1. How are genes inherited?
Mendelian Genetics
Genetics – The Study of Heredity
Heredity – Passing traits from parents to offspring
How are genes inherited?

2. Each Chromosome Is Made of Millions of Base Pairs that Compose 100’s to 1000’s of Different Genes.
Chromosome Maps Link

3. Chromosomes / Genes / Proteins / Traits
ATGC sequence
Proteins
folded amino acid sequence
Traits
blue eyes
contain
produce
determine
But what are the rules that allow us to predict the likelihood of inheriting certain traits over others?

4. Important Vocabulary
Allele = A gene, but in a specific form. Each characteristic (trait) is always determined by two alleles.
Hair color is either brown (B) or blonde (b)

5. Dominant Allele = A gene that, if present, always produces a
Dominant Allele = A gene that, if present, always produces a protein (and therefore a trait).
Brown hair is dominant = BB or Bb

6. Recessive Allele = A gene that produces a protein (and therefore a trait) ONLY if present with another recessive allele.
Blond hair is recessive = bb

7. Important Vocabulary
Genotype = symbols (capital and lower case letters) that represent genes on homologous chromosomes. BB, Bb and bb are all genotypes.

8. Blond hair and brown hair are both phenotypes.
Phenotype = the organism’s characteristic as a result of its genes.
Blond hair and brown hair are both phenotypes.

9. Homozygous = a genotype showing two of the same allele.
BB is homozygous dominant; bb is recessive.

10. Heterozygous = a genotype showing two different alleles.
Bb is heterozygous.

11. Laws of Heredity Law of Unit Characters Law of Dominance
Genes were discovered by Gregor Mendel.
Mendel developed four laws that govern how traits are passed down
Law of Unit Characters
Law of Dominance
Law of Segregation
Law of Independent Assortment

12. Law of Unit Characters Example: Height can either be TT, Tt, or tt
Law of Unit Characters – Characteristics of an individual are controlled by hereditary factors (genes) that occur in pairs (on homologous chromosomes)
Example: Height can either be TT, Tt, or tt

13. Law of Dominance
Law of Dominance – the dominant allele is expressed and the recessive allele can be hidden.
This is why offspring don’t look like a complete blend of each of the parents.

14. Law of segregation
Law of Segregation – a pair of genes are separated during the formation of gametes. You can inherit either allele with equal probability. Ex. T or t but not both.

15. The dominant allele is not passed on more often than the recessive allele.
Parent has a dominant allele and recessive allele for hair color. Which allele will he pass on to his offspring?
Answer: 50% chance for either allele

16. Law of Independent Assortment
Law of Independent Assortment – inheritance of most traits does not depend on other traits.
The fact that you have brown hair has no effect on the color of eyes that you have.
There are exceptions to this law, and these will be discussed later.

17. Inheritance Patterns
How can you determine what traits are being passed on?
How can you predict what certain offspring will look like?
Reginald Punnett, an English Biologist, developed an easy way to solve these problems.

18. Punnett Squares
Suppose a woman with blue eyes marries a man with brown eyes, what color eyes will their children most likely have?

19. Punnett Squares Determine the genotypes of the parents
Father has brown eyes.
His genotype is either BB or Bb. For the sake of this problem, we’ll say his genotype is BB.
Mother has blue eyes.
Her genotype is bb.
P1: BB x bb

20. Punnett Squares
Create the grid. The father’s alleles go on the top, and the mother’s alleles go on the side: B B b b

21. B B b B b B b B b B b b Punnett Squares
Fill in the grid. Combine the parent alleles inside the boxes. The letters show the genotypes of the offspring. B B b B b B b B b B b b

22. Punnett Squares B b Bb
Determine the phenotypes. Use the law of dominance to determine the phenotype and phenotype ratio of the offspring.
(brown)
(brown)
(brown)
(brown)
F1 Generation

23. B b Bb (brown) 4 : 0 0 : 4 : 0 F1 Phenotypic Ratio: Brown : blue
4 : 0
F1 Genotypic Ratio:
Homozygous Brown: Heterozygous Brown: Homozygous blue
BB Bb bb
: : 0

24. Question:
If one of these children were to have a child with a heterozygous brown-eyed spouse, what color eyes would their children have?

25. B b BB Bb bb Bb (brown) (brown) (brown) (blue) 3 : 1 1 : 2 : 1
To find the F2 generation:
F1 (x) F1
(brown)
(brown) bb Bb
F2 generation
(brown)
(blue)
3 : 1
F2 Phenotypic Ratio:
Brown : blue
: : 1
F2 Genotypic Ratio:
Homozygous Brown: Heterozygous Brown: Homozygous blue
BB Bb bb

26. Genotypes of offspring: B- Brown White
In mice, brown color is dominant over white. Two brown mice are mated several times. They have a total of 20 offspring, 14 brown, and 6 white. Determine the genotypes of the parents and all the offspring. B b
B= brown
b = white B b B b
brown (x) brown B BB Bb
14 brown and 6 white Bb bb b B bb
Genotypes of parents:
Bb (x) Bb bb
Genotypes of offspring: B-
Brown
White

27. T t tt Tt Tt t Tt tt Tt T- tt t Tt tt tt
In humans, tongue-rolling is controlled by a dominant allele. A non-tongue roller man (whose parents are both tongue rollers) marries a woman who can role her tongue. The woman’s father was a tongue roller but her mother was not. The couple have one child who is a non-tongue roller. Determine the genotypes of all the people in the problem (7 total).
T= tongue roller
t = non tongue roller tt T t T t T -
non tongue roller
(x) tongue roller
t t T t
Genotypes :
man= _____
His dad ______ His mom _____
woman = _____
Her dad ______ Her mom _____
Child = ________ T t tt Tt Tt t Tt tt Tt T- tt t Tt tt tt

28. Skipping a Generation
When a trait disappears (not seen in parents) and then reappears in offspring generation
Proof that trait is controlled by a recessive gene.
Ex. Two white sheep produce a black sheep. Determine the genotypes of the parents and offspring.

