Gregor Mendel and His Peas

1. Gregor Mendel and His Peas Inheritance of Traits

2. Mendel’s discoveries: • He began with pea plants that produced only yellow or only green peas. They were known as “true breeding” and are now called “homozygous” or “purebred” for their trait—pea color. • He crossed them to create a hybrid pea. But instead of seeing yellow and green peas in the hybrid, only yellow was visible. • When he bred the yellow hybrids, the results were about 3 yellow peas for every 1 green pea. (His actual numbers were 6,022 yellow and 2,001 green…that’s a lot of peas)

3. From this, he concluded that there were strong and weak traits. Traits are distinguishing characteristics that can be inherited. • Mendel discovered that there were variations in genes that could be inherited. • Various forms of a single gene are called alleles. • And some alleles are dominant over others. • A dominant (strong) allele will express itself over a recessive (weak) allele—this is the Law of Dominance.

4. The Significance of Alleles • Mendel assigned the alleles letters: • Capital for dominant - ‘A’ • Lowercase for recessive - ‘a’ • Each individual has two alleles (one from each parent) • The alleles can be the same—AA or aa—known as homozygous • Or different—Aa—known as heterozygous

5. Punnett squares were invented to better visualize mathematic probabilities related to inheritance • Punnett Square: a diagram that shows the probability of inheriting traits from parents with certain genes.

6. A A a Aa Aa Aa a Aa Mendel’s results shown in a Punnett square In the case of the yellow and green peas: • Yellow, true breeding = AA • Green, true breeding = aa • You can look at two things: • Genotype: the actual alleles inherited, the genetic makeup (Aa) • Or Phenotype: the physical traits of the organism (yellow) Parents: yellow x green or AA x aa

7. A a A AA Aa aa a Aa Then he crossed the offspring (first filial, F1) generation to make a second (F2) generation: • Genotypes: • 25% homozygous dominant, AA • 50% heterozygous, Aa • 25% homozygous recessive, aa • Genotype ratio: 1:2:1 • Phenotypes: • 75% yellow • 25% green • Phenotype ratio: 3:1 • Parents = Aa x Aa

8. He then tested his test with purebred smooth and wrinkled peas: • First generation: all smooth • Second generation: 5,474 smooth and 1,850 wrinkled • Which trait is dominant? • Which is recessive? • Draw a Punnett square for the first and second generations (F1 and F2) using the letter ‘B’ to represent pea texture.

9. Mendel’s Conclusions • Up until Mendel’s discovery, the world thought that traits blended, becoming more diluted with time. • He explained that since traits are inherited separately, then traits can persist in a population for long periods of time, even if they are not seen.

10. Mendel’s Conclusions • He drew two other conclusions, which are known together as the Law of Segregation: • Organisms inherit two copies of each gene, one from each parent. • Organisms donate only one copy of each gene in their gametes. Thus, the two copies must separate, or segregate, during gamete formation.

11. But Mendel did not stop at crossing single traits. He crossed two traits at once—a dihybrid cross—crossing true breeding yellow, smooth pea plants (AABB) with true breeding green, wrinkled pea plants (aabb). • There are four ways to sort each of these sets of alleles, so the Punnett square will need to be a four by four grid. • What do you think the genotype and phenotype of the first filial generation will be?

12. AB Ab aB ab AB AABB AABb AaBB AaBb Ab AABb AAbb AaBb Aabb aB AaBB AaBb aaBB aaBb ab AaBb Aabb aaBb aabb But what if you cross the first generation offspring? • What are the four possible sets of alleles for a heterozygous yellow, smooth pea plant? • AB, Ab, aB, and ab 9:3:3:1 ratio

13. Law of Independent Assortment • Regardless of the combination of traits, Mendel continued to find a 9:3:3:1 ratio whenever he tested two traits at once. • From this he formed the Law of Independent Assortment. • It states that allele pairs separate independently of each other during meiosis. So they appear to be inherited separately.

14. Complex Inheritance Patterns Based on the Chromosome Theory of Inheritance: a basic principle that states genes are located on chromosomes and that behavior of chromosomes during meiosis accounts for inheritance patterns.

15. Incomplete Dominance • The heterozygous phenotype is somewhere between the homozygous phenotypes. • Neither allele is completely dominant or completely recessive.

16. Codominance • Both traits are fully and separately expressed. • Example: blood type • A, B, and O (also called IA, IB, and i) • Blood type is also an example of multiple alleles, having more than two alleles for one gene.

17. Baby 1: O Baby 2: AB Baby 3: A Make a list of the possible baby genotypes. Then make a Punnet square (or two) for each set of parents. Mr. and Mrs. Al Leel AB and AB Mr. Aaron and Mrs. Dina Haye O and AB Mr. and Mrs. Gene Pool B and O Who’s Your Daddy? Genetics

18. Polygenic Traits • A trait produced by two or more genes. • Examples: skin color, eye color • Occasionally, there is one gene that can overshadow all the others--known as epistasis. • Examples: Albinism

19. T.H. Morgan and his Flies

20. XX XY XX XY His team discovered sex determination. • More specifically they discovered that there were different chromosomes in males and females. They named them ‘X’ and ‘Y’ • Females always have XX • Males always have XY • This is why there is a 50/50 chance of having a male or female. X Y X X

21. XW Y XR XR XR Y XR XRXW XRY XRXR XRY XW XRXW XRY XRXW XWY Males with white eyes!!! And they discovered traits linked to the sex chromosomes. F1 Generation • Morgan’s lab discovered a white eyed male. • It was bred with a red eyed (normal) female. • All the children had red eyes. • In the second generation, 25% had white eyes, but they were ALL MALES. F2 Generation

22. Gene Linkage and Crossing-over • Flies have 4 pairs of genes. • As the team bred more and more flies, they found that some genes (like wing shape and leg bristles) were always inherited together. They proposed that the genes were “linked” on the same chromosome.

23. Gene Linkage and Crossing-over • But sometimes, even these linked genes were mixed up. This was the result of crossing over during meiosis. • It created new allele combinations—genetic diversity!

24. Genetic Disorders and Mutations

25. There are two main types of mutations • 1. Gene mutations—happen during DNA replications—A change to the original DNA sequence • 2. Chromosome mutations—happen during meiosis—Changes the number or location of genes

26. Gene Mutations • Point mutations: substitute one nucleotide for another • ATTACC AATACC • Frameshift mutations: the insertion or deletion of a nucleotide • ATTACC  ATACC (deletion) • ATTACC  ACTTACC (insertion) • These will affect all the codons that come after the insertion or deletion

27. Chromosome Mutations • Duplication changes the size of chromosomes and results in multiple copies of a single gene • Translocation: pieces of non-homologous chromosomes exchange segments

28. Genetic Disorders are coded in Disease Alleles created by mutations • Like all alleles, they can be: • Dominant • Recessive • Or sex-linked • That means that a person with a single dominant disease allele will have the disease! • We trace family histories of diseases in charts called pedigrees