14.1 Human Chromosomes

Karyotypes

• A genome is the entire set of genetic information that an organism carries in its DNA

• To see human chromosomes clearly, cell biologists take pictures of cells during mitosis, when the chromosomes are condensed and can be seen clearly

  • Scientists then cut out the chromosomes from the photographs and arrange the chromosome images into a picture known as a karyotype

    • A karyotype shows the complete diploid set of chromosomes

    • They are lined up together in pairs and arranged from largest to smallest

    • A typical human cell has 46 chromosomes, arranged in 23 pairs

• Two of the 46 chromosomes in the human genome are known as sex chromosomes because they determine an individual’s sex

  • Females have two copies of the X chromosome and males have one X chromosome and one Y chromosome

• The other 44 are autosomal chromosomes, or autosomes

  • Most of the genetic information in the cell is on the autosomes

Transmission of Human Traits

• Many human traits follow a pattern of simple dominance

• Some alleles for human genes are codominant

  • One example is the ABO blood group, the set of alleles that determines your blood type

• The genes located on X and Y chromosomes show a pattern of inheritance called sex-linkage; a sex-linked gene is a gene on a sex chromosome

  • Only males have the genes on the Y chromosome; since only males have a Y chromosome, boys inherit it from their fathers

  • The X chromosome comes from their mother; the fact that males have only one X chromosome can cause genetic problems

• A cell only needs one X chromosome to function normally

  • Female cells adjust to having two X chromosomes by randomly inactivating genes on one of them

  • The inactive chromosome makes a dense area in the nucleus called a Barr body

Human Pedigrees

• A pedigree is a chart that shows the presence or absence of a trait according to the relationships within a family across several generations

  • A shaded circle or square indicates that a person expresses the trait

  • A horizontal line connecting a male and a female represents a marriage

  • A circle represents a female

  • A square represents a male

  • A vertical line and a bracket connect the parents to their children

  • A circle or square that is not shaded indicates that a person does not express the trait

• We can often use a pedigree to figure out the genotypes of family members

• The information learned from pedigrees helps determine the nature of genes and alleles associated with inherited human traits

14.2 Human Genetic Disorders

From Molecule to Phenotype

• Genotype and phenotype are directly connected since changes in a gene’s DNA sequence can change proteins

• Changes in a gene’s DNA sequence can change proteins by altering their amino acid sequences and may directly affect a person’s phenotype

  • Sickle-cell disease is caused by a flawed allele for a polypeptide in hemoglobin, the oxygen-carrying protein in red blood cells

  • Huntington’s disease is caused by a dominant allele for a protein found in brain cells; the allele for this disease has a long string of bases in which the codon CAG (which codes for the amino acid glutamine) repeats over and over again, more than 40 times

    • It is unknown why this long string of glutamine causes the disease

  • Cystic Fibrosis is usually caused by the deletion of three bases in the DNA of a single gene, causing the body not to make normal CFTR (a protein needed to transport chloride ions)

• Diseases such as sickle cell disease and CF are still common in human populations, even though the alleles for these diseases can kill those who have them; sometimes having just one recessive allele can be an advantage

  • For example, people with one copy of the sickle cell allele are resistant to the parasite that causes malaria

  • People with a single CF allele were less likely to die from typhoid

    • Typhoid is caused by a bacterium that enters the body through cells in the digestive system; the protein made by the CF allele helps block this bacterium from entering the cells

Chromosomal Disorders

• The most common error in meiosis occurs when homologous chromosomes do not separate, a mistake called nondisjunction

• If nondisjunction occurs during meiosis, gametes may receive an abnormal number of chromosomes, a change that can lead to a disorder of chromosome numbers

  • If two copies of an autosomal chromosome fail to separate during meiosis, an individual may be born with three copies of that chromosome, a condition known as a trisomy

• Nondisjunction of sex chromosomes also causes problems

  • A female who inherits only one X chromosome usually has Turner’s syndrome

  • A male with an extra X chromosome has Klinefelter’s syndrome

14.3 Studying the Human Genome

Manipulating DNA

• Scientists study DNA by using tools that cut, separate, and then replicate DNA base by base; now they can read the base sequences in DNA from any cell

  • To study DNA, scientists first cut it into smaller pieces using bacterial enzymes called restriction enzymes, which cut a DNA molecule into exact pieces called restriction fragments

  • After DNA is cut, scientists use a technique called gel electrophoresis, a procedure used to analyze DNA fragments by placing a mixture of DNA fragments at one end of a porous gel and applying an electrical voltage to the gel

  • After the DNA fragments have been separated, researchers read, or sequence, the DNA

The Human Genome Project

• The Human Genome Project had two main goals: to sequence all 3 billion base pairs of human DNA and to identify all human genes

  • First, scientists break up the whole genome into smaller pieces

  • Next, they determine the base sequences in regions of a DNA strand that are far apart

  • Using the markers, researchers can find and return to specific places in the DNA

  • Once researchers have marked the DNA strands, they cut them into random fragments and determine the base sequence of each fragment

  • Computer programs put the fragments in order using the markers in a method referred to as “shotgun sequencing”

• The Human Genome Project was completed in 2003

  • One of the key research areas of the project was a new field called bioinformatics, the application of mathematics and computer science to store, retrieve, and analyze biological data

  • Bioinformatics also began a similar field of study known as genomics—the study of whole genomes including genes and their functions

• Overall, the Human Genome Project pinpointed genes and matched some sequences in those genes with many diseases and disorders; it also identified about three million places where single-base DNA differences occur in humans