2.  DNA stands for “Deoxyribonucleic acid”  DNA is the genetic material of an organism; it is passed down from parents to offspring  DNA’s shape is called a “double-helix” (like a twisted ladder)

3.  DNA strands are made up of a few different substances: Deoxyribose Sugar, Phosphate, and Nitrogen Base  The “sides” of DNA are made up of Sugar and Phosphate; the “rungs” of DNA are made up of Nitrogen Bases (4)  Adenine  Thymine  Cytosine  Gunaine

4.  DNA strands make up something called “genes”  Genes are found on “chromosomes” and they call for a person to have a specific trait (like brown hair or blonde hair etc.)  A chromosome is a doubled rod of chromatin; chromosomes are inside the cell’s nucleus  A chromosome is made up of 2 identical rods called “chromatids”

5.  2 chromatids are held together by a “centromere”

6.  Humans have 23 pairs of chromosomes, 46 in all  22 pairs of chromosomes determine an individual’s traits; 1 pair of chromosomes determines if a person is male or female  Females have 2 “X” chromosomes; Males have an “X” and a “Y” chromosome  Chromosome #23 determines the gender of a human MALE FEMALE

7.  All the cells in your body come from a single fertilized egg; your body grows through a process of continuous cell divisions called “Mitosis”  Each cell in your body goes through the Cell Cycle which has 3 stages : Interphase, Mitosis, and Cytokinesis  During Mitosis, a body cell duplicates the chromosomes and then divides into 2 identical “daughter cells” each with 1 complete set of chromosomes

8.  Interphase is the period before cell division occurs  During interphase, the cell grows to its mature size, makes a copy of its DNA (DNA REPLICATION), and prepares to divide into 2 cells

9.  Mitosis is the stage in which the nucleus divides into 2 new nuclei  During Mitosis, one copy of the DNA is distributed into each of the 2 daughter cells  Mitosis occurs in “somatic” cells (body cells) Interphase

10.  The cytoplasm divides and organelles are distributed into each of the 2 identical daughter cells  Same number of chromosomes as parent cell (23 pairs=46 chromosomes Interphase

11.  Process where the # of chromosomes is reduced by half to produce sex cells a.k.a. “gametes” (Sperm and Eggs)  Meiosis occurs in sex cells  As in Mitosis, the chromosomes are copied in Meiosis and the parent cell divides into 2 “daughter cells” with the same # of chromosomes as the parent  Then, those 2 cells divide again and form 4 “daughter cells” with HALF the # of chromosomes as the original parent cell

12.  1 parent cell with 4 chromosomes produces 4 daughter cells with 2 chromosomes each  Chromosome # reduced in HALF!!

13.  So… Humans have 46 chromosomes but how many chromosomes do their sex cells have???  A sperm has 23 chromosomes from Dad  An egg has 23 chromosomes from Mom  Therefore, how many chromosomes does a zygote have? (a zygote is the initial cell formed when a sperm meets an egg)  The 23 chromosomes from the sperm combine with 23 chromosomes from the egg to create a new human with 46 chromosomes 46 23 46

14.  When a zygote is made it receives 23 chromosomes from each parent  Mom’s 23 rd pair of chromosomes are “XX”  Dad’s 23 rd pair of chromosomes are “XY”  So… Mom can only pass on an “X” chromosome to the zygote  But… Dad can pass on either an “X” or a “Y”  If Dad passes an “X” to the zygote, the baby will be a girl (XX)  If dad passes a “Y” to the zygote, the baby will be a boy (XY)  So… the 23 rd chromosome determines an individual’s gender, the other 22 determine the physical traits of a person

15.  Alleles are the different forms of a gene; there are 2 alleles per gene  There are dominant alleles represented with a capital letter i.e. “B”  There are recessive alleles represented with a lower-case letter i.e. “b”  A dominant allele will “mask” a recessive allele meaning that the dominant allele will be physically visible on the person

16.  EXAMPLE:  Brown eyes are dominant to blue eyes  “B” is the allele for Brown eyes and “b” is the allele for blue eyes  Lil’ Archie gets a “B” allele from his dad and a “b” allele from his mom. What color are his eyes????

17.  In our example, Lil’ Archie had a “B” allele and a “b” allele  Lil’ Archie’s Genotype (combination of alleles) is “Bb”  Lil’ Archie’s Phenotype (physical appearance) is brown eyes because “B” for brown is dominant over “b” for blue

18.  Lil’ Archie’s genotype is called “heterozygous” because he has 2 DIFFERENT alleles (Bb) for the eye color trait  Some people’s genotype is called “homozygous dominant” because they have 2 of the SAME dominant alleles for a trait (BB)  Some people’s genotype is called “homozygous recessive” because they have 2 of the SAME recessive alleles for a trait (bb)

19.  A punnett square is a chart that shows all of the possible combinations of alleles in a genetic cross  For example, Lil’ Archie (Bb) gets married to Lil’ Susie (BB) and we want to do a punnett square for their future children’s eye color BB (brown Eyes) Bb (brown Eyes) BB (brown Eyes) Bb (brown Eyes) Lil’ Archie’s Genotype Lil’ Susie’s genotype Bb B B