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Heredity / Genetics Early explanations for heredity? Gregor Mendel published 1865 Consequences of a monk proposing the nature of heredity? Early 1900’s accuracy recognized Grew up on a farm Educated in mathematics (probability) & science Why garden pea plant? P1 - Pure Breeding homozygous F1 seeds phenotypes F2 phenotypes Round x wrinkled All round 5,474 round 2.96 : 1 1,850 wrinkled Yellow x green All yellow 6,022 yellow 2,001 green 3.01 : 1 All gray 705 gray 224 white 3.15 : 1 Full x constricted All full 882 full 299 constrict 2.95 : 1 Green x yellow All green 428 green 152 yellow 2.82 : 1 Axial x terminal All axial 652 axial 207 terminal 3.14 : 1 All long 787 long 277 short 2.84 : 1 Gray x white pods stem Long x short F2 ratio Explanation Experimentation led to three basic conclusions: 1. 2. 3. inheritance of each trait is determined by "units" or "factors" (now called genes ) that are passed on to descendents unchanged an individual inherits one such unit from each parent for each trait that a trait may not show up in an individual but can still be passed on to the next generation. Law of Segregation: This says that of a pair of characteristics (e.g. blue and brown eye color) only one can be represented in a gamete. Law of Independent Assortment different pairs of alleles are passed to offspring independently of each other. The result is that new combinations of genes present in neither parent are possible. Gene for yellow seeds (Y), gene for green (y) P1 = Pure bred cross - YY x yy x y y Y Yy Yy Y Yy Yy x Y y Y YY Yy y yY yy Essential Terms •Homozygous vs Heterozygous •Dominant vs Recessive •Phenotype vs Genotype •Ratio •Alele •Hybrid •Monohybrid Cross •Dihybrid Cross •P1. F1. F2 •Punnet Square •Pedigree •Gamete •Probability •Product Rule Human examples of Mendelian Traits Listed below are human traits that reveal dominance & recessiveness All traits listed are the dominant phenotype •Tongue Curling •Widow’s Peak •Free Ear lobe •Brown Pigmented Iris •Hitch Hikers Thumb (45 degree angle) •Shorter Big Toe (shorter in length than second toe) •Mid Digital Hair •Freckles •Rh Factor •PTC taster •Left Thumb Over Top Right - (When Interlocking Fingers of both hands) •Short Palmar Muscle (Two tendons back of thumb to wrist is dominant, three is recessive) Non-Mendelian patterns of inheritance Incomplete Dominance: Hybrids produce a trait that reveals a blend of both aleles R = red W = white RR = red WW = White RW = Pink Co-Dominance Hybrids reveals trait of both aleles R = red W = white RR = red WW = White RW = Roan (red & white colors) Polygenic inheritance Multiple pairs of genes all influencing one trait Yao Ming 7’5” Earl Boykins 5’5” Many pairs of genes influence the height of a human. Efficiency of the digestive system, circulatory system, and endocrine system - each being controlled by many pairs of genes will all impact height. Multiple Aleles Traits in which within a population there exists more than two forms of a gene. Human blood type - RBC surface proteins must be compatible when initiating a transfusion. Some people have A proteins on their RBC’s, others have B proteins., and others have neither.Within the human population there exists three forms of the gene that controls surface proteins on RBC’s. Alele A (makes A proteins) Alele B (makes B proteins) Alele i (makes neither A or B) Four main blood types: (phenotypes) A - B - AB - O Genotypes: •AA, Ai - blood type A •BB, Bi - blood type B •AB - blood type AB •ii - blood type O Ai x Bi ? Sex Linkage Most animal species have a special pair of chromosomes that determine gender. In humans 22 pairs of autosomes and 1 pair of sex chromosomes Two types - X (large and has genes that control non gender related traits) Y (small and only promotes the formation of male genitalia) XX = female XX x XY = male XY Duchene’s Muscular Dystrophy Color Blindness Hemophilia X Y X XX XY X XX XY Sex linked traits are traits controlled by genes on the X chromosome. Females have two, males have only one. Females can carry but not show defective sex linked traits. Males CAN NOT be carriers. Other Non Mendelian Hereditary Conditions Non-Disjunction: Abnormal separation of chromosomes during meiosis. Gametes are formed having too many (too much information) or too few chromosomes (not enough information) and therefore unexpected characteristics arise. There are several human disorders associated with this circumstance many of them have been described by the CytoGenetic Gallery. Some human disorders caused by non-disjunction include: •Down Syndrome (three of the #21 chromosome) •Turner Syndrome (X_ - only one sex chromosome - always the X chromosome) •Klinefelter's Syndrome (XXY) •Trisomy X (XXX) Epistasis Effects of one gene override or mask the phenotype of a second gene. Epistasis is not dominance. Dominance only involves one gene which has two alleles (one dominant and the other recessive). Epistasis involves two different genes where one gene (having two alleles) impacts the expression of a different gene (also having two alleles). Albinism - Melanin Production vs. Amount of melanin produced - two separate genes directly impacting the color of skin Pleiotropy Pleiotropy occurs when a gene has more than one phenotypic expression. In a species of mouse a gene that controls fur color has two alleles. Y and y. The heterozygous genotype produces yellow fur. The hmozygous recessive produces gray fur. The homozygous dominant creates a lethal condition that terminates development. Therefore a single gene controls fur color and a critical stage during the embryonic development of the mouse. Gene Imprinting For imprinted genes, the gene copy that is turned on depends only on whether it came from the mother or father, rather than on the classic laws of Mendelian genetics, where genes are either dominant or recessive. It seems that certain genes are only functional with one active copy, not zero and not two. A gene is made inactive by adding a methyl groups that blocks access to RNA transcriptase and therefore makes the gene unable to be expressed. Fragile X Syndrome is a disorder that appears to be a consequence of gene imprinting. Extracellular Genes Not all DNA is associated with chromosomes in the nucleus of Eukaryotic cells. The mitochondria and plastids in plant cells have their own DNA which consists of small rings. This DNA is responsible for the production of proteins essential to the organization and function of their internal membranes (electron transport chain, ATP synthase).In humans the mitochondria in every cell of an individual originated from the egg. The sperm only contributes nuclear material.All mitochondrial DNA has maternal origin and this DNA may impact the expression of nuclear genes.