29. Gregor Mendel How did Mendel come up with his four laws?
The Laws of Heredity were discovered using PEA PLANTS, but they are the same laws for ALL living things!!!
Law of Unit Characters
Law of Dominance
Law of Segregation
Law of Independent Assortment
How did Mendel come up with his four laws?

30. Why Peas? Easy to care for Characteristics are easy to identify.
Short life span, so you can get several generations in a short amount of time.

31. Large number of offspring at one time, providing lots of data.
Easily control the mating of pea plants in a green house.
Pea offspring can be mated with their parents and with one another.

33. Mendel’s Experiments
(mated with)
Purebred tall plants were crossed with purebred short
P1 Generation (original parents) X
All F1 offspring are tall. Short trait disappeared!
F1 Generation X
Short trait reappears in a consistent ratio of tall : 1 short
F2 Generation

34. Mendel’s Experimental Results
Purebred or Homozygous
P1 Generation: Tall (x) Short
TT (x) tt
F1 Generation: All Tall
F2 Generation: 3 Tall and 1 Short T t Tt Tt Tt Tt F1 T t TT Tt Tt tt F2

35. Mendel’s Results

36. Incomplete Dominance
When there is no dominant allele for a trait and blending occurs in the heterozygous condition

37. Example: Flower in Four- O’Clocks and Snapdragons
Red (RR), Pink (Rr), White (rr)
Pink Flower (x) Pink Flower
Rr (x) Rr R r RR Rr Rr rr
1 : 2 : 1
Phenotypic Ratio:
Red : Pink : White
Genotypic Ratio:
: :
Homozygous RR : Heterozygous Rr : Homozygous rr
Phenotypic & Genotypic Ratios are the same!

38. Pedigree Chart Show relationships in a family (family tree)
Shaded individuals have different phenotype from unshaded
Circles – females
Squares - males

39. WRONG! - B = Brown eyes b = blue eyes - - bb B b B bb bb B b B b B b B

40. DIHYBRID CROSSES – Two factor inheritance
Crosses involving two traits
Demonstrates Law of Independent Assortment

41. First, Outside, Inside, Last
TtRr – Tall and Round
outside
first
( T x t ) ( R x r )
inside
last
F O I L
First, Outside, Inside, Last

42. Key 1 Tall = TT or Tt, thus T- Short = tt Round= RR or Rr, thus R-
Wrinkled= rr
Key 2
Tall, Round = T-R-
Tall, Wrinkled= T-rr
Short, Round = ttR-
Short, wrinkled = ttrr
P1 Pure Tall, Pure Round (x) Pure Short, Pure Wrinkled
P TTRR (x) ttrr
Possible Gametes:
TR, TR, TR, TR
(x) tr, tr, tr, tr TR
(x) tr

43. Possible Gametes: TR (x) tr
TtRr F1
1 : : : 0
Phenotypic Ratio:
Tall, Round : Tall, Wrinkled : Short, Round : Short, Wrinkled
We will not be doing genotypic ratios for dihybird problems.

44. Find the F2 phenotype ratio: F1 (x) F1
Heterozygous Tall, Heterozygous Tall,
Heterozygous Round (x) Heterozygous Round
TtRr (x) TtRr
Possible
Gametes:
TR, Tr, tR, tr
(x)
TR, Tr, tR, tr

45. Possible Gametes: TR, Tr, tR, tr (x) TR, Tr, tR, tr
TTRR
TTRr
TtRR
TtRr Tr
TTRr
TTrr
TtRr
Ttrr tR
TtRR
TtRr
ttRR
ttRr tr
TtRr
Ttrr
ttRr
ttrr
Phenotypic Ratio:
Tall, Round : Tall, Wrinkled : Short, Round : Short, Wrinkled

46. Possible Gametes: TR, Tr, tR, tr (x) TR, Tr, tR, tr
TTRR
TTRr
TtRR
TtRr Tr
TTRr
TTrr
TtRr
Ttrr tR
TtRR
TtRr
ttRR
ttRr tr
TtRr
Ttrr
ttRr
ttrr
9 : : : 1
Phenotypic Ratio:
Tall, Round : Tall, Wrinkled : Short, Round : Short, Wrinkled

47. Heterozygous Tall, Homozygous Tall,
Heterozygous Round (x) Homozygous Round
TtRr (x) TTRR
Possible
Gametes:
TR, Tr, tR, tr
(x) TR

48. Possible Gametes: TR, Tr, tR, tr (x) TR
TTRR
TTRr
TtRR
TtRr
4 : : : 0
Phenotypic Ratio:
Tall, Round : Tall, Wrinkled : Short, Round : Short, Wrinkled

49. Practice Law of Unit Characters Law of Dominance Law of Segregation
Law of Independent Assortment
A. Characteristics of an individual are controlled by hereditary factors (genes) that occur in pairs
B. States that a pair of genes are separated during the formation of gametes. You can inherit either allele with the same probability.
C. The dominant allele is expressed and the recessive allele can be hidden.
D. Most traits do not depend on other traits.