Mendel and Meiosis

1. Mendel and Meiosis 6.3-6.6

2. DNA, Genes, Traits DNA • Deoxyribonucleic Acid • The major nucleic acid in organisms • Carries genetic information and is responsible for the transmission of traits. Gene • A segment of DNA that codes for a specific trait in an organism Trait • A characteristic that can be passed from parent to offspring

3. Gregor Mendel • Father of modern genetics • Researched genetics with pea plants • Researched with pea plants • Developed ideas of inheritance and traits

4. Mendel laid the groundwork for genetics. • Traits are distinguishing characteristics that are inherited. • Genetics is the study of biological inheritance patterns and variation. • Mendel showed that traits are inherited as discrete units. • Many in Mendel’s day thought traits were blended.

5. Mendel’s data revealed patterns of inheritance. • Mendel made three key decisions in his experiments. • use of purebred plants • control over breeding • observation of seven“either-or” traits

6. Mendelian Genetics • Mendel studied a number of characteristics in pea plants including: • Height - short or TALL • Seed color - green or YELLOW • Seed shape - wrinkled or ROUND • Seed coat color - white or GRAY • Pod shape - constricted or SMOOTH • Pod color - yellow or GREEN • Flower position - terminal or AXIAL

7. Mendel controlled the fertilization of his pea plants by removing the male parts, or stamens. He then fertilized the female part, or pistil, with pollen from a different pea plant. Mendel and Heredity • 1. Mendel used pollen to fertilize selected pea plants. • P (parent) generation crossed to produce F1(filial) generation • interrupted the self-pollination process by removing male flower parts

8. Mendel and Heredity • 2. Mendel allowed the resulting plants to self-pollinate. • Among the F1 generation, all plants had purple flowers • F1 plants are all heterozygous • Among the F2 generation, some plants had purple flowers and some had white

9. purple white Mendel and Heredity • 3. Mendel drew three important conclusions. • Traits are inherited as discrete units. • Organisms inherit two copies of each gene, one from each parent. • The two copies segregateduring gamete formation. • The last two conclusions arecalled the law of segregation.

10. Traits Genes and Alleles • A gene is a piece of DNA that directs a cell to make a certain protein. • The same gene can have many versions. • Each gene has a locus, a specific position on a pair ofhomologous chromosomes.

11. Traits Genes and Alleles • An allele is any alternative form of a gene occurring at a specific locus on a chromosome. • Each parent donates one allele for every gene. • Homozygous describes two alleles that are the same at a specific locus. • Heterozygousdescribes two alleles that are different at a specific locus

12. Traits Genes and Alleles • Genes influence the development of traits. • All of an organism’s genetic material is called the genome. • A genotype refers to the makeup of a specific set of genes. • A phenotype is the physical expression of a trait.

13. Traits, Genes, and Alleles • Alleles can be represented using letters. • Dominant alleles are represented by uppercase letters (Ex. A orBor D or E); recessive alleles by lowercase letters (Ex. a or b or d or e).

14. Traits, Genes, and Alleles • A dominantallele is expressed as a phenotype when at least one allele is dominant. • Ex. AA (Homozygous Dominant) or Aa (Heterozygous) • Heterozygous – A dominant allele is expressed and the recessive allele is hidden • A recessiveallele is only expressed when both alleles are recessive • Ex. aa (Homozygous recessive)

15. Traits, Genes, and Alleles • Both homozygousdominant (DD) and heterozygous (Dd) genotypes yield a dominant phenotype. • Most traits occur in a range and do not follow simple dominant-recessive patterns. • Ex. Height, Skin Color, Hair Color

16. Traits and Probabilities • The inheritance of traits follows the rules of probability. • The Punnett square is a grid system for predicting all possible genotypes resulting from a cross.

17. Traits and Probabilities • The axes representthe possible gametesof each parent. • The boxes show theratio of possible genotypes and phenotypesof the offspring.

18. Heterozygous x Heterozygous Male A a Female A A A A a a a A a a

19. Heterozygous x Heterozygous Male A a Female A A A A a a a A a a

20. Traits and Probabilities • Genotypic ratio – 1:2:1 • 1 Homozygous Dominant (AA) • 2 Heterozygous (Aa) • 1 Homozygous Recessive (aa)

21. Homozygous R x Heterozygous Male a a Female A A a A a a a a a a

22. Homozygous R x Homozygous D Male a a Female A A a A a A a a A A

23. Traits and Probabilities • Monohybridcrosses examine the inheritance of only one specific trait. • A testis a cross between an organism with an unknown genotype and an organism with the recessive phenotype.

24. Probability = number of ways a specific event can occur number of total possible outcomes Heredity patterns can be calculated with probability. • Probability is the likelihood that something will happen. • Probability predicts an average number of occurrences, not an exact number of occurrences. • Probability applies to random events such as meiosis and fertilization.

25. 6.5 Traits and Probabilities • A dihybrid cross involves two traits.

26. Traits and Probabilities • Mendel’s dihybrid crosses with heterozygous plants yielded a 9:3:3:1 phenotypic ratio. • Mendel’s dihybrid crosses led to his second law, the law of independent assortment. • The law of independent assortment states that: allele pairs separate independently of each other during meiosis.

27. Sexual reproduction creates unique combinations of genes. • Genetic diversity by sexual reproduction creates unique combination of genes. • independent assortment of chromosomes in meiosis • random fertilization of gametes • Unique phenotypes may give a reproductive advantage to some organisms.

28. Crossing over during meiosis increases genetic diversity. • Crossing over is the exchange of chromosome segments between homologous chromosomes. • occurs during prophase I of meiosis I • results in new combinations of genes

29. Meiosis and Genetic Variation • Chromosomes contain many genes. • The farther apart two genes are located on a chromosome, the more likely they are to be separated by crossing over. • Genes located close together on a chromosome tend to be inherited together, which is called genetic linkage.