7.01SC | Fall 2011 | Undergraduate

Fundamentals of Biology

Genetics

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a diagram of a human pedigree chart

A diagram of a human pedigree chart (image by Michelle Mischke).

This unit will cover the background and assumptions that Mendel made regarding the inheritance of particular traits, the hypotheses he developed, the experiments he performed to test the hypotheses, and the conclusions he made. We will explore why inheritance does not always follow Mendel’s rules, or the rules predicted for the chromosome theory of inheritance, and how the pattern of inheritance depends upon the location of the genes studied with respect to one another. We will study the inheritance of genes in humans, using pedigree analysis. Finally, we will cover how genetics can be used to study biochemistry, and how a collection of mutants with the same phenotype can be used to study a biochemical pathway.

During this unit, you will be introduced to common genetic terms and learn to use them accurately. You will predict the outcome of genetic crosses involving one, two or three unlinked genes, as well as one, two, or three linked genes. In addition, you will design genetic crosses that can determine the genotype of an individual, and whether a trait is dominant or recessive. You will then draw a simple genetic map based upon data from test crosses. You will also construct a pedigree based on a family history, and analyze the information in the pedigree by applying the laws of inheritance.

By the end of this unit, you will understand the relationship of meiosis to Mendelian inheritance and Punnett Squares, and why Mendel’s laws of inheritance do not apply to linked genes. You will understand the linear arrangement of genes along a chromosome, and how meiosis and crossing over results in recombinant gametes. You will be able to determine the mode of inheritance of a trait based upon the information in a pedigree, and predict the probability that a child of particular parents would inherit the trait in question.

Finally, you will become familiar with the life cycle of yeast and understand the difference between a genetic screen and a genetic selection. You will have learned why mutants with mutations in different genes can have the same phenotype. You will understand tests of dominance, complementation, and epistasis, and why they are performed.

Looking for something specific in this course? The Resource Index compiles links to most course resources in a single page.

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Session Overview

Genetics_sess_4.jpg

This session will explain how a collection of mutants with the same phenotype can be used to study a biochemical pathway. In effect, this session outlines how genetics can be used to study biochemistry. It includes a discussion of yeast as a model system. It outlines how to identify mutants with mutations in the same pathway, how to characterize the mutants into complementation groups, and how to use epistasis tests to infer details about a biochemical pathway.

Learning Objectives

  • To be familiar with the life cycle of yeast.
  • To understand the difference between a genetic screen and a genetic selection.
  • To understand why mutants with mutations in different genes can have the same phenotype.
  • To understand what a test of dominance is and why it is performed.
  • To understand what a test of complementation is and why it is performed.
  • To understand what a test of epistasis is and why it is performed.

Session Activities

Lecture Video

For this session, watch the video lecture called “Genetics 3” by Prof. Eric Lander recorded in 2004. Then watch the first 7 ½ minutes of the lecture called “Human Genetics.”

Check Yourself

Question 1

A mutant haploid yeast cell that cannot grow without added histidine (mutant 1) is crossed to a wild-type haploid yeast cell (can grow without added histidine). The resulting diploid yeast cell CANNOT grow without added histidine. Which of the following statements is correct?

The phenotype seen in the heterozygotic diploid is dominant.

Question 2

The phenotypes of two haploid tryptophan auxotrophs (trp1 and trp2) are both found to be recessive. These two mutants are crossed and the resulting diploid CAN grow on minimal media. Which of the following statements is correct?

Session Activities

Help Session Videos

First, watch the short video of Genny Gould explaining the concept of Complementation.

Next, watch Genny work a Complementation Problem (PDF) using fruit flies as an example.

Practice Problems

Further Study

Suggested topics for further study in an introductory-level Biology textbook

  • Test of dominance
  • Test of complementation
  • Test of epistasis

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Session Overview

For this exam, you will need to know how to predict the outcomes of genetic crosses involving one or more genes. You will need to know the possible genotypes and phenotypes, modes of inheritance, and mutant and wild-type genotypes and phenotypes. You will also need to know how to analyze the outcomes of genetic crosses involving linked and unlinked genes, as well as how to analyze the information in a pedigree chart.

Check Yourself

Exam and Solutions

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Session Overview

Genetics_sess_2.jpg

This session will explain why inheritance does not always follow Mendel’s rules, or the rules predicted for the chromosome theory of inheritance. Because thousands of different genes are found along a single chromosome, and only one of the pair of chromosomes found in the parent is passed to the offspring, Mendel’s law of independent assortment cannot apply to all genes. Because homologous chromosomes can exchange DNA, resulting in recombination, alleles of genes found on the same chromosome in the parent are not necessarily inherited together. Thus the pattern of inheritance depends upon the location of the genes studied with respect to one another. Mating experiments can be used to map the relative distance between linked genes.

Learning Objectives

  • To understand why Mendel’s laws of inheritance do not apply to linked genes.
  • To understand the linear arrangement of genes along a chromosome.
  • To understand how meiosis and crossing over results in recombinant gametes.
  • To predict the outcome of genetic crosses involving one, two or three linked genes.
  • To draw a simple genetic map based upon data from test crosses.

Session Activities

Lecture Video

Watch the lecture video

Check Yourself

Fruit flies can have red eyes or brown eyes. Fruit flies can have long body hairs or short body hairs. You cross a true-breeding female fly (P1) with red eyes and short hairs to a true-breeding male fly (P2) with brown eyes and long hairs. All of the F1 flies have red eyes and long hairs. Assume the E and H loci are linked.

Question 1

Which of the following phenotypes are dominant?

