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# Punnett squares

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### Transcript

• 1. Punnett Squares
• 2. Gregor Mendel
Austrian Monk
Famous for his work with pea plants
He is known as the father of genetics
• 3. Mendel’s Work
Mendel used true-breeding plants which means if they were left to breed with themselves they would produce offspring identical to themselves.
Mendel studied 7 different traits in pea plants.
• 4. The Principle of Dominance
The principle of dominance states that some alleles are dominant and others are recessive.
Dominant alleles are always expressed.
Recessive alleles are only expressed if both alleles are recessive.
• 5. Segregation & Independent Assortment
The separation of alleles during meiosis to form gamates (sex cells).
The Law of Independent Assortment states that genes for different traits can segregate independently during the formation of gamates.
• 6. Probability & Genetics
Probability is the likelihood that an event will happen.
The principle of probability can be used to predict the outcomes of genetic crosses.
• 7. Punnett Squares
Diagram used to predict genetic crosses.
• 8. How to use Punnett Squares
Choose a letter to represent the alleles in the cross.
Write the genotypes of the parents.
Determine the possible gametes (reproductive cells) that the parent can produce.
Enter the possible gamete at the top and side of the Punnett square.
• 9. More Punnett Squares
Complete the Punnett square by writing the alleles from the gametes in the appropriate boxes.
Determine the phenotypes of the offspring.
Using the results of step E and F write down the genotypic and phenotypic ratios.
• 10. Punnett Squares
• 11. Mendel’s Principles
The inheritance of biological characteristics are determined by genes.
For two or more forms of a gene, dominance and recessive forms may exist.
Most sexually reproductive organisms have two sets of genes that separate during gamete formation.
Alleles segregate independently.
• 12. Punnett Squares Monohybrid Crosses
Ratios and percentages
Genotype
1:2:1
25%: 50%: 25%
Phenotype
3:1
75%:25%
• 13. Punnett Squares Dihybrid Crosses
Ratios
Genotype
1:2:2:1:4:2:1:2:1
Phenotye
9:3:3:1
• 14. BeyondDominantand RecessiveNon-mendelian genetics
Incomplete Dominance – One allele is not completely dominant over the other. White flower crosses with a red = pink flower.
• 15. Incomplete dominance
With incomplete dominance, a cross between organisms with two different phenotypes produces offspring with a third phenotype that is a blending of the parental traits.
It's like mixing paints, red + white will make pink.  Red doesn't totally block (dominate) the pink, instead there is incomplete dominance, and we end up with something in-between.
• 16. We can still use a Punnett square
You just have to remember that the heterozygous genotype produces the “blended” phenotype.
• 17. Codominance – Both alleles contribute to the phenotype. Red cow X White Cow = Roan Cow.
• 18. Codominance
Similar to incomplete dominance because a hybrid organism shows a third phenotype --- not the usual "dominant" one & not the "recessive" one ... but a third, different phenotype.
BUT instead of a blending of the dominant & recessive traits BOTH traits appear together
• 19. You can still use Punnett squares
There are a couple of different ways of representing the genes – it is not really important which one you choose.
• 20. Sex-linked - genes that occur on the X or Y chromosome. (there are not as many genes on the Y chromosome)
• 21. Multiple Alleles – More than two alleles control the phenotype. Blood type.
• 22. Polygenic traits – Several genes control the trait. Skin color in humans.