Punnett Square Notes Gregor Mendel Father of Genetics
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Punnett Square Notes
Gregor Mendel Father of Genetics 1 st important studies of heredity Identified specific traits in the garden pea and studied them from one generation to another
Mendel’s Conclusions 1. Law of Segregation – Two alleles for each trait separate when gametes form; Parents pass only one allele for each trait to each offspring 2. Law of Independent Assortment – Genes for different traits are inherited independently of each other
What is Genetics? Genetics is the scientific study of heredity
What is a Trait? A trait is a specific characteristic that varies from one individual to another. Examples: Brown hair, blue eyes, tall, curly
What is an Allele? Alleles are the different possibilities for a given trait. Every trait has at least two alleles (one from the mother and one from the father) Example: Eye color – Brown, blue, green, hazel Examples of Alleles: B = Brown Eyes b = Blue Eyes G = Green Eyes g = Hazel Eyes
What are Genes? Genes are the sequence of DNA that codes for a protein and thus determines a trait.
Dominant vs. Recessive Dominant - Masks the other trait; the trait that shows if present Represented by a capital letter R Recessive – An organism with a recessive allele for a particular trait will only exhibit that trait when the dominant allele is not present; Will only show if both alleles are present Represented by a lower case letter r
Incomplete Dominance - Situation in which one allele is not completely dominant over another. Example – Red and white flowers are crossed and pink flowers are produced.
Codominance - Situation in which both alleles of a gene contribute to the phenotype of the organism. Example – A solid white cow is crossed with a solid brown cow and the resulting offspring are spotted brown and white (called roan). +
Dominant & Recessive Practice Upper case letters represent Dominant traits. Lower case letters represent Recessive traits. TT - Represent offspring with straight hair Tt - Represent offspring with straight hair tt - Represents offspring with curly hair T – straight hair t - curly hair
Homozygous vs. Heterozygous Homozygous – Term used to refer to an organism that has two identical alleles for a particular trait (TT or tt) Heterozygous - Term used to refer to an organism that has two different alleles for the same trait (Tt) RR rr Rr
Genotype vs. Phenotype Genotype – The genetic makeup of an organism; The gene (or allele) combination an organism has. Example: Tt, ss, GG, Ww Phenotype – The physical characteristics of an organism; The way an organism looks Example: Curly hair, straight hair, blue eyes, tall, green
Punnett Squares Punnett Square – Diagram showing the gene combinations that might result from a genetic cross Used to calculate the probability of inheriting a particular trait Probability – The chance that a given event will occur
Punnett Square Parent Offspring
How to Complete a Punnett Square
Y-Yellow y-white Genotype: 1: 2: 1 (YY: Yy: yy) Phenotype: 3 Yellow 1 White
Let’s put a couple examples in our notebook!
You Try It Now! Complete the following punnett square problems in your SISN. Create a punnett square and give the genotype and phenotype for the following cross: TT x tt (T = Tall and t = Short)
TT x tt Step One: Set Up Punnett Square (put one parent on the top and the other along the side) t T T
TT x tt Step Two: Complete the Punnett Square T t Tt Tt
TT x tt Step Three: Write the genotype and phenotype T t Tt Tt Remember: Each box is 25% Genotype: 4 - Tt Phenotype: 100% Tall
You Try It. Complete the following punnett square problems in your SISN. Give the genotype and phenotype for the following cross: Tt x tt
Tt x tt Step One: Set Up Punnett Square (put one parent on the top and the other along the side) t t T t
Tt x tt Step Two: Complete the Punnett Square T t Tt tt
Tt x tt Step Two: Complete the Punnett Square T t Tt tt Remember: Each box is 25% Genotype: Tt - 2 (50%) tt - 2 (50%) Phenotype: 50% Tall 50% Short
Work through the practice!!!
Punnett Square Pop Quiz Take out a blank sheet of notebook paper and something to write with.
1. What is the dominant gene? What is the recessive gene? B – Brown eyes b – Blue Eyes
2. Label which is heterozygous and which is homozygous. a. BB b. Bb c. bb
3. A scientist crossed a homozygous tall daisy with a homozygous short daisy. Each of the daisies has two of the same alleles that make them homozygous. The tall trait of the daisy is the dominant, and is represented with a “D”. The short trait of the daisy is the recessive trait, and is represented with a “d”. Use a Punnett Square to cross a homozygous tall daisy (DD) with a homozygous short daisy (dd).
4. In the first question, what is the genotype and phenotype of all the 1 st generation offspring? Genotype ________ Phenotype ________
5. Black fur (B) in guinea pigs is dominant over white fur (b). Create a punnett square that displays the offspring if a heterozygous black fur guinea pig is crossed with a homozygous white fur guinea pig.
Multiple Alleles Multiple Alleles- Three or more alleles of the same gene. Even though three or more alleles exist for a particular trait, an individual can only have two alleles - one from the mother and one from the father.
Examples of Multiple Alleles 1. Coat color in rabbits is determined by a single gene that has at least four different alleles. Different combinations of alleles result in the four colors you see here.
Examples of Multiple Alleles Blood Type – 3 alleles exist (IA, IB, and io), which results in four different possible blood types What are the four main blood types? Hair Color – Too many alleles exist to count There are over 20 different shades of hair color.
Blood type punnett squares
Blood Type Punnett Square A man who is AB marries a woman who is heterozygous type B. If they had four children, what would their probable genotypes and phenotypes be? *hint – you will need to create a punnett square.
Multiple Alleles There Always Multiple Alleles! Genetic inheritance is often presented with straightforward examples involving only two alleles with clear-cut dominance. This makes inheritance patterns easy to see. But very few traits actually only have two alleles with clear-cut dominance. As we learn more about genetics, we have found that there are often hundreds of alleles for any particular gene. We probably know this already - as we look around at other people, we see infinite variation.
Blood type
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