Handbook of Genetic Counseling/Genes and Chromosomes

Genes and Chromosomes

Assess the client's knowledge of DNA, genes, and chromosomes

• Reassure that we will start with the basics (will go back to H.S. biology)
  • Explain that I have some pictures that we can look at
  • Feel free to interrupt me with questions at any time

Chromosomes

• Our bodies are made of hundreds of thousands of cells
  • We have skin cells, muscle cells, stomach cells
  • Take one of those cells and break it open
    • inside the nucleus of the cells are our chromosomes
    • chromosomes are the packets that contain all of the genetic information that we inherit from our moms and dads
    • we can look at them by using a microscope (show metaphase spread)
    • explain that these chromosomes are the same in every one of our cells

Karyotypes

• This looks very messy and it is hard to see exactly what is going on, so we match up the chromosomes in pairs and line them up from biggest to smallest to make a karyotype (show pictures)
• 23 pairs of chromosomes (46 total)
  • we have 2 copies of each chromosome, 1 from mom and 1 from dad
    • the first 22 pairs are called autosomes (body chromosomes) and are the same in both men & women
    • the last pair are the sex chromosomes (XX women, XY men)
• There should always be 46 chromosomes and they should always come in pairs
  • Eggs and sperm only have 23 chromosomes so that when they join during fertilization, they will make a zygote that has 46 chromosomes (with half coming from the mom and half from the dad)
  • Sometimes there can be an error when the egg or sperm are being made (nondisjunction) & a baby can be born with too many or too few chromosomes
    • show karyotypes of Trisomy 21 and Turner syndrome

Genes & Mutations

• Chromosomes are made up of a lot of genes that are lined up along them, so our chromosomes the packages that hold all of our genes
• Our genes are made up of DNA that has all the instructions for how our body is formed and how it grows
  • genes have the recipe that tells our body how to make the proteins that do all the work in our cells & are what makes us have our eye color
  • any change in our genes will change the way that our bodies form and grow
  • so, there can be a change in the number of chromosomes or the way that they are arranged (by changing the genes that make them up)
    • either of these changes can result in birth defects

Analogy - Set of Encyclopedias

• Think of the chromosomes as being a set of encyclopedias lined up on a shelf.
  • You have 2 copies of book 1 (one that your mom gave you and one that your dad gave you), 2 copies of book 2, 2 copies of book 3, etc.
  • The genes are like each of the words on the pages.
  • Each of the words is made up of letters, just like each of the genes is made up of DNA.
• Sometimes there can be an extra copy of a book (trisomy) or there can be a book missing (monosomy)
  • this is a numerical change
• Other times, there could be a typo within one of the words inside the book
  • this is a structural change within a gene
    • a word could be repeated (duplication) or a word could be missing (deletion) or the words could be in the wrong order (inversion) or a word could be misspelled (point mutation)

Notes

The information in this outline was last updated in 2001.