Mendel and Heredity

1. Mendel and Heredity • Gregor Mendel (1850’s)– Austrian monk that bred pea plants and from his experiments he formed the basis of GENETICS: study of heredity • Used peas because they had easily distinguishable forms of various traits: flower color, pod shape/color, seed shape/color, plant height and flower placement • Easy to grow and matured quickly • Traits are determined by genes received from each parent

2. Mendel’s Experimental Design • 1. Allowed peas to self-pollinate for several generations • Purple flowering plants would produce only purple flowering plants (same for white flowering plants) • These were the P or parent generation—Mendel started his experiments with these • 2. Cross-pollinated 2 varieties that had contrasting traits (purple flowers X white flowers)—pollen from white plant was placed on the stigma of the purple flower • Offspring of this cross would be the F1 generation (filial)--only 1 flower color was present: purple • 3. Allowed the F1 plants to self-pollinate, planted seeds and the offspring were the F2 generation; rec trait (white) showed up again • Mendel named the trait that “disappeared” the recessive trait and the one that showed he called the dominant trait • After counting all the F2 offspring he found that there was always a 3:1 ratio of purple:white • Mendel found this to be true of ALL the pea’s identifiable traits

3. Mendel’s Theory of Heredity • Parents pass on genes to offspring—not actual traits • For each trait, an individual has 2 genes governing that trait: 1 from Mom and 1 from Dad • If both genes carry the same info (purple, purple) then the individual is HOMOZYGOUS for that trait • If the genes are different (purple, white) then the individual is HETEROZYGOUS for that trait • Each copy of a gene is called an allele; set of alleles that an individual has is called a genotype : PP, Pp or pp—shows genes from parents as capital or lower case letters • Capital letters are dominant traits, lower case are recessive traits (ALWAYS use the first letter of the dom trait) • Phenotype (purple/white flowers) is the physical appearance • Dom allele (capital letter) is expressed, rec allele (lower case letter) is still present but is unexpressed; this rec allele CAN still be passed on to offspring where it might be expressed

4. Laws of Heredity • Law of Segregation---alleles separate when gametes are formed during meiosis and the chromosomes separate • Law of Independent Assortment—pairs of alleles separate independently of one another during meiosis—for example, if the gamete is heterozygous (Pp) before meiosis, the dom allele (P) goes into 1 new gamete and the rec allele (p) goes into another • What phase of meiosis would this occur?

5. Legend steps for Genetic Crosses • 1. Read the word problem and determine WHAT is being crossed • 2. Determine dom and rec traits from the problem or your text book—ALWAYS use the first letter of the dom trait in the problem; capitalize it if dom, lower case if rec • 3. Using steps 1 & 2 write the parents’ genotypes • 4. Draw a punnett square to show the cross--♂’s genotype goes on top of square, ♀’s on the side; fill in the boxes with the offspring’s possible genotypes • 5. Write the offspring’s possible genotypes in a ratio, always starting with the homozygous dom, then hetero, then homozygous rec • 6. Write the possible phenotypes in a ratio

6. Probability Lab or the PENNY LAB! • Intro: Mendel’s crosses can be explained by the rules of probability—the likelihood that a specific event will occur, or to put it another way: • # of 1 kind of possible outcomes • Total # of all possible outcomes • Ex : probability that a baby will be a girl? Kind of possible outcomes is 1 and the total # of outcomes is 2 (either boy or girl) so the probability is ½ • Purpose: To relate probability to genetic crosses • Procedure: Pair up, make a chart, take 2 coins of the same type, toss coins 100 times each, at the same time, record #’s in the chart with tally marks, figure % error, place your results in the class chart on the whiteboard • Conclusion: 1. How does the probability change with the increasing # of tosses? 2. What parent genotypes were present?

7. Other types of crosses • We have been practicing monohybrid crosses—those that deal with only 1 trait (flower color or pod shape, etc) • Dihybrid crosses involve 2 different traits; the steps are all the same, except the punnett square has 16 boxes instead of 4! Let’s try one! • In guinea pigs the allele for short hair (S) is dom over long hair (s) and the allele for black hair (B) is dom over brown hair (b). So if the guinea pig farmer mated a hetero short haired brown male g.p. with a hetero short haired brown female g.p., the steps would look like this: • 1. hetero short, homo brown X hetero short, homo brown • 2..S=short, s=long B=black, b=brown • 3. Ssbb X Ssbb • 4. Use “foil” method to determine the parents’ gametes: Sb Sb sb sb Sb SSbb SSbb Ssbb Ssbb Sb SSbb SSbb Ssbb Ssbb sb Ssbb Ssbb ssbb ssbb sb Ssbb Ssbb ssbb ssbb • 5. 4 SSbb : 8 Ssbb : 4 ssbb • 6. 12 short haired brown: 4 long haired brown

8. Incomplete Dominance • Complete dom is when the dom trait completely masks the rec trait (Rr = red) • Incomplete dom is when you have an intermediary trait in the hetero phenotype (Rr = pink) • Common in some flowers like snapdragons • Book uses R and R‘ instead of R and r– we will use R (red) and r (white) and you will be told that the problem is Inc dom • Still supports Mendel’s Laws of Heredity and the steps are the same • Cross a pink snapdragon with a red one

9. Codominance • With codominance you have 2 dominant traits that are both expressed • Both the letters are used and both are capitals • Roan coat in horses and cattle is an example—Red (R) is dom and so is white (W), so when both are present in the genotype (RW) the phenotype is not spotted, but both colors are expressed • Try crossing a roan bull with a white cow • Blood types are this sort of genetic problem—more about that later……..