1. Chapter 12 Inheritance Patterns and Human Genetics

2. 12-1 Chromosomes and Inheritance

3. Remember… DNA in chromosomes has instructions for protein synthesis
Chromosomes are transmitted from one generation to the next

4. Sex Determination
1900s: Thomas Hunt Morgan bred Drosophila melanogaster (fruit flies)
Determined:
Females: XX
Males: XY
“Y” chromosome is smaller and hook-shaped
Autosome: all other chromosomes

5. Sex chromosomes segregate (form pairs) during meiosis
SO! Each sex cell either has an “X” or a “Y”
Male gametes: can contain “X” OR “Y”
Female gametes: ONLY “X”
Why????

6. Sex Linkage
Morgan also hypothesized that more genes are carried by X chromosome than the Y
X-linked: genes carried on the X chromosome
Y-linked: genes found on the Y chromosome
Sex linkage: presence of gene on sex chromosome

7. Sex Linked in Drosophila
Normal: Red eyes
Mutation: WHITE eyes
P1 x P1  F1
Red eye female x White eye male ALL RED EYES
F1 x F1  F2
Red female x red male  3(red):1(white)
HOWEVER! All white eyes on MALES ONLY

8. No White Eyed Females Trait for eye color is carried on X
Red eye female: XR XR
White eye male: Xr Y

9. Linkage Group
Thousands more genes than chromosomes  many genes on same chromosome
Linkage group: multiple genes on same chromosome
Linked genes tend to be inherited together
Why?

10. Linked Groups in Drosophila
Morgan looked at fly body color and wing size
Gray body, Long wing x Black body, Short wing
BBLL x bbll
If on separate chromosomes:
9:3:3:1
If on the same chromosome:
3:1

11. So, what actually happened?
Results:
several Gray, long (Ggll)
Several black, short (bbLl)
HOW????

12. Crossing Over Exchange of pieces of DNA between homologous chromosomes
Accounts for unexpected phenotypes in the F2 generation of Morgan’s experiment

13. Chromosome Mapping
Likelihood of genes crossing over depends on distance from each other on chromosome
Farther apart the genes, the more likely that they will cross
If more offspring show the new combination of traits, the farther the genes are on a chromosome

14. Scientist conduct breeding experiments to determine how frequently genes separate from one another
THUS! They measure distance of genes on chromosomes
THUS! They prepare chromosome maps: diagram used to show linear sequence of gene on chromosome

17. Alfred H. Sturtevant Morgan’s student
Used crossing over data to construct chromosome map of Drosophila
Genes that are separated by crossing over 1% of the time are 1 map unit apart

18. Mutation MANY Different types: Germ-cell mutations: in gametes
Chromosomal: Whole chromosome
Genetic: Single nucleotide
Germ-cell mutations: in gametes
Do NOT affect organism
Affect organism’s offspring
Somatic mutations: in body cells
affect organism (skin cancer, leukemia)
NOT passed on to offspring

19. Chromosome Mutations
Deletion: loss of a piece of chromosome due to chromosomal breakage
All info of that chromosome lost
Inversion: piece of chromosome broken off and reattached upside-down
Translocation: piece of chromosome breaks off and attaches to another, nonhomologous chromosome

20. Nondisjunction: failure of chromosome to separate from homologue during meiosis
Result: one gamete receives EXTRA COPY of a chromosome

21. Gene Mutations Point mutation: single based mutated
Substitution, deletion, insertion
Substitutions: single nucleotide replaced with a different nucleotide ( ATC TTC)
* REMEMBER: DNA mRNA  amino acid

22. Results: 1- base codes for same A.A.  NO CHANGE
2- base codes for different A.A.  Protein change
3- base code for STOP codon  Protein change

23. Sickle Cell Anemia Point substitution mutation
Substitutes adenine for thymine
Protein changed: hemoglobin
Result: irregular shaped red blood cells
Circulatory problems: heart, brain, lungs and many other organs and tissues damaged
Widespread in African Americans ( 1 of 500 in US)

24. Heterozygous: produced both normal and mutated forms of hemoglobin
In US 1 of every 10 African Americans are heterozygous for sickle cell anemia
Heterozygous: produced both normal and mutated forms of hemoglobin
Usually healthy

