Human Heredity

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

Human Heredity

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

Human Karyotype (image/svg+xml)

Caption: : 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

Autosomal Dominant Pedigree Chart (image/svg+xml)

Caption: : 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

1280px Nondisjunction Diagrams.Svg (image/png)

Caption: : 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