1. Extending Mendelian Genetics

2. Disorders caused by recessive alleles
Two copies of the recessive allele must be present for a person to have the disorder
Often appear in offspring of parents who are both heterozygous (carriers)

3. Sickle Cell Anemia Recessive genetic disorder
Mutated allele that produces a defective form of the protein hemoglobin
Hemoglobin
Found in red blood cells (RBCs)
Binds oxygen and transports it throughout the body

4. Sickle Cell Anemia
Sickle cell anemia causes a defective form of hemoglobin that causes many RBCs to bend into a sickle shape
Less oxygen can be carried in the blood
Cells tend to get stuck in blood vessels, cutting off blood supply to organs

5. Malaria Benefits of sickle cell anemia?
Protects the cells of heterozygous individuals from the effects of malaria
A disease caused by a parasitic protozoan that invades RBCs
Sickled cells kill the parasite
The individual’s normal RBCs can deliver oxygen
In this case, heterozygous individuals have a better survival rate in areas plagued by malaria

6. Cystic Fibrosis (CF)
Most common fatal, hereditary, recessive disorder amongst Caucasians
1/25 Caucasian individuals has at least one copy of a defective gene that makes a protein necessary to move chloride into and out of cells
1/2,500 Caucasian infants in the USA is homozygous for the cf allele

7. Cystic Fibrosis (CF)
The airways of the lungs become clogged with thick mucus, and the ducts of the liver and pancreas become blocked
There is no known cure

8. Disorders caused by dominant alleles are far less common than recessive disorders

9. Huntington’s Disease (HD)
Genetic disorder caused by a dominant allele
Causes a repeat of a gene sequence, causing cell death in the brain area
First symptoms: mild forgetfulness and irritability
Appears in victims in the 30’s and 40’s

10. Huntington’s Disease (HD)
Secondary symptoms: loss of muscle control, uncontrollable physical spasms, severe mental illness…eventually death
Most people do not know they have the disease until after they have had children
Unknowingly the gene is passed on

11. Males and females can differ in sex-linked traits
Sex-linked genes: genes located on the sex chromosomes

12. Autosomal Autosomal gene: will appear in both sexes equally
Found on chromosomes other than the X or Y

13. Sex-linked Usually males are the only sex affected
Why? Males only have one X
A female carrying a recessive allele on one chromosome will not exhibit the condition if there is a dominant allele on the other one

14. Treating Genetic Disorders
Most genetic disorders cannot be cured
Gene Therapy
Replacing defective recessive genes with copies of healthy ones
Outlook is promising

15. Expression of sex-linked genes
X chromosome inactivation: process that occurs in female mammals in which one of the X chromosomes is randomly turned off in each cell

16. Incomplete Dominance
In some organisms an individual displays a phenotype that is intermediate between the two parents
Examples
Snapdragon flowers: red and white flowers produce pink flowers
Caucasian child: parent with curly hair and parent with straight hair has wavy hair

17. Incomplete dominance

18. Codominance Two dominant alleles are expressed at the same time
Different than incomplete dominance because both traits are displayed
Example: blood type

19. Multiple Alleles Genes with three or more alleles
i.e. blood type in humans
ABO blood types are determined by 3 alleles
IA, IB, and i

20. Multiple Alleles
A and B are carbohydrates on the surface of red blood cells
i allele has no carbohydrates on the surface
IA, IB, are dominant to i but neither IA, IB dominant over each other (they are codominant)
Possible blood types:
A (IA IA or IAi)
B (IB IB or IBi)
AB (IA IB)
O (ii)

21. Polygenic trait When several genes influence a character
The genes may be on the same chromosome or on different chromosomes
Determining the effect of any one of these genes is difficult

22. Polygenic traits
Due to independent assortment and crossing-over during meiosis, many different combinations appear in offspring
Polygenic characters in humans: eye color, height, weight, hair and skin color

23. epistasis
Process by which a epistatic gene can interfere with the expression of other genes
Albinism: a single epistatic gene interferes with the expression of other genes

24. The environment interacts with genotype
Phenotype often depends on conditions in the environment
The plant hydrangea has flowers that can be blue to pink, depending on the pH of the soil
Acidic soil: blue flowers
Neutral to basic: pink flowers
Temperature can determine sex of sea turtles and other species
In humans
Height is affected by nutrition
Skin color is affected by exposure to the sun

25. Gene linkage and mapping
Gene linkage was explained through fruit flies
Linked genes are on the same chromosome
The closer two genes are, the more likely it is that they will be inherited together
The closer two genes are, the more likely they are to stay together when cross-overs in meiosis take place

26. Linkage maps estimate distances between genes
Maps of relative locations of genes on a chromosome

27. Human genetics & pedigrees
Human genetics follows the patterns seen in other organisms
Females can carry sex-linked genetic disorders

28. Hemophilia Recessive genetic disorder
Impairs the blood’s ability to clot
Sex-linked trait
More than a dozen genes code for the proteins involved in blood clotting
A mutation of one of these genes on the X chromosome causes hemophilia A

29. A pedigree is a chart for tracing genes in a family
Shows how a trait is inherited over several generations
Particularly helpful in finding out if a person is a carrier for a genetic disease

30. Several methods help map human chromosomes
Karyotype: An array of the chromosomes found in an individual’s cells at metaphase of mitosis
Arranged in homologous pairs and in order of diminishing size

31. Trisomy 3 copies of chromosomes instead of 2
3 copies of chromosome #21 instead of 2?
Down syndrome, or trisomy 21

32. What causes trisomy?
Older mothers are more likely to have children with Down syndrome
Mothers under 30 years old: 1/1500 births
Mothers over 37 years old: 1/290 births
Mothers over 45 years old: 1/46 births

33. Trisomy
Eggs are present in the ovaries since birth and over time can accumulate damage
(What about males? They produce new sperm throughout their adult lives)
Older pregnant women may be advised to undergo prenatal testing that includes fetal karyotyping

34. Disjunction
When sperm and eggs cells form, each chromosome and its homologue separate

35. What causes trisomy?
Nondisjunction: If one or more chromosomes fail to separate properly
One new gamete ends up receiving both chromosomes and the other gamete receives none
A normal gamete and one with 2 copies fuse during fertilization, causing trisomy

36. Mutations Changes in an organism’s chromosome structure 4 Main Types
Deletion
Duplication
Inversion
Translocation