Biology 100/101
Lecture 11
Heredity and Meiosis
(Print Version)

Announcements &
Assignments

Lecture Objectives

Web Resources

Glossary of
Genetic Terms

More
on Alleles

Chromosomes as CDs

The Study of
Heredity

Mendel's Law
of Segregation
(single genes)

Mendel's Law of
Independent Assortment
(two genes)

Lecture Syllabus

IB 100/101 Home Page


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Text readings in Hoefnagels

Chapter 10, Patterns of Inheritance

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Moodle

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Objectives:

The content of this lecture will help you complete these assignments:

After studying this material you should be able to:

  1. Explain what chromosomes are, what their role is, and how organisms get them.

  2. Explain which events in meiosis contribute to genetic variability, and how these events relate to "Mendel's laws" of segregation and independent assortment.

  3. Understand the relationships among these genetic terms:

    4. homologous
    chromosomes    		 sister chromatids genes
    dominant and
    recessive alleles    		 genotype phenotype
    homozygous
    (dominant and recessive)    		 heterozygous autosome
    single gene cross two gene cross pedigree

  4. Describe the functionality of proteins produced by dominant allele and recessive alleles.

  5. Given the phenotypes or genotypes of two mating individuals, predict the genotypes and phenotypes of offspring that they might produce.

  6. Given the phenotypes or genotypes of the offspring of two parents, predict the genotypes and phenotypes of the parents giving rise to them.

  7. Interpret the inheritance of cases of human genetic conditions that are autosomal dominant or autosomal recessive.

General Web resources:

Glossary of Genetic Terms

Sister Chromatids: The two halves of a replicated chromosome. Each chromatid is an identical copy of the DNA of the original chromosome before DNA replication.  Single or Replicated Chromosomes? Hoefnagels Page 157, figure 8.3

Homologous chromosomes: Two chromosomes which have the same sequence of genes (controlling the same traits). One chromosome of each pair comes from each of the parents by way of the gametes. Human Chromosomes Hoefnagels Page 158, figure 84.5

    Interactive Chromosome/Gene Poster from Gene Gateway - Exploring Genes and Genetic Disorders. Check out a few chromosomes to get an idea of the variety of gene locations found on the different homologous chromosomes.

Autosomes and sex chromosomes: Chromosomes that do not determine sex. Humans have 44 of them (22 pairs). The two remaining chromosomes determine sex and are known as the sex chromosomes - X and Y.

*Gene: A sequence of DNA specifying the sequence of amino acids of a particular protein involved in the expression of a trait.

*Gene Locus (pl. loci): The specific location of a sequence of DNA on a particular chromosome that specifies the sequence of amino acids of a particular protein involved in the expression of a trait. For example, this segment of DNA located on chromosome #7 is the gene locus of the Cystic Fibrosis membrane protein (CFTR) gene. (A more detailed look at Chromosome 7 - The Cystic Fibrosis gene locus is noted 17th from the bottom of the right-hand list of loci.)

*Allele: An alternate form of a gene. An alternate sequence of DNA at a particular Gene Locus. Alleles are formed by mutation of a pre-existing gene. As with Cystic fibrosis above, there may be several hundred mutated forms of the CFTR gene - several hundred alleles.

Genotype: The combination of alleles in an individual (an individual's genetic make up). The term can be used to describe an individual's combination of alleles for one gene locus, or more broadly, an individual's entire genetic make up.

Phenotype: The observable expression of an allele combination for a particular trait. The term may also be used to describe all the physical characteristics of an individual.

Homozygous: Possessing two identical alleles of a gene. An individual with two dominant alleles is homozygous dominant. If both alleles are recessive, the individual is homozygous recessive.

Heterozygous: Possessing two different alleles of a gene (an individual with one dominant allele and one recessive allele).

* Probably the three most frequently confused terms in biology

More on alleles

Alleles can be recognized as being "dominant" or "recessive"

A dominant allele masks the expression of a recesive allele (and is commonly symbolized by a capital letter - e.g. "A"). Dominant alleles produce proteins that result in the visible expression of a trait, regardless of the allele present on the homologous chromosome. For instance, the allele for dark eye color produces a protein that results in the presence of a dark pigment in the iris of the eye. If the allele for eye color on the homologous chromosome is recessive, it produces a lighter colored pigment that is "hidden" by the darker pigment produced by the dominant allele.

Dominant alleles may be "good" and produce a healthy condition, even in the presense of a defective recessive allele. However, some dominant alleles can produce diseased conditions. In that case, it takes only one dominant allele to make a person sick.

A recessive allele is masked (or hidden) by the presence of a dominant allele (and is commonly symbolized by the corresponding lower case letter - e.g. "a"). The expresion of recessive alleles are usually only visible when a person has inherited recessive alleles from both parents. Some recessive alleles produce variations of characteristics that do not affect a person's health.

Recessive alleles at gene loci for important proteins may produce proteins that are not functional and unable to do the job they are supposed to do. If the other allele on the homologous chromosome is dominant, it often produces enough of the needed protein to do the necessary job. However, if a person has two recessive alleles for a gene locus he/she may not be able to produce the necessary protein at all.

The Chromosome as a music CD

Music CDs are like chromosomes in that they store information in the form of microscopic pits and high places in the groves on the disk. Chromosomes store information in the sequence of nucleotides (As, Cs, Gs, and Ts) in the DNA.

As we develop the analogy, work with a partner to answer these questions.

If we think of a chromosome as a music CD:

  1. What is a sister chromatid?

  2. What is a gene locus?

  3. What is a homologous pair of chromosomes?

  4. What is an allele?

  5. What is the product of a music CD "allele"?

  6. What is the product of a real chromosome allele?

Print your name and sign your name and pass your paper to the aisles when requested.

The Study of Genetics, or Heredity

  • The study of genetics, or heredity, seeks to answer two basic questions:

    • Can we predict the genetic make up and physical characteristics of our children?

    • AND, Can we decipher the gentic make up and physical characteristics of our ancestors?

  • Understanding the way the various forms of a gene (alleles) are expressed gives us information to make hypotheses about a person's genetic make up based on the person's physical appearance. (sometimes we are talking about a person's chemical make up that we can not directly "see".)

  • Understanding meiosis allows us to determine the possible combinations of chromosomes and the genes located on them in our eggs or sperm.

  • Understanding the random nature of fertilization (random combinations of eggs and sperm) allows us to make predictions concerning the possible combinations of genotypes and phenotypes we should expect to see in our children.

Mendel's Law of Segregation (The Inheritance of Single Genes)

Mendel's Law of Independent Assortment The Inheritance of Two (or more) Genes