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
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Presentation Transcript
01
Punnett Squares:
Genetic Crosses<br>
02
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.<br>
03
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)<br>
04
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<br>
05
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.<br>
06
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<br>
07
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<br>
08
Punnett Squares B B b
b Bb = Brown
All of the
offspring will
have brown eyes,
but they will be
heterozygous
individuals.<br>
09
Punnett Squares Eye Color
– Father has Brown eyes and is heterozygous (Bb).
– Mother has Brown eyes and is heterozygous (Bb).<br>
10
Punnett Squares B
b B b BB = Brown
Bb = Brown
bb = Blue<br>
11
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<br>
12
Punnett Squares One parent is heterozygous for the trait the other parent is homozygous
Eye Color
– Father (Bb) Brown
– Mother (bb) blue<br>
13
Punnett Squares b
b B b<br>
14
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%<br>
15
Mendel’s Work Pea Color:
Yellow: Y
Green: y
P Generation
YY x yy<br>
16
Mendel’s Work Crossed two of the F1 to
see what the F2 would
look like.<br>
17
Multiple Characteristics Punnett Squares work with multiple characteristics.
Crosses can be done with an assortment of genes.<br>
18
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<br>
19
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)<br>
20
BE Be bE be BE
Be
bE
be<br>
21
Phenotype Ratios Phenotypes
– Brown Hair Brown Eyes _________
– Brown Hair Blue Eyes _________
– Blonde Hair Brown Eyes _________
– Blonde Hair Blue Eyes _________<br>
22
Phenotype Ratios Phenotypes
– Brown Hair Brown Eyes ____9____
– Brown Hair Blue Eyes ____3____
– Blonde Hair Brown Eyes ____3____
– Blonde Hair Blue Eyes ____1____<br>
23
Phenotype Percentages Phenotypes
– Brown Hair Brown Eyes ____56%___
– Brown Hair Blue Eyes ____19%___
–Blonde Hair Brown Eyes ____19%___
– Blonde Hair Blue Eyes ____6%____<br>
