Genetics

8th Grade ScienceGenetics

DNA

A = AdenineT = ThymineC = CytosineG = Guanine

The structure of DNA

Watson & CrickWatson & Crick did NOT discover DNADNA was discovered in the late 1860s by Swiss chemist Friedrich MiescherOther 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 = GWatson and Crick made a three-dimensional, double-helix model for the structure of DNA.How they did itWatson 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 MendelThe basic laws of heredity were first formed during the mid-1800s 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

TallShortPod colorSeed ShapePod ShapeSeed Color

GreenYellowGreenYellowRoundWrinkledSmoothPinchedMendels ExperimentsMendel 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.

XPurebred Short ParentsPurebred Tall Parents

X

Short OffspringTall OffspringMendels First ExperimentMendel 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.

Mendels Second ExperimentMendel then crossed two of the offspring tall plants produced from his first experiment.

Tall

X

34 Tall & 14 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 PlantsOffspringMendels ConclusionsEach plant must have two sets of instructions (genes) for each traitThe 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 chromosomes22 pairs are body chromosomes called autosomes1 pair are the sex chromosomes X and YFemales will have XXMales will have XYMales = XYFemales = XX

TERMS TO KNOWALLELESDIFFERENT FORMS OF A TRAIT THAT A GENE MAY HAVET,tHOMOZYGOUSAN ORGANISM WITH TWO ALLELES THAT ARE THE SAMETT, ttHETEROZYGOUSAN ORGANISM WITH TWO DIFFERENT ALLELES FOR A TRAITTt, GgTERMS TO KNOW

HYBRIDSAME AS HETEROZYGOUSTt, GgDOMINANTA TRAIT THAT DOMINATES OR COVERS UP THE OTHER FORM OF THE TRAITREPRESENTED BY AN UPPERCASE LETTERT OR GRECESSIVETHE TRAIT BEING DOMINATED OR COVERED UP BY THE DOMINATE TRAITREPRESENTED BY A LOWER CASE LETTERt or gTERMS TO KNOWPHENOTYPETHE PHYSICAL APPEARANCE OF AN ORGANISM(WHAT IT LOOKS LIKE)TALL, SHORT, GREEN, WRINKLEDGENOTYPETHE GENE ORDER OF AN ORGANISM(WHAT ITS GENES LOOK LIKE)TT, GG, Tt, ggGg, ttRATIOTHE RELATIONSHIP IN NUMBERS BETWEEN TWO OR MORE THINGS3:1, 2:2, 1:2:1(You can also express as a percentage.)A GOOD TOOL TO KNOWA PUNNET SQUARE IS A TOOL USED TO PREDICT THE POSSIBLE GENOTYPES FOR THE OFFSPRING OF TWO KNOWN PARENTS.PARENTS GENESPARENTS GENESHOW TO USE A PUNNETT SQUARETHE PARENTS ALLELES GO ON THE OUTSIDE OF THE SQUAREBBbbParents = BB X bbHOW TO USE A PUNNETT SQUARETHE PARENTS ALLELES GO ON THE OUTSIDE OF THE SQUAREBBbbBBBBDROP THE LETTERS ON THE TOP, INTO EACH SQUARE MOVE EACH LETTER ON THE SIDE, INTO EACH SQUARETHE ORDER DOES NOT MATTER IN THE BOXES, BUT UPPERCASE FIRST IS A GOOD RULEbbbbHOW TO USE A PUNNETT SQUARETtTtWHAT ARE THE RESULTS?PHENOTYPE: 75% TALL 25% SHORT3 TO 1 RATIO: TALL TO SHORTGENOTYPE:1TT: 2Tt: 1tt 1:2:1 RATIO 25 %TT, 50% Tt, 25% tt LETS LOOK AT ANOTHER PUNNETT SQUARE AND PREDICT THE OUTCOMETTTtTtttT IS THE DOMINANT ALLELE FOR TALLNESSt 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 itH hHh43122. WHICH OF THE FOLLOWING HAS THE hh GENOTYPE?

1 & 324NONE3. WHICH OF THE FOLLOWING IS A TRUE STATEMENT?

INDIVIDUAL 4 IS RECESSIVEINDIVIDUALS 1 & 3 ARE HETEROZYGOUSINDIVIDUAL 2 IS DOMINANTALL INDIVIDUALS ARE FEMALE

Try itANSWER: 2. B3. B

25B bBbBBBbBbbb4. IF B IS THE ALLELE FOR BLACK FUR AND b IS THE ALLELE FOR WHITE FUR, WHAT PERCENT WOULD BE BLACK?

25%50%100%75%5. WHAT FRACTION IS HOMOZYGOUS DOMINANT IN THE ABOVE CROSS?

1/2C.1/41/3D. 3/4Try itANSWER:4. D. 75%5. 1/4 B BBbBBBbBBBb6. IN THIS CROSS, WHAT IS THE RATIO OF BB TO Bb?

3 : 14 : 12 : 20 : 4Try itANSWER:6. C. 2:2