GN 301 Module 4: Variation of Chromosome Number

**2** n = **46**

* First blank (2) stands for **ploidy number**
* Second blank (36) stands for **interphase cell**

Aneuploidy

an individual has more or fewer chromosomes than normal, but not extra sets of chromosomes. When there’s an aneuploidy, the number of chromosomes in an interphase cell isn’t an exact multiple of the haploid number. Humans haploid number is 23; aneuploids don’t have a chromosome number that is a multiple of 23.

Monosomic

an individual is missing one chromosome

Trisomic

an individual has 1 extra chromosome

Tetrasomic

2 extra copies of the same chromosome, so they have a total of 4 copies of that chromosome type

Nullisomic

missing both copies of one chromosome

Double monosomic

an individual is missing 1 chromosome from 2 different chromosome pairs

Double trisomic

an individual has 3 copies of 2 different chromosome types

Discuss the role of Nondisjunction in producing aneuploids

Nondisjunction in the first or second meiotic division can result in an aneuploid progeny, since it produces gametes with additional or fewer chromosomes than normal.

Down Syndrome

a trisomy (1 extra copy) of chromosome 21. There is an increased risk of Down Syndrome as a mother’s age increases. 47 chromosomes, 21+

Turner Syndrome

females have only one X chromosome and tend to lack normal development at puberty

Klinefelter Syndrome

men have an extra X chromosome (XXY), resulting in some feminized characteristics

Edward Syndrome:

trisomy 18 (47, 18+), resulting in severe intellectual challenges

Patau Syndrome:

trisomy 13 (47, 13+)

What proportion of conceptions result in spontaneous abortions?

15%

What proportion of spontaneous abortions are due to chromosome abnormalities?

7.5%

What are some common chromosome abnormalities that have a high spontaneous abortion rate?

Trisomies, XO, triploids, and tetraploids are common chromosome abnormalities with a high spontaneous abortion rate

What are some of the more common chromosome abnormalities among live births?

Among live births, many have sex chromosome aneuploids, autosomal trisomics, inversions, etc

Ploidy Levels

number of complete chromosome sets

Haploid

only one copy of each chromosome

Diploid

two copies of each chromosome

Triploid

four copies of each chromosome

Tetraploid

four copies of each chromosome

Pentaploid

five copies of each chromosome

Autopolyploidy

polyploids with multiple copies of the SAME set of chromosomes

Allopolyploidy

a combination of sets of chromosomes from 2 or more species

How can polyploids arise?

* Polyploidy can arise during nondisjunction in anaphase of mitosis, resulting in a segment of the organism having twice as many chromosomes as normal
* This is common in plants

Mitosis nondisjunction

In the lab, colchicine can be used to double the number of chromosomes by destroying spindle fibers so that chromosomes aren’t pulled to opposite poles in mitosis

Meiosis nondisjunction

Nondisjunction in meiosis can also result in a gamete with extra sets of chromosomes

Autopolyploids

Autopolyploids have multiple sets of chromosomes from one species. They tend to be sterile

Interspecific hybrids

* Eg. Rabbage: an unsuccessful breeding of radish and cabbage
* Eg. Triticale: Triticale is a man-made cereal grain species derived from hybridization of wheat and rye. It was developed to combine favorable traits from both parents; growth vigor, cold tolerance and high protein from rye, and good baking characteristics of wheat gluten.
* Eg. Wheat: Early bread wheat was the result of the crossing of goat grass (Aegilopstauschii ) with Triticum turgidum. Modern bread wheat varieties have forty-two chromosomes and evolved from crosses between emmer and goat grass, which is the source of the unique glutenin genes that give bread dough the ability to form gluten.
* Others: What is an example of interspecific hybrid? Interspecific Animal Hybrids
* Mule – The mule is a progeny of a female horse and a male donkey. Liger – An offspring of a female tiger and a male lion. Zebroid or Zonkey – An offspring of a female donkey and a male horse.