1. Single Gene Inheritance
How Traits are Passed On

2. Heredity/Classical Genetics
2 Genetics Units
Heredity/Classical Genetics
Molecular Genetics
Inheritance patterns
Based on the work of Gregor Mendel in the mid 1800s
DNA/Chromosomal explanations for inheritance patterns
Defines our modern understandings of genetics

3. *Note We are covering the genetics of sexually reproducing mammals
Bacteria/Archaea and asexual eukaryotes are a little different

4. Human Chromosomes

5. So you have 2 alleles, which determines what you look like??
It depends. This is where genetics starts to get complicated
Sometimes only 1 of the 2 alleles gets expressed
Sometimes they both express themselves

6. Case 1 – Complete Dominance
Only 1 trait is expressed
The dominant trait OR the recessive trait is expressed

7. Human Examples You have dimples or you don’t
You have freckles or you don’t
You are a dwarf or you aren’t
You have attached earlobes or you don’t
You have cystic fibrosis or you don’t

8. Complete Dominance Is expressed if it is present
Dominant Trait
Recessive Trait
Is expressed if it is present
You can have 1 or 2 dominant alleles
Represented by capital letters
Is expressed only if there is no dominant allele
You must have 2 recessive alleles (usually)
Represented by lower case letters

9. What’s Actually Happening?
One version codes for a functional protein, the other makes nothing
If one set of instructions makes enough proteins for the trait to be present, that trait is “dominant”
If one set of instructions does not make enough proteins for the trait to be present, that trait is “recessive”

10. The Dominant Phenotype
Phenotype = appearance
Is shown by individuals who have 1 or 2 dominant alleles
Called homozygous dominant (AA) or heterozygous (Aa) genotypes

11. The Recessive Phenotype
Is only shown if the individual is homozygous recessive (aa)

12. “Genotypes” – Combinations of Alleles
= homozygous dominant (shows dominant trait) =heterozygous (shows dominant trait) = homozygous recessive (shows recessive trait)

13. Punnett Square Show the possible alleles each parent could give.
This example shows an Aa (heterozygous) person mating with an aa (homozygous recessive individual)
Dad
Parent 1
Potential sperm
Parent 2
Mom
Potential zygotes
Potential eggs

14. Say A codes for green eyes a codes for black eyes

15. Draw a Punnett Square for Two Heterozygotes Mating

16. Case 2
If 2 different alleles are both present, their affects are combined
i.e. red allele + white allele  pink flowers
Straight + curly hair  Wavy hair

17. “Genotypes” – Incomplete Dominance
= AA (red flowers, or straight hair) =Aa (pink flowers, or wavy hair) = aa (white flowers, or curly hair)

18. Incomplete Dominance R r Rr R= red allele, r= white allele
RR = red flowers, Rr=pink flowers, rr= white flowers
If red flowers are mated with white flowers, what will the offspring be? R r Rr

19. What’s actually happening?
One set of instructions produces some proteins, but the fewer proteins cause a change in the phenotype

20. Draw a Punnett Square for 2 pink flowers being crossed

21. Case 3
Both alleles are expressed at the same time and do not mix/form intermediate

22. Codominance E.g. blood types IA and IB alleles are both dominant
There is a 3rd, recessive allele called i (codes for blood type O)

23. Blood Types 4 possible types A (IAIA or IAi individuals)
B (IBIB or IBi) individuals
AB (IAIB individuals)
O (ii individuals)

24. What’s Actually Happening
Both sets of instructions code for functional proteins

25. What parents could have children with type A or type B blood only?

26. Summary
You have 2 copies of every gene, one from your mom, one from your dad
Which version you express depends on the gene, sometimes only 1 (complete dominance), sometimes a mixture (incomplete dominance) and sometimes both (co-dominant)
The “traits” are due to which protein is produced

27. X-Linked Genes The X chromosome contains many non-sex related genes
E.g. an important gene for color vision- without individuals suffer color blindness
Since males have no second X chromosome, they express whatever allele is present on their single X chromosome
Punnett Squares are drawn with XA or Xa to show dominant/recessive traits and Y to represent the Y chromosome

28. X-Linked Punnett Square
Eggs XA Xa
XAXA
XAXa Y
XAY
XaY
Sperms
All daughters show dominant phenotype
Half of sons show recessive phenotype

29. Examples of X-linked genes in humans
Hemophilia
Color-blindness
Duchenne’s Muscular Dystrophy

30. What has to happen for a girl to be color blind? (draw the square)

31. Pedigrees

32. Pedigree Take 2

33. Sex-linked Pedigree

34. What if the data doesn’t fit these patterns?

35. Possibility 1– the Trait is Polygenic
Multiple genes play a role in determining phenotype
Skin color
Height
Eye color
Hair color

36. Possibility 2– Environmental Effects
Environment plays huge role in gene expression
E.g. weight, strength
May also directly affect phenotype
E.g. hydrangeas

37. Possibility 3 - Epistasis
Another gene affects the expression of this gene
E.g. mice need to have a “color gene” to express the agouti or black color