Punnett Squares 1
Monohybrid Crosses
TEKS
• 6F: Students will…
predict possible outcomes of
genetic combinations such as
monohybrid crosses, dihybrid crosses,
and non-Mendelian inheritance
Gregor Mendel
• Studied the different physical characteristics of pea plants
• Noticed characteristics of the offspring resembled their parents
• This passing of traits from parents to offspring is called heredity
• Genetics = scientific study of heredity
Genetics Vocabulary
• Gene = segment of DNA where genetic
instructions for traits are found
Genetics Vocabulary
• Allele = different versions of genes (ex: the gene
for eye color can be determined by a brown allele or a blue allele)
Genetics Vocabulary
• Dominant = the allele that is stronger and ALWAYS expressed (covers up the recessive allele)▫ Represented with a capital letter
• Recessive = the weaker allele; only seenwhen found on BOTH chromosomes▫ Represented with a lowercase letter
• *An allele being recessive or dominant has nothing to do with how common it is
Genetics Vocabulary
• Homozygous = both alleles are the same(dominant OR recessive)
• Heterozygous = each allele is different (one dominant, the other recessive)
Genetics Vocabulary
• Genotype = represents the genetic code (letters) for each allele
Three types Homozygous dominant (BB)
Homozygous recessive (bb)
Heterozygous (Bb)
• Phenotype = physical appearance seen BECAUSE of the genetic code
Example: Blue or brown eyes
Genetics Vocabulary
Genetics Vocabulary
• Monohybrid: mating between two individuals with different alleles fora single gene
• Dihybrid: mating between two individuals with different alleles for two genes
Mendel’s Laws
• Law of Segregation
▫ During the formation of gametes, the two allelesresponsible for a trait separate from each other
▫ Alleles for a trait are then “recombined” at fertilization, producing the genotype for the traits of the offspring
Mendel’s Laws
• Law of Segregation
Mendel’s Laws
• Law of Independent Assortment
▫ Alleles for different traits are distributed to sex cells (and therefore offspring) independently of one another
▫ This law can be illustrated using dihybrid crosses
▫ Dihybrid cross = a breeding experiment that tracks the inheritance of two traits
Number of gametes formed = 2n
*(n = # of heterozygous traits)
Mendel’s Laws
• Law of Independent Assortment
Mendel’s Laws
• Law of Dominance
▫ In a cross of parents that are homozygous forcontrasting traits, only one form of that trait will appear in the next generation
▫ All the offspring will be heterozygous andexpress only the dominant trait
▫ Ex: one parent is homozygous for brown eyes and the other is homozygous for blue eyes. All offspring will have brown eyes
BB x bb Bb
Mendel’s Laws
• Law of Dominance
Probability and Independence of Events
• Probability▫ The number that describes how likely it is that an
event will occur
▫ Usually written as a percentage
▫ Example: probability of a coin toss – heads or tails
There are two possibilities each time you toss the coin – heads or tails –each is as likely as the other
The probability of heads is 1 in 2, which is the sameas the probability for tails
This can also be expressed as a fraction – ½
That fraction can be converted into a percent – 50%
Probability and Independence of Events
• Probability (cont.)▫ Keep in mind that this is a prediction of what is
LIKELY to occur, not necessarily what WILL occur
The more tosses you make, the closer your actualresults will be to the prediction
• Independence of Events▫ One event has no effect on another
▫ For our coin toss example – the results of the first toss do not affect the results of the second, and so on
Punnett Square
• Also called a genetic cross
• One parent’s traits are placed at the top of the box
• The second parent’s traits are placed along the side of the box
• The traits of the offspring are placed inside the box
Let’s Try One Together
• Parents: Bb x Bb
Let’s Try One Together
• Parents: Bb x Bb
Let’s Try One Together
• Parents: Bb x Bb
Let’s Try One Together
• Parents: Bb x Bb
Let’s Try One Together
• Parents: Bb x Bb
Let’s Try One Together
• Parents: Bb x Bb
Let’s Try One Together
• Parents: Bb x Bb
Let’s Try One Together
• Parents: Bb x Bb
Let’s Try One Together
• Parents: Bb x Bb
Let’s Try One Together
• Parents: Bb x Bb
Let’s Try One Together
• Parents: Bb x Bb
Let’s Try One Together
• Parents: Bb x Bb
Let’s Try One Together
• Parents: Bb x Bb
Determining Ratios - Genotypic
• Parents: Bb x Bb
• Offpspring:
▫ ¼ = 25% BB
▫ 2/4 = ½ = 50% Bb
▫ ¼ = 25% bb
• 1:2:1 genotypic ratio
Determining Ratios – Phenotypic
• Parents: Bb x Bb
• Offspring:
▫ ¾ = 75% brown
▫ ¼ = 25% blue
• 3:1 phenotypic ratio