3.2.2 Monohybrid Crosses

• What is a monohybrid cross?

  Inheritance of a single trait
• What is a dihybrid cross?

  Inheritance of two traits
• What is the term for the condition where an individual has two identical alleles for a trait?

  Homozygous
• What type of letters are used to represent dominant alleles in a Punnett square?

  Uppercase
• What is a Punnett square used for?

  Predicting offspring genotypes
• In the Punnett square for parents Tt x Bb, what is the genotype of one possible offspring?

  TB
• When completing a Punnett square, the possible genotypes of the offspring are shown in the grid.
• To complete a Punnett square, create a 2x2 grid.
• A monohybrid cross examines the inheritance of one gene with two alleles.
• What type of letter is used to represent a dominant allele?

  Uppercase
• What type of grid is created for a monohybrid cross?

  2x2
• Steps in a monohybrid cross

  1️⃣ Identify parental genotypes

  2️⃣ Create a Punnett square

  3️⃣ Determine offspring genotypes

  4️⃣ Determine offspring phenotypes
• A recessive allele is only expressed when two copies are present.


  True
• Steps in a monohybrid cross

  1️⃣ Identify parental genotypes

  2️⃣ Create a Punnett square

  3️⃣ Determine offspring genotypes

  4️⃣ Determine offspring phenotypes
• In codominance, both alleles are expressed equally in the phenotype.


  True
• Steps to create a Punnett square

  1️⃣ Identify parental genotypes

  2️⃣ Create a 2x2 grid

  3️⃣ Label the top row with gametes from one parent

  4️⃣ Label the left column with gametes from the other parent

  5️⃣ Combine gametes to show offspring genotypes
• What are the four possible genotypes for offspring in the Punnett square for Tt x Bb?

  TB, Tb, tB, tb
• What is the first step in completing a Punnett square for a monohybrid cross?

  Identify parental genotypes
• A monohybrid cross examines the inheritance of a single trait with two alleles.


  True
• In a monohybrid cross, the inheritance of a trait is determined by dominant and recessive alleles.

  True
• The recessive allele is only expressed when two copies are present in the genotype.

  True
• When filling in a Punnett square, you combine the gametes from the top row and left column.

  True
• What type of cross uses a 4x4 Punnett square?

  Dihybrid cross
• If both parents have heterozygous Bb genotypes, their offspring will have a genotypic ratio of 1:2:1.


  True
• A monohybrid cross examines the inheritance of a single trait
• Steps to determine genotypic and phenotypic ratios in a monohybrid cross

  1️⃣ Identify the parental genotypes

  2️⃣ Create a Punnett square

  3️⃣ Determine the offspring genotypes

  4️⃣ Determine the offspring phenotypes

  5️⃣ Calculate genotypic ratios

  6️⃣ Calculate phenotypic ratios
• Dominant alleles require two copies for expression, while recessive alleles require only one copy.

  False
• What is a dihybrid cross concerned with the inheritance of?

  Two characteristics
• What does a dominant allele do to the expression of a recessive allele?

  Masks or overrides
• The dominant allele determines the observed physical characteristic, also known as the phenotype
• Steps to set up a Punnett square for a monohybrid cross

  1️⃣ Identify the parental genotypes

  2️⃣ Create a 2x2 grid

  3️⃣ Label the top row with gametes from one parent

  4️⃣ Label the left column with gametes from the other parent

  5️⃣ Combine gametes to show offspring genotypes
• The Punnett square for parents with Tt x Tt genotypes shows possible offspring genotypes TT, Tt, and tt.


  True
• What is the phenotypic ratio if both parents have heterozygous Bb genotypes, where B represents brown hair and b represents blonde hair?

  3:1
• The phenotypic ratio is the same as the genotypic ratio in a monohybrid cross.

  False
• Dominant alleles are represented by lowercase letters.

  False
• Recessive alleles require one copy for expression.

  False
• The first step in a monohybrid cross is to identify the parental genotypes
• A dominant allele is represented by an uppercase letter.
• The dominant allele determines the observed physical characteristic, or phenotype.
• What is the phenotype of an individual with the genotype Bb if B represents brown hair and b represents blonde hair?

  Brown hair