Chapter 11-2 Probability and Punnett Squares Notes
Every time Mendel performed a cross with his pea plants, he carefully counted the offspring (over 20,000 plants) his why he noticed there was a pattern! LARGE NUMBERS
Whenever he crossed 2 plants that were hybrid for stem height (t), about ¾ (75%) of the resulting plants were all and ¼ (25%) were short (3:1 ratio) He realized that the principles of probability could be used to explain his results!
Probability Definition: the likelihood that a particular event will occur Examples: Flip a coin: ½ or 50% chance it will come up heads
If you flip a coin 3 times in a row, what is the probability it will come up heads 3 times in a row? Each coin flip is an independent event therefore it is: ½ x ½ x ½ = 1/8
Past outcomes do not influence future outcomes he principles of probability can be used to predict the outcomes of genetic crosses
Punnett Squares he gene combinations that MIGH result from a genetic cross can be determined by drawing a Punnett Square
1. he parents go on the outside of the square along the side (male alleles) and top (female alleles 2. he possible outcomes of the cross are on the inside of the square 3. he letters in the Punnett Square represent alleles
Alleles in a Punnett Square - represents the dominant allele: ALL - t represents the recessive allele: short
Organisms with 2 identical alleles ( or tt) for a trait are said to be HOMOZYGOUS Mendel called these: true-breeding
Organisms with 2 different alleles for a particular trait are said to be HEEROZYGOUS Mendel called these hybrids
Phenotype: the physical characteristics of the organism (what the organism looks like) Ex: all; short
Genotype: the actual genetic makeup of the organism (the 2 alleles the organism inherited) Ex: ; t: tt
Probabilities Predict Averages Probabilities predict the outcomes of a LARGE number of events
Probabilities cannot predict precise outcomes of an event -Ex: flip a coin twice you may get 100% heads -You need to flip the coin many, many times to get close to 50%
Punnett Squares he t genes from t the female parent go here. he t genes from t the male parent go here.
Punnett Squares t t t t t t
Punnett Squares t t t t t t
Punnett Squares t t t t t t
Punnett Squares t t t t t t
Punnett Squares t t t t t t
Punnett Squares t t t F 1 generation t t t
Interpreting the Results he genotype for all the offspring is t. he genotype ratio is: t 4:0 = 100% he phenotype for all the offspring is tall. he phenotype ratio is: tall 4:0 = 100% t t t t t t
Punnett Squares t Your urn!!???? t????
Punnett Squares t t F 2 generation t t tt
Punnett Squares Next, give the genotype and phenotype ratios of the offspring (F 2 generation). t t t t tt
Punnett Squares Genotype ratio: - 1 t t t t tt
Punnett Squares Genotype ratio: - 1, t - 2 t t t t tt
Punnett Squares Genotype ratio: - 1, t - 2, tt - 1 t t t t tt Genotype ratio: 1: 2: 1
Punnett Squares Genotype ratio: - 1, t - 2, tt - 1 Phenotype ratio: all - 3 t t t t tt
Punnett Squares Genotype ratio: - 1, t - 2, tt - 1 Phenotype ratio: all - 3, short - 1 t t t t tt
Punnett Squares One his is a monohybrid cross. We worked with t only one trait. he height of the plant. t t t tt
Punnett Squares his is a monohybrid cross. We worked with t only one trait. he height of the plant. two t Later we will work with a dihybrid cross, using the height of the plant and either seed color or seed shape. t t tt
Male tt 3. What is the expected ratio for this crosses? Assume the first written is from the female. t x tt Female t t t t t t t t t t
t x tt Genotypic Ratio: 0, 2 t, 2 tt = 2 t : 2 tt 50% t : 50% tt Phenotypic Ratio: 2 all : 2 short or 50% all : 50% short
XX and XY Male XY X Female XX X X XX XX Y XY XY
XX x XY Phenotypic Ratio: 50% Females and 50% Males
he estcross Homozygous dominant and heterozygous individuals are indistinguishable (phenotypically) A testcross allows us to determine the genotype of an individual that has the dominant trait. We cross the parent showing the dominant trait with a recessive parent.
-If all (100%) of the offspring are dominant type then we assume the initial parent was homozygous dominant. -If even one offspring shows the recessive type then we know the initial parent was heterozygous.