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

Announcements
Text readings in Life by Ricki Lewis, et. al. Testing Your Knowledge Thinking Scientifically
Chapter 10,
How Inherited Traits are Transmitted		 Pg. 199 Questions 1, 2, 3, 5, and 6 Pg. 199; 1-4

The "Mastering Concepts" boxes are valuable summaries of the main ideas in these sections of the text.

Answers to many of these questions can be found at the Text On-Line Learning Center

You may also ask questions and see answers to your classmates' questions in Web Crossing in the "Talk to Steve and Ed" discussion.

Objectives:

After studying this material you should be able to:

  1. Understand 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     		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 genotypes of two mating individuals, predict the genotypes and phenotypes of offspring that they might produce.

  6. Given the 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

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 Lewis, et. al. Page 201

Autosomes: 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.

*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 gene.

*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 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 either "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 small letter - e.g. "a"). The expresion of recessive alleles are usually only visible when a person has inherited a recessive allele form both parents. Some recessive alleles produce variation 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 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 can not 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 gene locus?

  2. What is a sister chromatid?

  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, sign your name, and include your TA's name.

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)

  • During meiosis, homologous pairs of chromosomes (and the genes that compose them) separate from one another and are packaged into separate gametes. At fertilization, gametes combine at random to form the individuals of a new generation.

  • Overview of Meiosis Lewis, et. al. pg. 163, fig. 9.6

  • By knowing the genotypes of the parents or the phenotypes of the offspring (assuming that there are enough to see all, or most of the possible phenotypes), it is possible to understand inheritance patterns, determine dominance of traits, and other genetic characteristics.

  • This sort of analysis was pioneered by Gregor Mendel (the "Father of Genetics") using peas. He observed different genetic lines for such characteristics as flower color, plant height, seed texture, and followed their inheritance patterns for many generations to determine the basic rules of inheritance that are stil in use today.

  • Punnett Square diagram of a cross involving only ONE GENE (Monohybrid)

  • AUTOSOMAL RECESSIVE INHERITENCE
    • Xeroderma Pigmentosum (Autosomal recessive located on Chromosome 9.

      • Affected people lack a critical enzyme that repairs DNA damaged by ultrafiolet light. This results in rapid and severe sunburn and skin cancer.

      Can affect both sexes; (aa) has disorder; can skip generations; carriers (Aa) show no symptoms

    • Diagram of autosomal recessive inheritance
  • AUTOSOMAL DOMINANT INHERITENCE

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

  • A gene on one chromosome does not influence the inheritance of a gene on a different (nonhomologous) chromosome because meiosis packages chromosomes randomly into gametes.

  • Independent assortment refers to the RANDOM arrangement of paired homologous chromosomes during the middle part of the first division of MEIOSIS. Independent assortment Lewis, et. al., pg. 188, figure 10.12(see Fig. 4.9, Human Genetics, pg. 76)

  • Punnet square analysis of a cross involving TWO GENES. (Dihybrid)