Name: ________________________________________________ Period: _________________

 

Investigating Inherited Human Traits

Dominant, Incomplete Dominant and Recessive Traits

Background:

Heredity is the passing on of traits, or characteristics, from parent to offspring. The units of heredity are called genes. Genes are found on the chromosomes in a cell. The combinations of genes for each trait occur by chance.

When one gene in a pair is stronger than the other gene, the trait of the weaker gene is masked or hidden. The stronger gene is the dominant gene, and the gene that is masked is the recessive gene. Dominant genes are written as capital letters and recessive genes are written as lowercase letters. If both genes in a gene pair are the same, the trait is said to be homozygous or pure. If the genes in a gene pair are not the same, the trait is said to be heterozygous or hybrid. Sometimes genes are neither dominant nor recessive. They are equally strong and the result of such a situation is a blending of the traits in a heterozygous individual. This is called incomplete dominance.

The genetic makeup of an individual is known as its genotype. The observable physical characteristics of an individual that are the result of its genotype are known as its phenotype. In humans, the sex of an individual is determined by the particular combination of the two sex chromosomes. Individuals that have two X chromosomes (XX) are females, whereas those with an X and Y chromosome (XY) are males. In this investigation, you will observe how the results of different gene combinations produced certain traits.

 

Problem: How are traits inherited?

 

Materials: 2 different coins (a nickel and a penny)

      Pencil

Procedure:

1.      Whoever has the nickel is the male parent (dad) and whoever has the penny is the female parent (mom). Remember that there are two genes per trait.

2.      Have the partner who is representing the dad flip the nickel to determine the sex of the offspring. If the coin lands heads up, the offspring is a female. If the coin lands tails up, the offspring is a male. Record the sex of the offspring in Observations. Come up with a name for your offspring and record it in Observations.

3.      For all the remaining coin tosses you will now make, heads will represent the dominant gene and tails will represent the recessive gene.

4.      You and your partner should flip your coins at the same time to determine the shape of face. If you both have heads, the genotype is RR and the baby’s face will be round. If one of you has heads and the other has tails, then the genotype is Rr and the baby’s face will be round. If you both have tails, the genotype is rr and the baby’s face will be square. NOTE: You will flip the coins only once for each trait.

5.      Continue to flip coins for each trait listed in the table in Figure 1. After each flip, record the trait of you offspring by circling the appropriate box in the table.

6.      Using the recorded traits, draw the facial features for your offspring in the space provided in Observations. Please use colored pencils.

 

           

 

Observations:

            Sex of the offspring: ______________________________________________________

            Name of the offspring: ____________________________________________________

Drawing of the face of your offspring:

  

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Analysis and Conclusions:

1.      What was the sex of your offspring? How is the sex of an offspring determined in real life?

 

2.      What percent chance did you and your partner have of “producing” a male offspring? A female offspring?

 

 

3.      There have been cases in history where a king divorces his queen because she produces only daughters. Using your knowledge of genetics, explain why this was an incorrect move.

 

4.      If a woman who is homozygous for almond-shaped eyes (AA) marries a man who is heterozygous for almond-shaped eyes (Aa), what are the possible phenotypic and genotypic ratio of their offspring. Complete the Punnett square to help you.

 

 

 

 

 

 

 

Phenotypic ratio:

Genotypic ratio:

5.      What are the possible genotypes of the parents of a child who has curly hair (HH)? Complete the following Punnett squares by filling in the correct genotypes of the parents to help you.

 

HH

 

HH

 

HH

Hh

Hh

 

Hh

 

Hh

hh

 

HH

 

HH

HH

 

HH

 

Possible parent genotypes: _____________ X _____________

Possible parent genotypes: _____________ X _____________

Possible parent genotypes: _____________ X _____________

6.      List the seven traits in this investigation that show incomplete dominance in the heterozygous individuals. NOTE: the heterozygous phenotype is a blending of the two traits.

 

7.      Find the phenotypic and genotypic ratio of the offspring for a cross between man with normal shaped lips and a woman with thick shaped lips. Complete a Punnett square to help you. This is a case of INCOMPLETE DOMINANCE.

 

 

 

 

 

 

 

Phenotypic ratio:

 

Genotypic ratio:

 

8.      Find the phenotypic and genotypic ratio of the offspring for a cross between man and a woman both heterozygous for a cleft in the chin. Complete a Punnett square to help you.

 

 

 

 

 

 

 

Phenotypic ratio:

 

Genotypic ratio:

 

9.      How might is be possible for you to show a trait when neither of your parents shows it?

 

 

10.  If you were to repeat this investigation with your partner to “produce” a second offspring, would you expect the offspring to look like your first offspring? Explain your answer.