2. Mendel and the Gene Idea

3. A question… Mendel was wise (or lucky) in his choices of which characters of pea plants to follow from generation to generation. Why were his choices so good? Could Mendel have drawn the same conclusions if he had followed hair color in humans from generation to generation?

4. What Mendel did. Explained inheritance. Came up with two big ideas. The law of segregation and the law of independent assortment. What is being segregated and assorted independently? Any questions about the vocab (true- breeding, hybridization, the different generations, etc.)?

5. There are four big ideas that come out of Mendel’s model of inheritance: 1. Alternative versions of genes (alleles) account for variations in inherited characters (its in the DNA). 2. Two alleles are inherited for each character, one from each parent. 3. In heterozygotes, the dominant allele is always expressed over the recessive. 4. The two alleles segregate or separate during gamete production (go Punnett squares!). Thank you meiosis.

7. Check it out! That’s tongue folding baby!

8. Let’s do a Punnett square for Diana’s parents. Remember that Punnett squares show possible arrangements of alleles in the zygote. If Diana’s parents had hundreds of children, what would their Genotypic ratio be? Phenotypic ratio be? Without knowing it, Diana’s parents did a test cross for the tongue folding character. What do we mean by this?

9. The law of independent assortment The law of segregation applies when we talk about a single character (like tongue folding). If we only look at tongue folding, then Diana was born. But what if we looked at two characters together? Are they connected in some way or do their alleles “assort independent” of one another into separate gametes? Let’s see.

11. So, When Mendel came up with the idea that alleles assort independently of one another, he got lucky. Why was he lucky (think about where alleles are located)?

12. A question… You have the following cross – RR x rr. What is the likelihood that you will produce offspring with each of the following genotypes: RR? Rr? rr?

13. Mendel laws and probability Segregation and independent assortment explain heritable variation in terms of alleles that are passed along from one generation to the next, according to rules of probability. So we need to know some probability rules.

14. The rule of multiplication. Let’s use the rules of multiplication to see the probability of Diana’s tongue being able to do what it can do.

15. It’s an “and” or “or” thing… Use the rule of multiplication when “and” is what you are looking for (there is a probability of ½ of Diana getting a recessive allele from her dad and a probability of 1 for a recessive allele from mom). Use the rule of addition when “or” is an issue. Let’s say Diana’s parents were both heterozygotes - Tt – and we want to know the probability of Diana also being a heterozygote. We need to account for the possibility of each parent being able to contribute the dominant or recessive allele. Let’s work it out.

16. OK, now Mendel was really lucky. Usually more complicated than a character (like flower color) being determined by one gene with two variations (alleles). Many kinds of dominance ranging from complete (what Mendel observed) to codominance to incomplete dominance (look at those F2 individuals).

17. Incomplete dominance After hybridizing true- breeding parents, the F1 hybrids show an intermediate phenotype. The original colors show up again in the F2 generation (so it’s not blending because those original alleles are still present) In a heterozygous situation, some alleles just aren’t dominant enough.

18. Codominance is different. In codominance, both phenotypes are in present in equal amounts when two dominant alleles are inherited. So, in codominance you have only two alleles for a gene present in the species, but they’re both dominant and both expressed if inherited together.

19. The relationship between dominant and recessive for a given locus can vary depending on the level at which you study the individual Tay-Sachs disease Only homozygous recessives get the disease (suggesting complete dominance since heterozygotes appear healthy). But heterozygotes have enzyme activity levels that inbetween HD and hr individuals(suggesting incomplete dominance) Looking at enzyme concentration, the good one and the messed up one are present in equal amounts (codominance).

20. Then you got… Multiple alleles (ABO blood groups – where we also see that some of these alleles are codominant). How does this help in criminal investigations or for identification purposes? Pleiotropy (one gene can have multiple effects). How? Epistasis (a gene at one locus alters the phenotypic expression of a gene at another locus) Polygenic inheritance (multiple genes contributing to one phenotype like skin color)

21. For you visual learners

22. So, Gina, Brianna, do you feel you’ve accumulated phenotypic differences based on your unique experiences? Some phenotypes (and the genes responsible for them) are influenced by environmental factors, but some gene loci (like ABO blood groups) aren’t. If we observe a range of phenotypes, it is called the norm of reaction (most obvious for polygenic characters). Experience, physical activity, sunlight, nutrition, altitude are some factors that can cause norms of reaction.

23. Mendelian inheritance in humans Relevant for recessive and dominant genetic disorders. Recessive disorders can be an irritation (albinism) or fatal (tay-sachs, cystic fibrosis, sickle-cell disease). Dominant disorders are more rare. How are they perpetuated in the gene pool?