The Human Genome Chapter 14
Human Chromosomes • Karyotype—picture of paired homologous chromosomes • Humans have 46 chromosomes • 2 of those are sex chromosomes (X or Y) • Females have XX; Males have XY • The remaining 44 are called autosomes.
Gametes • All egg cells contain 22 autosomes and 1 X. • 50% of sperm contain 22 autosomes and 1 X; the other 50% contain 22 autosomes and 1 Y • The sperm determines the sex of the offspring!
Human Traits • Pedigree Chart—shows relationships within a family; can be used to determine how a trait is passed from one generation to the next • Many traits are polygenic • Many traits are influenced strongly by environmental factors • Environmental effects on gene expression are not inherited • The gene may be expressed in a later generation under the right conditions
Human Genes • The human genome contains tens of thousands of genes. • Example: Chromosome 22 is one of the smallest human autosomes. • 22 contains approximately 43 million DNA bases and contains as many as 545 different genes. • Many human traits are controlled by more than one gene (polygenic) or have incomplete or co-dominant alleles.
Blood Typing • ABO blood groups • Controlled by a gene that codes for a hemoglobin protein, either A, B, or none • The protein is what makes different blood types incompatible • A & B are co-dominant • O is recessive • Rh factor—controlled by an additional gene, Rh+ is dominant; Rh- is recessive
Sex-Linked Genes • Genes that are located on either the X or Y chromosome are called sex-linked. • The X chromosome is much larger and contains hundreds more genes than the Y. • Males have just one X chromosome. All X-linked alleles are expressed in males, even if they are recessive. • Examples: Colorblindness, Hemophilia, and Muscular Dystrophy
X-chromosome Inactivation • In females, one X chromosome is randomly switched off. It is called a Barr body. • Different cells can switch off different X chromosomes. • Example: All calico cats are female. Their patches of different colors are controlled by different X chromosomes. Male cats can have only one color of spots because they have only one X chromosome.
Chromosomal Disorders • The most common error in meiosis occurs when homologous chromosomes fail to separate. This is called nondisjunction. • If nondisjunction occurs, abnormal numbers of chromosomes may find their way into gametes and a chromosomal disorder may result. • Example: Down Syndrome (trisomy 21), Turner’s syndrome, and Klinefelter’s syndrome
Human Genetics • DNA fingerprinting—a tool that analyzes sections of DNA that vary widely from individual to individual • The Human Genome Project—started in 1990, scientists from around the world successfully mapped and sequenced all human DNA by the end of 2000. • Gene Therapy—process of changing the gene that causes a genetic disorder. An absent or faulty gene is replaced by a normal, working gene.