1. Genetic Crosses

2. Homozygous – both alleles are the same
Heterozygous – alleles are different
Monohybrid Crosses – cross using only one
trait

3. Punnett Square

4. Steps for Solving Monohybrid Crosses
Read the problem carefully
Identify dominant trait and assign is a CAPTIAL letter symbol
Assign the recessive trait a lower case letter of the dominant symbol
Identify the genotypes of the parents
Form the gametes
Fill in the square
Count the genotypes and phenotypes

5. Types of Monohybrid Crosses
Homozygous X Homozygous
a. AA X AA  Homo. Dominant
b. aa X aa  Homo. Recessive
Heterozygous X Heterozygous
a. Aa X Aa

6. Example
In a pea plant, tall is dominant to short. Cross a homozygous tall with a heterozygous tall plant.

7. Dihybrid Crosses
“crossing 2 traits”

8. Example
In pea plants, tall is dominant to short and green seeds are dominant to yellow seeds. Cross a plant that is short and has heterozygous green seeds with a homozygous tall with heterozygous green seeds.

9. Incomplete & Complete Dominance and Codominance

10. Incomplete v. Complete Dominance
Complete Dominance – one allele is completely dominant over another
* PP and Pp (both dominant)
Incomplete Dominance – phenotype in F1 generation that is between that of the parents
*occurs when 2 or more alleles influence the phenotype
*RR – dominant
rr – recessive
Rr - intermediate

11. Example of Incomplete Dominance

12. Codominance
neither allele is dominant; both alleles for a gene are expressed in a heterozygous offspring
Example: R – represents red
R` - represents white
RR` - represents roan

13. Genetic Studies

14. Sex Determination Early 1900s – Thomas Hunt Morgan Female – X-X
Male – X-Y
Sex chromosomes – chromosomes which determine sex

15. Sex-Linked Traits “traits who genes are found on the X chromosome”
Genotypes
1. XX – normal female
2. XX1 – normal female who is a carrier
3. X1X1 – abnormal female
4. XY – normal male
5. X1Y – abnormal male

16. Example
1. A mother that was normal/non carrier for hemophilia married a man that was a hemophiliac. What is the probability that they will have a child that is a hemophiliac?

17. Polygenic Traits “traits that have more than one gene” Most common
Skin color, height, weight, eye color, fur color

18. Multiple Allele Traits
“traits whose genes have more than 2 alleles”
Human Blood Types
4 blood types
1. Type A, Type B, Type AB, Type O
Genotypes
1. IAIA, IAIO Type A
2. IBIB, IBIO Type B
3. IAIB Type AB
4. IOIO Type O
Almost to the end

19. RH Factor in the Blood RH+ = have the protein
RH- = does not have the protein
Genotypes Phenotypes
IRH+/IRH RH+
IRH+/IRH-
IRH-/IRH RH-

20. Example
Tommy has Type AB+ (homozygous RH+) blood. He marries Susie who is homozygous B-.

21. More Examples 
1. Bob has homozygous A blood. He marries Jane who has AB blood. What are the possible blood types of the offspring?

22. 2. Betty has type O blood. She has a child that has type A blood
2. Betty has type O blood. She has a child that has type A blood. She claims Jim is the father. He has type B blood. Can he be the father? Show all the work to make your determination.

23. Human Genetic Studies

24. Pedigrees
“a family record that shows how a trait is inherited over several generations”

25. Practice What are the genotypes of both sets of parents?
What are the genotypes of each child from both sets of parents?
What are the genotypes for the F2 generation?
What is the relationship between offspring 8 and 9?
What is the relationship between offspring 13 and 1?

26. Mutations “any change in an organism’s DNA”
Change DNA  Change RNA  change protein
Somatic Cell Mutations
1. cannot be passed to offspring
2. ex) Cancer (human skin cancer, leukemia)
Germ-Cell Mutations
1. occur in organism’s germ cell (gametes)
2. can be passed on to offspring

27. Chromosome Mutations “change in structure of the chromosomes” Types
1. Deletion – part of the chromosome is lost
and breaks off
2. Inversion – part of a chromosome breaks
off and reattaches backwards
3. Translocation – part of chromosome breaks
off and reattaches to another
chromosome

29. Nondisjuntion Mutation
“failure of chromosome to separate from it’s homologue during meiosis”
Monosomy v. Trisomy

30. Examples of Nondisjunction Disorders
Down Syndrome (Trisomy 21)
1. extra chromosome 21
2. Symptoms – mental retardation, muscle
weakness, large forehead, large
tongue, heart defects, short
stature

31. Klinefelters Syndrome 1. male with extra X chromosome (XXY)
2. Symptoms: sometimes mental retardation,
infertile, some feminine
characteristics, undersized
testicles
Turners Syndrome
1. single X chromosomes (XO)
2. female appearance, infertile
due to not maturing
sexually

32. Karyotypes of Individuals with Genetic Disorders

33. Gene Mutations “something changes in the genes”
Point Mutation – one nitrogenous base is replaced by a different nitrogenous base
* Sickle cell Anemia – results in defective
form of the protein hemoglobin

34. Frame-Shift Mutation – deletion of nucleotide causing the remaining codons to be incorrectly grouped

35. Genetic Disorders “diseases which are expressed on the chromosomes”
Single-Allele Traits
1. Cystic Fibrosis – failure for the chloride ion
transport mechanism
* mucus clogs the liver, lungs, and pancreas

36. X-Linked Traits
1. Colorblindness
2. Hemophilia
3. Duchenne muscular Dystrophy

37. More Genetic Disorders
1. Tay-sachs
2. PKU

38. Detecting Genetic Disorders
Genetic Screening
1. examining a person’s DNA
2. Karyotype
Ways to get a cell sample
1. amniocentesis
2. chorionic villi sampling
Finding deformities
1. ultrasound