Human Heredity

Human Heredity

• Miller Levine Textbook:
• Section 14.1 Human Chromosomes
• Section 14.2 Genetic Disorders
• Section 14.3 Studying the Human Genome

Karyotypes (14.1)

• Genome: Complete set of genetic information carried in the DNA of an organism
• - Chromosomes involved in studying them
• - DNA and protein bundles inside nucleus of cells of eukaryotes
• Karyotype: Picture that allows for chromosomes to be seen clearly coming out from photographs of cells in mitosis (chromosomes easy to see), chromosomes cut out and arranged
• - Full diploid set of chromosomes in pairs are shown, chromosomes from largest to smallest in size
• - 46 total chromosomes in human cell, arranged into 23 pairs
• - Pairs from egg and sperm
• Image credit: https://upload.wikimedia.org/wikipedia/commons/e/e0/Human_karyotype.svg
• 
• Bildunterschrift: Karyotype Illustration

Sex Chromosomes and Autosomes (14.1)

• Sex Chromosomes: Two chromosomes that determine the sex of an individual
• Females --> XX (two copies of X chromosome)
• - 46, XX (summarization of total chromosomes in cell)
• Males --> XY (one X chromosome, one Y chromosome)
• - 46, XY
• - If you put XX and XY in a Punnett Square, you will see why boys and girls are born at a ratio of about 50:50
• - Human egg cells carry a single X chromosome while half of sperm cells carry an X and half carry a Y
• - 1200+ genes on X chromosome
• - Y chromosomes much smaller; 140 genes mostly related to male development and determination of sex
• Autosomes (or Autosomal Chromosomes): The 44 chromosomes that are not sex chromosomes
• - 46 chromosomes in human genome
• - 44 autosomal and 2 sex

Transmission of Human Traits (14.1)

• Dominant and Recessive Alleles
• - A bunch of human genes follow simple dominance pattern
• - Rh blood group --> Rh+ dominant and Rh- recessive
• Regular dominant/recessive rules: HH, Hh, hh
• Multiple Alleles and Codominance
• - Quite a few human genes show they are inherited by codominance
• Codominance: Phenotypes of both alleles are expressed
• - ABO blood group
• - Three alleles (I^A, I^B, and i)
• - I^A and I^B display codominance
• - Make blood type AB (producing both antigens)
• - i is recessive (two of these alleles produce type O blood)

Sex-Linked Inheritance (14.1)

• Sex-Linked Gene: A gene located on a sex chromosome
• - These chromosomes (X and Y) determine the sex of an individual, so sex-linkage is the pattern displayed by the genes located on them
• - Males have only one X chromosome, while females have two
• - Some genes are found on the X chromosome
• - Colorblindness
• - Hemophilia
• - Muscular dystrophy
• As a result of these being recessive conditions, they are far more common in males, since females can mask up the gene with a dominant one of their other X chromosome.

X-Chromosome Inactivation (14.1)

• Mary Lyon, British geneticist
• - Found that most of the genes in one random X chromosome are switched off in female cells
• - Creates dense nucleus region called Barr body
• - This is how the cell adjusts to the extra X chromosome in females
• - Calico cats with three fur colors
• - Most likely female
• - Coat color spot gene located on X chromosome
• - Maybe one has an allele for orange and the other for black
• - Different X chromosomes switched off in different parts of the body
• - Produces the color pattern

Human Pedigrees (14.1)

• Pedigree: Chart used to analyze inheritance of a specific trait by showing relationships within a family
• - Presence or absence of trait using relationships between family members
• - Applies for any species
• - Lets us assume genotypes of family members
• - Dominant/recessive
• - Lets us determine nature of genes and alleles connected to traits
• - Dominant, recessive, sex-linked, autosomal, etc
• Image Credit: https://upload.wikimedia.org/wikipedia/commons/6/63/Autosomal_Dominant_Pedigree_Chart.svg
• 
• Bildunterschrift: Pedigree Chart (red has trait, blue does not)

From Molecule to Phenotype (14.2)

• - If a gene doesn't work properly, which it can, there can be consequences
• - Genotype --> Phenotype
• - Clear link
• - For example, the difference between wet and dry earwax is caused by just one base change
• - The connection between molecule and phenotype is similar
• - If a DNA sequence changes, it can affect amino acids, and therefore change proteins
• - Affects traits of individual
• - Genetic disorders have a molecular basis

Disorders, From Individual Genes (14.2)

• Sickle Cell Disease:
• - Caused by a defective allele in a hemoglobin polypeptide
• - Defect causes hemoglobin less soluble, making the molecules stick together when oxygen levels plunge
• - Long fibers created, eventually forcing into the sickle cell shape
• - These blood cells are more rigid than normal, meaning they can get stuck in capillaries, causing damage
• - More common in people of African ancestry
• Cystic Fibrosis (CF):
• - Deletion of three bases from CFTR gene
• - Deletes amino acid, causing protein to fold improperly
• - Protein is destroyed
• - Chloride ions unable to transport through cell membranes
• - Digestive and breathing issues
• - More common in people of European ancestry

Genetic Advantages (14.2)

• - Sickle Cell disease
• - Allele carried by 1/12 people of African ancestry in US
• - Cystic Fibrosis
• - Allele carried by 1/25 people of European ancestry in US
• - Why?
• - In west central Africa, malaria is common. Malaria is caused by a parasite living in red blood cells.
• - Individuals with only one copy of the allele are healthy and resistant to parasite
• - Protects against malaria
• - Typhoid fever in medieval Europe
• - Caused by digestive system bacterium
• - CF allele gives advantage, blocking its entry
• - Individuals with just one CF allele are protected and do not suffer from CF

Chromosomal Disorders (14.2)

• Nondisjunction: The failure of homologous chromosomes to separate during meiosis
• - May result in abnormal amount of chromsomes, causing a numerical chromosome disorder
• - Nondisjunction of autosome
• - Three copies of chromosome
• - Trisomy
• - Most common form is Down Syndrome, trisomy 21
• - Nondisjunction of X chromosome
• - Turner's syndrome or Klinefelter's syndrome
• 
• Bildunterschrift: Nondisjunction

Manipulating DNA (14.3)

• - Scientists can use tools that cut, separate, then replicate DNA base by base to read DNA sequences from any cell
• Cutting DNA:
• - Difference of nucleic acids to other macro-molecules
• - Molecules of DNA too large for analyzing
• - Need to be cut into smaller pieces
• Restriction Enzymes: Enzymes produced by bacteria that cut DNA into precise pieces (restriction fragments)
• - Many, with each one cutting at a different nucleotide sequence
• - EcoRI restriction enzyme recognizes only the base sequence GAATTC
• - Cuts between G and A bases leaving AATT
• Sticky ends - Overhangs left by restriction enzymes that can "stick" or bond to complementary sequences (AATT is sticky end)
• Blunt ends - Straight ends left by a restriction enzyme that cannot bond onto a complementary sequence

Separating and Reading DNA (14.3)

• Separating DNA
• - Gel Electrophoresis: Technique to separate and analyze differently sized fragments of DNA
• - A mix of DNA fragments is put on one end of a gel, and an electric voltage is applied. The negatively charged fragments move across the gel and create a size-based pattern (smaller fragments move faster and therefore move farther on the gel)
• Reading DNA
• - Fragments put into test tube with DNA polymerase
• - The four bases are also put into the tube
• - Enzyme uses unknown strand to make new strands of DNA
• - Bases dyed with chemicals are added, so that when added to a strand the strand stops synthesis
• - Results in color-coded fragments of DNA