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Unit3 genetics
 

Unit3 genetics

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    Unit3 genetics Unit3 genetics Presentation Transcript

    • Unit 3: Genetics
    • What Are Genes? - GENES are segments of DNA that contain the information for an hereditary trait. They are located in the chromosomes - Genes are transmitted from parents to offspring through the gametes. - Each gene has alternatives called ALLELES. In the population there may be many, but in an organism there are two, one in each homologous chromosome. - The position a gene has in a chromosome is called LOCUS (plural: loci)
    • Individuals that have two identical alleles for a gene are called HOMOZYGOUS (AA or aa) If the two alleles are different, they are HETEROZYGOUS or HYBRID for that trait (Aa)
    • GENOTYPE is the group of genes of an individual. For a trait it may be AA, Aa or aa. PHENOTYPE is the external expression of the genotype, that is, the observable trait. Phenotype does not only depend on the genotype, but also on the environment in which the individual develops.
    • Multiple alleles make possible multiple phenotypes in a population
    • For each gene, there are three possible genotypes. For the gene that regulates the production of melanine, for example: Genotypes Phenotypes MM-------------------------------------- normal (pigmented skin) Mm-------------------------------------- normal (pigmented skin) mm--------------------------------- ---- albino This happens because the allele M is DOMINANT over the allele m, that is RECESSIVE. Dominant alleles have the same effects whether they are in homozygosis (MM) or heterozygosis (Mm). Recessive alleles are expressed only in homozygosis (mm)
    • Sometimes there is no complete dominance of an allele over the other, but an INCOMPLETE DOMINANCE, or INTERMEDIATE INHERITANCE. Then the heterozygous phenotype is intermediate between the dominant and the recessive. Sometimes both the alleles express simultaneously (CODOMINANCE). An example of this happens with the bloodtypes.
    • GAMETES are HAPLOID (n), so they only have one chromosome for each homologous pair. This means they only have one alelle for each gene. Genotype of the individual AA aa Gametes it produces A a (100%) (100%) A (50%) Aa a (50%) A heterozygous will produce 50% of the gametes with each possible allele.
    • In fertilization, gametes combine randomly X P (parents) PHENOTYPE normal GENOTYPE albino Mm GAMETES PRODUCED M mm m m FERTILIZATION F1 (Offspring) GENOTYPES PHENOTYPES Mm normal (50%) mm albino (50%)
    • Mendel's Good Move: Mendel's success in discovering the laws of inheritance depended on his wise decision to choose the pea (Pisum sativum) for his experiments. This was because: - it has a short life cycle - it produces many offspring in one cross - it allows both self and cross-fertilization and it is easy to manipulate - it has easily observable traits. He focused on 7, and studied its transmission with the aid of statistics:
    • The traits Mendel studied in peas (Pisum sativum):
    • MENDEL'S FIRST LAW: UNIFORMITY OF THE FIRST GENERATION When crossing two homozygous parents (or pure lines), the resulting offspring (F1 generation) is uniform. 100% phenotype R
    • MENDEL'S SECOND LAW: SEGREGATION During gamete formation each allele of a pair is separated (=segregated) from the other member, and will join again with another one after fertilization. 75% phenotype R 25% phenotype r
    • First Law: Uniformity of F1 Second Law: Segregation
    • First Law: Uniformity of F1 Second Law: Segregation
    • What happens with more than one trait? To study this, Mendel crossed two pure lines (homozygous) for two traits: Seed colour: yellow (A) > green (a) Seed shape: round (B) > wrinkled (b) P: AABB x (yellow,round) F1: F2: aabb (green,wrinkled) AaBb (yellow,round) yellow,round yellow,wrinkled 9 : 3 : green,round 3 green,wrinkled : 3
    • MENDEL'S THIRD LAW: INDEPENDENT ASSORTMENT Genes for each trait are transmitted and inherited independently.
    • Punnett Squares - They are diagrams used to predict the outcome of a breeding or cross experiment. - In these diagrams, we represent in a table the possible alleles produced by the two individuals that are being crossed. We assume that the probability of inheriting copies of each parental allele is independent (according to Mendel's Laws). One trait Two traits
    • How Is Sex Determined?
    • Multiple Alleles: Blood Types
    • Sex-linked (X-linked) recessive traits: Hemophilia, color-blindness Some traits (diseases like hemophilia or color-blindness are determined by alleles whose loci are in the X chromosome. Males will inherit only one allele from their mother (and always express it), while females will receive two alleles, one from their mother and other from his mother.
    • Tracing Hemophilia through Queen Victoria's Pedigree
    • Chromosomal abnormalities Down Syndrome: trisomy 21
    • Abnormalities in sex chromosomes: Turner syndrome: X_ Klinefelter syndrome: XXY