Understanding Genetics: Mendel's Discoveries and Inheritance Principles

1. Genetics – Chapter 10 Dr. Bill Stafford 2012

2. Genetics “Genetics explains why you should look like your father, and, if you don’t, why you should.

7. Genetics –Chapter 10.1 • Gregor Mendel – Austrian Monk – 1850 – “Father of genetics” • Studied garden pea plants – advantage for experimental study of genetics • They reproduced sexually • Had seven different traits that were visually identifiable • Grew quickly – short generation time • Could easily control which plants bred with which plants

10. Mendel stated that physical traits are inherited as “particles” Mendel did not know that the “particles” were actually Chromosomes & DNA Particulate Inheritance

11. Genetics – Chapter 10.1 • Plant reproductive parts • Male parts • Stamen – made up of anthers that produce pollen • Pollen – male gamete of the plant • Anther – part of the plant that produces pollen • Female parts • Pistil – has a stigma at the top to accept the pollen and the pollen is carried down to the ovary by a stem-like structure called a style • The ovule is the female gamete of the plant and is found in the ovary

13. Genetics – Chapter 10.1 • Plant reproduction • The pollen (male gamete) is deposited on the stigma and carried by the style down to the ovary where the pollen unites with the ovule (female gamete) – when they unite the process is called pollination • The united gametes form a zygote that is placed in a seed that will grow into a new plant • Mendel’s pea plants had closed petals and usually self-pollinated – Mendel had to do special things to get the plants to cross- pollinate

14. Genetics – Chapter 10.1 • Definitions: • Heredity – passing on of traits from parents to offspring • Genetics – branch of biology that studies heredity • Traits – visible characteristics that are inherited • True breeding plant (homozygous) – plants that all have the same characteristic for several generations and always produce offspring with that same trait • Hybrid (heterozygous) – a cross between true breeding parents that have different traits

15. Genetics –Chapter 10.1 • Mendel’s monohybrid cross • Monohybrid – the parents differed by only one trait – “mono = one” • P1 generation – Mendel cross-pollinated a true breeding short pea plant with a true breeding tall pea plant • All the offspring came out as tall as the tall parent – as if the short pea plant never existed – WOW!!!!!! • F1 generation – Mendel allowed all the tall plants produced from the P1 generation to self-p0linate

17. Genetics – Chapter 10.1 • Mendel’s monohybrid cross (cont.) • F2 generation – produced from the self-pollination of the tall plants in the F1 generation • The offspring from the all tall plants in the F1 generation were produced in a 3:1 ratio – ¾ were tall and ¼ were short • Amazingly, the short plants showed up again in the F2 generation from all tall plants in the F1 generation – WOW!!!!! • Mendel did similar monohybrid crosses with a total of seven different traits with the same results.

20. Genetics – Chapter 10.1 • Mendel’s studies • Rule of unit factors – Mendel concluded that there were two factors that controlled each trait • These factors are genes– a short segment of a chromosome that codes for a protein • Different gene forms for a given trait are called alleles • One gene or allele is inherited from each parent to give each offspring two alleles or genes for each trait

22. Genetics – Chapter 10.1 • Mendel’s studies • Rule of dominance– Mendel called the observed trait that resulted from the monohybrid cross in the P1 generation dominant and the trait that disappeared recessive • We now know that the tall allele is dominant to the short allele • Another way to say it is that the short allele is recessive to the tall allele • Use the same letter for both alleles for a trait and make it upper case for a dominant allele and lower case for a recessive allele

23. Designer “Genes” • Alleles - two forms of a gene (dominant & recessive) • Dominant - stronger of two genes expressed in the hybrid; represented byacapital letter (R) • Recessive - gene that shows up less often in a cross; represented by alowercase letter (r)

24. Genetics – Chapter 10.1 • Law of segregation – Mendelian principle explaining that because each plant has two alleles, it can produce two different types of gametes if the alleles are different. During fertilization, male and female gametes randomly unite to produce four combination of alleles. parents Tt x Tt offspring gametes T or t T or t  TT, Tt, Tt, or tt

25. Genetics – Chapter 10.1 • Phenotype – the way an organism looks or behaves – the trait you see - Tall • Genotype – the gene or allele combination an organism contains – TT or Tt • You cannot know genotype by looking at its phenotype – a dominant phenotype might be due to a genotype of TT or Tt - only one allele has to be dominant for the phenotype to show the dominant trait • The only way to get a recessive phenotype is with a genotype of tt – or both alleles have to be recessive

27. Genetics – Chapter 10.1 • Homozygous – an organism is homozygous for a trait if both alleles for that trait are the same – TT or tt – also known as true breeding • Heterozygous – an organism is heterozygous for a trait if the two alleles are different – Tt – also known as hybrid – the phenotype of a heterozygous organism will be the dominant trait

28. Genetics – Chapter 10.1 • Mendel’s dihybrid cross – a cross involving two different traits – “di = two” – hybrid = heterozygous for both traits – TtAa x TtAa • The question is will the two traits be inherited together or will they be inherited independently of each other? – the answer is independently of each other • The P1 generation was a cross of a true breeding parent that was dominant for both traits and a true breeding parent that was recessive for both traits • TTAA x ttaa

29. Genetics – Chapter 10.1 • Mendel’s dihybrid cross (cont.) • The F1 generation was a cross by self-pollination by the resulting offspring of the P1 generation – TtAa x TtAa • The F2 generation offspring results showed phenotypes in a ratio of 9:3:3:1 • This led to Mendel’s law of independent assortment – genes for different traits are inherited independently of each other – allows for a combination of traits in four different ways

32. Genetics – Chapter 10.1 • Punnett Squares – Reginald Punnett – 1905 • Law of segregation – fertilization occurs at random • If you know the genotypes of parents, you can determine the possible gametes produced by both parents and use that to predict the genotype of the offspring and their frequency • Monohybrid cross – Tt x Tt – both parents will produce gametes with a genotype of either T or t in equal amounts • After genotypes are determined 1:2:1, the phenotypes of the offspring can be determined – 3:1

33. Punnett Square Used to help solve genetics problems

34. Genetics – Chapter 10.1 • Punnett squares (cont.) • Dihybrid cross – TtAa x TtAa – both parents will produce the following gametes in equal amounts: TA, Ta, tA, ta which will go down the side and across the top of the Punnett square to make a sixteen square Punnett square. • Gives a genotypic ratio of 1:2:2:4:1:2:1:2:1 • Gives a phenotypic ratio of 9:3:3:1

36. Genetics – Chapter 10.1 • Probability – Punnett squares show the probability of a certain ratio of genotype or phenotype in offspring from a certain genotypic cross of two parents • This is due to the randomness of fertilization • An example is flipping a coin – a 50:50 chance of getting heads each time you flip the coin • In reality, the ratio is not 50:50 unless you flip it an infinite number of times • Same 50:50 chance to get a girl each time a couple has a baby – could get 4 girls and no boys