Today is Monday, December 9 th , 2013

1. In This Lesson: Punnett Squares and Monohybrid Crosses (Lesson 2 of 6) Today is Monday,December 9th, 2013 Pre-Class Predict the possible genotypes and phenotypes from the following cross: AA xaa In this situation, A is dominant for normal skin, while a is recessive for albinism (white skin/hair). Other stuff: Turn in your Making Babies Activity.

2. Today’s Agenda • Further explore the idea of dominant/recessive alleles. • Learn how to do problems like the pre-class MUCH faster. • RapidTriviaKinda • Where is this found in my book? • Academic: P. 267 and following… • Honors: P. 156 and following…

3. First, Mouse Genetics • ExploreLearning has a great activity that clearly illustrates how dominant and recessive genes interact. • In the activity, you’ll be pairing different-colored mice and seeing how the offspring turn out. • You can pair two purebred starter individuals, or you can put the offspring of one cross into holding cages and breed them later. • There is an accompanying Quia quiz. Also complete the questions right under the gizmo. • [Log-in Instructions]

4. Back to Mendel • Mendel established two true-breedingpopulations of pea plants (the parents). • True breeding means they only produce their own phenotypes (homozygous genotypes). • The population with purple flowers produced only purple flowers, the population with white flowers produced only white flowers. • When Mendel crossed a purple parent flower with a white parent flower, he only got purple offspring plants (the first generation). • Which allele is dominant?

5. But then… • Mendel let the first generation plants self-pollinate to form the SECOND GENERATION. • Now here’s the drama: Out of 929 total second generation plants (he had a lot of free time, remember?), 705 were purple, and 224 were white!!!!!11

6. In other words… • Parents: • All purple and all white (true-breeding/homozygous). • First Generation: • Offspring of parents ↑ is all purple. • Second Generation: • Offspring of first generation ↑ is mostly purple, some white. • What gives?

7. Look Closer at the Numbers

8. So what’s the ratio, then? • The ratio of purple flowers to white flowers is 3:1 • This number is so cool I wasted a whole slide for it.

9. Why a genetic ratio is important… • For Mendel, 3:1 was just a cool number. He used it to formulate four hypotheses that were further developed by later scientists: • For each trait, each individual has two copies of the gene. • There is more than one allele for these genes. • When two alleles are put together, one may be expressed, while the other may not have any visible impact on the organism’s phenotype. • Since gametes only have one set of chromosomes, they only have one allele each. Therefore, each gamete contributes one allele during fertilization.

10. Back to Mendel • As it turned out, doing that sort of cross Mendel did (in the same order) will always produce roughly the same numbers of purple/white plants, even with different total numbers of plants. • Scientists wanted a way to predict these numbers, and to do it easily. • Their solution?

11. Reginald Punnett *Probably not true The Punnett Square • Meet Reginald Punnett: • British Biologist • June 20, 1875 - January 3, 1967 • Champion foosball player* • Created the Punnett Square, amongst other cool discoveries. http://www.nndb.com/people/167/000100864/reginald-punnett-1.jpg

12. The Punnett Square • The Punnett Square is a tool for: • Determining which possible genotypes and phenotypes a cross can produce. • Determining the probability a certain genotype or phenotype will occur.

13. The Punnett Square • The Punnett Square cannot: • Predict the order of offspring and their traits. • Predict exact numbers (remember, the pea plants weren’t exactly 75/25). • Account for mutations.

14. How The Square Works • Mendel, monk that he was, didn’t know about alleles. We do, so let’s use them. • Let’s set our allele letters [write this down] • F = Purple Flowers • f = White Flowers • I’m using F instead of P because it’s easier to tell lower/upper case Fs apart.

15. How The Square Works • Mendel’s “parent generation” was true-breeding, so they were probably homozygous. • If that’s the case, let’s cross two homozygous individuals: one purple, one white. • FF x ff • By the way, that ___ x ___ notation is the way to write a cross. I’m going to do that a lot. • One last thing: this cross is a monohybrid cross. • Monohybrid crosses are those that analyze only one trait. • We’re not looking at flower color and height – that would be different.

16. FF x ff • To analyze this cross with a Punnett Square, first draw a box and divide it in fourths. • Then, put one parent’s genotype across the top and one across the side (doesn’t matter which). F F f f

17. FF x ff • Put arrows in like so. • Now, “drag” each allele in the direction of the arrows. • What color are the offspring? • F = Dominant allele, purple flowers • f = Recessive allele, white flowers F F F F F F ↓ ↓ f f f Purple Purple → f f f Purple Purple →

18. Mendel’s Parent Generation • Remember that Mendel then took two of his resulting plants from the first generation and crossed them to make his second generation generation. • In other words, he took two heterozygous individuals. • Now we will take two heterozygous individuals from the first generation and cross them.

19. F = Dominant allele, purple flowers • f = Recessive allele, white flowers Ff x Ff • What color are the possible offspring? f f f F F F ↓ ↓ F F F Purple Purple → f f f Purple White →

20. Statistics • Percent Chance of Purple Flowers • 75% • Percent Chance of White Flowers • 25% • Phenotype Ratio • Dominant : Recessive • 3:1 • Genotype Ratio • Hom. Dom. : Heterozygous :Hom. Rec. • 1:2:1

21. One BIG caveat… • Remember 3:1? That magic ratio Mendel found? • The 3:1 phenotype ratio only happens when you cross two heterozygous individuals for a trait that exhibits complete dominance. • Try crossing Ff x ff. What’s the phenotype ratio? • 2:2 (50% Purple, 50% White) • What about FF x ff? What’s the phenotype ratio? • 4:0 (100% Purple)

22. F = Purple; f = White • Using our example cross, solve this problem in your notebook: • I’m a plant with purple flowers (heterozygous) and my plant girlfriend (she’s got white flowers) have had 3 purple-floweredbabies. • What is the probability our fourth baby will have white flowers? • Answer: 50%. • Remember, PunnettSquares only lead you to probability, not the order of children or any kind of guarantee.

23. F = Purple; f = White • Here’s another example (same genotypes): • Whew! It’s now the next season and my new plant girlfriend and I (plants aren’t monogamous) have had a busy spring. We’ve now had 25 plant babies, all with purple flowers! • What is the likelihood our 26th will have white flowers? • Answer: 50%. • No matter how many offspring, as long as the parents have the same genotypes, this number will not change.

24. Punnett Squares Backward? • How about we try a backwards Punnett Square problem? • Let’s imagine two parents. One parent has brown eyes, one parent has blue eyes. • B = Brown, b = Blue • They have a child with blue eyes. • What are the parents’ genotypes? • Bb and bb

25. Hey wait a second… • Remember our guiding questions? • Suggested by a former student: • “My Dad has blue eyes. My Mom has brown eyes. My siblings and I all have brown eyes. How did this happen?” • Dad: bb; Mom: BB or Bb (likely BB) • Suggested by my former teacher: • “My husband and I both have brown eyes. Our children have blue eyes. How?” • Husband: Bb; Wife: Bb

26. Last note… • Notice how some people could have the allele for a certain trait but not show it? • That’s called being a carrier.

27. Closure and Practice • Your assignment is to practice monohybrid crossing. I will be grading these problems closely. • Genetics Worksheets packet page 99, #19-21 ONLY.