1. Introduction to Mendelian genetics
Who is Gregor Mendel?
Introduction to Mendelian genetics

2. TRUE OR FALSE?
Girls inherit more traits from their mother than their father
You have inherited traits that are not apparent
Color blindness is more common in males than females
Identical twins are ALWAYS the same sex
A person can transmit genetic traits to their offspring which they themselves DO NOT show
The father determines the sex of a child
The total number of male births exceeds female births each year
Acquired characteristics, like mathematical skills, can be inherited
Fraternal twins are more closely relates to each other than to other siblings

3. Answers
False
True

4. Genetics
The field of Biology devoted to understanding how characteristics are passed from parents to offspring

5. Gregor Mendel
In the 19thcentury, Mendel studied heredity-which is the transmission of characteristics from parent to offspring
Mendel is most famous for studying pea plants
He studied what he called “factors” in pea plants
Factors would be things like tall or short (height), or yellow or green (pod color)

6. Some of Mendel’s Factors

7. Gregor Mendel First, Mendel grew true-breeding plants
According to Mendel, true-breeding plants are plants that will always produce offspring with the same traits
So a true-bred pea plant with purple flowers will only produce plants with purple flowers because it only has the “factors” for purple (not white).

8. Gregor Mendel: P generation
Mendel bred two opposite true-breeding plants
For example, he bred a true-breeding purple flower pea plant and a true-breeding white flower pea plant
He called this his P generation – parent generation

9. Gregor Mendel: F1 generation
All of the offspring of the P generation (which he called the F1 generation) turned out purple
Mendel called purple flower color the dominant factor
He hypothesized that when the dominant factor was present, the recessive factor(white color) did not show.

10. Gregor Mendel: F2 generation
Next, Mendel crossed the offspring from the F1 generation (he called this the F2 generation)
–He observed that about 75% of the flowers were purple and about 25% were white
–This is equal to about a 3:1 ratio

11. Mendel P Generation(true-breeding parents) Purple White flowers
F1 Generation(hybrids)
All plants had
purple flowers
F2 Generation

12. Mendel’s Real Results

13. Mendel’s Laws
Keep in mind that Mendel knew nothing of Punnett squares, genes, alleles, or even DNA!!!
All he could do was observe phenotypes and record ratios and other statistics
He came up with 2 important laws as a result of his observations.

14. Mendel’s Laws: Law of Segregation
Mendel concluded that each plant gets two factors (alleles) for a characteristic and when the plant reproduces, these two factors separate or segregate. So…
Each gamete (sex cell) gives one factor (allele) AND therefore…
Each offspring gets one factor from each parent

15. Law of Segregation: Punnett Squares
Alleles separate
Alleles separate

16. Mendel’s Laws: Law of Independent Assortment
Mendel did experiments using more than one trait (like height and seed color)
He noticed that one trait did not influence the inheritance of another trait
In other words, different factors separate independently of each other during the formation of gametes

17. Mendel’s Laws: Law of Independent Assortment
Examples:
Pea plants can be short or tall
Their seeds can be green or yellow
Short plants can have green or yellow seeds
Tall plants can have green or yellow seeds
So the inheritance of one does not affect the inheritance of the other.
Mendel noticed this with all the traits he studied

18. Independent Assortment

19. Mendel’s Laws Independent Assortment is not always true-
If different genes are located on the same chromosome, then they will most likely be inherited together
These are called Linked Genes

20. What were Mendel’s factors in reality?
We call these alleles today
–Alleles are alternative forms of a gene
Alleles for flower color were purple and white
The characteristics (like height) are caused by genes on DNA
Genes are segments of DNA that code for one protein
Each gene has 2 alleles, or versions (1 from mom and one from dad)

21. What were Mendel’s factors in reality?
The reason alleles come in pairs is because chromosomes come in pairs (homologous pairs)!!
One allele on each chromosome!
WHAT A COINCIDENCE!!!

22. The Genetics of Mendel’s Experiments
Some Vocab
Dominant trait-masks the recessive
Shown with capital letters
Recessive trait-only shows if dominant is not present
Shown with lower case letters
Phenotype-physical appearance
For example purple, wrinkled, tall, etc

23. The Genetics of Mendel’s Experiments
Some Vocab
Genotype-genetic makeup
This is usually abbreviated with letters like Gg, FF, or hh
Genotypes for a trait are usually2 letters because you get 2 alleles (1 from mom and 1 from dad)
Homozygous-two of the same alleles (like HH or hh)
Heterozygous-two different alleles (like Hh)

24. The Genetics of Mendel’s Experiments
Mendel’s P generation had the genotypes FF (for purple) and ff (for white)
True breeding is also homozygous
FF is homozygous dominant
ff is homozygous recessive

