1. Inheritance Patterns & Human Genetics
Sex chromosomes form pairs and segregate during meiosis I just like the 22 autosomes
In mammals and most insects, males are XY and females are XX
(Also XX/X0 system, ZW system, haploploidy and non-genetic systems)
Leads to 50/50 chance male or females

3. Sex Linkage
Presence of a gene on a sex chromosome
X-linked or Y-linked (X is larger so has more)
Eye color in Drosophila an example:
Cross red eyed female with a white eyed male
All F1 red eyed flies
F2 showed 3:1 ratio expected, but white eyed flies were all male

4. Gene Linkage
Morgan’s research on fruit flies led him to principle of linkage
Identified over 50 Drosophila genes
Discovered many seemed “linked” in a way that violated independent assortment
Ex.- reddish-orange eyes and miniature wings inherited together

5. Observed so many linked genes, Morgan could group them into 4 linkage groups
Linkage groups assorted independently, but all genes in 1 group inherited together
Drosophila has 4 linkage groups
Also has 4 chromosomes

7. Two conclusions:
-each chromosome is a group of linked genes
-principle of independent assortment still holds true
Chromosomes assort independently, not individual genes

8. Gene Maps
Crossing-over sometimes separates and exchanges linked genes
Produces new combinations of alleles
Helps generate genetic diversity
Can use rates of crossing-over events in meiosis to produce a gene map

9. Shows relative distances between genes on a chromosome
Further apart two genes are, more likely they are to recombine (during crossing-over)
Closer together, the less likely they are to become “unlinked”

10. Mutations
Changes in the genetic material
Germ-cell mutations occur in gametes
Somatic mutations occur in body cells
Lethal mutations cause death, often before birth
Kinds of Mutations
Changes in a single gene known as gene mutations
Changes in a whole chromosome known as chromosomal mutations

11. Gene Mutations
Mutation involving changes in one or a few nucleotides are called point mutations
Include:
Substitutions- one base changed to another
Insertions- extra base inserted
Deletions- base is removed from sequence

12. Substitutions usually only affect one amino acid in protein
Insertions/deletions can affect whole protein
Sequence read in codons (groups of 3)
If base added or deleted, “shifts” the codons that are read
Called frameshift mutations
May change every amino acid after mutation

13. mRNA: AUG-CGU-ACC-UUA Amino acids: Met-Arg-Thr-Leu Substitution
DNA: TAC-GCA-TGG-AAT
mRNA: AUG-CGU-ACC-UUA
Amino acids: Met-Arg-Thr-Leu
Substitution
DNA: TAC-GTA-TGG-AAT
mRNA: AUG-CAU-ACC-UUA
Amino acids: Met-His-Thr-Leu

14. mRNA: AUG-CGU-ACC-UUA Amino acids: Met-Arg-Thr-Leu Insertion
DNA: TAC-GCA-TGG-AAT
mRNA: AUG-CGU-ACC-UUA
Amino acids: Met-Arg-Thr-Leu
Insertion
DNA: TAT-CGC-ATG-GAA-T
mRNA: AUA-GCG-UAC-CUU-A
Amino acids: Ile-Ala-Tyr-Leu

15. DNA: TAC-GCA-TGG-AAT-GGA mRNA: AUG-CGU-ACC-UUA-CCU
Amino acids: Met-Arg-Thr-Leu-Pro
Deletion
DNA: TGC-ATG-GAA-TGG-A
mRNA: ACG-UAC-CUU-ACC-U
Amino acids: Thr-Tyr-Leu-Thr

16. Chapter 14- Human Genome Sickle Cell Anemia
Common genetic disorder found in African Americans
Have bent, twisted red blood cells
Cells more rigid, tend to get stuck in capillaries
Produces physical weakness and damage to brain, heart, and spleen

18. Chapter 14- Human Genome Protein affected is hemoglobin:
Just one base change causes protein to be abnormal
Abnormal protein causes sickle shape of cells
Why so many African Americans?
Tropical regions have high prevalence of malaria

19. Chapter 14- Human Genome
People who are heterozygous normally healthy, and are resistant to malaria
Sickle cell allele is beneficial in these areas
A Mutation Story

