Parent = Gametes = Rr 1/2 R 1/2 r How do scientists predict the probability of inheritance? Punnett Squares can be used to visualize crosses between two individuals. Possible gametes of individual are listed along horizontal side. And possible gametes of the other individual are listed along the vertical side. Genotypes of potential offspring are represented by cells within square. Frequency expressed as probability. Genotype Rr ovule produced with one allele R and one allele r Genotype rr pollen produced with one allele r and one allele r YyRr Y Y y y Y YY YY Yy Yy Y YY YY Yy Yy rr y Yy Yy yy yy y Yy Yy rr yy yy rr YyRr YyRr – heterozygous Parental heterozygous cross phenotypic ratio? Yellow round seeds (any with Y or R) Yellow wrinkled seeds (any with Y and rr) Green round seeds (any with yy and R) Green wrinkled seeds (any with yy and rr) Incomplete Dominance Some alleles produce a heterozygote phenotype that is intermediate between those of the parents. Epistasis - Interaction between products of two genes where one gene modifies the other gene’s phenotypic expression. Emerson - To produce pigment, a Zea mays plant must possess at least one functional copy of each enzyme gene. Often each allele has its own effect and the alleles are considered codominant. Human ABO Blood type Type A individuals only galactosamine Type B individuals add only galactose Type AB individuals add both sugars Type O individuals add neither sugar Rh Blood Group Rh cell surface marker A trait determined by a gene on the sex chromosome is said to be sex-linked. In Drosophila, sex is determined by the number of copies of the x chromosome. Mutations in Human Heredity Mutations are accidental changes in genes. Rare, random, and usually result in recessive alleles. Pedigrees used to study heredity. Hemophilia - Inherited condition where blood is slow to clot or does not clot at all. Only expressed when individual has no copies of the normal allele. Royal Hempohilia - Sex-linked Sickle-Cell Anemia Sickle-Cell Anemia is a recessive inherited disorder in which afflicted individuals have defective hemoglobin, and thus are unable to properly transport oxygen to tissues. Heterozygotes usually appear normal. Homozygotes have Sickle-Cell, but are resistant to malaria. Other Disorders Tay-Sachs Incurable heredity disorder which causes brain to deteriorates. Nonfunctional form of hexosaminidas A enzyme. Huntington’s Disease Inherited condition caused by dominant allele causing progressive brain deterioration. Symptoms usually develop late in life. Genetic Counseling and Therapy Process of identifying parents at risk of producing children with genetic defects and assessing genetic state of early embryos. Amniocentesis Ultrasound Chorionic Villi Sampling
