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Intro Bio Week 7

by: thersh

Intro Bio Week 7 BIOL 1010


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Notes from week 7 of class, genetics unit
Introduction to Biology
Stephanie Hutchins
Class Notes
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This 6 page Class Notes was uploaded by thersh on Thursday March 10, 2016. The Class Notes belongs to BIOL 1010 at Rensselaer Polytechnic Institute taught by Stephanie Hutchins in Fall 2016. Since its upload, it has received 17 views. For similar materials see Introduction to Biology in Biology at Rensselaer Polytechnic Institute.

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Date Created: 03/10/16
3/7/16 Session 2.2 Human Genetics I: Meiosis and Mendel Meiosis ­ Haploid gametes:  o 22 autosomal chromosomes o 1 sex chromosome, either X or Y ­ Multicellular diploid adults 2n = 46 chromosomes, 23 pairs ­ Meiosis is the basic biological process where the number of chromosomes shifts from diploid to  haploid.  This is the process used in formation of gametes.  Gametes are the only cells that can be  used to propagate the species of sexually reproducing organisms. ­ Major Principles o Where and when does it occur?  Reproductive organs (Animals: testes, ovary; Plants: anther, ovules)  Spermatogenesis in testes, oogenesis in ovary o Meiotic division and fertilization are the key to understanding the inheritance of  chromosomes  Fertilization  Somatic cells are diploid.  Gametes are specialized to contain a haploid set of chromosomes.  Fusion of two gametes at fertilization results in a diploid zygote. Meiosis I: Meiosis II:  o Meiosis and fertilization provide genetic variation  Homologous chromosomes carry different versions of genes.  Independent orientation of chromosomes at metaphase.  Crossing over at Prophase I.  Fertilization:  random Mendelian Genetics ­ Gregor Mendel o Discovered the fundamental principles of genetics. o Invented quantitative approach for the study of inheritance o Used the garden pea as an experimental model. o Critical findings:  1) heritable factors (genes) retain their individuality and do not blend  and 2) genes permanently retain their identities. o Darwin published the first edition of The Origin of Species in 1859, 7 years before  Mendel’s paper was published. ­ Mendel’s Law of Segregation o There are alternative versions of a trait (which today we call alleles of a gene). o If two alleles of an inherited pair differ, one can determine the appearance (phenotype)  and the allele related to this phenotype is called dominant; the other allele is called  recessive. o Each individual has two alleles of a given gene. Thus an individual can have two  identical alleles (i.e. is homozygous) or two different alleles (i.e. is heterozygous) o Each parent contributes only one of its two alleles to each of the offspring of a cross  between two parents. This is called the law of segregation. We now know this is due to  the separation of homologous chromosomes during meiosis, but Mendel did not know  about meiosis ­ Mendel’s Second Law: Independent Assortment o In addition to these groundbreaking ideas, Mendel also found that genes controlling different traits in the organism, such as height and flower color, are inherited independently of one another. o This is called the law of independent assortment. It depends on genes being on different chromosomes or being far apart if they are on the same chromosome. o For many genes this law does not hold, because some genes are in fact closely linked to Figure 1If genes for diff. traits are on diff. chromosomes, they are inherited  each other and are inherited independently together more often than not.  ­ Crossing Over – Prophase I o •Crossing over occurs between homologous chromatids. Nonsister chromatids  o •Centromeres holding each pair of sister chromatids together do not divide.  Sister  Chromatids stay together.     o •Homologous chromosomes align randomly. o Crossing over leads to genetic recombination ­ Meiosis is critical for Diploid Sexual Reproduction  o •Gametes receive either the maternal or the paternal chromosomes from each  homologous pair. o •Crossing over adds further genetic variation. The chromosome has mixture of maternal  & paternal derived DNA. o •Meiosis is responsible for   ­Maintenance of a consistent genomic complement in successive generations.  ­Genetic variation within populations. ­ Autosomal recessive Inheritance  o 50% probability of being heterozygous: not expressed. o 25% probability of homozygous recessive: expressed. o Both parents aa, each child will exhibit same phenotype. o Ex.  Tay­Sachs Disease ­ Autosomal Dominant Inheritance  o The allele is expressed in heterozygotes even though it is abnormal. o Trait if present in a parent is likely to appear in offspring. o 50% probability of being heterozygous. o Example:  Huntington’s disease 3/10/16 Session 2.3 Human Genetics II: Non­Mendelian Examples Sex Domination in Humans ­ Female egg: X ­ Male sperm: X or Y ­ Y chromosome: o SRY gene is one of 307 genes on Y o    Master gene for male sex determination. o    SRY gene triggers testosterone synthesis ­ Early embryo neither male nor female ­ XX Embryo:  o No SRY gene o Much less testosterone o Ovaries, estrogen ­ X Chromosome carries 1,336 genes o Few are involved in sexual traits X­Linked Inheritance ­ Mendel died in 1884, but chromosomes were not discovered until 1900’s. ­ Thomas H Morgan – Drosophila melanogaster. ­ Each gene has specific location on a chromosome. ­ Discovered gene for eye color on X­chromosome. ­ Drosophila as model system: o Female lays hundreds of eggs in a few days. o < 2 weeks, flies emerge. o Amenable to rapid genetic analysis. ­ Think about how inheritance would differ for a recessive gene found on the Y vs the same gene  found on the X chromosome. Morgan’s Analysis of Eye Color ­ Eye color alleles carried on X­chromosome. ­ Phenotype dependent upon X.  Normal Mendelian expression female because there are two  copies of X. ­ Male is homozygous and demonstrates whatever phenotype is on the X chromosome X­linked Inheritance Patterns and Human Disease ­ Examples of human diseases or conditions that are X­linked recessive include:   o Hemophilia, variations of Color­blindness, Duchene muscular dystrophy.  ­ There are >300 examples of disorders associated with X­linked genes. ­ X­linked disorders, often called sex­linked disorders, affect mostly males.  Why? Incomplete dominance results in intermediate phenotypes Multiple alleles at the same locus ­ Many genes have more than two alleles in the population whereas what we have discussed thus  far are genes with only two differing alleles ­ ABO blood group phenotype. o 3 alleles of a single gene. o 6 possible genotypes. o Both A and B alleles are expressed if present and the A and B alleles are co­ dominant. Pleiotropy ­ A single gene may affect many phenotypic characters ­ E.g. sickle cell anemia ­  Hutchinson­Golford progeria syndrome, 1 per 8 million o Accelerated rate of aging & sharply reduced life span. o Symptoms start at age 2.  Oldest lived to 20. o Mutant gene for lamin A o    Lamins are intermediate filaments. o    Provide support for inner surface of  nuclear membrane Epistasis: Interactions between more than one genes impacting a single trait ­ Two genes involved:  pigment formation (B and b) and pigment deposition (E and e). ­ The pigment genes encode B for black which is dominant to b for brown. ­ The pigment deposition genes are E for maximum deposition of pigment into the hairs and e for  minimal pigment deposition. Polygenic Inheritance ­ A single character may be influenced by many genes. ­ Examples: Human skin color, height, and eye coloration ­ Neurobiological disorders: depression, schizophrenia, bipolar disorder ­ Schizophrenia:  mutant alleles chromosomes 1, 3, 5, 6, 8, 11­15, 18, 22 o Environmental factors may contribute


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