Meiosis and Punnett Square Notes

Primary Learning Objectives To understand the differences between meiosis and mitosis. To be able to use a Punnett square to predict two parents’ offsprings’ phenotypic and genotypic percentages for one trait.

Chromosomes Where are they found? Why are they important?

Chromosomes Found in the nucleus Important because they contain the directions for making proteins. What are Proteins? And why are Proteins so important?

Proteins The building blocks of all organisms. Most cell organelles are made up mostly of proteins.

Proteins The human body contains more than 100,000 different types of protein, each with its own function. The type of proteins that are produced within an organism will determine 1. what an organism will look like 2. how it will behave

Back to the Chromosomes Chromosomes contain the directions for which and how many proteins should be produced. This information changes from organism to organism. Therefore different organisms have different numbers of chromosomes.

Each type of organism (species) has a set number of chromosomes Fruit Flies have 8 chromosomes Humans have 46 chromosomes Potatoes have 48 chromosomes Note: The number of chromosomes is NOT dependent on the complexity of the organism.

The Structure of Chromosomes are made of DNA

DNA DNA stores the information for making proteins in a pattern, much like a book stores information in a pattern of words and letters.

An Analogy comparing Chromosomes and DNA Chromosomes would be like a library and the sections of DNA, called genes, would be the books inside the library containing information for building proteins.

Reproduction When an organism reproduces it has to pass on the information about how to build proteins In other words the chromosomes must be duplicated and passed on to the next generation of organisms. There are two ways in which this is done.

Two Types of Reproduction Asexual Sexual We have learned about one type of reproduction already!

Asexual Reproduction Mitosis is one type of asexual reproduction Mitosis is a form of asexual reproduction because offspring have DNA that is identical to their parents. In asexual reproduction there is only one parent.

Mitosis is One Example of Asexual Reproduction Offspring Parent Cell One Parent cell produces 2 identical offspring cells

Sexual reproduction usually involves two different parents. Sexual reproduction involves the combining of DNA from two separate cells to begin a new organism.

Sexual Reproduction The two cells that merge during sexual reproduction are special sex cells called GAMETES. In most living things, gametes come in two different forms: Male and Female.

Sexual Reproduction GAMETES Egg and sperm cells are gametes. The merging of two gametes is called FERTILIZATION. The result of fertilization is a ZYGOTE.

Sexual Reproduction   Sperm Fertilization Gametes Egg Zygote Gametes have ½ the normal number of chromosomes. When fertilization occurs, the resulting zygote has a complete set of chromosomes to make a new organism!

Sexual Reproduction Gametes DO NOT divide by the process of mitosis. IF an egg cell contained 46 chromosomes and a sperm cell contained 46 chromosomes then the new offspring would have 92 chromosomes. THAT’S TOO MUCH INFORMATION!!

Sexual Reproduction Humans gametes (egg & sperm cells) contain only 23 chromosomes instead of the normal 46. HOW DID THAT HAPPEN?

Sexual Reproduction Instead of going through mitosis (Where the same number of chromosomes are produced) Gametes go through the process of MEIOSIS (Where ½ the number of chromosomes are produced)

Meiosis

Mitosis Meiosis

Punnett Squares

Consider a plant with the following phenotype and genotype. Phenotype: red flowers Genotype: RR (pure dominant) If a sperm (pollen) from this plant fertilizes (pollinates) an egg (ovule from a plant that has Phenotype: white flowers Genotype: rr (pure recessive) What are the possible phenotypes and genotypes of the offspring?

Genotypes: Rr Phenotypes: Red flowers

Male: phenotype: red flowers genotype: Rr (hybrid) What if… Male: phenotype: red flowers genotype: Rr (hybrid) Fertilizes (pollinates) Female: phenotype: red flowers What are the possible phenotypes and genotypes of the offspring?

Genotypes: RR, Rr, rr Phenotypes: Red or White flowers

There is an easy way to determine the answers for the last two questions. To make it easier to answer questions that ask about predicting the possible phenotype and genotypes of offspring you can use what is called a Punnett square. Let’s use the second question to show you how much easier it can be.

Male: phenotype: red flowers genotype: Rr (hybrid) What if… Male: phenotype: red flowers genotype: Rr (hybrid) Fertilizes (pollinates) Female: phenotype: red flowers What are the possible phenotypes and genotypes of the offspring?

A Punnett square looks like the following. Possible Female Genes R r Possible R Male Genes r

Now you can line up the possible combinations of genes. Let’s take a look at how the answers in each box are determined. Possible Female Genes R r Possible R Male Genes r

Now looking at the Punnett square it is easier to determine the possible genotypes and phenotypes. Possible Female Genes R r Possible R Male Genes r RR Rr rr

Genotypes: RR, Rr, rr Phenotypes: Red or White flowers

Using a Punnett square you can also predict what percentage (%) of offspring will have which phenotypes and genotypes.

What percentage of the offspring would be pure dominate, pure recessive, and hybrid? RR Rr rr

What percentage of the offspring would be red flowers and white flowers? RR Rr rr

Punnett Square Problem Solving Today we will be predicting the genotypic and phenotypic outcomes of hypothetical mice breeding.

Problem 1 What would the possible phenotypic and genotypic outcomes be of the offspring if two black mice, both with the genotype (BB) were mated (crossed)?

Problem 2 What would the possible phenotypic and genotypic outcomes of the offspring be if a pure dominant black mouse was crossed with a hybrid?

Problem 2, Part II What is the phenotype of the hybrid? What percentage of offspring would you predict to be pure dominant? What percentage of offspring would you predict to be pure recessive? What percentage of offspring would you predict to be hybrid?

Problem 2, Part III What percentage of the offspring would you predict to be black? What percentage of the offspring would you predict to be white?

Problem 3 Create a Punnett square to determine the phenotypic percentage of black and white offspring mice if a pure dominant mouse is mated with a pure recessive mouse. Part II Determine the genotypic percentage of offspring that will be pure dominant, pure recessive and hybrid.

Problem 4 Predict the phenotypic and genotypic percentages of offspring if a hybrid mouse if crossed with a pure recessive mouse.

Problem 5 Predict the phenotypic and genotypic percentages of offspring if two hybrid mice are crossed.