Genetics

1. Genetics “The real key to explaining Inheritance”

2. Objectives To understand the experiments and conclusions of Gregor Mendel To compare Monohybrid and Dihybrid crosses To compare Complete, Incomplete and Co-dominance To understand how Multiple Alleles work

3. Gregor Mendel • Austrian Monk in the 1800’s • Experimented with pea plants • Kept detailed records of several characteristics for generations • Height • Flower color • Pod color • Shape of peas and pods • Position of flowers on the plant

4. Mendel’s Observations • If he had tall plants in one section of the garden, they always produced tall offspring • Similarly if he had short plants in another section the same would occur (short offspring) • He called these plants Pure Lines • Tall x Tall = Tall • Short x Short = Short • Mendel's Experiment

5. Mendel’s Experiment He decided to cross (breed) these pure lines Pure Line Tall x Pure Line Short The result was ALL tall plants He decided to cross these plants with each other To his surprise, this resulted in both tall and short plants The ratio was exactly 3 to 1 (tall to short)

6. Mendel’s Conclusions(4) • All characteristics are controlled by factors inherited from your parents • There are two factors for every trait (characteristic) • These traits can be hair color, length of nose, skin tone, height, etc. • One factor can dominate another • ‘Mask’ the other • These factors separate from each other during sex cell formation (meiosis)

7. Modernizing Mendel’s Conclusions • We now call these ‘factors’ Alleles • Alleles can be Dominant or Recessive • We represent Alleles with letters: • Capitals = Dominant Allele • Lowercase = Recessive Allele • *Dominant Alleles always go first • Genotype is the combination of alleles you have • Phenotype is the expressed trait • Homozygous – 2 alleles that are the same. • Heterozygous – 2 alleles that are different

8. Types of Genotypes Genotypes can also be expressed in words ‘Homo’ – Latin prefix for ‘the same’

9. The Punnett Square • He then took those hybrids (Tt) and crossed them with one another Mendel started his experiments crossing a pure line of tall plants (TT) with a pure line of short plants (tt)

10. Sample Problem # 1 • What genotype (in words) can we expect from a homozygous dominant, homozygous recessive cross? • ALL offspring will be heterozygous Widow’s peak is dominant to straight hairline A homozygous woman with widow’s peak reproduces with a man with a homozygous man with straight hairline What hairline will their children (F1) have?

11. Sample # 1 Continued Now if an offspring of that cross reproduces with another of the same genotype, what hairlines can we expect for their children? What genotype(s) (in words) can we expect from a cross such as this? What ratio of phenotype?

12. Our Findings • Homozygous dominant crossed with homozygous recessive yields heterozygous offspring (100%) • A heterozygous crossed with heterozygous yields three options: • Homozygous Dominant • Heterozygous • Homozygous Recessive • The ratio of those options are: • A genotype Ratio of 1:2:1 (HD:H:HR) • A phenotype Ratio of 3:1 (Dominant Trait : Recessive)

13. Sample Problem # 2 Attached earlobes are recessive to un-attached (hanging) lobes A homozygous dominant man (for the trait) mates with a heterozygous woman (for same trait) What are the chances of their offspring having attached earlobes? The chances of their child having a child with attached earlobes is 0%, 0 in 4, 0/4

14. Let’s follow Mendel’s Experiment • P1 – • F1 – • F2 –

15. Let’s try some problems • 1. Genetics Practice Problems • 2. Bikini Bottom Genetics

16. Variations in Dominance All of the eight traits Mendel looked at had two alleles (dominant & recessive) This ‘all-or-nothing’ expression is known as complete dominance Another form is incomplete dominance This is where neither of the alleles can be considered dominant or recessive The phenotype is a mixing of the two different alleles The classic case is with flowers Red flowers crossed with white flowers gives us pink flowers

17. Incomplete Dominance In order to represent these alleles we need to make a change Since we’re talking about color we’ll use the letter C For each color we add the capital superscript to the allele Capital designation always indicates Incomplete Dominance Therefore: CR = Red Allele CW = White Allele What the genotypes mean: CR CR = Red CW CW = White CR CW = Pink CW CR = Pink

18. Codominance with Flowers With codominance neither allele is dominant, they both show through Lets look at flower color In codominance a plant with red flowers mating with one having white flowers produces a plant with what? Both red & white flowers So again we write the alleles differently Lower case designation indicates Codominance For flower color: Cr Cr Red Flowers Cw Cw = White Flowers Cr Cw = Red & White Flowers Lets cross a homozygous red plant with a heterozygous one What are the chances of our baby plant having red & white flowers?

19. Multiple Alleles • Some traits are controlled by multiple alleles • It still only takes two alleles to make a gene • However there are more than two options for alleles • For example, the eye color of fruit flies have four options for alleles • E1 = Red • E2 = Apricot • E3 = Honey • E4 = White • Each allele is dominant to the remaining • E1 is dominant to E2, E3, & E4 • E2 is dominant to E3 & E4 • E3 is dominant to E4 • E4 is the recessive allele • Options for genes would be: • E1 E4 = Red Eyes • E2 E3 = Apricot • E4 E4 = White • Examples of Multiple Alleles: • Blood Types • Types of Corn • Feather Color (in some cases)

20. Let’s try a Multiple Allele Cross

21. Monohybrids & Dihybrids • Everything we talked about so far have been examples of monhybrid crosses • Monohybrids – crosses that are looking at one set of alleles • We can also do dihybrid crosses • Dihybrids – crosses that are looking at two sets of alleles • The following are two examples • A monohybrid cross • A dihybrid cross • The color is just coding for different phenotypes

22. Independent Assortment

23. Dihybrid Example • In peas plants tall is dominant to short and purple flowers are dominant to white • So if a pea plant is heterozygous for height and heterozygous for flower color what is their genotype? • TtPp • During the creation of sex cells (meiosis) there are four combinations of alleles the gamete (sex cell) could hold: • TP (Dominant & Dominant alleles) • Tp (Dominant & Recessive alleles) • tP (Recessive & Dominant alleles) • tp (Two recessive alleles) • If a plant that is heterozygous for both traits reproduced with a plant having the same genotype, what might the phenotype of their offspring be?

24. Dihybrid Cross Summary If we look at the combinations of phenotypes possible from a dihybrid cross between two heterozygous organisms for two traits we find a ratio of = 9:3:3:1 9 = Tall plants with purple flowers 3 = Tall Plants with white flowers 3 = Short Plants with purple flowers 1 = Short Plant with white flowers

25. Test Cross

26. Dihybrid Cross Practice • Dihybrid Cross Problems • Guinea Pig Activity • Mendel's Peas and the Pedigree Chart