8 th Grade Science Genetics

DNA A = Adenine T = Thymine C = Cytosine G = Guanine

The structure of DNA

Watson & Crick Watson & Crick did NOT discover DNA DNA was discovered in the late 1860s by Swiss chemist Friedrich Miescher Other scientists over the years notably, Phoebus Levene and Erwin Chargaff--carried out a series of research efforts that revealed additional details about the DNA molecule, including its primary chemical components and the ways in which they joined with one another. Chargaff realized that A = T and C = G Watson and Crick made a three-dimensional, double- helix model for the structure of DNA.

How they did it Watson and Crick's discovery was possible because of advances in model building based upon known molecular distances and bond angles, a technique advanced by American biochemist Linus Pauling. Watson and Crick were worried that they would be "scooped" by Pauling, who proposed a different model for DNA just months before they did. Pauling's prediction was incorrect. Using cardboard cutouts representing the four bases Watson and Crick shifted molecules around on their desktops, as though putting together a puzzle.

Gregor Mendel The basic laws of heredity were first formed during the mid-1800’s by an Austrian botanist monk named Gregor Mendel. Because his work laid the foundation to the study of heredity, Mendel is referred to as “The Father of Genetics.”

Mendel’ Pea Plants Mendel based his laws on his studies of garden pea plants. Mendel was able to observe differences in multiple traits over many generations because pea plants reproduce rapidly, and have many visible traits such as: Plant Height Tall Short Pod color Seed Shape Pod Shape Seed Color GreenYellow GreenYellow RoundWrinkled SmoothPinched

Mendel’s Experiments Mendel noticed that some plants always produced offspring that had a form of a trait exactly like the parent plant. He called these plants “purebred” plants. For instance, purebred short plants always produced short offspring and purebred tall plants always produced tall offspring. X Purebred Short Parents Purebred Tall Parents X Short Offspring Tall Offspring

Mendel’s First Experiment Mendel crossed purebred plants with opposite forms of a trait. He called these plants the parental generation. For instance, purebred tall plants were crossed with purebred short plants. Parent Tall Parent Short X Offspring Tall Mendel observed that all of the offspring grew to be tall plants. None resembled the short parent.

Mendel’s Second Experiment Mendel then crossed two of the offspring tall plants produced from his first experiment. Tall X 3⁄4 Tall & 1⁄4 Short To his surprise, Mendel observed that this generation had a mix of tall and short plants. This occurred even though none of the parents were short. Parent PlantsOffspring

Mendel’s Conclusions Each plant must have two sets of instructions (genes) for each trait The two forms of a gene are called alleles. Genes can be dominant or recessive.

How gender is determined. Gender is determined by sex chromosomes. Humans have 23 pairs of chromosomes 22 pairs are “body” chromosomes called autosomes 1 pair are the “sex” chromosomes – X and Y Females will have XX Males will have XY

Males = XYFemales = XX

TERMS TO KNOW ALLELES DIFFERENT FORMS OF A TRAIT THAT A GENE MAY HAVE T,t HOMOZYGOUSAN ORGANISM WITH TWO ALLELES THAT ARE THE SAME TT, tt HETEROZYGOUSAN ORGANISM WITH TWO DIFFERENT ALLELES FOR A TRAIT Tt, Gg

TERMS TO KNOW HYBRID SAME AS HETEROZYGOUS Tt, Gg DOMINANT A TRAIT THAT DOMINATES OR COVERS UP THE OTHER FORM OF THE TRAIT REPRESENTED BY AN UPPERCASE LETTER T OR G RECESSIVE THE TRAIT BEING DOMINATED OR COVERED UP BY THE DOMINATE TRAIT REPRESENTED BY A LOWER CASE LETTER t or g

TERMS TO KNOW PHENOTYPE THE PHYSICAL APPEARANCE OF AN ORGANISM (WHAT IT LOOKS LIKE) TALL, SHORT, GREEN, WRINKLED GENOTYPE THE GENE ORDER OF AN ORGANISM (WHAT ITS GENES LOOK LIKE) TT, GG, Tt, gg Gg, tt RATIO THE RELATIONSHIP IN NUMBERS BETWEEN TWO OR MORE THINGS 3:1, 2:2, 1:2:1 (You can also express as a percentage.)

A GOOD TOOL TO KNOW A PUNNET SQUARE IS A TOOL USED TO PREDICT THE POSSIBLE GENOTYPES FOR THE OFFSPRING OF TWO KNOWN PARENTS. PARENT’S GENES

HOW TO USE A PUNNETT SQUARE THE PARENTS’ ALLELES GO ON THE OUTSIDE OF THE SQUARE BB b b Parents = BB X bb

HOW TO USE A PUNNETT SQUARE THE PARENTS’ ALLELES GO ON THE OUTSIDE OF THE SQUARE BB b b BB BB DROP THE LETTERS ON THE TOP, INTO EACH SQUARE MOVE EACH LETTER ON THE SIDE, INTO EACH SQUARE THE ORDER DOES NOT MATTER IN THE BOXES, BUT UPPERCASE FIRST IS A GOOD RULE bb bb

HOW TO USE A PUNNETT SQUARE Tt T t WHAT ARE THE RESULTS? PHENOTYPE: 75% TALL 25% SHORT 3 TO 1 RATIO: TALL TO SHORT GENOTYPE: 1TT: 2Tt: 1tt 1:2:1 RATIO 25 %TT, 50% Tt, 25% tt LET’S LOOK AT ANOTHER PUNNETT SQUARE AND PREDICT THE OUTCOME TT Tt Tt t t T IS THE DOMINANT ALLELE FOR TALLNESS t IS THE RECESSIVE ALLELE FOR SHORTNESS

1. IN DROSOPHILA MELANOGASTER (FRUIT FLIES), RED EYE COLOR (R) IS DOMINANT OVER BROWN EYE COLOR (r). IF THE FLIES IN THE PICTURE WERE CROSSED, WHAT PERCENT OF THEIR OFFSPRING WOULD BE EXPECTED TO HAVE BROWN EYES? ANSWER: 1. 50% Try it

H h H h WHICH OF THE FOLLOWING HAS THE hh GENOTYPE? A.1 & 3 B.2 C.4 D.NONE 3. WHICH OF THE FOLLOWING IS A TRUE STATEMENT? A.INDIVIDUAL 4 IS RECESSIVE B.INDIVIDUALS 1 & 3 ARE HETEROZYGOUS C.INDIVIDUAL 2 IS DOMINANT D.ALL INDIVIDUALS ARE FEMALE Try it ANSWER: 2. B 3. B

B b B b BB Bb bb 4. IF B IS THE ALLELE FOR BLACK FUR AND b IS THE ALLELE FOR WHITE FUR, WHAT PERCENT WOULD BE BLACK? A.25% B.50% C.100% D.75% 5. WHAT FRACTION IS HOMOZYGOUS DOMINANT IN THE ABOVE CROSS? A.1/2C.1/4 B.1/3D. 3/4 Try it ANSWER: 4. D. 75% 5. 1/4

B B b BB Bb BB Bb 6. IN THIS CROSS, WHAT IS THE RATIO OF BB TO Bb? A.3 : 1 B.4 : 1 C.2 : 2 D.0 : 4 Try it ANSWER: 6. C. 2:2