Probability, Heredity and Punnett Squares

Coin Toss I call “Heads” What are my chances that I win?

Probability A number that describes how likely it is that an event will occur Example: My coin toss ▫Probability that a tossed coin lands with heads up is 1 in 2  ½ or 50%

Independence of Events When you toss a coin more than once, the results of one toss do not affect the results of the next toss ▫Each event occurs independently ▫The results of previous tosses do NOT affect the result of the next toss

Probability and Genetics Mendel was the first scientist to recognize that the principles of probability can be used to predict the results of genetic crosses ▫Cross TALL (Tt) and SHORT (tt) pea plant  2 out of 4 were TALL  2 out of 4 were SHORT ▫Cross TALL (Tt) and TALL (Tt)  3 out of 4 were TALL  1 out of 4 were SHORT

Punnett Squares A chart that shows all the possible combination of alleles that can result from a genetic cross ▫To determine the probability of a particular outcome

How to make a Punnett Square 1.Draw a box and divide it into four squares 2.Write the male parent’s alleles along the top and the female parent’s alleles along the left side 3.Copy the Female alleles into the boxes and copy the Male alleles into the boxes

Let’s try one together…. Create a Punnett square to show the cross between a Black guinea pig (BB) and a white guinea pig (bb) BBBB b BbBb BbBb BbBbBbBb Probability 4 out of 4 are Bb 100% of the Guinea pigs will be Black

Practice on your own…. 1.Create a Punnett square to show the cross between a Purple flower (Pp) and Purple flower (Pp) 1.Create a Punnett square to show the cross between yellow seed (Yy) and green seed (yy)

Phenotype vs. Genotype Phenotype: physical appearance Genotype: genes or allele combination Example: G (green pea pod) g (yellow pea pod) GenotypePhenotype GGGreen pea pod GgGreen pea pod ggYellow pea pod

Homozygous and Heterozygous Homozygous: has two identical alleles for that trait Heterozygous: has two different alleles for that trait (sometimes called hybrid) GenotypePhenotype GGGreen pea podHomozygous dominant GgGreen pea podHeterozygous ggYellow pea podHomozygous recessive

Codominance: alleles are neither dominant nor recessive. As a result, both alleles are expressed in the offspring

Incomplete Dominance – one allele is not completely dominant over the other. Results in a third phenotype

Sex-linked traits 23 rd pair of chromosomes are the sex chromosomes ▫XX – Female ▫XY – Male Sex-linked genes ▫Ex. Red- green colorblindness, hemophilia, muscular dystrophy, Fragile X X and Y chromosomes are different and most of the genes on the X chromosome are not on the Y chromosome. ▫In males, there is usually not matching allele on their Y chromosome. ▫Any allele on their X chromosome (even a recessive) will be seen in that male.

Multiple Alleles When more than one allele determines a trait ▫ex. Hair and skin color ▫The more active alleles present, the more color pigme nt

Multifactorial “Many factors” Both genetic and environmental Example: Height, neural tube defects (spina bifida)