Name

Period Date

INTRODUCTION

As you know from your studies in genetics, chromosomes work in pairs. The members of each pair of chromosomes are called homologous chromosomes and are approximately the same length and shape and carry alleles for the same genes. Each chromosome of a pair comes from a different parent: one from the “mother” through the egg, and the other from the “father” through the sperm. Humans have 23 pairs of chromosomes, or a total of 46 chromosomes per cell. Only 22 of these pairs are truly homologous. The twenty-third pair, the sex chromosomes, may or may not match, depending on whether the individual is female (XX) or male (XY).

The diagram to the rights shows a generalized view of one pair of homologous chromosomes from an individual human. The two chromosomes are the same shape and size, and carry alleles for the same genes at equivalent positions. Assume the chromosome on the left is from the father and the one on the right is from the mother.

You should be aware that any two alleles of a gene have different effects on the trait that they control. For example, the M allele causes more melanin to be made in the skin, giving a darker color; the m allele causes less melanin, resulting in lighter colored skin. Even though the effects are different, the two alleles in an allele pair control the same trait  in this case skin color.

SAMPLE CHROMOSOMES

Father / Mother / Characteristic
F  freckled / f  unfreckled / Freckled skin
T  can taste / t  can’t taste / Ability to taste bitter
B  brown / b  blond / Hair color
h  no disease / H  has disease / Genetic disease
Rh+ / Rh - / Rh blood factor (+ is dominant)
M  more / m  less / Melanin skin color
c  color blind / C  color vision / Color perception

Determine the genotypes and phenotypes of the person whose chromosomes are shown above.

Trait / Genotype / Phenotype
Having freckles
Ability to taste bitter
Hair color
Genetic disease
Rh blood factor
Skin color
Color vision

ACTIVITY

  • Determine the genotypes and phenotypes of the people whose chromosomes appear below.
  • Then answer the questions that follow about them and their families.

Dave / Father / Mother / Genotype / Phenotype
f / F / Ff / freckled
t / t
b / B
h / h
Rh+ / Rh+
m / m
C / C
Juanita / Father / Mother / Genotype / Phenotype
f / F
T / T
b / b
h / h
Rh- / Rh-
m / m
C / c
Phil / Father / Mother / Genotype / Phenotype
f / f
t / t
B / B
h / H
Rh- / Rh-
M / M
c / c

QUESTIONS

In some cases where the child inherited a recessive allele, you may not have enough information to answer the question  Check NEI for “Not enough information”

YES / NO / NEI
  1. Was Dave’s mother freckled?
/  /  / 
  1. Was Dave’s father freckled?
/  /  / 
  1. Does FF show more freckles than Ff?
/  /  / 
  1. Were either of Dave’s parents tasters?
/  /  / 
  1. Are all of Juanita’s brothers and sisters blond?
/  /  / 
  1. When Juanita has children of her own, will they be tasters?
/  /  / 
  1. Do Juanita’s parents have a genetic disease?
/  /  / 
  1. Is Phil’s hair dark?
/  /  / 
  1. Do Dave’s parents have a positive (+) blood type?
/  /  / 
  1. Were both of Phil’s parents color-blind?
/  /  / 

ANALYSIS

  1. Would Juanita like the taste of baker’s chocolate? Explain why or why not genetically.
  2. Color-blindness is a recessive trait associated with the X chromosome of the 23rd pair. Why are males more likely to show the color-blindness trait than females?
  3. In this lab, “M” stands for the occurrence of melanin. How can you account for all the variations in skin color if “M” is dominant over “m”?