15.4: Errors and Exceptions in Chromosomal Inheritance

When the members of a pair of homologous chromosomes do not separate properly during meiosis I, or sister chromatids don't separate properly during meiosis II, nondisjunction occurs.

As a result of nondisjunction, one gamete receives two copies of the gene while the other gamete receives none. In the next step, the faulty gametes engage in fertilization, the offspring will have an incorrect chromosome number. This is called aneuploidy.

Fertilized eggs that have received three copies of the chromosome in question are said to be trisomic; those that have received just one copy of a chromosome are said to be monosomic for the chromosome.

If nondisjunction occurs during mitosis (and early in embryonic development) it will be passed on to a large number of the organism's cells and have a significant effect on the organism.

Trisomy and monosomy often occur on the 21st and 18th chromosome pairs. When a pair of 21st chromosomes in one of the gametes fails to separate, the zygote ends up with three chromosomes, resulting in Trisomy 21 (aka Down Syndrome). Down syndrome is the most common chromosomal error. It occurs in 1 of every 733 births. Ninety percent of Down syndrome babies are miscarried. This is why Mrs. Lyon says that people with Down syndrome are a gift to us; they have overcome amazing odds to be alive.

*Above is a karyotype if a person who has Down syndrome.

People with Down syndrome are more likely to have congenital heart defects, respiratory and hearing problems, Alzheimer's disease, childhood leukemia, and thyroid conditions.

They also often have low muscle tone, small stature, an upward slant to the eyes, and a single deep crease across the center of the palm. The severity of each case varies so some people with Down syndrome may demonstrate all of the above traits while others may show very few.

People with Down syndrome all have cognitive delays, but some are much worse than others in the same way that physical problems vary.

Polyploidy is the condition of having more than two complete sets of chromosomes. This condition is somewhat common in plants. Terms such as triploidy (3n) and tetraploidy (4n) indicate that there are three or four chromosomal sets. Bananas are triploid and wheat is hexaploid (6n-meaning it has 6 chromosomal sets).

The following conditions are just based on chromosomes. You should know how to recognize them but you do not need to know what they do:

Deletion refers to a chromosome segment that has no centromere. It is broken off and lost during segregation The cell that receives the partial chromosome will be missing all of the genes located on the chromosome fragment.

-How bad the situation is depends on what has been deleted.

-If the centromere is deleted, the entire chromosome will be lost.

If the chromosome fragment that broke off (causing the deletion) becomes attached to its sister chromatid, a duplication occurs. In this case, the zygote will get a double dose of the genes located on that chromosome.

-Good Example: If the extra DNA codes for the production of a protein that is safe and helpful in excess.

-Bad Example: If the extra DNA codes for a disease, the person is more likely to get the disease.

An inversion refers to a chromosome fragment breaking off and then reattaching to its original position, but backwards so that the part of the fragment that was originally at the attachment point is now at the end of the chromosome.

-Example: abcdefghi becomes abcfedghi (the portion def in the pattern was inverted)

A translocation occurs when the chromosome fragment joins a nonhomologous chromosome. This moves a segment of one chromosome to a nonhomologious chromsome. A translocation can be reciprocal, meaning that the nonhomologous chromosomes can exchange segments.

Quiz:

1. Which of the following is an inversion of ABCDEFGHI?

a) ABFGHI

b) ABCDEHGFI

c) ABDCFGHIE

d) ABCABCDEFGHI

2. Which of the following might cause nondisjunction?

a) the breakage of chromosomes

b) centromere duplication

c) problems in Meiosis II

d) failure of synapsis in Meiosis I

3. A somatic cell that does not have an exact multiple of the haploid chromosome number is called:

a) aneuploid

b) polyploid

c) diploid

d) non-disjunctioned

Answers: 1-b, 2-d, 3-a

**If anyone wants to practice linkage map problems, this is a great website: http://library.thinkquest.org/20465/g_linkagemap.html.

I hope everyone had a wonderful, relaxing Thanksgiving Break!

Here are some reminders for this week:

Monday: 15.1 Quiz

Tuesday: Chapter 9 Vocab Check

Wednesday: Chapter 9 Vocab Quiz and Unit III Test

Friday: Lab 3 Mitosis