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Mendelian Genetics What a monk and his peas taught us about ourselves Gene Expression Talk with your lab partner about the following questions: • What is a gene? • What does a gene code for? • What is gene expression? Gene Expression • Mechanism: Protein Synthesis – Protein synthesis is regulated by other genes Human Traits Roller Non-Roller Right Handed Left Handed Dimples Non-dimples Taster Non-taster Widow’s Peak Non-Widows Peak PTC: one gene – one protein- one trait (taster or non-taster) • Phenylthiocarbamide (PTC) is not found in nature, but the ability to taste it correlates strongly with the ability to taste other bitter substances that occur naturally, especially toxins. Eons ago, the ability to discern bitter tastes developed as an evolutionary mechanism to protect early humans from eating poisonous plants Human Traits • Either the trait was expressed or it wasn’t – why? • This map shows the prevalence of the ability to taste a bitter chemical, phenylthiocarbamide (PTC), • The darker the area in this map the higher the percentage of the native population of the area who can taste PTC. • As you can see, the percentage varies considerably, being over 85% in the darkest areas, and as low as 5% in the lightest. Chromosomes • Chromosomes are located in the nucleus • Chromosomes are made up of DNA – coded (or divided) into genes • Each organisms gets 1 set of chromosomes from one parent and 1 complimentary set from another parent • In humans there are 46 chromosomes in each cell – 23 from each parent Meiosis: Cell division in sex cells Meiosis Animation Showing Crossing Over http://www.youtube.com/watch?v=1-RKH5eVJVM http://www.pbs.org/wgbh/nova/miracle/divi_flash.html Meiosis: creates sex cells (eggs & sperm) Mitosis: creates all cell types except for sex cells Meiosis • Cell division in sex cells (eggs, sperm, pollen etc.) • Results 4 separate, haploid (1N) gametes • Creates variation in a population from – Crossing over – Different orientations of chromosomes Mendel worked out the rules of genetics by breeding pea plants. Mendels Experiments Law of Independent (Random) Assortment -Chromosomes sort independently of each other during meiosis 1 -Alleles on the same chromosome will “move together” Law of Segregation • One allele from each gene pair (one from mom/one from dad) will separate during meiosis and end up in separate sex cells • Occurs during anaphase I How did Mendel infer that alleles must separate independently? What about alleles on the same chromosome? • A monohybrid cross represents the inheritance pattern of a single trait, whereas a dihybrid cross represents the inheritance patterns of two traits that are linked because they are on the same chromosome. Traits are coded for in DNA • Taster or Non-taster • A gene codes for the tasting trait • Each person receives a copy of the PTC gene from each parent – so you have two copies. • Each copy is called an allele of the gene Alleles An allele is an alternate form of a gene (PTC) you either have two taster alleles (TT), one taster and one non-taster allele (Tt), or two non-taster alleles (tt) Genes (alleles) are either dominant or recessive • Recessive copies of a gene can be masked by a dominant gene • Dominant genes mask recessive genes PTC GENE • Tasting=Dominant form of gene • Non-tasting=Recessive form of gene If you have two recessive genes (tt) you cannot taste PTC If you have two dominants (TT), or one dominant and one recessive (Tt) you can taste PTC What PTC genotypes could you have inherited from your biological parents? TT - taster Tt - taster Tt – non-taster Principles of Dominance – Some alleles mask the presence of other alleles Bay (brown with black hair) is dominant in horses over chestnut (brown with blonde hair) - Some traits are controlled by multiple genes Skin Pigment and Height are examples Genetic Crosses A genetic cross can be done using a punnett square to help determine the chances of offspring. Let cross the two original parents above using a punnett square Genetic Crosses Blue eyes: Recessive Brown eyes: Dominant 1. A blue eyed man has a child with a brown eyed woman. What are the chances their child will have blue eyes? 2. White coat color: Dominant Black coat color: Recessive A white mare with a black father is crossed with a black stallion. What are the chances for a white foal? Freckles are a dominant trait in humans 3. Cross a homozygous dominant male with a heterozygous female for the freckle trait. What are all of the possible genotypes and phenotypes for this couple?