The Chromosome Theory states that chromosomes are rod-shaped carriers of genetic information: DNA CHROMOSOMES

WHAT IS A CHROMOSOME?  Chromosomes form when CHROMATIN (nuclear material containing DNA) condenses to form rod- shaped structures  Chromosomes are found in the nucleus but are only visible when the cell is dividing

WHAT ARE THE PARTS OF A CHROMOSOME?  Each replicated copy of DNA is called a CHROMATID  The two chromatids that make up a chromosome are held together by a point called a CENTROMERE

HOW DO CHROMOSOMES, DNA, AND GENES RELATE?  DNA is the genetic directions for the cell  Chromosomes contain a portion of an organism’s DNA  Each segment of DNA that contains directions for a particular trait is a GENE

CHROMOSOMES, DNA, GENES… DNA replicates (copies Itself) prior to cell Division…so every new Cell gets a complete Copy of DNA

GENETIC DIVERSITY

 Crossing Over is one of the Two major occurrences of Meiosis  (The other is Non-disjunction) During Crossing over chromatids chromatid During Crossing over segments of non-sister chromatids break and re-attach to the other chromatid. DURING PROPHASE I “CROSSING OVER” OCCURS.

 Creates variation (diversity) in the offspring’s traits. CROSSING OVER nonsister chromatids Tetrad

CROSSING OVER

INDEPENDENT ASSORTMENT

INDEPENDENT ASSORTMENT AT ANAPHASE 1 Lots of different combinations are possible! This is why you don’t look exactly like your brothers and sisters even though you share the same parents!

 Non-disjunctions usually occur in one of two fashions.  The first is called Monosomy, the second is called Trisomy. If an organism has Trisomy 18 it has three chromosomes in the 18 th set, Trisomy 21…. Three chromosomes in the 21 st set. If an organism has Monosomy 23 it has only one chromosome in the 23 rd set. NONDISJUNCTION

COMMON NONDISJUNCTION DISORDERS  Down’s Syndrome – Trisomy 21  mental retardation, webbed fingers & toes, slanted eyes, short stature  Turner’s Syndrome – Monosomy 23 (X)  the fertilized egg is missing the X chromosome, short statured, wide neck, many are miscarried before birth  Kleinfelter’s Syndrome – Trisomy 23 (XXY)  – the presence of a Y indicates a male, but at puberty, the XX leads to a lot of female hormones produced resulting in a sterile male  Edward’s Syndrome – Trisomy 18  Small head, low, malformed ears, heart defects, kidney malformations, protruding intestines (majority die before birth)

 (picture of an individual’s chromosomes) KARYOTYPE What genetic disorder does this karyotype show?

 Alterations of the DNA sequence that occur during DNA replication.  Source of new genetic material.  Can cause changes in the organisms appearance or function. GENE MUTATION

 The random nature of fertilization also adds to the genetic variation arising from meiosis.  Any sperm can fuse with any egg.  A zygote produced by mating of a woman and man has a unique genetic identity.  An ovum is one of approximately 8 million possible chromosome combinations (actually 2 23 ).  The successful sperm represents one of 8 million different possibilities (actually 2 23 ).  The resulting zygote is composed of 1 in 70 trillion (2 23 x 2 23 ) possible combinations of chromosomes.  Crossing over adds even more variation to this. FERTILIZATION