9.7.1 Polygenic Inheritance

Question 1

Polygenic Inheritance

Answer 1

• Mendel’s experiments trace characters that vary in an either/or manner. Quantitative characters are characters that vary in a continuous manner. Height is one example.
• Because of polygenic inheritance, the parental phenotypes may vary greatly from offspring phenotypes.
• Many traits result from a combination of polygenic inheritance and interaction with the environment, making the prediction of phenotypes difficult.

Question 2

note

Answer 2

• Review: Mendel’s experiments involved characters that
  varied in an either/or manner.
• Quantitative characters vary in populations along a
  continuum because many variations of phenotype exist.
• The influence of two or more genes may have an additive effect on one phenotypic character. This phenomenon is called polygenic inheritance.
• Genes inherited in a Mendelian (either/or) fashion can be traced by constructing family pedigrees such as the one on the left.
• Characteristics resulting from polygenic inheritance are
  difficult to trace using this method.
• Several genes influence the phenotype of skin color. In the example on the left, three loci contribute to skin color. The heterozygous condition “AaBbCc” produces moderately dark skin. What is the probability that two heterozygotes would have light-skinned offspring?
• Using the multiplicative rule, determine the probability that the cross will produce an individual with the aabbcc
  genotype. The probability of receiving each allele is 1/2, and there are six alleles. 1/2 X 6 = 1/64.
• The same process is used to determine the probability that the offspring will have the AABBCC genotype and be very dark skinned. The probability is 1/64.
• When three genes code for skin color, a range in skin color results. Two heterozygous individuals can produce six different phenotype classes. The environment interacts with the genotypes, causing an even greater number of phenotypes.
  The result is that skin color can be represented by a
  bell-shaped curve, with most of the population falling in the middle.
• Other human characteristics—like height and weight—are the result of these combined influences and can also be represented by a bell-shaped curve.

Question 3

What occurs when two or more genes influence a phenotype in an additive manner?

Answer 3

• Polygenic inheritance

Question 4

Mendel’s experiments involved which of the following kinds of traits?

Answer 4

• Traits that are either present or absent

Question 5

When a trait is governed by two or more sets of alleles, possibly located on different pairs of chromosomes, the phenomenon is called __________ inheritance.

Answer 5

• polygenic

Question 6

A common distribution of phenotypes for polygenic traits is a pattern of

Answer 6

• a bell-shaped curve.

Question 7

Which of the following best describes a quantitative characteristic?

Answer 7

• A characteristic that varies in a continuous manner.

Question 8

Which of the following statements concerning the sun’s effect on skin color is not true?

Answer 8

• The production of melanin is not affected by the environment.

Question 9

Two individuals of the genotype “AaBbCcDd” are crossed. What is the probability of producing an offspring of the genotype AaBBccdd?

Answer 9

• 2/256.