Probability and Genetics Pages 118 123 What are

Probability and Genetics Pages 118 -123

What are the chances? • Probability is the likelihood that a particular event will occur.

Flip A Coin! • When you flip a coin there are only two possible outcomes that could occur. – Heads – Tails Each side has an equal chance of showing up every time you flip the coin. Roll a Die • How many sides are on a die? • What is the chance that when you roll the die it will land on 4? On 2?

Mendel and Probability • Mendel carefully counted the offspring of his pea plants and the traits they showed. – ¾ dominant trait – ¼ recessive trait • Mendel was the first scientist to recognize that probability can be used to predict the results of genetic crosses.

Punnett Squares • A Punnett square is a chart that shows all the possible combinations of alleles that can result from a genetic cross. • Created by Reginald C. Punnett, a geneticist, who used Mendel’s experiments to show the passing of traits from one generation to the next.

Punnett Squares • Each parent can pass either of their two alleles to their offspring. • The possible alleles are written across the top and left hand side of the box. – Each allele gets its own box.

Punnett Squares • To figure out the possible allele combinations of the offspring the traits are followed from the top of the box and the side of the box. t t T t In a hybrid the Dominant trait goes first.

What are the chances? • The allele that will be passed from parent to offspring is based on chance. Just like the flip of a coin. • There are four possible allele combinations. t t Purebred Tall - TT Tt Tt tt tt Hybrid Tall – Tt/t. T Purebred Short - tt

What is the probability? t t Tt Tt tt tt 2 out of 4 – Tt What is the percentage that each allele combination will occur?

What is the Probability? • What is the chance (in percent) of getting: – A hybrid black guinea pig? – A purebred white guinea pig? BB bb

Phenotypes and Genotypes • Phenotype: an organisms’ physical appearance, or its visible traits. – Example: tall/short, purple/white, yellow/green • Genotype: an organisms’ genetic makeup, or allele combinations – Example: TT, tt, Tt

Homozygous and Heterozygous • Homozygous: When an organism has two identical alleles for a trait. – Example: Purebred – TT or tt • Heterozygous: An organism that has two different alleles for a trait. – Example: Hybrid – Tt, Rr, Ll, Bb

Co-dominance • In pea plants there were only two possible alleles. – Ex. Green OR Yellow – Ex. Smooth OR Pinched • Sometimes, genes have alleles that are NEITHER dominant nor recessive. • This is a co-dominant trait. In co-dominance, neither trait is masked which means that both traits are equally as dominant and the hybrid will show both. – Ex. Feather color in chickens

Co-dominance • When crossing co-dominant traits we use superscripts to tell us the different allele for the trait. – Example: Feather color • purebred black = FBFB • purebred white = FWFW • mixed = FBFW – Example: Flower color • purple = CPCP • white = CWCW • striped = CPCW

Example #1 • Cross a purebred black rooster with a purebred white hen. • F BF B X F W • Report all genotypes and phenotypes

Example #2 • In petunias, petal color is a co-dominant trait. Flowers that are heterozygous and have BOTH alleles (one for purple and one for white) will phenotypically look striped. • Cross a purple petunia with a striped petunia. • Report all genotypes and phenotypes. • Is it probable to grow any white petunias from this cross?

Incomplete dominance • There are times when traits show a BLEND of both the dominant and the recessive form. This is called INCOMPLETE DOMINANCE. • An organism that is a hybrid will show a phenotype that is in the “middle” of the dominant or the recessive form. – Ex. Hair texture in humans: – HH = curly, hh = straight, Hh = wavy

Example #1 • In humans, lip size shows incomplete dominance. There are large lips, small lips, and hybrids, will have medium-sized lips. Cross two parents; one with small lips and one with mediumsized lips. Use the letter “L” for alleles. Report all genotypes and phenotypes. ll X Ll

Example #2 • In humans, lip size shows incomplete dominance. There are large lips, small lips, and hybrids, will have medium-sized lips. Cross two parents; one with large lips and one with mediumsized lips. Use the letter “L” for alleles. Report all genotypes and phenotypes.