Lecture 25 Punnett Squares Honors Biology Ms Day

Lecture #25 Punnett Squares Honors Biology Ms. Day

What is “Probability”? ¡ ¡ ¡ Why did Mendel get his results? What is the probability that Mendel would get his results EACH time? PROBABILITY = likelihood (chance) that an event will happen

2 Probability Rules Multiplication Rule l What are the chances of _? _ AND _? _ happening at the same time? l MULTIPLY probability of EACH event happening TOGETHER Addition Rule l What are the chances of _? _ OR _? _ happening at same time? l ADD the probability of EACH event happening TOGETHER

Probability Example Problem Ms. Day has 4 sisters. What is the probability that this will happen (her mother having 5 girls in a row)? ¡ ½ x ½ x ½ =? ? ¡ l 0. 03125 or 3. 125 % chance

Monohybrid Cross ¡a cross between 2 individuals that looks at 1 trait ¡ There are 4 different types l Ex: freckles OR dimples Dihybrid Cross ¡a cross between 2 individuals that looks at the possibilities of inheriting 2 DIFFERENT traits at one time l Ex: Freckles AND dimples

What is a Punnett Square? ¡a diagram (box) used to predict the probabilities of the possible outcomes for offspring that will result from a cross between 2 parents l SHOWS EXPECTED RESULTS (not necessarily actual!!!!!)

Law of Segregation Germ Line Cell Gamete (sperm or egg) A A a a

Monohybrid Punnett Square Mom’s genotype (Hh) x Dad’s genotype (hh) Tall Short Mom’s allele #1 H Mom’s allele #2 h Dad’s Allele #1 h Hh hh Dad’s Allele #2 h Hh hh

Genotypic Ratio vs. Phenotypic Ratio ¡ Genotypic Ratio (Probability or Frequency) = the GENOTYPE probabilities (expected results) of offspring ¡Ex: 50 % AA and 50 % Aa ¡ Phenotypic Ratio (Probability or Frequency) = the PHENOTYPE probabilities (expected results) of offspring ¡Ex: 100% Tall

Probabilities can be expressed as a: l DECIMAL ¡Ex: 0. 75 and 0. 25 l FRACTION ¡Ex: ¾ and ¼ l PRECENTAGE ¡Ex: 75% and 25% l RATIO ¡Ex: 3: 1

Punnett Square Genotypic ratio = 50 % Hh, 50% hh Phenotypic ratio = 50 % Tall, 50% short Hh hh

Ex#1: Homozygous Dominant x Homozygous Recessive Mom’s genotype (HH) x Dad’s genotype (hh) Tall Short H H h Hh Hh Genotypic ratio = 100% Hh (4: 0) Phenotypic ratio = 100% Tall (4: 0)

Ex #2: Heterozygous x Heterozygous Mom’s genotype (Hh) x Dad’s genotype (Hh) Tall H h H HH Hh hh Genotypic ratio = 25% HH, 50% Hh, 25% hh (1: 2: 1) Phenotypic ratio = 75% Tall, 25% short (3: 1)

3 Phenotype Genotype Purple PP Purple (homozygous) Pp (heterozygous) Pp Purple 1 Figure 14. 6 1 2 (heterozygous) pp White (homozygous) Ratio 3: 1 Ratio 1: 2: 1 1

The Testcross ¡ Allows us to determine the genotype of an organism with the dominant phenotype, but unknown genotype l Genotype is not immediately obvious ¡ Cross: l dominant phenotype individual x homozygous recessive for same trait (H _ ? _ x hh)

Test Cross Mom’s genotype (H? ) x Dad’s genotype (hh) Tall Short H ? h h HH ? h Hh ? h Genetypic ratio = ? Phenotypic ratio =? IF THERE ANY SHORT OFFSPRING, THEN WE KNOW THE UNKNOWN GENOTYPE HAS TO BE Hh

Mendel’s 2 nd Law: law of independent assortment that the genes are inherited independently of other genes ¡ Occurs during Metaphase I ¡ Mendel assumed traits occur on different chromosomes! ¡ says

DIHYBRIDS Review… Mendel’s 1 st Law: law of segregation says that the each gamete gets 1 allele PER GENE Mendel’s 2 nd Law: law of independent assortment says that the genes are inherited independently of other genes

Dihybrid Crosses ¡ crosses involving crossing 2 DIFFERENT traits at one time l Example: Mate 2 parents and look at the probability of seeing 2 traits, such as: ¡eye color AND hair color ¡freckles AND dimples

How do You Do Dihybrid Crosses? 2 ways 1. one BIG Punnett Square (16 boxes instead of 4) 2. Make 2 separate Punnett Squares ¡ 1 square for EACH trait ¡Then use PROBABILITY RULES and MULTIPLY

One BIG Punnett Square 1. Find the possible gametes using F. O. I. L method (ONLY 1 “letter” per gene) Aa. Bb x AABb AB AB Ab Ab a. B AB ab Ab

2. Place the gametes in the Punnett Square & fill in boxes AB Ab a. B ab AABB AABb Aa. BB Aa. Bb AAb. B AAbb Aab. B Aabb

Genotypic Ratio AAb. B = 8/16 or 1/2 Aa. BB = 2/16 or 1/8 Aabb = 2/16 or 1/8 AABB = 2/16 or 1/8 AAbb = 2/16 or 1/8

Phenotypic Ratio A= freckles, a= no freckles B = Dimples, b= no dimples AAb. B = 8/16 (50%) freckles, dimples Aa. BB = 2/16 (12. 5%) freckles, dimples Aabb = 2/16 (12. 5%) freckles, no dimples AABB= 2/16 (12. 5%) freckles, dimples AAbb = 2/16 (12. 5%) freckles, no dimples So… 12/16 (75%) freckles AND dimples 4/16 (25%) freckles AND no dimples

Now…the easier way! ¡ Write out a key for EACH trait l l ¡ ¡ ¡ AA, Aa= Freckles, aa= No freckles BB, Bb= Dimples, bb= No dimples Figure out the genotypes of EACH parent and put them together Draw 2 Punnett squares & figure out the FRACTION ratio of possible genotype and phenotypes l Draw punnett square #1 for trait #1 l Draw punnett square #2 for trait #2 Figure out genotype/phenotype possibilities of offspring by multiplying fractions

Aa. Bb x aabb AA, Aa= Tall aa= short b BB, Bb= Black bb= white b Bb Bb bb bb ½ Bb ½ bb ½ Black ½ white So… babies could look: 25% Black & Tall (½ * ½) 25% Black & short (½ * ½) a a A Aa Aa a aa aa ½ Aa ½ Tall ½ aa ½ short 25% white & tall (½ * ½) 25% white & short (½ * ½)

Probability Rules and Dihybrid Crosses ¡A dihybrid or other multicharacter cross l Is equivalent to two or more independent monohybrid crosses occurring simultaneously (AT THE SAME TIME)