Download MODELING POLYGENIC INHERITANCE

MODELING POLYGENIC INHERITANCE : a do -it-yourself-lesson!
Polygenic traits are determined by several genes scattered over several chromosomes. Because many polygenic traits
are also affected by environmental factors, they are described as multifactorial. Examples in humans include height,
skin color, and fingerprint pattern. We’ll make these assumptions:
•
•
•
•
There is an active allele and an inactive allele for each gene involved,
The alleles are additive in effect; i.e. no dominance is exhibited.
The effect of each active allele is small and equal to that of each of the others.
Phenotype is determined by the sum total of all active alleles present.
Directions for using our model
1. Assume 3 genes carried by 3 chromosomes (represented by 3 pennies) determine human height. Heads (H) represent
active alleles (A, B, C), while tails (T) represent inactive alleles (a, b, c). Tossing the coins represents meiosis;
combining their outcomes represents fertilization.
2. Each active allele (heads) adds 3 inches of height to a base height of 4 ft. 9 inches (the height of an aabbcc
person). Fill in the middle row of the table below to check your understanding of the model.
3. For maximum variety, we’ll model the situation in which each parent is heterozygous for all 3 genes; that is, AaBbCc.
To model two AaBbCc people having a family, toss six pennies simultaneously! And have a big family: 64 kids!
Recopy the chart below into your journal and keep track of the kids’ heights with tally marks.
Coin combinations
Height
Number of offspring
0 H, 6 T
4ft.9in.
1 H, 5 T
2 H, 4 T
3 H, 3 T
4 H, 2 T
5 H, 1 T
6 H, 0 T
Follow-Up
1. Make a bar graph of height on the x-axis and number of offspring on the y-axis. Glue it in your journal.
2. Explain why medium parents usually have medium kids, but can have offspring who are SHORTER or TALLER than
they are.
MODELING POLYGENIC INHERITANCE : a do -it-yourself-lesson!
Polygenic traits are determined by several genes scattered over several chromosomes. Because many polygenic traits
are also affected by environmental factors, they are described as multifactorial. Examples in humans include height,
skin color, and fingerprint pattern. We’ll make these assumptions:
•
•
•
•
There is an active allele and an inactive allele for each gene involved,
The alleles are additive in effect; i.e. no dominance is exhibited.
The effect of each active allele is small and equal to that of each of the others.
Phenotype is determined by the sum total of all active alleles present.
Directions for using our model
1. Assume 3 genes carried by 3 chromosomes (represented by 3 pennies) determine human height. Heads (H) represent
active alleles (A, B, C), while tails (T) represent inactive alleles (a, b, c). Tossing the coins represents meiosis;
combining their outcomes represents fertilization.
2. Each active allele (heads) adds 3 inches of height to a base height of 4 ft. 9 inches (the height of an aabbcc
person). Fill in the middle row of the table below to check your understanding of the model.
3. For maximum variety, we’ll model the situation in which each parent is heterozygous for all 3 genes; that is, AaBbCc.
To model two AaBbCc people having a family, toss six pennies simultaneously! And have a big family: 64 kids!
Recopy the chart below into your journal and keep track of the kids’ heights with tally marks.
Coin combinations
Height
Number of offspring
0 H, 6 T
4ft.9in.
1 H, 5 T
2 H, 4 T
3 H, 3 T
4 H, 2 T
5 H, 1 T
6 H, 0 T
Follow-Up
1. Make a bar graph of height on the x-axis and number of offspring on the y-axis. Glue it in your journal.
2. Explain why medium parents usually have medium kids, but can have offspring who are SHORTER or TALLER than
they are.