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Notes: Genetics Mendel & Human Genetics (Honors/Gifted) Genetics & Heredity What is genetics? the science of heredity What is heredity? passing of traits from parents to offspring Gregor Mendel Austrian Monk interested in inheritance Studied offspring from different “matings,” or crosses, of pea plants. He began his research with pea plants that were considered true-breeding. Plants that have the ability to produce offspring identical to themselves, through the use of their own gametes = “Self-Pollination” = true breeding Gregor Mendel Most plants reproduce via cross-pollination, the combining of gametes from two separate plants. Mendel DID NOT want to have this extra variable in his research, so he prevented his plants from “self-pollinating” by crossing the pea plants himself. He began his research looking at 7 different pea plant TRAITS a characteristic that varies from one organism to another Mendel’s Research All 7 traits Mendel studied occurred in one of 2 forms. He cross-pollinated plants with different forms of the same trait (i.e. purple x white) to examine offspring. After crossing these plants Mendel discovered that… 1. Inheritance is determined by genes. Segments of DNA that determine a specific trait 2. Some alleles (different forms of a gene) are considered Dominant and some are Recessive: The Principle of Dominance. Dominant alleles that are expressed (usually capital letters) Recessive alleles that are hidden (usually lower-case) His Experiments Mendel conducted Monohybrid Crosses (mating involving only one pair of traits). He cross-pollinated 2 pea plants with opposing traits The original mating pair he used was called the Parental or P – Generation. Ex Purple flowering plant & white flowering plant Mendel - Monohybrid Crosses The offspring of this Parental cross is called the F1 Generation, meaning “first filial.” Results: ALL PLANTS’ FLOWERS WERE PURPLE! Mendel – Monohybrid Crosses Mendel’s 2nd Experiment… Mendel allowed the F1 Generation (all purple) to self-pollinate. Results: The F2 Generation (“second filial”) produced flowers in a ratio of 3 purple: 1 white This 3:1 ratio occurred for each of the 7 traits in the F2 Generations! Alas…Mendel’s Conclusions 1) Every inherited trait has 2 copies of the gene – one from each parent. 2) There are alternative versions of genes (alleles). 3) When 2 different alleles occur together, one can be completely expressed (dominant) while the other can be hidden (recessive). 4) Gametes (sperm and eggs) each carry one allele for a given trait; during fertilization, the offspring receives 1 allele from each parent. Now let’s review some basic Terms… Gene: Segment of DNA that determines a specific trait Trait: a characteristic that varies from one organism to another Allele: Different forms of a gene Dominant Allele: an allele that hides a recessive trait; usually characterized by a capital letter. Recessive Allele: an allele that can be “masked” or hidden by a dominant allele; usually characterized by a lower-case letter Homozygous vs. Heterozygous Homozygous: Offspring have two identical alleles (PP = homozygous dominant; pp = homozygous recessive); also known as “pure-bred” Heterozygous: Offspring has one of each allele (Pp = heterozygous); also known as “hybrid” Genotype v. Phenotypes Genotype: the set/ combination of alleles an organism has for a certain trait…the “letters” (ex: PP, pp, or Pp) Phenotype: the physical appearance of a trait in an organism…. the “looks” (ex: blonde hair) Genotype Phenotype The Law of Segregation 2 alleles for a trait segregate (separate) when gametes are formed; each offspring receives one trait from their parents The Law of Independent Assortment Alleles for different genes separate independently of each other during gamete formation. One trait, like flower color, does not influence the inheritance of another trait, like plant height. Probability & Genetics Probability = the likelihood that a particular event will occur. Principles of probability can be used to predict the outcomes of genetic crosses. The more trials conducted, the closer the result will come to the EXPECTED ratio. The Punnett square can predict the “probability” of outcomes resulting from a genetic cross. Predicting Results: Punnett Squares Punnett Squares are diagrams that use the Laws of Segregation and Independent Assortment to predict offspring Possible gametes for 1 parent are placed along the top of the square; possible gametes for the other parent are written on the left of the square. The genotypes are predicted by combining alleles R r from each parent. R RR Rr r Rr rr For example… We can use ratios to express genotypes and phenotypes. Genotypic Ratio = ___ YY: ____ Yy :___yy Phenotypic Ratio = ____ yellow : _____ green Predicting Results: Punnett Squares Genotypic Ratio = 0 YY : 4 Yy : 0 yy 4 Yy or 100% Yy Phenotypic Ratio = 4 yellow : 0 green 100% yellow Predicting Results: Punnett Squares Dihybrid crosses (crosses involving 2 traits) are a little more complex. Possible combinations for the different types of alleles are placed at the top and sides of the square. Example: (Round, Yellow) (wrinkled, green) RrYy rryy Possible allele combos RY Ry rY ry ry RrYy Rryy rrYy rryy ry RrYy Rryy rrYy rryy ry RrYy Rryy rrYy rryy ry RrYy Rryy rrYy rryy BEYOND DOMINANT & RECESSIVE ALLELES Part II: Beyond Dominant & Recessive Alleles… Some traits are Polygenic, meaning that more than one gene determine the phenotype. Examples human hair color, eye color, weight, skin color Incomplete Dominance One form of a trait is NOT dominant or recessive to the other. Results in an “in-between” phenotype three different phenotypes are possible. Example Four O’Clocks flower Red (RR) X White (WW) Pink (RW) Co-dominance When two dominant alleles are expressed at the same time in an offspring Example: A homozygous red horse mates with a homozygous white horse to produce a horse with BOTH red and white hair (called a Roan coat). BLOOD TYPES Part III: Blood Types: A Result of Multiple Alleles Multiple Alleles – Genes with 3 or more possible alleles determining a trait Each individual receives only 2 alleles, but there are more than 2 in the population Example = Blood Type Blood Phenotypes = A, B, AB, and O Blood Alleles = IA, IB, and i IA and IB are both dominant (codominant when together), & i is recessive Blood Typing Chart Blood Type Possible Genotypes Can DONATE A ;AB Can RECEIVE Type A IAIA IAi Type B IB I B IAi B ;AB B;O Type AB IAI B AB Universal RECEIVER Type O ii Universal DONOR O A;O Blood Type Frequency in Population HUMAN GENETICS Part IV: Human Genetics Sex Chromosomes The two chromosomes that determine an individuals sex are XX (mom) or XY (dad). Autosomal Chromosomes The other 44 chromosomes, not sex chromosomes Karyotype = a picture of chromosomes arranged in 23 matching pairs; Sex chromosomes are ALWAYS on pair #23. 2 Types of Disorders Sex-Linked Disorders Chromosomal Disorders ***But first, let’s talk a little about SexLinked Traits…*** Sex-Linked Traits All eggs carry an X chromosome Females are XX; Males carry XY In females, if a defective gene rides on one of the X chromosomes, the other X is likely to have a good copy of the gene that can take over for the “bad” gene Males do not carry the backup copy of the X chromosome, so the gene is expressed Sex-Linked Disorders The information/ traits on the sex chromosomes are called sex-linked genes. Because these chromosomes determine sex, disorders caused by these genes are sex-linked disorders. Typically, these genes are found on the X chromosome. Examples Colorblindness Hemophilia Duchene Muscular Dystrophy Chromosomal Disorders Nondisjunction (failure of homologous chromosomes to separate) occurs during meiosis. The resulting individual has an abnormal number of chromosomes and that results in a disorder! Examples: Down Syndrome (Trisomy 21) Turner’s Syndrome (XO) sterile Klinefelter’s Syndrome (XXY)