Monohybrid Crossesscienceinthemaking.weebly.com/.../1._punnett_squares_-_monohybri… · monohybrid crosses, dihybrid crosses, and non-Mendelian inheritance. Gregor Mendel •Studied

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