25. The Genetics of Mendel’s Experiments
We can show the results Mendel observed using a Punnett Square:
A Punnett Square shows possible genetic combinations in the zygotes
Mendel crossed his true breeding purple and white flower pea plants
We write this as FF x ff
LET’S DO THIS ON THE BOARD

27. The Genetics of Mendel’s Experiments
What Mendel did not know:
All of F1 pea plant flowers heterozygous (two different alleles), or Ff
That is why they were all purple
Remember dominant alleles mask recessive alleles
So with one purple allele present and one white, only purple would show as it is dominant

28. F2 generation LET’S EXAMINE EACH RATIO FOR EACH CROSS: F2 Generation
What genotypes do you start with?
How are they crossed?
What are your results?
What is the genotypic ratio (genes)?
What is the phenotypic ratio of purple (F) to white (f)?

29. Punnett Square Examples
Let’s do a Punnett square for BB x Bb
B= black fur in bunnies
b= white fur in bunnies
Black fur is dominant
What is the genotypic ratio?
What is the phenotypic ratio?
What are the chances for a white bunny?

30. Punnett Square Examples
Let’s look at a heterozygous cross
Bb x Bb
What is the genotypic ratio?
What is the phenotypic ratio?
What are the chances for a white or black bunny?

31. Predicting the Results of Heredity
What do these ratios and percents mean?
If we flip a coin, there is a 50% chance that it will land on heads. But it is still possible to get 5 tails in a row (although it is highly UNLIKELY!)
The more times you flip it, the more likely your results will be 50:50
If Bb and Bb bunnies mate, there is a 1:4 chance the offspring will be white (this does NOT mean that they will or will not have white bunnies)
If they have LOTS of children, about 25% of them will be white

32. REMEMBER… Homozygous dominant means 2 BIG letters
Heterozygous means one big one little
Homozygous recessive means 2 little letters
If an organism shows the dominant trait, then the can be either heterozygous OR homozygous dominant

33. Test Cross
When genotypes are not known, a test cross can be performed to figure it out
The organism with an unknown genotype is crossed with a homozygous recessive individual.
Test crosses are often used in breeding (like dog breeding) to determine is organisms are really “pure bred” (homozygous) for desired characteristics

34. Test Cross
Problem:
Let’s say you want to breed black bunnies and you do not want any white bunnies
What would be the only parents’ genotypes to produce black bunnies?
BB x BB
There are 2 ways to know for sure which black bunnies are homozygous and which are heterozygous: expensive genetic testing, or test crosses

35. Test Cross
Solution:
We take some black bunnies and mate them with white bunnies (homozygous recessive)
Let’s look at the Punnett Square results to see the possible results
Remember, black bunnies can be either BB or Bb

36. Test Cross – Punnett Squares (try each cross)
If a BB is crossed with bb, no white bunnies are produced
If a Bb is crossed with bb, then white bunnies may be produced
If a test cross produces white bunnies, we know the unknown genotype is Bb; if not the genotype is BB
The cross would be performed multiple times to be sure of the results

38. Predicting Dihybrid Crosses
When 2 traits are being looked at…
Let’s do a cross between two heterozygous tall, heterozygous purple flowered pea plants
So, TtFfx TtFf
For each plant, we now look at genotype for color and height

39. Predicting Dihybrid Crosses
Instead of 2 possible gametes, there will be 4
So, the Punnett Square will be 4 x 4
Phenotypic Ratios
Tall, purple : tall, white : short, purple : short, white
Keep same letters together, capitals 1st
You will not be asked for genotypic ratios for dihybrid crosses
What are the phenotypic ratios?
LET’S DO IT ON THE BOARD

41. Complex Inheritance
Mendel observed monogenic traits and no linked genes…It’s not usually that simple….

42. Other Types of Inheritance
Incomplete Dominance
The phenotype of the heterozygote is intermediate between phenotypes of the dominant and recessive traits
Example: when a homozygous red carnation is crossed with a homozygous white carnations, then pink carnations are produced
We usually don’t use lower case letters in this type of inheritance because nothing is really dominant

43. Incomplete Dominance Let’s look at the cross on the board RR = Red
RW= pink
WW= white
Let’s look at the cross on the board

44. Other Types of Inheritance
Codominance
Occurs when both alleles for a trait are expressed in heterozygous offspring
Codominant alleles are often symbolized with different letters

45. Codominance BB = Brown BW= Roan WW= White
Notice both brown and white are present in the heterozygous genotype

46. Codominance LET’S EXAMINE THE PUNNETT SQUARE ON THE BOARD Roan x Roan
BW x BW
What are the ratios for each phenotype?

