Heredity

The genetic passing of characteristics from parent to offspring

• Eye color• Hair color and texture• Facial features• Height

It’s all in the DNA….

• DNA – DeoxyriboNucleic Acid– Unique individual sequence of nucleotides– Only identical twins have the same sequence

• Gene– A segment of DNA that codes for a protein– Humans are estimated to have 50,000 genes

Chromosomes• DNA wrapped around proteins• Contain genes

Humans have 46 chromosomes in somatic (body) cells

That equals 23 pairs of chromosomes

• the pairs are called: Homologous Chromosomes

Homologous Chromosomes

Pairs of chromosomes with the same genes on each individual in the pair

Which means…..We have two copies of each gene

Those copies of the genes are called alleles

Where do we get our chromosomes?

• We inherit 23 chromosomes from each parent

But…somatic cells have 46 chromosomes…..if two somatic cells from our parents are put together that would be 96

chromosomes!

• Gametes are the solution!– Sex cells (eggs and sperm)– Have 23 chromosomes– When two gametes are put together = 46 chromosomes!– Created during Meiosis – a reduction division

Karyotype• A visualization of the chromosomes of a

somatic cell.• Used as part of amniocentesis tests• Cells are “caught” with their chromosomes

condensed and duplicated during prophase.• Chromosomes are matched by size• Can see chromosome disorders using a

karyotype

Special Names for the Number of Chromosomes

• Diploid– A cell with a double set of chromosomes– All somatic cells are diploid– 46 chromosomes in humans– 2n

• Haploid– A cell with a single set of chromosomes– All gametes are haploid– 23 chromosomes in humans– n

Diploid Cells

• Pair of chromosomes – 2n • Each individual of the pair has a copy of the

same set of genes• We all have two alleles (copies) of a single

gene– We got one allele from Mom and one from Dad

Can’t use Mitosis to get a haploid cell!

Meiosis• Cell division that reduces the number of

chromosomes – Reduction Division• Involves two consecutive cell divisions called

Meiosis I and Meiosis II

Meiosis I• Interphase– Carrying out metabolic processes– Replicating DNA – condensing it into chrmosomes

• Prophase I• Metaphase I• Anaphase I• Telophase I

Prophase I• Chromosomes appear– sister chromatids are joined in the

middle• Synapsis– Replicated homologous

chromosomes pair up and form bonds

• Crossing over– Chromosome segments are

exchanged between homologous chromosomes

– Results in greater genetic variation

Metaphase I

• Homologous chromosomes and their sister chromatids line up on the equator (middle) of cell

• Spindle fibers attach

Anaphase I

• Homologous chromosomes separate• Sister chromatids go to the poles of the cell

Telophase I• Sister chromatids reach poles• Only one of the original homologous

chromosome pair (and it’s sister chromatid –the replicated DNA is still attached) are a part of this cell

• Creates two haploid cells

Meiosis II• During the Interphase between Meiosis I and

Meiosis II –The DNA does not replicate• The cell undergoes another round of phases:– Prophase II– Metaphase II – sister chromatids line up on

equator– Anaphase II – sister chromatids pull apart– Telophase II – 4 haploid cells all genetically

different are created