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Punnet Square Notes. Phenotype & Genotype Phenotype – the physical characteristics of an organism (example: blue eye color)

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Presentation on theme: "Punnet Square Notes. Phenotype & Genotype Phenotype – the physical characteristics of an organism (example: blue eye color)"— Presentation transcript:

1 Punnet Square Notes

2 Phenotype & Genotype Phenotype – the physical characteristics of an organism (example: blue eye color)

3 Phenotype & Genotype Genotype – the alleles possessed by an organism (example: inheriting two blues alleles on from each parent)

4 Dominant & Recessive Allele Dominant Allele – An allele that dominants over a recessive allele. Represented in genetics with a capital letter (example the capital letter B could represent the dominant allele for brown eyes).

5 Dominant & Recessive Allele Recessive Allele - An allele that does NOT dominate over other traits. Represented in genetic by a lower case letter (example the lower case letter b could represent the allele for blue eyes)

6 Types of Genotypes Homozygous Dominant: An organism having two identical dominant alleles. Represented in genetic by having two upper case letters. (example: BB represents a person that has two alleles for brown eyes)

7 Types of Genotypes Homozygous Recessive: An organism having two identical recessive alleles. Represented in genetic by having two lower case letters. (example: bb represents a person that has two alleles for blue eyes)

8 Types of Genotypes Heterozygous: An organism having two different alleles for a trait, one recessive and one dominant. Represented in genetic by having one upper case and one lower case letters. (example: Bb represents a person that has a dominant alleles for brown eyes and a recessive allele for blue eyes).

9 Punnet Square/ Grid Punnet Square / Grid is a grid or table that shows all the possible outcomes between a cross between a male in female organisms

10 Example Cross a Heterozygous pea plant with smooth seeds with a homozygous recessive plant with wrinkled seeds. 1.Select Symbols the alleles: 2.Write the cross (alleles parents can give): 3.Make a punnet square. –What percent are heterozygous? –What percent are smooth? –What percent are wrinkled?

11 Example Cross a Heterozygous pea plant with smooth seeds with a homozygous recessive plant with wrinkled seeds. 1.Select Symbols the alleles: S = Smooth; s= wrinkled 2.Write the cross (alleles parents can give) :Ss X ss 3.Write down what each parent can give, also called Gamete or sex cell S; s x s ; s 1.Make a punnet square. –What percent are heterozygous? 50% –What percent are smooth? 50% –What percent are wrinkled? 50% ss S SsSsSsSs sss

12 P-square Practice Question #1 Let's say that in seals, the gene for the length of the whiskers has two alleles. The dominant allele (W) codes long whiskers & the recessive allele (w) codes for short whiskers. Cross two long-whiskered seals, one that is homozygous dominant and one that is heterozygous? –The Symbols (letters you will use) ______________________ –The Cross (the parent’s genotypes)_________________________ –The Punnett square:

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14 P-square Practice Question #2 In purple people eaters, one-horn is dominant and no horns is recessive. Draw a Punnet Square showing the cross of a purple people eater that is heterozygous for horns with a purple people eater that does not have horns. –The Symbols (letters you will use) ______________________ –The Cross (the parent’s genotypes)_________________________ –The Punnett square:

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16 Test Crosses Problem: When an organism has a shows a dominant phenotype (example: Brown Eyes) it is not always possible to know if this organism is has a heterozygote genotype (Bb) or Homozygote dominant genotype (BB) because the phenotype (physical appearance) it the same for both genotypes.

17 Test Cross Solution: To determine the genotype of an organism showing a dominant phenotype a test cross is used.

18 What is a test cross?: In a test cross an individual that might be heterozygous or homozygous dominant is crossed with an individual that is homozygous recessive.

19 How it works? If you cross a homozygous recessive organism with a homozygous dominant organism all (100%) the offspring will have the dominant phenotype.

20 Example: Black fur color is dominant to white fur color in sheep. A homozygous dominant black sheep is cross with a homozygous recessive white sheep. The Symbols (letters you will use) ______________________ The Cross (the parent’s genotypes)_________________________ The Punnett square:

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22 If you cross a homozygous recessive organism with a heterozygote 50% of the offspring will have the dominant phenotype and 50% will have the recessive phenotype.

23 Example: Black fur color is dominant to white fur color in sheep. A heterozygous black sheep is cross with a homozygous recessive white sheep. The Symbols (letters you will use) ______________________ The Cross (the parent’s genotypes)_________________________ The Punnett square:

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25 Practice 3 problem. First try this problem on your own then check your answer with the next slide. P-square practice Question #3 A green-leafed luboplant (I made this plant up) is crossed with a luboplant with yellow-striped leaves. The cross produces 185 green-leafed luboplants. Summarize the genotypes & phenotypes of the offspring that would be produced by crossing two of the green-leafed luboplants obtained from the initial parent plants.

26 A green-leafed luboplant (I made this plant up) is crossed with a luboplant with yellow-striped leaves. The cross produces 185 green-leafed luboplants. Summarize the genotypes & phenotypes of the offspring that would be produced by crossing two of the green-leafed luboplants obtained from the initial parent plants. Note: Green plant is crossed with yellow plant results in all green plants. THUS, green is dominant, and initial green parent’s genotype homozygous dominant (GG). Initial yellow parent is homozygous recessive (gg), because the only way you can show the recessive phenotype is if you are homozygous recessive. Cross of initial parents: GG x gg ALL offspring from the initial parents are green and heterozygous (Gg). This question asks you to cross the offspring from the initial parents. Thus you will cross brother and sister produced from initial parents (Gg x Gg) The Symbols (letters you will use) G= green g = yellow The Cross (the parent’s) genotype: Gg x Gg The Punnett square: Summary of offspring phenotypes –Percentage that will have yellow-striped leaves: 25% –Percentage that will have green-leaves: 75%

27 Problem 4 First try this problem on your own then check your answer with the next slide Mendel found that crossing wrinkle-seeded plants with pure round- seeded plants produced only round-seeded plants. What genotypic & phenotypic ratios can be expected from a cross of a wrinkle-seeded plant & a plant heterozygous for this trait (seed appearance)?

28 NOTE: Crossing a wrinkled seed plant with a round seed plant produced all round seeds. Thus you can assume that round is dominant to wrinkled. Remember the only way the recessive phenotype will show is if the organism is homozygous recessive for that trait. –The Symbols (letters you will use) R = round; r = wrinkled –The Cross (the parent’s genotype): Rr x rr –The Punnett square: Summary of offspring –Percentage that will have heterozygous: 50% –Percentage that will have rounded-seeds: 50% –Percentage that will have wrinkled-seeds: 50%

29 Problem 5 & 6 Do these problems on your own.


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