4.2 Probability and Genetics

POINT > Review some genetics vocabulary POINT > Define genotype and phenotype POINT > Define homozygous and heterozygous POINT > Use a Punnett Square to predict genotypes of offspring POINT > Describe codominance

POINT > Review some genetics vocabulary Genetics is the scientific study of heredity

POINT > Review some genetics vocabulary A trait is a physical characteristic that is different from one individual to another. Ex. Brown hair, blue eyes, purple flowers, large leaves

POINT > Review some genetics vocabulary Genes are the sequence of DNA that determines a trait

POINT > Review some genetics vocabulary Alleles are the different genes possible for a single trait ex. In pea plants, the gene for flower color has one allele for purple flowers, one allele for white flowers Every trait in an individual has two alleles (one from the mother and one from the father) A parent passes one of its alleles to each offspring

POINT > Review some genetics vocabulary Dominant alleles – Covers up the other trait; the dominant trait is always expressed Represented by a capital letter Recessive alleles - The recessive trait is only expressed when the dominant allele is not present Represented by a lower case letter R r

POINT > Review some genetics vocabulary T - tall plant (dominant) t - short plant (recessive) So, the possibilities for one plant: TT - Two dominant alleles Tt - One dominant allele and one recessive allele tt - Two recessive alleles

POINT > Review some genetics vocabulary P – Original parents F1 – First generation F2 – Second generation P x P = F1 F1 x F1 = F2

POINT > Define genotype and phenotype The genotype shows which alleles the organism has: ex. Tt, ss, GG, Ww The phenotype is the physical characteristics of an organism; things you can see or measure. Ex.: curly hair, straight hair, brown eyes, tall, green seeds, etc

POINT > Define genotype and phenotype The genotype determines the phenotype!

CHECK: What is a phenotype? What is a genotype? Is “red hair” a genotype or a phenotype? Is “Gg” a genotype or phenotype?

POINT > Define homozygous and heterozygous RR Homozygous –two identical alleles for a trait (TT or tt) Heterozygous - two different alleles for a trait (Tt) rr Rr

CHECK: What is a phenotype? What is heterozygous? What is homozygous? What is a genotype?

POINT > Define homozygous and heterozygous Example: R allele is for red eyes (dominant) r allele is for white eyes (recessive) Homozygous dominant -Two dominant alleles (ex. genotype = RR) This individual (RR) will show the dominant phenotype ( red eyes )

POINT > Define homozygous and heterozygous R allele is for red eyes (dominant) r allele is for white eyes (recessive) Homozygous recessive - Two recessive alleles (ex. genotype = rr) This individual (rr) will show the recessive phenotype (white eyes)

POINT > Define homozygous and heterozygous R allele is for red eyes (dominant) r allele is for white eyes (recessive) Heterozygous - One dominant allele (R) and one recessive allele (r) (ex. genotype = Rr) This individual (Rr) will show the dominant phenotype (red eyes)

POINT > Use a Punnett Square to predict genotypes of offspring A Punnett Square is a diagram that shows the allele combinations that might result from a genetic cross We use it to calculate the probability of inheriting a particular trait Probability – The chance that a given event will occur

CHECK: What is a Punnett square? What is probability?

POINT > Use a Punnett Square to predict genotypes of offspring Remember Mendel’s first cross: Purebred purple flowers X purebred white flowers Purebred purple flowers = PP (homozygous dominant) Purebred white flowers = pp (homozygous recessive) PP x pp Purple Parent P P p Pp Pp White Parent F1 Offspring p Pp Pp

POINT > Use a Punnett Square to predict genotypes of offspring F1 offspring = Pp (heterozygous) = 100% purple flowers Remember Mendel’s second cross: F1 x F1 Pp x Pp Purple Parent P p P PP Pp Purple Parent F2 Offspring p Pp pp

POINT > Use a Punnett Square to predict genotypes of offspring F2 offspring = 25% PP (homozygous dominant) = 50% Pp (heterozygous) = 25% pp (homozygous recessive) = 75 % purple flower plants 25% white flower plants Purple Parent P p P PP Pp Purple Parent F2 Offspring p Pp pp

POINT > Use a Punnett Square to predict genotypes of offspring TT x tt Step One: Set Up Punnett Square (put one parent on the top and the other along the side) T T t

POINT > Use a Punnett Square to predict genotypes of offspring TT x tt Step Two: Complete the Punnett Square T T t Tt

POINT > Use a Punnett Square to predict genotypes of offspring TT x tt Step Three: Write the genotype and phenotype T T t Genotype: 100% Tt Phenotype: 100% Tall Tt Remember: Each box is 25%

POINT > Use a Punnett Square to predict genotypes of offspring Test ex. The allele for black berries in a plant is dominant (B) and the allele for red berries is recessive (b). What is the genotype of a heterozygous plant? What is the phenotype of a heterozygous plant? If a heterozygous plant is crossed with a plant with red berries, what percentage of offspring will have black berries? What percentage will have red berries? Bb Black 50% 50%

POINT > Describe codominance All of the traits Mendel studied were dominant or recessive (this is not always the case) Codominance is a situation in which both alleles contribute to the phenotype of the organism Ex. A white cow is crossed with a brown cow and the offspring are spotted brown and white x

CHECK: What is codominance?

Homework: Finish Study Guide Read pages 118-123 S.A. #1-3 GRAS pages 58-60