Punnett Squares
DOMINANT/RECESSIVE Dominant allele is represented by a capital letter. (usually the first letter of the trait) Recessive allele is represented by the SAME letter except in lower-case. EX: (Trait: height) Tall = T Short = t; not s for short
Misconceptions About Dominant Alleles
The genetic makeup (set of 2 alleles) of an organism is its genotype Genotype & Phenotype The genetic makeup (set of 2 alleles) of an organism is its genotype The physical appearance of an organism is its phenotype
Genotype vs. Phenotype
Talk to a Partner: Determine the genotype and phenotype of each flower
HOMOZYGOUS & HETEROZYGOUS When both alleles in the pair are the same, the organism is homozygous (purebred) EX: TT or tt When both alleles in the pair are different, the organism is heterozygous (hybrid or carrier) Ex: Tt
Which chromosome set is heterozygous?
Punnett Squares (PSq) Developed in 1905 by Reginald Punnett. Used to predict & compare possible genetic variations resulting from a cross Probabilities, not exact results
PROBABILITY ____________________ is the liklihood that a particular event will occur It can be written as a: Fraction ____ Percent ____ Ratio ____ (dominant : recessive) 1/4 25% 1:3
Symbols used in PSq Original parents P1 generation Offspring of the parent plants F1 generation Offspring of the F1 generation F2 generation
Explaining Mendel’s Results using Punnett Squares Creating Pure breeding P generation PP X PP pp X pp
Explaining Mendel’s Results using Punnett Squares Creating F1 generation: PP X pp
Explaining Mendel’s Results using Punnett Squares Creating F2 generation: Pp X Pp
Practice Problem: Monohybrid Crosses: Working w/ PS #1
Now try problems on: Punnett Squares Practice: Monohybrid Crosses STOP !!
Non-Mendelian Genetics Patterns that don’t follow Mendel’s Rules
1) Incomplete Dominance Two different alleles BLEND to form a new phenotype Ex: Colors in flowers, red + whitepink
2) Codominance Two different alleles of the same trait are completely and fully expressed EX: Red flower & white flower produces a red and white striped flower
Examples
2. Codominance & Multiple Alleles Classic Ex: Human blood types - A, B, AB, O Alleles: IA, IB, i IA & IB are codominant i - recessive (ii = type O blood)
Genotype IA IA or Iai IB IB or Ibi IA IB ii Phenotype
IA IA or IAi A IB IB or IBi B IA IB A and B AB ii none O Genotypes Surface Proteins Phenotypes IA IA or IAi A IB IB or IBi B IA IB A and B AB ii none O
A and AB see A as “like me” as Different! DONOR BLOOD A and AB see A as “like me” B and O see A as Different! IMMUNE SYSTEM ATTACKS! Body images modified from: http://www.new-fitness.com/images/body_shapes.jpg
B and AB see B as “like me” as Different! DONOR BLOOD B and AB see B as “like me” A and O see B as Different! IMMUNE SYSTEM ATTACKS! Body images modified from: http://www.new-fitness.com/images/body_shapes.jpg
O ____ can donate to EVERY BLOOD TYPE = _____________________ DONOR BLOOD O ____ can donate to EVERY BLOOD TYPE = _____________________ Nothing on surface to recognize as “NOT SELF” UNIVERSAL DONOR YOU DON’T HAVE ANYTHING I DON’T HAVE! Body images modified from: http://www.new-fitness.com/images/body_shapes.jpg
AB can only GIVE to AB BUT . . . ______ can RECEIVE FROM EVERY BLOOD TYPE = ________________________ UNIVERSAL RECIPIENT Body image modified from: http://www.new-fitness.com/images/body_shapes.jpg
Comparison Codominance Incomplete dominance
Practice Problem: Incomplete Dominance Working w/ PS #3,4 & 6
Now try problems on: Punnett Squares Practice: Incomplete/Codominant STOP !!
Non-Mendelian Genetics Patterns that don’t follow the rules Incomplete Dominance Codominance
3) Sex-linked traits Traits carried on the sex chromosomes (mostly the x chromosome)
Ex. Colorblindness, hemophilia
4) Polygenic traits Trait produced from many alleles on more than one chromosome Ex. Hair color, skin color, eye color, height
The more genes that contribute to the trait, the more complicated the pattern of expression
Practice Problem: Incomplete Dominance Working w/ PS #2, 5
Now try problems on: Punnett Squares Practice: Sex-Linked STOP !!