1. Mendelian Genetics and Meiosis

2. Gregor Mendel (1822 – 1884) Father of Genetics
Austrian monk who worked with pea plants in monastery garden.
Developed first theories on genetic inheritance.

3. Why Experiment With Pea Plants?
Grow quickly
Many kinds available
Self-pollinate so they have both male and female reproductive parts on the same flower and can pollinate themselves. They are true-breeding meaning offspring will have same traits as parent.
Can cross-pollinate so one plant can pollinate another plant to produce offspring.

4. What was Mendel studying?
Inherited characteristics (features with different forms in a population) that are passed from parents to offspring.
EX: Flower color or seed shape
Studied one characteristic at a time to determine which traits appeared in offspring.

5. Mendel's First Experiment
Manually crossed true breeding plants for each characteristic.
Example: crossed purple flower plant & white flower plant.
All offspring displayed the same trait of one parent. In this case, all had purple flowers. White flowers seemed to disappear.

6. Conclusions:
The trait that showed up most often in the offspring was the DOMINANT trait.
The trait that seemed to disappear or fade away was the RECESSIVE trait.
To determine what happened to the recessive trait, Mendel decided to do another set of experiments.

7. Mendel's Second Experiment
Allowed the plants produced by his first experiment to self-pollinate.
All purple flowered plants self pollinated:
75% of offspring were purple flowered
25% of offspring were white flowered

8. Conclusions
Recessive trait did not disappear, it was masked by the dominant trait as it showed up again in the second generation.
Each plant had 2 sets of instructions (one from each parent) for each characteristic.

9. Mendel's Principles
Inheritance of traits is predetermined by genes. Genes are passed on from parents.
Some forms of genes are dominant and others are recessive.
Organisms have 2 copies of each gene (one from each parent).
Alleles(different forms for a gene) for different genes segregate independently of one another. (Monohybrid cross)

10. Alleles Different forms of a gene
EX: freckles or no freckles
Dominant allele – expressed with an UPPER CASE letter.
Recessive allele – expressed with a lower case letter.
NOTE: The same letter is used to express an allele – variations are expressed with the upper or lower case.

11. Phenotype vs. Genotype
PHENOTYPE: Physical characteristic – the characteristic that you can see.
EX: Purple flowers
GENOTYPE: The two inherited alleles for a trait. (Cannot be seen)
EX: PP or Pp

12. Types of Genotypes Homozygous Dominant: Two dominant alleles
PP or DD or BB
Homozygous Recessive: Two recessive alleles
pp or dd or bb
Heterozygous: One dominant and one recessive allele
Pp or Dd or Bb

13. Punnett Square
Organizes all possible genotype combinations for offspring from particular parents.
How to make a Punnett Square crossing a homozygous recessive white flowering pea plant with a heterozygous purple flowering pea plant.

14. Exceptions to Mendel's Principles
Incomplete Dominance: One allele is not completely dominant over the other allele. Each allele contributes to the phenotype produced.
EX: Snapdragons (white and red produce pink)
One gene may influence more than one trait.
EX: in white tigers, one gene codes for fur color and eye color.
Several genes may work together to produce a trait.
EX: human skin, hair and eye color

15. MEIOSIS Creates the sex cells
It is a copying process that produces cells with ½ the number of chromosomes.
Helped Walter Sutton determine genes are located on chromosomes in the nucleus of the cell. Prior to this no one knew where the genetic traits (genes) were located.

16. Steps of Meiosis Meiosis I Meiosis II Prophase I Metaphase I
Anaphase I
Telophase I and Cytokinesis
Meiosis II
Prophase II
Metaphase II
Anaphase II
Telophase II and Cytokinesis

17. http://www. cps. ci. cambridge. ma

18. Prophase I
Homologous chromosomes find each other and pair up (one chromosome from each parent
Crossing over may occur
Centrioles move toward the poles
Nuclear membrane begins to dissolve

19. Metaphase I Spindle fibers attach to homologous chromosomes.
Homologous chromosomes line up at the equator

20. Anaphase I
Homologous chromosomes are separated so each chromosome moves toward opposite poles.

21. Telophase I
Homologous chromosomes are completely separated with one chromosome at each pole.
Nuclear membrane re- forms
Cytokinesis takes place and cell divides to form two cells.

22. Prophase II Centrioles move to poles Nuclear membrane dissolves
NOTE: CHROMOSOMES ARE NOT COPIED AGAIN PRIOR TO PROPHASE II

23. Metaphase II Spindle fibers form and attach to chromosomes
Chromosomes line up at the equator.

24. Anaphase II
Chromosomes are pulled apart so each chromatid moves toward opposite poles.

25. Telophase II Chromatids reach the poles. Nuclear membrane re-forms
Cytokinesis occurs

26. MEIOSIS Results 4 new cells
Each cell has ½ the number of chromosomes as parent cell (haploid – N)
New cells are NOT identical to each other or to the parents as a result of crossing over.

27. Meosis vs Mitosis Meiosis Mitosis Sex cells Two divisions
4 genetically different cells produced
Cells produced have half the number of chromosomes (haploid) 2n  n
Somatic cells
One division
2 genetically identical cells produced
Cells produced have the same number of chromosomes as parents 2n  2n

28. Sex Chromosomes Chromosomes that carry the genes that determine sex.
In humans:
Females: two X chromosomes (XX)
Males: one X chromosome and one Y chromosome (XY)
Sex of offspring is determined by the male:
Egg fertilized by sperm with X chromosome = FEMALE
Egg fertilized by sperm with Y chromosome = MALE

29. Sex-Linked Disorders
Examples:
Color blindness
Hemophilia
Males have an X and Y chromosome. The Y chromosome does not have all the genes found on the X chromosome, so they only have one copy of those genes on the X. If those genes are damaged, they do not have a backup while females do – they have two X chromosomes. Therefore, males are more likely to inherit these disorders.