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6.3 Genetics and Predictions

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You will be able to: 1.Explain how probability is used in genetic predictions 2.To construct a Punnett Square for monohybrid and dihybrid crosses 3.To infer genotype by using a testcross

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Probability the measure of how likely an event is. Measure how likely a species will inherit a trait Probability Of An Event P(A) = The Number Of Ways Event A Can Occur The Total Number Of Possible Outcomes

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Represent probability 1.Fraction ½ 2.Percent 50% 3.Ratio (1:2)

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Punnett Square a simple graphical way of discovering all of the potential combinations of genotypes that can occur in children, given the genotypes of their parents. genotypes It also shows us the odds of each of the offspring genotypes occurring.

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Sample Problems In pea plants, tall pea plants are dominant over short pea plants. Using Punnett Squares, you can predict the genotypes and phenotypes of the offspring of a cross between a homozygous (purebred) tall pea plant and a homozygous (purebred) short pea plant.

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Step 1 You begin by drawing a grid

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Step 2 Designate letters which will represent the genes/traits. Capital letters represent dominant traits lowercase letters represent recessive traits. T = tall t = short

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Step 3 Write down the genotypes (genes) of each parent. These are often given to you or are possible to determine. TT X tt

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Step 4 List the genes that each parent can contribute. Parent 1 : T Parent 2: t

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Step 5 Write the possible gene(s) of one parent across the top The gene(s) of the other parent along the side

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Step 6 Fill in each box of the Punnett square by transferring the letter above and in front of each box into each appropriate box. As a general rule, the capital letter goes first and a lowercase letter follows.

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Step 7 List the possible genotypes and phenotypes of the offspring for this cross. GenotypesPhenotypes TT - none Tt - 4Tall – 4 tall tt - noneShort – 0

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Monohybrid Cross only one trait is crossed. A tall pea plant TT is crosses with a short pea plant tt.

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Dihybrid Cross Two traits are crossed. A tall, green flowered plant is crossed with a short, yellow flowered plant. TtPp x ttpp

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Test Cross A simple method to confirm the genotype of the F1 hybrid. F1 hybrid is crossed with the homozygous recessive

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Test Cross Example Tall x Dwarf Tall Parent = F1 (Tt) Dwarf Parent = homozygous recessive (tt)

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If the F1 tall is heterozygous with (Tt) genotype and, if Mendel's law of segregation of characters is correct, then the F1 should produce two types of gametes, (T) and (t) in equal proportion. The recessive dwarf parent produces only (t) type of gametes. The resulting test cross progeny should be 50% Tall and 50% Dwarf or in 1:1 ratio as follows

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50% Tall and 50% Dwarf 1:1 ratio 1 tall for every 1 dwarf The resulting test cross should be

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