Chapter 6 Notes

1. Chapter 6 Notes Sections 6.4, 6.5 & 6.6

2. Section 6.4- Traits, Genes, Alleles • Mendel’s “discrete units” are now called genes. • Genes are pieces of DNA that provides a set of instructions to a cell to make a certain protein. • Most genes exist in many forms • When looking at Mendel’s experiments this is evident: • Yellow or green seed color • Tall of short plant height

3. Section 6.4- Traits, Genes, Alleles • Any alternative form of a gene is an allele and we represent these alleles with letters. • T = tall • t= short • Every trait has 2 genes (alleles), one from mom and one from dad. • When both of the alleles are the same (TT) or (tt), we call this a homozygous condition. • When the alleles are different (Tt), we call this a heterozygous condition.

4. Section 6.4- Traits, Genes, Alleles • When we talk about an organism’s gene make-up, we are talking about its genotype. • While both (TT) or (Tt) plants would be tall, they have a different genotype. • This refers to the actual alleles that make-up a trait.

5. Section 6.4- Traits, Genes, Alleles • When we talk about an organisms physical appearance, we are talking about its’ phenotype. • In the previous example, both TT and Tt would be tall, so we don’t care that the alleles are different • We only care about the physical appearance when talking about phenotype.

6. Section 6.4- Traits, Genes, Alleles • In Mendel’s studies, he found that when a trait was in the heterozygous condition, the phenotype of the plant showed only one of the two traits • NOT BOTH AND NOT A MIXTURE OF BOTH!

7. Section 6.4- Traits, Genes, Alleles • Mendel quickly learned from his crosses that certain genes masked other genes. • He called genes that masked other genes dominant genes. • The genes that are being masked are the recessive genes.

8. Section 6.4- Traits, Genes, Alleles • Remember- dominate alleles are not necessarily better or occur more. It simply means when 2 different alleles are together, one masks the other.

9. Section 6.4- Traits, Genes, Alleles • We use an uppercase letter for dominant traits and a lowercase letter for recessive traits. • Because some alleles are dominant over others, 2 different genotypes can produce the same phenotype! • TT= tall and Tt =tall

10. Section 6.4- Traits, Genes, Alleles • As you know not all plants are tall or short or just have purple or white flowers. Environmental factors like water and sun affect plants as well.

11. Section 6.5: Traits and Probability • Mendel soon concluded after some of his initial findings that through probability laws he could determine possible outcomes. • A scientist by the name of R.C. Punnett took some of Mendel’s work and developed a grid system to figure possible outcomes. • The punnett square is a grid system used to predict all possible genotypes and phenotypes resulting from a cross.

12. Section 6.5: Traits and Probability • By using this grid system, possible outcomes of certain crosses can be predicted. • Remember the punnett square is a prediction of possible outcomes, it is not what will actually happen.

13. Section 6.5: Traits and Probability • The first type of cross is called a monohybrid cross or one-factor cross. • This deals with only one trait!! • The traits can be either homozygous or heterozygous. • Remember, every trait ALWAYS has 2 genes, you get one gene from mom and one gene from dad.

14. Section 6.5: Traits and Probability • When doing actual crosses, it is important to know what type of organism you are starting with. • If you don’t know the genotype of an organism, you can do a test cross • A test cross involves crossing an organism with an unknown genotype with a recessive organism.

15. Section 6.5: Traits and Probability • If you cross an unknown plant with a recessive plant and get offspring that are recessive, than you know your unknown is hybrid!

16. Section 6.5: Traits and Probability • Dihybrid crossesinvolve 2 traits! • Up until now we have dealt with crosses involving one trait. • You can, however, cross two traits at a time- this is called a dihybrid cross.

17. Section 6.5: Traits and Probability • Mendel did dihybrid crosses and wondered: • “would traits always stick together or if they would express themselves independently?” • In other words- Mendel wanted to know if you took a plant that was both tall and axial and crossed it with a short, terminal plant would you only get offspring that look like the parents, or would you get recombination (tall & terminal) and (short & axial).

18. Section 6.5: Traits and Probability • When Mendel did his first dihybrid cross, his results were similar to the results he got in his first monohybrid crosses • Crossing a purbred (Tall, Axial) x (Short, Terminal) = ALL tall, axial • He then let the F1 fertilize themselves (TtAa x TtAa) • His results showed that all possible combinations are possible • Tall, axial • Short, axial • Tall, terminal • Short, terminal

19. Section 6.5: Traits and Probability • This led him to his 2nd law which is the Law of Independent assortment. • This law states that given 2 traits each of these traits separate independently so all combinations are possible. • Traits don’t stick together!

20. Section 6.5: Traits and Probability • After years of study, Mendel started to figure out that genetics has a probability component to it. • Probability is the likelihood that a particular event will occur • Probability= # of desirable outcomes/total # of all possible outcomes

21. Section 6.5: Traits and Probability • This type of probability can be applied to gamete formation. • Suppose you have a plant hybrid for height (Tt). • The likelihood of a (T) is 1 over the total possible outcomes (T) & (t)is two • So probability = ½

22. Section 6.6: Variation • Genetic variation is the essence of all life. • Sexual reproduction allows for different genetic variations daily. • When chromosomes line up in the middle during metaphase I of meiosis, there is about 8 million different combinations that can be formed.

23. Section 6.6: Variation • Since an egg & sperm combine, the possible different combinations increase to about 70 trillion • In other words: • A human couple can produce a child with one of about 70,000,000,000 different combinations of genes!

24. Section 6.6: Variation • Genetic diversity is affected by other things as well. • A lot of variation comes from a process called crossing-over • This is when chromosomes exchange parts during metaphase I of meiosis.

25. Section 6.6: Variation • As you can see, genes have exchanged places on like chromosomes. • What this does is increase the number of different combinations possible now that genes have crossed-over. • What Mendel didn’t know was that some genes are on the same chromosomes! • How will that affect crosses??? • FIND OUT NEXT CHAPTER!!!!