1. Modern Genetics
Part 5

2. History Gregor Mendel: “Father of Genetics” Austrian Monk (1822-1884)
Published the results of scientific research on Garden Peas (Pisium sativum) in 1865.
Why study peas?
Great natural variation- stem length, seed color, pod shape ,pod color, small, edible, easy to grow, many offspring, easy to cross fertilize

3. Useful Terms Trait: characteristic of an organism.
Gene: piece of DNA that codes for a protein.
Allele: different forms of a gene.
Genotype: The allele combination of an individual.
Phenotype: The visible characteristics that result from a genotype.

4. Chromosomes and Human Genetics
Human Chromosomes
Types
Sex Chromosomes – 1 pair – carry the genes that determine male and female features (also some non-sex traits)
X and Y do not look alike but behave as a homologous pair at meiosis
XX = female, XY = male
Autosomes – non-sex chromosomes (22 pairs) – genes are unrelated to sex determination

5. PUNNETT SQUARE
Method for determining possible allele combination for the offspring
Gametes outside
Offspring Inside

6. Determining Sex…
MOM X X X X X X X
DAD X Y X Y Y

7. One Example of Mendel’s Work
Tall x
Dwarf
Phenotype P DD dd
Genotype
Homozygous
Dominant
Homozygous
Recessive
All Tall
Clearly Tall is Inherited…
What happened to Dwarf? F1 Dd
Tall is dominant to Dwarf
Use D/d rather than T/t for symbolic logic
Heterozygous
F1 x F1 = F2
possible gametes
Punnett Square: D d
3/4 Tall
1/4 Dwarf F2 D
Tall DD
Tall Dd
possible gametes d
Tall Dd
Dwarf dd
Dwarf is not missing…just masked as “recessive” in a diploid state… there IS a female contribution.

8. Sample Problem
In hamsters, white fur color (W) is dominant to brown fur color (w). If you cross a heterozygous female with white fur color (Ww) with a male that has brown fur (ww), what genotypes and phenotypes would you see and in what ratios?

11. Genetics After Mendel P F1 F2 Red Yellow x PRPR PYPY All Orange PRPY
After 1900 several scientists tried to replicate Mendel’s crosses using other species including snapdragon.
Genetics After Mendel
Red
Yellow P x
PRPR
PYPY
When these alleles go walking, they both do some talking (codominance)!
OK, so we cannot use R/r nor Y/y so we pick a third letter…P for the petal color gene.
Notice: we do NOT mix R/Y or r/y!
All Orange F1
PRPY
F1 x F1 = F2
possible gametes
Punnett Square: PR PY F2 PR
Red
PRPR
Orange
PRPY
This F2 will NOT have a 3:1 ratio of phenotypes.
Instead it shows a 1:2:1 ratio!
The exception here proves the rule.
possible gametes PY
Orange
PRPY
Yellow
PYPY

12. Sample Problem
In horses, black and white coat colors are codominant. Heterozygous horses have gray coats.
Black horse genotype: HBHB
White horse genotype: HWHW
Gray horse genotype: HBHW
What would be the possible genotypes and phenotypes of the following crosses?
Black x White
Gray x Gray

14. Blood Types Blood Type: A B AB O Genotype IAIA , IAi IBIB , IBi IAIB

15. Try These
If a male is homozygous for blood type B and a female is heterozygous for blood type A, what are the possible blood types in the offspring?

16. Is it possible for a child with Type O blood to be born to a mother who is type AB? Why or why not?

17. Chromosomes and Human Genetics
Human Chromosomes
Types
Sex Chromosomes – 1 pair – carry the genes that determine male and female features (also some non-sex traits)
X and Y do not look alike but behave as a homologous pair at meiosis
XX = female, XY = male
Autosomes – non-sex chromosomes (22 pairs) – genes are unrelated to sex determination

18. B. Determining Sex…
MOM X X X X X X X
DAD X Y X Y Y

19. II. Gene Location
Linked – Linkage Groups – genes located so close together on a chromosome that the traits always seem to appear together
Ex. Red hair and freckles
Ex. Colorblindness and Hemophilia
X X

20. Sex-linked Traits – genes on the sex chromosomes
Expression of certain genes often appears more in one sex than the other
Males require only one copy of a gene since they only have one X chromosome
See Royal Families of Europe Pedigree
Ex. Eye color in fruit flies, hemophilia, color-blindness

21. Colorblindness Tests

23. X-Linked/Sex Linkage – do not write
Genes present on the X chromosome exhibit unique patterns of inheritance due to the presence of only one X chromosome in males.
X-linked disorders show up rarely in females
X linked disorders show up in males whose mothers were carriers (heterozygotes)

24. Practice Sex-linked Problems….
What will the result of mating between a normal (non-carrier) female and a hemophiliac male?

