Chapter 3Genetics: The science of heredity Miss Luzma Fabre
IntroductionToss a coin and describe the probability2 possibilities: heads or tailsEach of these events is equally likely tooccur (50%)
Section 2: Probability and heredityProbability is a number that describeshow likely it is that an event will occurIt can be described as a fraction orpercentageExample: ½ or 50%
Independence of eventsSuppose you toss a coin 5 times and itlands with heads up each time. What isthe probability that it will land withheads up on the next toss?1 in 2, or 50%Each event occurs independently
Probability and geneticsMendel recognized that the principles ofprobability can be used to predict theresults of genetic crossesA Punnett square is a chart that showsall the possible combinations of allelesthat can result from a genetic cross
How to make a Punnett square Male parent´s allelesFemale parent´s alleles
Using a Punnett squareYou can use a Punnett square tocalculate the probability that offspringwith a certain combination of alleles willresultIn a genetic cross, the allele thateach parent will pass on to itsoffspring is based on probability
What tool can be used to predict theresults of a cross?
Phenotypes and genotypesAn organism´s phenotype is itsphysical appearance, or visible traits.Ex: blue eyesAn organism´s genotype is itsgenetic makeup, or allelecombinations. Ex: bb
An organism that has 2 identicalalleles for a trait is said to behomozygous. Ex: SS, ssAn organism that has 2 differentalleles for a trait is heterozygous(hybrid). Ex: Ss
Describe the genotypes given as:Homozygous recessive, homozygous dominant or heterozygous Dd dd DD
In humans, brown eyes (B) are dominant overblue (b)*. A brown-eyed man marries a blue-eyed woman and they have three children,two of whom are brown-eyed and one ofwhom is blue-eyed. Draw the Punnett squarethat illustrates this marriage.What is the man’s genotype? What are thegenotypes of the children?
BbbbbBb Genotype of male parent: Bb Phenotype: brown eyes Genotype of children: Bb and bb Phenotype of children: brown and blue
What are the phenotypes of thefollowing genotypes? In other words,what eye color will they have?BBBbbb
CodominanceFor some alleles, an inheritance patterncalled codominance existsIn codominance, the alleles areneither dominant nor recessive. As aresult, both alleles are expressed inthe offspring.