The F1 flies show the dominant phenotypes, so red eyes are dominant to brown eyes and long hairs are dominant to short hairs.

Question 2

Give the genotypes of the gametes produced in the F1 fly that result from a meiotic event where no recombination occurs between the two loci.

The F1 fly has the genotype EeHh, because E and h were inherited together from mom, and e and H were inherited together from dad. If no recombination occurs during meiosis in the F1 cell, only two types of gametes will be produced, eH and Eh.

Session Activities

Practice Problems

Further Study

Suggested topics for further study in an introductory-level Biology textbook

  • Mendel’s laws of inheritance
  • Common genetic terms
  • Genetic crosses
  • Mendelian inheritance
  • Punnett Squares

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Session Overview

Genetics_sess_1.jpg

This session will cover the background and assumptions that Mendel made regarding the inheritance of particular traits, the hypotheses he developed, the experiments he performed to test the hypotheses, and the conclusions he made. In addition, how offspring acquire genes (and thus traits) from parents by inheriting chromosomes, and how the movement of chromosomes during meiosis is related to Mendel’s rules of inheritance will be emphasized.

Learning Objectives

  • To understand how experimentation resulted in Mendel’s laws of inheritance.
  • To accurately use common genetic terms.
  • To predict the outcome of genetic crosses involving one, two or three unlinked genes.
  • To design a genetic cross that can determine whether a trait is dominant or recessive.
  • To design a genetic cross that can determine the genotype of an individual.
  • To understand the relationship of meiosis to Mendelian inheritance and Punnett Squares.

Session Activities

Lecture Video

Watch the lecture video excerpt

Check Yourself

You are working with the following pea plants:

  • Plant 1: Purple flowers
  • Plant 2: Purple flowers
  • Plant 3: White flowers

You set up three mating experiments using these plants and obtain the following results:

  Ratio of plants with following Phenotype
Experiment Purple flowers White flowers
#1: P1 with P2 100 0
#2: P1 with P3 100 0
#3: P2 with P3 50 50
Question 1

Which phenotype is dominant?

Experiment #2 demonstrates that purple flower color is dominant to white flower color.

Question 2

What is the genotype of Plant 1 with respect to flower color?

Question 3

What is the genotype of Plant 2 with respect to flower color?

Question 4

What is the genotype of Plant 3 with respect to flower color?

Session Activities

Lecture Video

Watch the lecture video excerpt

Check Yourself

You are studying two true-breeding varieties of plants. Flower size is controlled by the A gene and flower color is controlled by the B gene.

Parent 1: small yellow flowers
Parent 2: large red flowers

Question 1

You cross the two true-breeding parental plants, and find that **all** the F1 progeny have small red flowers. Which of the following phenotypes are dominant?

Question 2

You cross the two true-breeding parental plants, and find that **all** the F1 progeny have small red flowers. What is the genotype of Parent 1 with respect to flower color and size? Use A and B for the dominant traits, a and b for the recessive traits.

Question 3

You cross the two true-breeding parental plants, and find that **all** the F1 progeny have small red flowers. What is the genotype of Parent 2 with respect to flower color and size? Use A and B for the dominant traits, a and b for the recessive traits.

Question 4

You cross the two true-breeding parental plants, and find that **all** the F1 progeny have small red flowers. What is the genotype of F1 progeny with respect to flower color and size? Use A and B for the dominant traits, a and b for the recessive traits.

Question 5

You cross an F1 plant with a true breeding plant that has large yellow flowers and obtain 1000 progeny. Assume that the two genes assort independently. The progeny from this cross could show how many different phenotypes?

Question 6

You cross an F1 plant with a true breeding plant that has large yellow flowers and obtain 1000 progeny. Assume that the two genes assort independently. Of the 1600 progeny plants, approximately how many would be expected to have large yellow flowers?

Session Activities

Practice Problems

Further Study

Study Guides

Suggested topics for further study in an introductory-level Biology textbook

  • Mendel’s laws of inheritance
  • Common genetic terms
  • Genetic crosses
  • Punnett Squares
  • Meiosis, and its relationship to Mendelian inheritance and Punnett Squares.

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Session Overview

Genetics_sess_3.jpg

Genetics in humans cannot be studied by performing controlled crosses rather, analysis of inheritance patterns in an existing population must be used. An approach, called pedigree analysis, is used to study the inheritance of genes in humans. This session will outline how to construct a family pedigree, and how to interpret the information in a pedigree using Mendel’s laws of inheritance and an understanding of the chromosome theory of inheritance.

Learning Objectives

  • To construct a pedigree based on a family history.
  • To analyze the information in a pedigree by applying the laws of inheritance.
  • To determine the mode of inheritance of a trait based upon the information in a pedigree.
  • To predict the probability a child of particular parents would inherit the trait in question.

Session Activities

Lecture Video

For this session, watch the video lecture called “Human Genetics” by Prof. Eric Lander recorded in 2004, starting partway into the lecture until the end.

Check Yourself

Question 1

The following pedigree represents the inheritance pattern of a specific genetic trait in humans. Assume the trait is common in the population. ![gensession3_quizzes.gif](/courses/7-01sc-fundamentals-of-biology-fall-2011/2550e3b6eb2d7737c03c6626fa4c3352_gensession3_quizzes.gif) This disorder could be inherited in which of the following ways?

Session Activities

Help Session Video

Watch the short video of Genny Gould explaining how to analyze a human pedigree (Question 2 in the practice problems below).

Practice Problems

Further Study

Suggested topics for further study in an introductory-level Biology textbook

  • Constructing a family pedigree
  • Pedigree analysis
  • Chromosome theory of inheritance

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