Chapter 14: Human Heredity

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

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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

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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

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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

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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

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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

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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

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