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 cellDivision…so every newCell 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 overDuring Crossing over segments of non-sister chromatidschromatids break and re-attach to the other chromatidchromatid.

DURING PROPHASE I “CROSSING OVER” OCCURS.

Creates variation (diversity) in the offspring’s Creates variation (diversity) in the offspring’s traits.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 18th set, Trisomy 21…. Three chromosomes in the 21st set. If an organism has Monosomy 23 it has only one chromosome in the 23rd set.

NONDISJUNCTION

COMMON NONDISJUNCTION DISORDERS

Down’s Syndrome – Trisomy 21 mental retardation, webbed fingers & toes, slanted eyes,

short statureTurner’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 223). The successful sperm represents one of 8 million

different possibilities (actually 223). The resulting zygote is composed of 1 in 70 trillion (223 x

223) possible combinations of chromosomes. Crossing over adds even more variation to this.

FERTILIZATION