Genetic Variation

Chromosome theory of inheritance

states that genes are located on chromosomes, and that the behavior of chromosomes during meiosis and fertilization accounts for inheritance patterns.

Homologous chromosome

chromosomes that are similar in shape, size, and genetic content

Sex Linked gene

a gene that is located on either chromosome 

X linked gene

genes on the X chromosome  

• Recessive disorders are much more common in males than in females 

• have genes for many unrelated to sex  

• For a recessive __ trait to be expressed 

• A female needs two copies of the allele (homozygous)  

• A male needs only one copy of the allele (hemizygous)  

Y linked gene

genes on the Y chromosome

- have genes help determine sex  

Nondisjunction

pairs of homologous chromosomes do not separate normally during meiosis I or sister chromatids do not separate during meiosis I or sister chromatids do not separate during meiosis II 

• As a result, one gamete receives two of the same type of chromosome, and another gamete recieves no copy  

Aneuploidy

results from fertilization involving gametes in which nondisjunction occurred  

• Offspring with this condition have an abnormal number of a particular chromosome

Monosomic

a ___ zygote has only one copy of  a particular chromosome 

Trisomic

a ___ zygote has three copies of a particular chromsome

Polyploidy

a condition in which an organism has more than two complete sets of chromosomes 

• Triploidy (3n) is three sets of chromosomes 

• Tetraplody (4n) is four sets of chromosomes 

• Is common in plants but not in animals  

Deletion

removes a chromosomal segment 

Duplication

repeats a segment 

Inversion

reverses orientation of a segment within a chromosome

Translocation

moves a segment from one chromosome to another