Biology 100/101
Lecture 12:
Chromosomes and Traits

Answers to many of these questions can be found on the "Answers to End-of-Chapter Questions" page at the text website.

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• Primate Cytogenetics Network

• MendelWeb

• National Institute of Health Gene Testing Glossary

• The Dictionary of Cell Biology

• Mendelian Genetics Directory

• Blazing A Genetic Trail

• University of Virginia Biology 121 Slide List This has a LOT of information, some of the "slides" have been linked below.

1. Use drawings or models of chromosomes to explain why genes are considered to be linked and demonstrate how linked genes are recombined by crossing over during meiosis.

2. Explain how sex is determined in humans.

3. Define a sex-linked trait and illustrate how human pedigree analysis is used to analyze its inheritance.

4. Describe normal and abnormal (aneuploid or polyploid) chromosome combinations affecting the expression of characteristics in humans.

5. Use common objects such as paper clips to model the changes in and movements of chromosomes during meiosis that would result in aneuploidy.

6. Describe some of the ways physicians can sample and examine fetal chromosomes.

Autosomes: Chromosomes that are non-sex determining (that is, in those organisms, such as humans, which have sex chromosomes).

Aneuploid: A cell with one or more extra or missing chromosomes. Monosomy is the absence of one chromosome; Trisomy is the presence of one extra chromosome.

Polyploid: A cell with extra chromosome sets.

Nondisjunction: The unequal partitioning of chromosomes into gametes during meiosis.

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, the segment of DNA located at position 393 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). A dominant allele masks the expression of another allele (and is commonly symbolized by a capital letter). The masked allele is recessive (and is commonly symbolized by the corresponding small letter).
The normal Cystic Fibrosis membrane protein allele produces the normal membrane protein. Other slightly different DNA sequences at this location produce variations of the protein or no protein at all, resulting in the disease.

Synapsis: The gene-by-gene alignment of homologous chromosomes during meiosis.

Homologous chromosomes: Chromosome pairs within cells which have the same sequence of genes. One chromosome of each pair comes from each of the parents through its gamete.

• What is a chromosome?

• Relationship between cells, chromosomes, genes, and DNA.

• Genes located on the same chromosome are linked, or inherited together, and do not assort independently.

• Crossing over between the loci separates linked genes

• A new Crossing Over Diagram From Access Excellence

• Crossing over is more likely to separate two genes if they are widely spaced on the chromosome.
  • Example of crossing over using the HyperCard Program, Cross Over.

  • Example of linked genes using corn. Genes for traits kernel color (yellow or purple) and kernel shape (wrinkled or smooth) lie very close together on a chromosome. Most yellow kernels are wrinkled and purple kernels are unwrinkled. How do you explain the origin of a smooth yellow kernel?

• Linked Genes: Secretor and Myotonic Dystrophy

  • Chromosome 19

• Human X Chromosome
  Note: Any gene on the X chromosome of a male human is expressed, because he lacks a second allele for that gene, which might mask its expression. He either has the trait or he does not--he cannot be a carrier. An allele on an X chromosome in a female may or may not be expressed, depending upon whether it is dominant or recessive and on the nature of the allele on the second X chromosome.

• Human Y Chromosome. Note the difference in the number genes present between this chromosome and the X chromosome.

• XX = female

• XY = male

• XX males and XY females

• SRY gene (Sex-determining Region of the Y chromosome)
  The SRY gene (Fig. 14.6, text) produces a protein that switches on other genes that direct the embryo to develop male structures. Once the rudimentary testes begin to secrete testosterone, cascades of other gene activities follow. The SRY protein also activates a gene that encodes a protein that destroys rudimentary female structures. XX males have just the very small SRY region of the Y chromosome. XY females lack just the SRY region on the Y chromosome, and thus the factors which would make them male instead.

• Mode of Activity of the SRY Gene

• Review of an autosomal dominant trait

• Review of an autosomal recessive trait

X-linked Recessive Inheritance (mother to son)
• X-linked disorders

• A pedigree of hemophilia in the Royal Families of Europe and see Fig. 14.8, text.
  Can a women get hemophilia?
  Can a man be a carrier for hemophilia?
  • Color blindness

  X-linked dominant inheritance
  A woman with the allele has the associated trait or illness. Males with the allele are usually more severly affected (and may never be born).

  Y-linked inheritance (father to son)

  Hairy ears?

• Normal gametogenesis and fertilization results in a zygote which is diploid.

• Under abnormal conditions, meiosis and fertilization result in unexpected, and often undesirable, chromosome numbers.
  • Polyploidy - Extra SETS of chromosomes. In humans and other mammals, polyploidy is lethal, and almost always before birth. In plants, polyploidy is quite common and has given rise to many new species. It is tolerated in as many as 50% of all species, and may even be responsible for desirable characteristics in crop and horticultural plants. Examples include wheat, cotton, bananas, sugarcane, potatoes, tobacco, and pineapples. Triploids have 3 sets of chromosomes. Tetraploids have 4 sets of chromosomes. Some plants have 1,200+ chromosomes!!

  • Aneuploidy - Extra or missing chromosomes.

    Autosomal aneuploids:

    In humans, autosomal aneuploidy often results in mental retardation because do many genes contribute to brain function. Extra genetic material is apparently less dangerous than missing material. (Sex chromosome aneuploidy is less severe than autosomal aneuploidy.)

    See The Cytogenetics Gallery for karyotypes of various chromosomal aneuploids.

    • Down syndrome and its karyotype. An extra chromosome 21. Incidence increases dramatically in moms over 40 years of age.

    • Patau syndrome. An extra chromosome 13.

    Sex chromosome aneuploids and other sex abnormalities:

    • XO=Turner Syndrome

    • YO

    • XXY=Klinefelter Syndrome

    • XXX=Triplo-X Syndrome

    • XYY=Jacob Syndrome

• The failure of a chromosomal pair to separate, either in meiosis I or meiosis II, is called nondisjunction. Any of the 22 autosomes may have an extra or missing chromosome. Sex chromosomes may appear as above. However, only 9 of these actually occur in newborns. About half of all spontaneous abortions are associated with aneuploidy.

• The result is a sperm or oocyte with either two copies of a particular chromosome or none at all.

• See Fig. 14.15, text, for illustrations.

• Ultrasound and Obstetric ultrasound

• Amniocentesis

• Chorionic Villus Sampling

• Fetal Cell Sorting (a related article concerning presence of fetal cells in a mother's blood)

• See page 287, text.

Sample Questions:

An individual has the genotype AaBb. Use chromosome models to demonstrate the chromome movement during MEIOSIS and determine all possible GAMETE genotypes.

• Did you put the A (or a) and B (or b) gene locus on different homologous chromosomes?

• What would happen if they were linked on the same chromosome?

• What would happen if A (or a) were located on the X chromosome?

• What GAMETE genotypes would you get if the individual's genotype was AaBB?

• What OFFSPRING genotypes would you get if one parent were AaBb and the other were AaBB?

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