What we know so far…

• DNA is the instructions for your cells

• DNA is like a cookbook – the recipes are GENES

• Each GENE provides instructions for making a specific protein

• Specific proteins have specific shapes –mutations in the instructions (DNA) can result in changes in the protein.

Blonde hairBlue eyes

Hitchhiker’s thumbdetattached / attached earlobes

widow’s peak

How are these traits

passed from one generation to the next?

MEIOSIS!!

A VERY SPECIAL TYPE OF CELL DIVISION.

Words you need to know:

• Mitosis• Diploid• Haploid• Homologous

Chromosome• Meiosis• Sperm

• Egg• Sexual Reproduction• Crossing Over• Genetic

Recombination• Nondisjunction• Allele

You have 46 Chromosomes

• Each of your body cells has 46 chromosomes, or 23 pairs.

• Each pair is made of HOMOLOGOUS CHROMOSOMES

Homo = same

These chromosomes contain the same genes in the same order

• Half of your chromosomes (23) came from your mom

• Half of your chromosomes (23) came from your dad

You have 46 Chromosomes

Each parent gave you one chromosome from a homologous pair.

Homologous Pairsgene

From Mom

From Dad

From Mom

From Dad

MITOSIS Review

• When your BODY cells (autosomal) divide, they make exact copies

• Results = two cells with the same type and number of chromosomes as the parent cell.

• Many animals reproduce ASEXUALLY – through mitosis

ex: flatworms, sponges, jellyfish

This is a problem for most other animals.

If humans were created through mitosis:

1st generation46 from mom + 46 from dad = 92 chromosomes for baby

then

2nd generation92 from mom + 92 from dad = 184 chromosomes for baby

And so on, and so on, and so on…

MEIOSIS

• A second type of cell division that occurs in the SEX CELLS (gametes: sperm and egg)

• Division creates four cells with half the number of chromosomes as the parent cell.

Haploid vs Diploid

• Body cells (autosomal cells) are DIPLOID• DI = 2; Each cell has 2 copies of each

chromosome• These are considered “2n”

– “n” is the chromosome number

• Sex cells are HAPLOID

• Each sex cell has HALF the normal number of chromosomes

•These are considered “n”

Sperm and Egg

• Sperm are the sex cells in males

• Eggs are the sex cells in females

Both are produced through MEIOSIS

Any reproduction that involves SPERM and EGGS is considered SEXUAL

REPRODUCTION.

Steps of MEIOSIS

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

• Looks a lot like Mitosis, huh?

• Meiosis has 2 DIVISIONS

How does this result in unique individuals?

Division 1

Division 2

Crossing Over

Homologous chromosomes exchange sections =

Genetic Recombination

• Prophase 1: Homologous Chromosomes line up next to each other – CROSSING OVER

Crossing Over

• Crossing over creates new genetic combinations

• This results in new and unique individuals

This is why you look similar to, but not identical to, your brothers and sisters!

Mendel’s Laws of Heredity

Gregor Mendel worked with pea plants in the mid – nineteenth century

When he crossed tall pea plants and short pea plants, all offspring were tall.

These were F1 generation

When he crossed the F1 generation, the offspring were mixed Tall and Short.

hmmmmmmmmmm.

These were the F2 generation

Mendel’s Laws of Heredity

1. The rule of unit factors

each organism has two “factors” that control the traits

Now, we call those factors GENES.

A homologous pair can have two variations of the same genes.

Mendel’s Laws of Heredity

Mendel’s Laws of HeredityIdentical ALLELES Different ALLELES

TALL GENE

TALL GENE

TALL GENE

SHORT GENE

mom momdad dad

2. The Rule of Dominance

Alleles (variations of a gene) can be DOMINANT or RECESSIVE

Dominant genes contain instructions for a functional protein

Recessive genes contain instructions for a non-functioning protein

Mendel’s Laws of Heredity

For Example:

Cystic Fibrosis is caused by a non-functional protein (recessive gene)

To have Cystic Fibrosis you must have 2 recessive alleles – one from each parent.

This is called HOMOZYGOUS recessive.

HOMO = same

Mendel’s Laws of Heredity

It is possible to have one dominant allele (a functional protein) and one recessive allele (non-functioning protein).

This is called HETEROZYGOUS.

HETERO = different

These individuals are carriers.

Mendel’s Laws of Heredity

3. The Law of Segregation

During meiosis, homologous chromosomes separate and each sex cell receives one of the homologues.

Mendel’s Laws of Heredity

Long chromosomes are HOMOLOGUES

Short chromosomes are HOMOLOGUES

Errors in MEIOSIS

• Nondisjunction – homologous chromosomes do not separate during meiosis; results in gametes with too many chromosomes

EX. Down Syndrome

Individuals with an unusual number of chromosomes are POLYPLOID.

Polyploidy

• Some chromosome abnormalities do not cause disease, however abnormal numbers of chromosomes can be lethal or give rise to genetic disorders.– Cri-du-chat– Down Syndrome– Edwards Syndrome– Patau Syndrome

Punnett Squares

• Allow you to make predictions of offspring GENOTYPE and PHENOTYPE

Genotype = the actual genes an individual has

Phenotype = the physical appearance of an individual

• Genotype can be HOMOZYGOUS or HETEROZYGOUS

Homozygous = both alleles (copies of the gene) are the same – both genes for eye color are “blue”

Heterozygous = both alleles are different – one gene for “blue,” one gene for “brown”

Punnett Squares

• Genotypes are written using letters. Each letter represents one allele

All genotypes will have 2 letters

• Dominant genes (functional protein) are CAPITALIZED

• Recessive genes (non-functional protein) are lowercase.

Punnett Squares

Example: Brown eyes are dominant, blue eyes are recessive.

Genotype of an individual with brown eyes:

BB (homozygous) or Bb (heterozygous)

Genotype of an individual with blue eyes:

bb (only homozygous)

Punnett Squares

To have a recessive trait, you need to have both recessive alleles – one from mom, one from dad.

Recessive traits are always HOMOZYGOUS.

• Phenotypes are written as descriptions; blonde hair, brown hair, brown eyes, blue eyes, etc.

Punnett Squares

Example: Black fur is dominant to white fur in rabbits.

What are the possible genotypes for a black rabbit?

What are the possible genotypes for a white rabbit?

Punnett Squares

BB, Bb

bb

Cross 2 white rabbitsb b

b

b

bb bb

bb bb

• Monohybrid cross: only work with ONE gene/trait

• Determine what gametes are formed– Homologous pairs are split up, one goes to

each gamete– Gametes from male and female are combined

Punnett Squares