Download MODELING POLYGENIC INHERITANCE Polygenic traits are

MODELING POLYGENIC INHERITANCE
MODELING POLYGENIC INHERITANCE
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.
• 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 coins)
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 feet, 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 coins 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
combination
0H
6T
Height
4ft.
9in.
1H
5T
2H
4T
3H
3T
4H
2T
5H
1T
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:
6H
0T
Number of
offspring
Follow-Up
1. Make a bar graph of height (histogram) on the x-axis and number of
offspring on the y-axis. Glue it in your journal.
2. What is the OVERALL shape of this graph? Why?
3. Explain why medium parents usually have medium kids, but can have
offspring who are SHORTER or TALLER than they are.
4. Why is it not appropriate to say that being tall is “dominant” over being
short? Explain.
Directions for using our model
1. Assume 3 genes carried by 3 chromosomes (represented by 3 coins)
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 feet, 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 coins 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
combination
0H
6T
Height
4ft.
9in.
1H
5T
2H
4T
3H
3T
4H
2T
5H
1T
6H
0T
Number of
offspring
Follow-Up
1. Make a bar graph of height (histogram) on the x-axis and number of
offspring on the y-axis. Glue it in your journal.
2. What is the OVERALL shape of this graph? Why?
3. Explain why medium parents usually have medium kids, but can have
offspring who are SHORTER or TALLER than they are.
4. Why is it not appropriate to say that being tall is “dominant” over being
short? Explain.