Human Chromosomes Section 14 2 This section describes

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. n n 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-andcontrast table for sex-linked genes.

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

Punnett Square below illustrates color blindness inheritance.

X-Chromosome Inactivation n n 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? n It is a dense region in the nucleus that is formed by the turned-off X chromosome.

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

n n 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? n Homologous chromosomes fail to separate during meiosis.

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

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

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

n 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? n Scientists do not exactly know for sure.

Chromosomal Disorders Circle the letter of each sentence that is true about sex chromosome disorders. 1. 2. A female with the karyotype 45, X has inherited only one X chromosome and is sterile. The Y chromosome contains a sexdetermining region that is necessary for male sexual development.