Understanding Genetic Heredity & Probability Through Punnett Squares

Question- Is it possible for the two Highland Cattle on the left produce either of the calves on the right?

Understanding Punnett Squares What is a Punnett Square? A chart that allows us to determine the different genotypes that offspring can inherit from their parents The size of the square depends on the number of genes you’re looking at

Let’s Try It! First we must get the genotype of each parent Example: B: allele for black hair b: allele for red hair Tt – Bull’s genotype Tt- Cow’s genotype

Set Up the Punnett Square B b B B B B b b b b B b

Now Let’s Talk About Probability What is probability? The chance that an event will occur How do you find the probability of an event? You must know: the total number of possible outcomes the number of each individual outcome

BB Bb Genotypic Ratio B b B b The offspring can inherit three different genotypes: BB, Bb, & bb B b BB Bb bb B The Genotypic Ratio would be: ¼ BB: ½ Bb: ¼ bb OR 25% BB: 50% Bb; 25% bb b

BB Bb Phenotypic Ratio B b B b The offspring can inherit three different genotypes, but those produce only two phenotypes. B b BB Bb bb B The Phenotypic Ratio would be: ¾ Black:¼ Red OR 75% Black: 25% Red b

Is it possible for two black cattle to produce a red calf? YES! If both parents are Bb and can pass on the recessive genes, there is a 25% chance of having a red calf! If they had 8 calves, you should expect (hope) that 2 of those are red!

Practice Wrinkled seeds are recessive to smooth seeds. Show a plant that always produces wrinkled seeds crossed with a heterozygous smooth seeds producing plant. Use S for allele Yellow seeds are dominant to green seeds in peas. Show a cross between a homozygous yellow seed with a green seed. Identify the genotypic and phenotypic ratios. Use Y for allele

What If We’re Looking at More Than One Trait? We use what’s called a dihybrid cross!

Dihybrid Example A male guinea pig that is heterozygous for both fur color and coat texture is crossed with a female that has light fur color and is heterozygous for coat texture. What possible offspring can they produce?

Start With Writing A Key Dark fur color (D) is dominant Light fur (d) is recessive. Rough coat texture (R) is dominant Smooth coat (r) is recessive

Figure Out the Genetics of Each The male guinea pig is heterozygous for both color and texture. Male = “DdRr”. The female guinea pig has light fur. Since that is a recessive trait the genotype = “dd”. It is also heterozygous for fur texture, which means a genotype of “Rr”. Female= “ddRr”.

DdRr X ddRr

Determine Gene Combinations DdRr X ddRr During the formation of sperm a “D” could go with a “R” producing a sperm “DR”, or a “D” could go with a “r” forming a sperm with “Dr”. DdRr Possible Combinations: DR, Dr, dR, or dr ddRr Possible Combinations: dR, dr, dR, or dr

Set Up Your Punnett Square DdRr DR Dr dR dr dR dr ddRr

DdRr ddRr Combine the Genetics DR Dr dR dr dR dr DdRR DdRr ddRR ddRr

Calculate Genotypic Ratio 2 DdRR : 4 DdRr : 2 ddRR : 4 ddRr : 2 Ddrr : 2 ddrr

Calculate Phenotypic Ratio 2 Dark/Rough : 4 Dark/Rough : 2 Light/Rough : 4 Light/Rough : 2 Dark/Smooth : 2 Light/Smooth OR 6 Dark/Rough, 6 Light/Rough, 2 Dark/Smooth, 2 Light Smooth 2 DdRR : 4 DdRr : 2 ddRR : 4 ddRr : 2 Ddrr : 2 ddrr

Try One On Your Own KEY: G= Black Hair, g= White Hair B= Black eyes, b= Red eyes A male rabbit with the genotype GgBb is crossed with a female rabbit with the genotype ggbb. Fill it out and determine the genotypic ratio and the phenotypic ratio of the offspring.