1. Bellwork: Why would the Y chromosome be unlikely to contain any of the genes that are absolutely necessary for survival?

2. Human Chromosomes
Section 14.1

3. In Today’s class…. We will look at Human chromosome and karyotypes
Autosomal and Sex chromosomes
How human traits are transmitted
How traits can be traced through entire families
Some genetic disorders which can commonly affect humans

4. Why are humans are very complicated when it comes to genetics?
Few offspring
Long life span
Can not be grown in a lab
All making piecing together knowledge of human genetics very complicated

5. Karyotypes
Human genome – full set of genetic information that a human carries in their DNA
Chromosomes photographed during mitosis
Chromosomes condensed and easy to view
A karyotype shows the complete diploid set of chromosomes grouped together in pairs, arranged in order of decreasing size
Pairs – due to diploid cells – half information from mother, half from father

6. Sex chromosomes
Most chromosomes are called autosomal chromosomes or autosomes
Sex chromosomes are different
Males : 46,XY, Females: 46,XX

7. Transmission of human traits
Human genes follow the same patterns of Mendelian inheritance as the genes of other organisms
Dominant and Recessive alleles
Many traits follow this pattern
MC1R gene codes for hair and skin color
Blood group Rh factor: simple dominance
Codominant and multiple alleles
ABO blood group: Codominant
Multiple possibilities

8. Sex-linked inheritance
X and Y chromosomes determine sex
Genes located on these chromosomes are referred to as sex linked
Genes on Y chromosome are only found in males and are passed father to son.
As men only have 1 X chromosome, also possibility for complications
Humans have 3 genes for color vision, all on x chromosome
Red-green color blindness affects 1 in 12 males, but 1 in females - why the difference?
Recessive phenotypes of sex linked genetic disorders are much more common in males than females

9. X-Chromosome inactivation
One X Chromosome is enough for men,
In females, the extra X chromosome is often switched off
Forms dense region of nucleus called the Barr Body
Common in other mammals
Explains why cats fur coats can exhibit different characteristics in different parts
Only possible with female cats
If a cats fur has three colors, it is almost certainly female

10. Human pedigree
A pedigree is a chart which shows relationships in a family tree
Shows the presence of absence of a trait
Can be used for any species
By analyzing a pedigree it is often possible to infer the genotype of family members
Pedigree analysis allows the application of Mendel’s genetic principles to humans
You can determine if an allele is dominant or recessive, autosomal or sex linked

11. Human pedigree example – freckles

12. Human genetic disorders
Section 14.2

13. How do small changes in DNA molecules affect human traits?
When a gene fails to work, or works improperly, there can be serious consequences
There is a direct link between genotype and phenotype
Changes in a gene’s DNA sequence can change proteins by altering their amino acid sequence, which can directly affect the phenotype of an organisms
Genetic disorders have a molecular basis

14. Sickle cell Disease
Caused by defective allele for beta- globin (a polypeptide in hemoglobin)
Resultant hemoglobin is less soluble
When bloods oxygen level decreased, hemoglobin molecules stick together
Clump into long fibers, giving distinctive sickle like shape
Sickle cells are more rigid, they get stuck in capillaries
This can lead to damage to cells, tissues or even organs

15. Cystic Fibrosis Most common in Europeans
Most cases result from deletion of just three bases in the gene for a protein called cystic fibrosis transmembrane conductance regulator (CTFR)
CTFR normally allows Cl- ions to pass through cell membranes
Without this protein, the cell cannot function properly
People with one copy of CF allele are unaffected (it is recessive)
Children with CF have digestive issues, and their lungs and breathing passageways can become blocked with this mucus

16. Huntington’s disease
Caused by a dominant allele for a protein found in brain cells
Codon CAG is repeated a lot (more than 40 times)
Unclear why long string of resultant glutamine causes the disease
Symptoms – mental deterioration, and uncontrollable movement normally do not appear until middle age
Age disease appears and severity of disease is related to number of CAG codons

17. Genetic advantages
Sickle cell disease and CF are still common amongst humans
1 in 12 people of African ancestry and 1 in 25 of European ancestry carry the sickle cell gene
Why hasn’t they both died out?
Sickle cell gene gives a natural resistance to malaria
CF allele can help prevent bacterium which cause typhoid – a disease which plagued medieval Europe
People heterozygous for CF had this genetic advantage, but did not have CF

18. Chromosomal disorders
Sometimes things can go wrong during meiosis
Nondisjunction (means not coming apart) leads to cells with an abnormal number of chromosomes
Leads to a disorder of chromosome numbers
Trisomy means three copies of a chromosome
Down syndrome is caused by three copies of chromosome 21
Turner’s syndrome is when a female only inherits one X chromosome
These women are sterile
Klinefelter’s syndrome – caused by an extra X chromosome
Again, normally sterile