25. A female carrier who is heterozygous for the recessive, sex-linked trait causing red-green colorblindness, marries a normal male. What proportions of their MALE progeny will have red-green colorblindness?

26. Hemophilia is inherited as an X-linked recessive
Hemophilia is inherited as an X-linked recessive. A woman has a brother with this defect and a mother and father who are phenotypically normal. What is the probability that this woman will be a carrier if she herself is phenotypically normal?

27. C. Gene Interactions
Polygenetic trait – many genes influence a single trait (ex. Height, intelligence)
Pleiotropic effect – one gene having many effects (ex. Gene to make testosterone)

28. Pleiotropy ?
Expression of a single gene has multiple phenotypic effects
Marfan Syndrome – abnormal gene that makes fibrillin (important in connective tissues)

29. III. Genetic Analysis
Karyotype – visualized chromosomes stained, squashed, and photographed at metaphase
- They are characteristic of the species or individual

30. B. Pedigree – chart showing family relationships (see worksheet)

31. Pedigree Analysis
Method of tracking a trait through generations within a family.
Good method of tracking sex-linked traits as well as autosomal traits.

33. Sex-Linked Pedigree
Shows gender bias with males exhibiting the trait more often than females

35. Autosomal Dominant Pedigree
Autosomal dominant traits do not skip a generation
Autosomal dominant traits do not show gender bias

36. Autosomal Recessive Pedigree
Autosomal recessive traits skip a generation
Autosomal recessive traits do not show gender bias

37. IV. Non-Mendelian Genetics
Incomplete Dominance – blended inheritance
Neither form of the gene is able to mask the other
Ex. Snap dragon petal color
R1R1 – RED
R1R2 – PINK
R2R2 - WHITE

38. Incomplete Dominance Neither allele is dominant
Heterozygotes are a blend of homozygous phenotypes = no distinct expression of either allele

39. Try these
In a plant species, if the B1 allele (blue flowers) and the B2 allele (white flowers) are incompletely dominant (B1 B2 is light blue), what offspring ratio is expected in a cross between a blue-flowered plant and a white-flowered plant?

40. What would be the phenotypic ratio of the flowers produced by a cross between two light blue flowers?

41. 2. Codominance No dominance and both alleles are completely expressed
Ex. Cat color
C1C1 – Tan
C1C2 – Tabby (black and tan spotted)
C2C2 - Black

42. Try These
Cattle can be red (RR = all red hairs), white (WW = all white hairs), or roan (RW = red & white hairs together.
Predict the phenotypic ratios of offspring when a homozygous white cow is crossed with a roan bull.

43. What should the genotypes & phenotypes for parent cattle be if a farmer wanted only cattle with red fur?

44. A cross between a black cat & a tan cat produces a tabby pattern (black & tan fur together).
What pattern of inheritance does this illustrate?
What percent of kittens would have tan fur if a tabby cat is crossed with a black cat?

45. 3. Multiple Alleles More than 2 alleles for one trait
Ex. Eye color, hair color, blood type, guinea pig fur color
ABO blood groups
Each individual is A, B, AB, or O phenotype
Phenotype controlled by marker on RBC
IA and IB alleles are dominant to the i allele
IA and IB alleles are codominant to each other

46. Blood Types Blood Type: A B AB O Genotype IAIA , IAi IBIB , IBi IAIB

48. Try These
If a male is homozygous for blood type B and a female is heterozygous for blood type A, what are the possible blood types in the offspring?

49. Is it possible for a child with Type O blood to be born to a mother who is type AB? Why or why not?

50. A child is type AB. His biological mother is also type AB
A child is type AB. His biological mother is also type AB. What are the possible phenotypes of his biological father?

51. Recessive can be common!
Human hair color follows a similar pattern:
Alleles: HBn = brown HBd = blonde hR = red hbk = black
HBnHBn = dark brown
HBnHBd = sandy brown
HBnhR = auburn
HBnhbk = dark brown
HBdHBd = blonde
HBdhR = strawberry blonde
HBdhbk = blonde
hRhR = red
hRhbk = red
hbkhbk = black
Dominant does NOT mean frequent!
Recessive can be common!