Mendelian Genetics

1. Hopefully a Review Mendelian Genetics

2. Gregor Mendel • German/Austrian monk in the mid 1800s • Father of genetics, heredity • Mendel's Laws • Tested the properties of inheritance using peas

3. Heredity Vocabulary • A character is a heritable feature, i.e. eye color • A gene codes for a character • A trait is a specific variant, i.e. brown eyes • An allele codes for a specific trait • P Generation (original parent generation) • F1 – first set of offspring • F2 – second set of offspring (produced by F1 generation)

4. Either-Or Traits • Mendel studied either-or traits, such as flower color • Flowers were either purple or white • Continuums (i.e. height) are usually caused by multiple genes

5. Mendel's Study • Mendel Crossed White and Purple flowers • All of the F1 generation were purple (not an intermediate) • F2 generation was 3/4th purple, 1/4th white

6. Mendel's Reasoning • The heritable factor for white flowers was present in the F1 generation, but not expressed • The heritable factor for purple flowers must dominate the heritable factor for white flowers

7. The Scientific Process • Mendel performed nearly a thousand trials on flower color • Also observed the same patterns in 6 other characters • Found similar results

8. Mendel's Hypotheses • Different versions of genes (alleles) account for variations in inherited characters • For each character, an organism inherits one allele from its mother, one from its father a) (Mendel made this assumption with no knowledge of chromosomes!) • If the two alleles are different one will be dominant and fully expressed, while the other will have no effect on the individual's appearance

9. Alleles • Alleles are the specific genes coding for a particular trait

10. Mendel's Hypotheses con. 4) The two alleles for each character separate when gametes are created (Mendel's law of segregation) • Mendel made this claim before chromosomes were discovered and long before meiosis/mitosis was understood • But he was mostly right, we do inherit one set of alleles from each parent

12. Dominant and Recessive Features Dominant Recessive Exhibited if both of a person’s genes are the recessiveallele A recessive trait can only be expressed with 2 recessive (0 dominant) alleles • Exhibited if either of a person’s genes are the dominant allele • Individuals can have 1 or 2 dominant alleles, in most cases it doesn’t matter

13. Punnett Squares • Let B code for the allele for Purple flowers and b code for the allele for white flowers • The original purple flowers were all BB (both alleles were the same) • The original white flowers MUST HAVE BEEN bb (the only way they could be white)

14. The F1 Cross

15. The F2 Generation Punnet Square

16. Phenotype vs. Genotype • The phenotype of an organism is its appearance for that character • The genotype is the genetic make up • Bb or BB bb

17. Testcross • We can determine the genotype of an organism by crossing (mating) it with an individual with recessive alleles • If the offspring are a mix of phenotypes, then the organism was heterozygous • If the offspring all exhibit the dominant pattern, then the organism was homozygous dominant

18. Dihybrid Crosses • What if we cross individuals differing in 2 characters? • Will the dominant alleles stick together and the recessive alleles stick together?

19. NO!!!! Alleles Sort Independently • Each allele is inherited independently (unless they are on the same chromosome) • RrYy x RrYy leads to 9:3:3:1 pattern

20. Probability/ The Multiplication Rule • The odds of 2 events both happening = the odds of one times the odds of the other • I.e. the odds of pulling a red jack out of a deck = 1/2 * 1/13 = 1/26 (or 2/52) • The odds of rolling 3 6’s in a row = 1/6 * 1/6 * 1/6 = 1/216

21. The Multiplication Rule • Assume heterozygous parents • Probability of both alleles ending up recessive = probability of receiving allele from mother * probability of receiving allele from father= ½ * ½ = ¼

22. Multiplication Rule • Assume parents are YyRr • The odds of giving a recessive trait are ½ for each character • The odds of a parent giving 2 recessive alleles (yr) is ¼ (1/2 * ½) • Thus the odds of offspring ending up with 2 recessive traits from both parents is ¼ * ¼ = 1/16

23. The Addition Rule • The odds of either of 2 events occurring is found by adding them • I.e. the odds of rolling a 1 or a 6 = 1/6 + 1/6 = 1/3 • The odds of pulling an Ace or a King = 1/13 + 1/13 = 2/13

24. The Rule of Addition • There are two possibilities to end up heterozygous with heterozygous parents • To find the probability of ending up heterozygous you add the individual odds ¼ + ¼ = ½

25. The other method • Make a dihybrid or trihybrid cross • List every possible combination of alleles each parent could give • So for 2 MmNn individuals, you would have:

26. Calculating the Probability • Imagine a tri-hybrid cross, with two parents who are MmNnOo. What are the odds of ending up recessive in all 3 characters? • Chances of ending up recessive for each character individually = ¼ • so the odds of ending up recessive in 3 is ¼ * ¼ * ¼ = 1/64

27. As a Group Calculate…(assume MmNnOo parents) • What are the odds of ending up MmNnOo • MMNNoo • MmNNOo • Expressing the dominant trait for M and N but recessive trait for o • Dominant in exactly 2 of the 3 traits

28. Quick Practice • A mother is AaBBDdee. What possible alleles can her eggs have? • Two AaBb individuals mate. What are the odds their offspring are homozygous dominant in both? • An AAbbDD man mates with a woman heterozygous in all 3 traits. What are the odds an offspring has the same genotype as one of their parents? • An AABBDDEEFFGG man mates with an aabbddeeffgg woman. What % of their offspring will be heterozygous?

29. Review • Some alleles dominate others • We inherit one allele from each parent • Alleles are inherited separately • Probability is observed in the long run

30. The Relationship Between Genotype and Phenotype is Rarely So Simple! • Mendel was lucky, he chose characters controlled by 1 gene, where 1 trait is completely dominant • Things are not always so simple • Doesn't mean Mendel is wrong!

31. Incomplete Dominance • For some characters, heterozygous individuals exhibit phenotypes that are a mix of the 2 alleles • i.e. red and white flowers can produce pink flowers • Pink flowers produce red, white and pink offspring • Not blending!

32. Codominance/ Multiple Alleles • The two alleles can both be expressed at the same time • i.e. Blood Types • A and B are dominant to O (3 alleles) • A and B are codominant

33. Blood Types • AA or AO individuals produce A antigens • BB or BO individuals produce B antigens • AB produce both • OO produce none • Individuals cannot receive blood with A or B antigens unless their blood produces them

34. 3 Important Points About Dominance • Ranges from complete dominance to incomplete dominance to codominance • The dominant allele does not suppress or interact with the other allele. The recessive allele simply isn't expressed by the cells • A trait being dominant does not necessarily mean it is more common in the population

35. Dominant but Rare Traits • 6 fingers • Dwarfism • Widow’s Peak • Dimples • Cleft chin • Ear wiggling • Hair on middle of fingers

36. Pedigrees • Obviously in humans we can't choose who breeds with whom • Mendelian patterns are observed using pedigrees • Geneticists collect information about a specific trait from a family

37. How to Read Pedigree Charts • Squares represent males, circles represent females • A line represents mating • Shading represents that the trait is present

38. How Can You Spot a Recessive Trait? • If it appears in offspring but neither parent had it • Both parents must have been carriers ff Ff Ff Ff Ff ff Ff ff Ff FF or Ff

39. How Can you Spot a dominant trait? • If two parents have the trait, but the offspring does not! • The parents must be heterozygous and have a recessive offspring Ff Ff Ff ff ff Ff Ff Ff Ff ff ff ff ff Ff