3vt monohybrid crosses

• Biologie
• Middelbare school
• vwo
• Leerjaar 3

Recap

Each individual contains 23 pairs of chromosomes.

You receive one from your mother and one from your father.

Every chromosome contains many genes also called alleles 

The alleles from your mother and father together determine the hereditary characteristics

Homozygous and heterozygous

Each individual contains two identical or two non-identical alleles.

Two identical alleles: homozygous

Two non-identical alleles: heterozygous

Dominant and recessive

In a heterozygous indivual only one allele can be expressed in the phenotype: this is called the dominant allele. 

The stronger allele

The other allele is called the recessive allele. This allele is only expressed in the phenotype when it comes in a pair.

Genotype

In genetics the genotype is expressed in capital and small letters:

AA: homozygous dominant

aa: homozygous recessive

Aa: heterozygous

Exercise 1

A father has brown eyes. He is heterozygous for eye colour. Mother has blue eyes. The gene for brown eyes is dominant.

a. Is mother homozygous or heterozygous for eye colour?

   

   b. Two children are born. Child 1 has brown eyes and child 2 has blue eyes. Show this in a cross. Use the letters B and b

P

P

genotype

phenotype

F1

sex cells

possibilities for F1:

Father has brown eyes. 

P

P

genotype:

phenotype:

F1

sex cells:

brown eyes

possibilities for F1:

Mother has blue eyes. 

P

P

genotype:

phenotype:

F1

sex cells:

brown eyes

blue eyes

possibilities for F1:

A father has brown eyes. He is heterozygous for eye colour. Mother has blue eyes. The gene for brown eyes is dominant.

What is the genotype of the father?

1. BB
2. Bb
3. bb

What is the genotype of the mother?

1. BB
2. Bb

P

P

genotype:

phenotype:

F1

sex cells:

brown eyes

blue eyes

bb                             Bb

possibilities for F1:

A father has brown eyes. He is heterozygous for eye colour. Mother has blue eyes. The gene for brown eyes is dominant.

a. Is mother homozygous or heterozygous for eye colour?

  

Is mother homozygous or heterozygous for eye colour?

1. homozygous
2. heterozygous

P

P

genotype:

phenotype:

F1

sex cells:

brown eyes

blue eyes

bb                             Bb

possibilities for F1:

And now the sex cells are made...

P

P

genotype:

phenotype:

F1

sex cells:

brown eyes

blue eyes

bb                             Bb

b

b

b

B

possibilities for F1:

P

P

genotype:

phenotype:

F1

sex cells:

brown eyes

blue eyes

bb                             Bb

b

b

b

B

possibilities for F1:

What eye colour do the children (F1) have?

P

P

genotype:

phenotype:

F1

sex cells:

	B	b
b
b

brown eyes

blue eyes

bb                             Bb

b

b

b

B

possibilities for F1:

spermcells 

egg cells

P

P

genotype:

phenotype:

F1

sex cells:

	B	b
b	Bb
b

brown eyes

blue eyes

bb                             Bb

b

b

b

B

possibilities for F1:

P

P

genotype:

phenotype:

F1

sex cells:

	B	b
b	Bb	bb
b

brown eyes

blue eyes

bb                             Bb

b

b

b

B

possibilities for F1:

P

P

genotype:

phenotype:

F1

sex cells:

	B	b
b	Bb	bb
b	Bb

brown eyes

blue eyes

bb                             Bb

b

b

b

B

possibilities for F1:

P

P

genotype:

phenotype:

F1

sex cells:

	B	b
b	Bb	bb
b	Bb	bb

brown eyes

blue eyes

bb                             Bb

b

b

b

B

possibilities for F1:

P

P

genotype:

phenotype:

F1

sex cells:

	B	b
b	Bb	bb
b	Bb	bb

brown eyes

blue eyes

bb                             Bb

b

b

b

B

genotype        

• 2x Bb (50%) 
  
• 2x bb (50%) 
  

possibilities for F1:

P

P

genotype:

phenotype:

F1

sex cells:

	B	b
b	Bb	bb
b	Bb	bb

brown eyes

blue eyes

bb                             Bb

b

b

b

B

genotype        

• 2x Bb (50%) 
  
• 2x bb (50%) 
  

phenotype

• brown eyes
  
• blue eyes
  

possibilities for F1:

Exercise 2

A man is right handed. He has two identical alleles for this trait. A woman is right-handed as well, but she is heterozygous.

When the man and woman will have a baby. What is the chance that the baby has the same phenotype as his/her parents.

P

P

genotype

phenotype

F1

sex cells

possibilities for F1:

Add a photo of your cross

Exercise 3

Two black guinea pigs that are both heterozygous for fur colour get offspring.  The black allele is dominant and the white allele is recessive

What can be the genotypes of the offspring?

What can be the phenotypes of the offspring? 

Give also the proportions in percentages.

P

P

genotype

phenotype

F1

sex cells

possibilities for F1:

Add a photo of your cross

A family tree

What phenotype is dominant?