1. Genetics and Heredity
9 Biology

2. Terminology Chromosome Allele Homozygous Heterozygous Dominant
Recessive
Monohybrid
Dihybrid
Incomplete dominance
Gene
Gamete
XX, XY
Phenotype
Genotype
Karyotype
Mutation
Autosomes

3. Chromosomes Contain genetic information Made of DNA
Humans have 23 pair
Undergo mitosis
Somatic cells, autosomes
Undergo meiosis
Gametes, sex cells, sperm and egg

4. Genes Encode for traits of an organism
Alternate form of a gene is an allele
You have 2 alleles for each trait
One from your mother
One from your father

5. Alleles Alleles can be dominant or recessive
Your genotype contains all the alleles that you carry
Your phenotype is the visible evidence of those alleles

6. Example of Allele Dominance
Eye color:
Brown eyes are dominant over lighter colored eyes
In order for brown eyed parents to have a blue eyed child, they both must carry the recessive allele for blue eyes

7. Dominant vs. Recessive
Dominant alleles are expressed in the organism’s phenotype
(visible)
Recessive alleles are only expressed if the organism has 2 copies of the recessive allele

8. Incomplete Dominance
Incomplete dominance results in an intermediate phenotype
Ex: Red flowers and white flowers
When red and white flowers are crossed, their offspring are pink

9. Mendelian Genetics and Monohybrid Crosses
Gregor Mendel
Monk who experimented with crosses of pea plants
Considered the “Father of Genetics”
Worked out genetic probabilities using Punnett squares
Worked with monohybrid and dihybrid crosses

10. Homozygous and Heterozygous
Both alleles are the same
Ex: AA or aa
AA indicates the dominant allele is expressed in phenotype
aa indicates the recessive allele will be expressed in phenotype
Heterozygous:
Both of the alleles are different
Ex: Aa
Aa indicates the dominant allele will be expressed in phenotype, but the individual is a carrier of the recessive allele

11. Monohybrid Crosses Using Punnett Squares
Crosses that involve one trait
Assign letters indicating dominant and recessive alleles
Ex: B (dominant purple)
b (recessive white)
Female genotype Bb
Male genotype Bb

12. Monohybrid Crosses Using Punnett Squares, cont.
Results:
Offspring genotypes:
BB, Bb, Bb, bb
Offspring phenotypes:
3 purple petals
1 white petals
Homozygous:
2 alleles that are the same (BB or bb)
Heterozygous:
2 alleles that are not the same ( Bb)

13. Monohybrid Practice:

14. Ratios and Percentages
Interpretation of genetic crosses often require either a ratio or a percentage.
Genotypic ratio:
1:2:1 (1 BB, 2Bb, 1 bb)
Phenotypic ratio:
3:1 (3 purple, 1 white)
Genotypic percentage:
25% BB, 50% Bb, 25% bb
Phenotypic percentage:
75% purple, 25% white

15. Dihybrid Crosses Dihybrid crosses use 2 traits
They also use Punnett squares, but have 16 squares instead of 4
Example: Seed color/ seed texture
Parent 1 RrYy
Parent 2 RrYy
Phenotype for both: yellow, smooth

16. Setting Up Dihybrid Crosses
1. Identify genotype of the parents.
RrYy and RrYy
Write:
&
R r Y y & R r Y y
Use this series of pairings to label your dihybrid Punnett square:
1-3, 1-4, 2-3, 2-4
1-3= RY 1-3= RY
1-4= Ry 1-4= Ry
2-3= rY 2-3= rY
2-4= ry 2-4= ry

17. Interpreting Dihybrid Crosses
Genotypes:
RRYY ( 1 )
RRYy ( 2 )
RrYY ( 2 )
RrYy ( 4 )
Rryy ( 3 )
rrYY ( 3 )
rryy ( 1 )

18. Interpreting Dihybrid Crosses, cont.
Phenotypes:
Yellow, round = 9
Green, round = 3
Yellow, wrinkled = 3
Green, wrinkled = 1
Phenotypic ratio:
9:3:3:1
Classic ratio for the dihybrid cross of 2 heterozygotes

19. Practice!

20. XX and XY Females are genetically XX
Female egg carries only X chromosomes
Males are genetically XY
Male sperm carries both X and Y chromosomes
Male sperm determines the sex of the baby
The sex chromosomes are the last pair of chromosomes in a karyotype.
The X chromosome is quite large, and the Y chromosome is quite small

21. X and Y Chromosomes

22. Karyotypes Karyotypes are photographs of chromosomes
Chromosomes are matched by hand or computer, in pairs depending on their size, and centromere placement
Karyotypes can tell us:
If the number of chromosomes is normal
If the fetus will have a genetic defect of chromosome number
Karyotypes cannot tell us:
the degree of limitations the fetus may have

23. Sex-Linked Disorders Carried only on X chromosome
Because of incomplete masking of the X chromosome by the Y, males are more frequently affected.
Examples of Sex-Linked Disorders:
Color-blindness
Hemophilia
Duchenne's Muscular Dystrophy

24. Co-Dominance Co-dominance occurs when neither allele is dominant
Both alleles are expressed
Example:
Blood Typing
There are 4 blood types:
A, AB, B, O
The alleles are:
IA, IB, i
Combinations are:
IAIA, IAi, IAIB, IBIB, IBi, ii
Blood types are either Rh+ or Rh–
Rhesus factor: a protein that is either present, or not present

26. Practice Blood Type Crosses