what we know so far… dna is the instructions for your cells dna is like a cookbook – the recipes...
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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.
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Blonde hairBlue eyes
Hitchhiker’s thumbdetattached / attached earlobes
widow’s peak
How are these traits
passed from one generation to the next?
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MEIOSIS!!
A VERY SPECIAL TYPE OF CELL DIVISION.
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Words you need to know:
• Mitosis• Diploid• Haploid• Homologous
Chromosome• Meiosis• Sperm
• Egg• Sexual Reproduction• Crossing Over• Genetic
Recombination• Nondisjunction• Allele
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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
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• 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.
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Homologous Pairsgene
From Mom
From Dad
From Mom
From Dad
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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
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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…
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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.
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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”
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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.
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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
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Crossing Over
Homologous chromosomes exchange sections =
Genetic Recombination
• Prophase 1: Homologous Chromosomes line up next to each other – CROSSING OVER
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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!
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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
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When he crossed the F1 generation, the offspring were mixed Tall and Short.
hmmmmmmmmmm.
These were the F2 generation
Mendel’s Laws of Heredity
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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
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Mendel’s Laws of HeredityIdentical ALLELES Different ALLELES
TALL GENE
TALL GENE
TALL GENE
SHORT GENE
mom momdad dad
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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
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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
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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
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3. The Law of Segregation
During meiosis, homologous chromosomes separate and each sex cell receives one of the homologues.
Mendel’s Laws of Heredity
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Long chromosomes are HOMOLOGUES
Short chromosomes are HOMOLOGUES
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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.
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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
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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
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• 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
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• 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
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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.
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• Phenotypes are written as descriptions; blonde hair, brown hair, brown eyes, blue eyes, etc.
Punnett Squares
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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
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Cross 2 white rabbitsb b
b
b
bb bb
bb bb
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• 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