Book cover for Biology

Biology

Sylvia S. Mader, Michael Windelspecht

ISBN #9780078024269

12th Edition

687 Questions

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Summary
Learning Objectives
Key Concepts
Example Problems
Explanations
Common Mistakes

Summary

This chapter covers the pioneering work of Gregor Mendel and the foundational genetic principles he established through his experiments. The text emphasizes Mendel's laws of segregation and independent assortment, detailing how Punnett squares are used to predict genetic outcomes. It also extends these concepts to complex inheritance patterns including autosomal dominant and recessive traits, X-linked characteristics, incomplete dominance, pleiotropy, and polygenic inheritance, highlighting both their historical impact and modern applications in understanding human diseases.

Learning Objectives

1

Explain how Gregor Mendel's experiments with garden peas led to the formulation of fundamental genetic principles.

2

Describe Mendel's laws including the law of segregation and the law of independent assortment.

3

Apply Punnett squares to predict genetic outcomes in various inheritance patterns.

4

Differentiate between Mendelian and non-Mendelian inheritance patterns including autosomal dominant/recessive traits, X-linked characteristics, incomplete dominance, pleiotropy, and polygenic inheritance.

Key Concepts

CONCEPT

DEFINITION

Gregor Mendel

The father of genetics whose experiments with garden peas led to the discovery of the particulate theory of inheritance.

Particulate Theory of Inheritance

The concept that inheritance is governed by discrete units (genes) that do not blend but rather retain their identity.

Blending Theory

An outdated concept of inheritance proposing that offspring are a uniform blend of parental traits.

Law of Segregation

Mendel’s principle stating that allele pairs separate or segregate during gamete formation, and randomly unite at fertilization.

Law of Independent Assortment

Mendel’s principle that genes for different traits can segregate independently during the formation of gametes.

Punnett Square

A diagram used to predict the outcome of a particular cross or breeding experiment by accounting for all possible combinations of alleles.

Autosomal Dominant

A pattern of inheritance where a single copy of a mutant allele on a non-sex chromosome can cause the phenotype to be expressed.

Autosomal Recessive

A pattern of inheritance where two copies of a mutant allele, one from each parent, are required for the phenotype to be expressed.

X-linked Inheritance

An inheritance pattern where a gene is located on the X chromosome, often affecting males more severely than females.

Incomplete Dominance

A form of inheritance in which the phenotype of a heterozygote is intermediate between those of the phenotypes of the homozygotes.

Pleiotropy

The phenomenon in which a single gene influences multiple, seemingly unrelated phenotypic traits.

Polygenic Inheritance

An inheritance pattern controlled by multiple genes, often resulting in a continuous distribution of phenotypes.

Example Problems

Example 1

Mendel’s work supported which of the following? a. blending theory of inheritance b. particulate theory of inheritance c. theory of acquired characteristics d. All of these are correct.

Example 2

Mendel’s success was based on a. use of the pea plant as a model organism. b. his ability to apply statistics to his studies. c. careful planning of his experiments. d. All of these are correct.

Example 3

The law of segregation states all of the following except a. factors separate during formation of the gametes. b. each individual has two factors for each trait. c. gametes contain a single factor for each trait. d. factors assort independently of each other by meiosis.

Example 4

In peas, yellow seed $(Y \text { is dominant over green seed }(y) . \text { In }$ the $\mathrm{F}_{2}$ generation of a monohybrid cross that begins when a dominant homozygote is crossed with a recessive homozygote, you would expect a. three plants with yellow seeds to every plant with green seeds. b. plants with one yellow seed for every green seed. c. only plants with the genotype Yy. d. only plants that produce yellow seeds. e. Both c and d are correct.

Example 5

In guinea pigs, smooth coat $(S)$ is dominant over rough coat $(s),$ and black coat $(B)$ is dominant over white coat $(b) .$ In the cross $S s B b \times S s B b,$ how many of the offspring will have a smooth black coat, on average? a. 1$/ 4$ b. about 9$/ 16$ C. 1$/ 16$ d. 6$/ 16$ e. 2$/ 6$
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Step-by-Step Explanations

QUESTION

How do you predict the genetic outcome of a cross between two heterozygous pea plants (Tt x Tt) for a single trait?

STEP-BY-STEP ANSWER:

Step 1: Identify the alleles for each parent. In this case, both are heterozygous (Tt).
Step 2: Write the alleles of one parent across the top of a Punnett square and the alleles of the other parent along the side.
Step 3: Fill in the squares by combining the alleles from the top and side of each cell.
Step 4: Determine that the combinations are TT, Tt, Tt, and tt.
Step 5: Conclude that the genotype ratio is 1 TT : 2 Tt : 1 tt and the phenotype ratio will depend on whether T is dominant over t.
Final Answer:

Punnett Square for a Monohybrid Cross

QUESTION

How do you approach a dihybrid cross (YyRr x YyRr) to see the independent assortment of alleles?

STEP-BY-STEP ANSWER:

Step 1: Recognize that each parent has two heterozygous gene pairs (Yy and Rr) that will assort independently.
Step 2: Set up a Punnett square with 16 boxes representing all possible allele combinations.
Step 3: Fill in the Punnett square by combining the gametes from each parent, generated using the FOIL method for each trait.
Step 4: Count the occurrence of each genotype and derive the phenotypic ratio.
Step 5: Understand that the expected phenotypic ratio is 9:3:3:1 for a dihybrid cross if the traits are independently assorting and exhibit complete dominance.
Final Answer:

Applying the Law of Independent Assortment

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Common Mistakes

  • Confusing the particulate theory of inheritance with the discredited blending theory.
  • Misinterpreting Punnett squares by not accounting for all possible allele combinations.
  • Overgeneralizing Mendelian patterns without considering non-Mendelian exceptions like incomplete dominance and polygenic traits.
  • Assuming that all traits strictly follow Mendel's laws, even when external factors or multiple genes are involved.