Human Chromosomes Section 14–2 This section describes the structure of human chromosomes. It also describes genetic disorders that are sex-linked, as well as disorders caused by nondisjunction.

Human Genes and Chromosomes Circle the letter of each sentence that is true about human genes and chromosomes. Chromosome 22 contains long stretches of repetitive DNA that do not code for proteins. Human genes located close together on the same chromosome tend to be inherited together.

Sex-Linked Genes What are sex-linked genes? They are genes that are located on the X or Y chromosomes.

Is the following sentence true or false? The Y chromosome does not contain any genes at all. False

Complete the compare-and-contrast table for sex-linked genes.

Is the following sentence true or false? All X-linked alleles are expressed in males, even if they are recessive. true

Punnett Square below illustrates color blindness inheritance.

X-Chromosome Inactivation How does the cell “adjust” to the extra X chromosome in female cells? In female cells, one X chromosome is switched off.

What is a Barr body? It is a dense region in the nucleus that is formed by the turned-off X chromosome.

Is the following sentence true or false? Barr bodies are found only in males.

If you saw a white cat with orange and black spots, is it most likely a male or a female? It’s probably a female. In cells in some parts of the body, one X chromosome that has the allele for orange spots is switched off, whereas in cells in other parts of the body, the other X chromosome with the allele for black spots is switched off.

Chromosomal Disorders What occurs during nondisjunction? Homologous chromosomes fail to separate during meiosis.

Chromosomal Disorders Is the following sentence true or false? If nondisjunction occurs, gametes may have abnormal numbers of chromosomes. True

Chromosomal Disorders The condition in which an individual has three copies of a chromosome is known as trisomy , which means “three bodies.”

Chromosomal Disorders Is the following sentence true or false? Down syndrome occurs when an individual has two copies of chromosome 21 False

Circle the letter of the characteristic of Down syndrome. b. mental retardation

Chromosomal Disorders Why does an extra copy of one chromosome cause so much trouble? Scientists do not exactly know for sure.

Chromosomal Disorders Circle the letter of each sentence that is true about sex chromosome disorders. A female with the karyotype 45,X has inherited only one X chromosome and is sterile. The Y chromosome contains a sex-determining region that is necessary for male sexual development.