25. Frame Shift Mutations
Deletion: one or more nucleotide deleted from sequence
Insertion: one or more nucleotides inserted
Frame shift mutation: one or more nucleotide deleted or inserted

26. MUCH more serious effects Entire rest of sequence mutated
More DNA mutated if deletion/insertion occurs closer to beginning of sequence
More amino acids inaccurately coded for

27. 12-2 Human Genetics

28. Quick Look: 23 pairs of chromosomes 100,000 genes
Scientists focus on disease causing genes
Easily traced from one generation to the next

29. Pedigree Analysis
Pedigree: a family record that shows how a trait is inherited over several generations
Study phenotypes of family to see how traits are inherited

30. Patterns of Inheritance
Certain phenotypes usually repeat in predictable patterns
Carriers: individuals who have one recessive autosomal allele

31. Genetic Disorders
Diseases or condition that has a genetic basis

32. Single-allele Traits
Controlled by single allele of a gene
200+

33. Huntington’s Disease (HD)
*“Autosomal-dominant pattern of inheritance”
Caused by dominant allele on autosome
Symptoms: mild forgetfulness; irritability
Results: loss of muscle control; uncontrollable physical spasms; severe mental illness; death
On-set: 40’s or later
*Usually unknown until after individuals have children
*THUS! Usually passed on without knowing

34. Genetic Marker
Short section of DNA that is known to have close association with particular gene
If marker is present, USUALLY gene is present
Sampling can show presence of gene (disorder)
Marker present: 96% chance of HD

35. Homozygous Recessive Single-allele Traits
Expressed ONLY in homozygous state (aa)
Show autosomal-recessive patt. of inherit.
250+
Ex: Cystic fibrosis (CF) and sickle cell anemia

36. Multiple-allele Traits
Controlled by three or more alleles of the same gene that code for a single trait
Ex: ABO blood groups
Three alleles: IA, IB, i
Codominant: IA, IB

37. Polygenic Traits Traits controlled by two or more genes
Show many degrees of variation
Skin color; 3-6 genes
Eye color;
Light blue: very little melanin
Dark brown: a lot of melanin
Many influenced by environment
Height: also influenced by nutrition and disease

38. X-Linked Traits
Not all are diseases
Ex: Colorblindness, Hemophilia

39. Sex-Influenced Traits
When presence of female or male hormones influence expression of certain human traits
Ex: Pattern Baldness
Male and female: BB  lose hair
Male: BB’  lose hair due to high testosterone
Female: B’  does NOT lose hair
Genes that code for trait on autosomes NOT gametes!

40. Due to Nondisjunction Lack a chromosome OR have an extra
Monosomy: only one copy of a chromosome
45 instead of 46
Trisomy: an extra copy of a chromosome
47 instead of 46

41. Down Syndrome AKA Trisomy-21 Extra chromo. 21
Severe mental retardation, characteristic facial features, muscle weakness, heart defects, short stature

42. Nondisjunction in Sex Chromosome
Klinefelters syndrome: XXY
Female appearance; mental retardation; infertile
Turner’s syndrome: XO
Female; do not mature sexually; infertile
Just Y CANNOT survive
X contains info vital to survival

43. Detecting Human Genetic Disorders
Family history of genetic disorders  Genetic screening before becoming a parent
Examination of genetic makeup
Make karyotype
Test blood for certain proteins
Genetic counseling: medical guidance to inform parents about problems that could affect offspring

44. Amniocentesis
Physician removes small amount of amniotic fluid from amnion (sac that surrounds fetus)
Fetal cells and proteins analyzed
Karyotype prepared

45. Chronic villi Sampling
Physician takes sample of chronic villi (tissue that grow between mother’s uterus and placenta)
Produce karyotype
Has same genetic make up as fetus

46. Sample Taken After Birth
Immediately after birth genetic testing done of fetus
Ex: PKU (phenylketonuria)
Disorder when infant cannot metabolize the amino acid phenylalanine
Excess phenylalanine can cause brain damage
Infants put on diet without phenylalanine