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Punnett Squares: Genetic Crosses. Making Genetic Predictions. Mendel found that you could predict the traits of a percentage of the offspring. He invented and used the tool of a Punnett square to illustrate these percentages. Punnett Squares. The Law of Dominance - PowerPoint PPT Presentation
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Punnett Squares:Genetic Crosses
Making Genetic Predictions
• Mendel found that you could predict thetraits of a percentage of the offspring.
• He invented and used the tool of a Punnettsquare to illustrate these percentages.
Punnett Squares
• The Law of Dominance Traits are shown with a Capital letter for dominant and a lower case for recessive
– Brown Eyes (B) Blue Eyes (b)
Punnett Squares
• Eye Color– Brown eyes B– Blue eyes b
• Purebred individual with brown eyes (BB)– Homozygous
• Purebred individual with blue eyes (bb)– Homozygous
• Hybrid individual with brown eyes (Bb)– Heterozygous
Punnett Squares
• Eye Color– Father is (BB)– Mother is (bb)
• Their Genes Are Crossed– BB x bb– Since we are looking at only 1 gene, it is
called a monohybrid cross.
Law Of Segregation
• Law of Segregation– Alleles for the same trait must be in
separate sex cells.– Father is (BB) as his diploid genotype– Mother is (bb) as her diploid genotype
• Somatic Cells BB bb • Sex cells B B b b
Monohybrid Crosses
• Monohybrid cross deals with one gene– We are working with only eye color
• Setting up a punnet square– The sex cells go on the outside of the square
Punnett Squares
B B
b
b
Bb = BrownAll of theoffspring willhave brown eyes,but they will beheterozygousindividuals.
Punnett Squares
• Eye Color– Father has Brown eyes and is
heterozygous (Bb).– Mother has Brown eyes and is
heterozygous (Bb).
Punnett Squares
BB Bb
Bb bb
B
b
B bBB = BrownBb = Brownbb = Blue
Ratios
• Genotype Ratio– 1 BB to 2 Bb to 1 bb
• Phenotype Ratio– 3 Brown eyes to 1 blue Eyes
• What is the chance of this couple having a child with blue eyes?
– 1/ 4 or a 25 % chance
Punnett Squares
• One parent is heterozygous for the trait the other parent is homozygous
• Eye Color– Father (Bb) Brown– Mother (bb) blue
Punnett Squares
Bb bb
Bb bb
b
b
B b
Ratios
• Genotype Ratio– 0 BB to 2 Bb to 2 bb (reduce)– 0 BB to 1 Bb to 1 bb
• Phenotype Ratio– 2 Brown eyes to 2 blue eyes (reduce)– 1 Brown eye to 1 blue eye
• What is the chance of this couple having a child with blue eyes?
– ½ or 50%
Mendel’s Work
• Pea Color:• Yellow: Y• Green: y• P Generation• YY x yy
Mendel’s Work
• Crossed two of the F1 tosee what the F2 wouldlook like.
Multiple Characteristics
• Punnett Squares work with multiple characteristics.
• Crosses can be done with an assortment of genes.
Dihybrid Crosses
• The Parents are heterozygous for both hair and eye color (BbEe: Brown hair and Brown Eyes).
– B: Brown hair - E: Brown eyes– b: Blonde hair - e : Blue eyes
• Cross = BbEe x BbEe
Mendel’s Laws
• The law of segregation– You can’t have the same letters in a Sex cell
• The law of independent assortment– The two genes will have their alleles placed into the same sex cells randomly.
• Work with one parent at a time– BbEe (B b) (E e)– Possible sex cells– (BE) (Be) (bE) (be)
BE Be bE be
BE
Be
bE
be
Phenotype Ratios
• Phenotypes– Brown Hair Brown Eyes _________– Brown Hair Blue Eyes _________– Blonde Hair Brown Eyes _________– Blonde Hair Blue Eyes _________
Phenotype Ratios
• Phenotypes– Brown Hair Brown Eyes ____9____– Brown Hair Blue Eyes ____3____– Blonde Hair Brown Eyes ____3____– Blonde Hair Blue Eyes ____1____
Phenotype Percentages
• Phenotypes– Brown Hair Brown Eyes ____56%___– Brown Hair Blue Eyes ____19%___–Blonde Hair Brown Eyes ____19%___– Blonde Hair Blue Eyes ____6%____
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