1. Gregor Mendel
Gregor Mendel was an Austrian monk who studied genetics and observed how these traits were inherited over several generations.
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2. Gregor Mendel
His work was REALLY important to understanding inheritance.
He studied what happened when he bred different kinds of pea plants (ex. green vs. yellow, short vs. tall, round vs. wrinkly)
From his experiments he came up with several conclusions.
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3. What did he learn from his studies?
Biological inheritance is determined by “factors” (genes within our DNA )that are passed on from one generation to the next.
Different forms of these genes determine our physical traits.
For every gene, we inherit two alleles (one allele from each parent).
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4. What did he learn from his studies?
trait: specific characteristic that varies from one individual to another
allele: one of a number of different forms of a gene
locus: the location of a gene on a chromosome
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5. Mendel’s three Principles
Principle (Law) of Segregation
states that the two different alleles for a gene segregate independenty from one another during meiosis
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6. Mendel’s three Principles
Principle (Law) of Independent Assortment
states that different traits sort and are inherited independently of one another when sex cells are formed
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7. Mendel’s Principles (A Summary)
For every gene, we inherit two copies (one allele from each parent)
Blonde Hair
Brown Eyes
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Blonde Hair
Blue Eyes
Brown Hair
Brown Eyes

8. If we have two sets of manuals, how do we know which directions to follow?
If we have two alleles for every trait, how does our body pick which one to express?
Blonde Hair
Brown Eyes
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Blonde Hair
Blue Eyes
Brown Hair
Brown Eyes

9. if we have two sets of manuals, how do we know which directions to follow?
When we get directions from each parent, it is a combination of the different directions that determine our traits.
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10. Mendel’s three Principles
Principle (Law) of Dominance
states that when two or more forms of a gene (alleles) exist for a single trait, some alleles are dominant while others are recessive
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11. Principle of Dominance
Some alleles are dominant and others are recessive .
Organisms with a dominant allele for a particular trait WILL exhibit that trait, because the dominant allele overpowers the recessive allele.
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12. Principle of Dominance
A recessive trait will only show in in the organism when BOTH alleles are recessive – that is, when the dominant allele is NOT present.
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13. PUNNETT SQUARES
A Punnett square is a useful tool used to determine or predict the probability that offspring will have certain traits after a genetic cross.
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14. PUNNETT SQUARES: probability
Probability: the likelihood that a particular event will occur
Because each organism has two alleles for each gene there is an equal (50/50) chance of giving either allele to an offspring
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15. PUNNETT SQUARES: terms
Genotype: genetic makeup of an organism (alleles)
Genotype sounds like genes
Phenotype: physical characteristics/appearance of an organism
Phenotype sounds like physical
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16. PUNNETT SQUARES: terms
Homozygous: an organism with two identical alleles for a particular trait
Heterozygous: an organism with two different alleles for the same trait
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17. Let’s use those words in an Example
Homozygous for the tall trait – “TT” is the genotype, “tall” is the phenotype
Heterozygous for the tall trait – “Tt” is the genotype, “tall” is the phenotype
Homozygous for the short trait – “tt” is the genotype, “short” is the phenotype
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18. PUNNETT SQUARES: Rules
Use letters to stand for alleles
Dominant alleles are noted with a CAPITAL ( D ) letter
Recessive alleles for the same trait are noted with the lowercase ( d ) of the same letter.
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19. PUNNETT SQUARES: Rules
Parental genotypes are shown along the top and side of the Punnett square.
Write one parent’s alleles on the top, and write the other on the side
Put one letter next to each cell, and drag alleles across or down so that there are two alleles in each square.
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20. PUNNETT SQUARES: Rules
Each of the four squares represents a possible genotype of the offspring
These are just probabilities!
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21. Punnett square practice
1. A homozygous tall (TT) pea plant is crossed with a homozygous short (tt) pea plant:
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22. Punnett square practice
A homozygous red (RR) flower is crossed with a heterozygous red (Rr) flower:
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23. Punnett square practice
A heterozygous funny (Ff) bird is crossed with a heterozygous funny (Ff) bird:
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24. Offspring Ratios When you have two homozygous parents?
Genotype Ratio: ________ Phenotype Ratio:________
When you have one homozygous and one heterozygous parent?
When you have two heterozygous parents?
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25. Multiple generations
Sometimes, we need to do a Punnett square to follow more than one generation.
Generations: P  F1  F parents first filial second filial
generation generation
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26. Test crosses
If you need to figure out the genotype of a parent with the dominant trait, you can do a test cross.
This is done by crossing that parent with a homozygous recessive organism.
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27. Test crosses
If any offspring show the recessive allele, you know that the dominant parent is heterozygous.
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28. Test crosses
If none of the offspring show the recessive allele, you know that the dominant parent is homozygous.
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29. Example:
A man with free earlobes wants to know if he is homozygous or heterozygous for that trait. He marries a woman with attached earlobes (the recessive trait). His first child has attached earlobes. The man has thus found out that he is _________________________.

30. Going Beyond Dominant and Recessive Alleles
Some alleles are neither dominant nor recessive – many traits are controlled by more than one allele or by multiple genes…
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31. Incomplete dominance
Incomplete dominance: a situation in which one allele is not completely dominant over another
Because neither trait dominates the other, sometimes the alleles are written differently (ex. RW, CrCw, DD’)
Examples are: flowers (red/pink/white), hair (curly/wavy/straight), fur (long/short)
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32. Identifying Incomplete dominance
Notice that the offspring show a third phenotype that is different from the parents
The offspring phenotype is a MIX or BLEND of the parental traits
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33. Identifying Incomplete dominance
What happens when we cross a red (RR) flower with a white (WW) flower?
The offspring look pink
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34. codominance
codominance: a situation in which both alleles of a gene contribute to the phenotype of the organism
What does the prefix “co-“ mean? to share
Because neither trait dominates the other, sometimes the alleles are written differently (ex. RW, CrCw, DD’)
Examples are: cows (red/roan/white), flowers (red/spotted/white), blood type (A/AB/B)
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35. Identifying codominance
Notice that the offspring show a third phenotype that is different from the parents
The offspring phenotype has both parental traits appearing TOGETHER
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36. Identifying codominance
What happens when we cross a pure red (RR) cow with a pure white (WW) cow?
The offspring look roan - spotted with red & white
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37. Identifying codominance
What happens when we cross a pure red (RR) cow with a pure white (WW) cow?
The offspring look roan - spotted with red & white
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38. Polygenic traits
Traits controlled by two or more genes are said to be polygenic.
Polygenic traits show a wide range of phenotypes because different combinations of alleles on these genes produce many different traits.
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39. Polygenic traits
For example, hair color and eye color are controlled by more than one gene and therefore have more than two phenotypes.
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40. Polygenic traits
And, the wide range of skin color in humans is due to more than four different genes that control this trait.
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