47. Other Types of Inheritance
Multiple Alleles:
Genes with 3 or more alleles (or variations)
Human blood type shows codominance and it has multiple alleles-A, B, and O

48. Blood Type Human blood types have 3 alleles A, B, and O.
Each person still only gets 2 alleles, but there are 3 possibilities
O is recessive to A and B,
A and B are codominant:
Genotype AO or AA = A blood
Genotype BO or BB = B blood
Genotype OO = O blood
Genotype AB = AB blood (both alleles expressed)

49. Blood Type

50. Terminology Genotype Heterozygous B BO Heterozygous A AO
Homozygous recessive OO
Homozygous A AA
Homozygous B BB
AB(technically heterozygous) AB

51. Codominance Punnett Square
LET’S EXAMINE THE PUNNETT SQUARE ON THE BOARD
Heterozygous A with Heterozygous B
AO x BO
What are the ratios for each phenotype?

52. Other Types of Inheritance
Sex-Linked Genes and Traits
Remember sex chromosomes are the chromosomes that determine the sex of an organism
So these are traits/genes carried on sex chromosomes
These traits are symbolized using a superscript on the X or Y, such as Xr or XR

53. Other Types of Inheritance
Sex-Linked Genes and Traits Examples:
In fruit flies, the gene for eye color is on the X chromosome. Red (XR) is dominant, white (Xr) is recessive.
To have white eyes, females must have the genotype XrXr, or in other words TWO white alleles
To have white eyes, males must have the genotype XrY, or in other words ONE white allele
This is why X chromosome sex-linked traits are more common in males

54. Try the Punnett Square Homozygous red eyed female x white eyed male
XRXR x XrY
What are the ratios for each phenotype?

55. Other Types of Inheritance
Polygenic Inheritance:
Traits that are controlled by more than one gene
Most human traits are polygenic
Examples are height, skin color, eye color, and hair color

57. Other Types of Inheritance
Complex Characters:
Characters that are influenced by genetics AND the environment
Skin color and height are examples

58. Other Types of Inheritance
Sex-Influenced Traits:
Traits in which males and females show different phenotypes even though they have the same genotypes
Baldness is an example- it is dominant in men, but recessive in women
The differences are mainly due to males and females producing different hormones (chemical signals)

59. Other Types of Inheritance
Single Allele Traits
Traits where there is only one allele
If you have the allele you have the trait-there is no recessive
Huntington’s disease is an example

60. Another way to show heredity….
Pedigrees
Another way to show heredity….

61. Pedigree is a chart or “family tree” that tracks which members of a family have a particular trait.

62. Pedigrees In pedigrees, carriers have one copy of the recessive allele
So they CARRY the trait, but they do not show it
Pedigrees can be used to make predictions about
Future offspring
The genotype of individuals in the pedigree

63. Pedigrees
The first pedigree tracks the widow’s peak, so the filled in shapes have a widow’s peak
Widow’s peak is a dominant trait
Carriers are not always shown on pedigrees
Think about what alleles their parents can give them

64. First generation(grandparents)Ww ww ww Ww
Second generation(parents plus aunts and uncles) Ww ww ww Ww Ww ww
Third
generation
(two sisters) WW ww or Ww
Dominant trait (widow’s peak)
ww= no widow’s peak
WW= widow’s peak

65. Chromosome Mutations
Chromosome mutations involve changes in the structure of a chromosome or the loss or gain of a chromosome.
–Deletion: The loss of a piece of chromosome due to breakage
–Inversion: A chromosomal segment breaks off, flips around, and reattaches
-Missence: A change in chromosomal arrangement by insertion of DNA segment

66. Translocation-A piece of chromosome breaks off and reattached to a nonhomologous chromosome

67. Chromosome Mutations
Nondisjunction-When a chromosome fails to detach from its homologue during meiosis, so one gamete gets an extra chromosome
Instead of a haploid number (n) or diploid (2n), the gamete has 3 chromosomes (3n)

68. Chromosome Mutations (Examples)
Down’s syndrome
Nondisjunction of chromosome 21 in the egg cell produces 3 copies of chromosome 21
Symptoms include: heart defects, stunted growth, mental retardation
Cystic fibrosis
Can be caused by several mutations on chromosome 7 (insertion, missence)
Symptoms include: problems with respiratory and digestive systems

69. Chromosome Mutations (Examples)
Klinefelter’s syndrome
A male receives an extra X chromosome (XXY) because of nondisjunction of egg cell
Symptoms: Boyish, rounded look (despite age) and often infertility
Trisomy 18 (Edward’s syndrome)
Nondisjunction of chromosome 18 results in 3 copies of chromosome 18
Much more severe problems than Down’s syndrome. Only 10% of births survive to their first birthday