20. Chromosomal Mutations
Involve changes in the number or structure of chromosomes
Can change locations of genes
Can change the # of copies of some genes

22. Mutations that cause dramatic changes in protein structure or gene activity often harmful
Mutations may produce proteins with new or altered activities that may be useful
Mutations in gametes can be passed along to offspring
Basis for new genetic variation in species

23. Studying Human Inheritance
Human genes inherited according to Mendel’s principles
Must identify an inherited trait is controlled by a single gene
First, must establish trait is inherited
Second, must study how trait is passed from generation to generation

24. Pedigree chart is used to show relationships within a family and follow a particular trait
Genetic counselors analyze pedigrees to infer the genotypes of family members
Only useful for single-gene traits
Many human traits are polygenic
Controlled by many genes
Many traits also influenced by environmental factors

26. Recessive Alleles
Many human genes become known through study of genetic disorders
Most cases, presence of a normal gene only revealed when abnormal allele affects phenotype
Autosomal recessive inheritance

27. Ex.- phenylketonuria (PKU)
Lack enzyme needed to breakdown phenylalanine
Can buildup in tissues in infants and cause mental retardation
Caused by autosomal recessive allele on chromosome 12

29. Dominant Alleles
If you have dominant allele for a genetic disorder, it will be expressed
Ex.-
Achondroplasia (form of dwarfism)
Huntington’s disease
Causes progressive loss of muscle control and mental function (begins in 30’s)
Autosomal dominant inheritance

31. Multiple phenotypes from multiple alleles
More common for multiple alleles to control a trait in a population
Only two alleles of a gene can exist within the cell, multiple alleles for a single gene can be found within a population
Blood Group Genes
Human blood comes in variety of genetically determined blood groups
Number of genes responsible, best known are ABO blood groups and Rh blood groups

32. Rh determined by single gene with two alleles
Either positive (dominant allele) or negative
ABO group more complicated
Three alleles: IA, IB, and i
IA & IB are codominant
Alleles produce proteins on red blood cell surface
i allele is recessive; no proteins produced

35. Polygenic Inheritance
Traits that are determined through expression of two or more genes
Polygenic traits show wide range of phenotypes
Ex- eye color, skin color, height

38. Sex-linked Traits
Traits controlled by genes found on the sex chromosomes
Any recessive allele found on the X chromosome of a male will not be masked (there is no other X in males)
X-linked dominant and Y-linked are very rare

39. Colorblindness
3 genes associated with color vision found on X chromosome
In males, defective version of any of these produces colorblindness
Most common form: red-green colorblindness; affects 1 in 10 males (U.S.)
Among females: 1 in 100

42. Males have only one X chromosome
So, all X-linked alleles are expressed in males (even if recessive)
In females, must have two copies of recessive allele to express trait
Pattern:
Males show recessive trait more often
Males pass along allele to daughters

44. Hemophilia
Two important genes on X chromosome help control blood clotting
Recessive allele in either may cause hemophilia (blood clotting protein missing)
1 in 10,000 males born with disorder (U.S.)
Treated by injections of clotting protein

46. Duchenne Muscular Dystrophy
Sex-linked disorder results in progressive weakening and loss of skeletal muscle
1 in 3000 males in U.S.
Caused by defective version of gene that codes for muscle protein
No treatment or cure to date

49. Chromosomal Disorders
Mechanisms that separate chromosomes in meiosis work well; sometimes fail
Most common error = nondisjunction
Occurs when homologous chromosomes fail to separate
Abnormal #’s of chromosomes in gametes may lead to genetic disorders

51. Down Syndrome
Individual may be born with 3 copies of a chromosome (called trisomy)
Most common form of trisomy is called Down Syndrome (Trisomy 21)
~1 in 800 born with Down Syndrome in U.S.
Produces mild to severe mental retardation

53. Detecting Human Disorders
Genetic screening or genetic counseling
-May use a karyotype or pedigree
Fetal cells obtained through an amniocentesis or chorionic villi sampling
Genetic tests have been developed to take advantage of small differences in DNA