1. Human Genetics
Mendelian Genetics

2. Inheritance Parents and offspring often share observable traits.
Grandparents and grandchildren may share traits not seen in parents.
Why do traits disappear in one generation and reappear in another?
Why do we still keep talking about Mendel and his peas?

3. Darwin Prior to Mendel
Scientists looked for rules to explain continuous variation.
The “blending” hypothesis: genetic material from the two parents blends together
Head size, height, longevity – all continuous variations - support the blending hypothesis
The “particulate” hypothesis: parents pass on discrete heritable units (genes)
Mendel’s experiments suggested that inherited traits were discrete and constant

4.
Why did Mendel succeed in seeing something that nobody else saw?
He counted
Chose a good system
Chose true-breeding characters
Gregor Mendel

5. The field of genetics started with a single paper!

6. Mendel is as important as Darwin in 19th century science
Mendel did experiments and analyzed the results mathematically. His research required him to identify variables, isolate their effects, measure these variables painstakingly and then subject the data to mathematical analysis.
He was influenced by his study of physics and having an interest in meteorology. His mathematical and statistical approach was also favored by plant breeders at the time.

7. Mendel used an Experimental, Quantitative Approach
Advantages of pea plants for genetic study:
There are many varieties with distinct heritable features, or characters (such as color); character variations are called traits
Mating of plants can be controlled
Each pea plant has sperm-producing organs (stamens) and egg-producing organs (carpels)
Cross-pollination (fertilization between different plants) can be achieved by dusting one plant with pollen from another

8. Self-fertilization Cross-pollination
Easy to cultivate and a short life cycle
- easy to control pollination
- keep unwanted pollen out
- cross-fertilize artificially
had discontinuous characteristics
flower color
seed texture
Mendel knew of at least 34
Self-fertilization
Cross-pollination

9. Mendel Planned Experiments Carefully
Mendel chose to track only those characters that varied in an “either-or” manner
He also used varieties that were “true-breeding” (plants that produce offspring of the same variety when they self-pollinate)
He spent 2 years getting “true” breeding plants to study
At least three of his traits were available in seed catalogs of the day

10. Mendel studied true breeding pea traits with two distinct forms

11. Terminology of Breeding
P1 (parental) - pure breeding strain
F1 (filial) – offspring from a parental cross
They are also referred to as hybrids – because they are the offspring of two 2 pure-breeding parents
F2 - produced by self-fertilizing the F1 plants

12. Phenotype Genotype PP (homozygous Purple 1 Pp (heterozygous 3 Purple 2
Because of the different effects of dominant and recessive alleles, an organism’s traits do not always reveal its genetic composition
Therefore, we distinguish between an organism’s phenotype, or physical appearance, and its genotype, or genetic makeup
In flower color in pea plants, plants have the same phenotype (purple), but different genotypes (PP and Pp) pp
(homozygous 1
White 1
Ratio 3:1
Ratio 1:2:1

13. 3 : 1 P Generation Appearance: Purple flowers PP White flowers pp
Genetic makeup:
Gametes P p
F1 Generation
Appearance:
Genetic makeup:
Purple flowers Pp
Gametes: 1 2 P 1 2 p
F1 sperm P p
F2 Generation P PP Pp
F1 eggs p Pp pp 3
: 1

14. The Testcross
How can we tell the genotype of an individual with the dominant phenotype?
This individual must have one dominant allele, but could be either homozygous dominant or heterozygous
The answer is to carry out a testcross: breeding the mystery individual with a homozygous recessive individual
If any offspring display the recessive phenotype, the mystery parent must be heterozygous

15. Mendel’s Model
Mendel developed a hypothesis to explain the 3:1 inheritance pattern he observed in F2 offspring
Four related concepts make up this model
These concepts can be related to what we now know about genes and chromosomes

16. The First Concept
Alternative versions of genes account for variations in inherited characters
For example, the gene for flower color in pea plants exists in two versions, one for purple flowers and the other for white flowers
These alternative versions of a gene are now called alleles
Each gene resides at a specific locus on a specific chromosome

17. Allele for purple color
Homologous
pair of
chromosomes
Locus for flower color gene
Allele for white color

18. The Second Concept
For each character, an organism inherits two alleles, one from each parent
Mendel made this deduction without knowing about the role of chromosomes
The two alleles at a locus on a chromosome may be identical, as in the true-breeding plants of Mendel’s P generation
Alternatively, the two alleles at a locus may differ, as in the F1 hybrids

19. The Third Concept
If the two alleles at a locus differ, then one (the dominant allele) determines the organism’s appearance, and the other (the recessive allele) has no noticeable effect on appearance
In the flower-color example, the F1 plants had purple flowers because the allele for that trait is dominant

20. The Fourth Concept Known as “the law of segregation”
Two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes
Thus, an egg or a sperm gets only one of the two alleles that are present in the somatic cells of an organism
This segregation of alleles corresponds to the distribution of homologous chromosomes to different gametes in meiosis

21. Mendel’s Laws Explain his Data
Mendel’s segregation model accounts for the 3:1 ratio he observed in the F2 generation of his numerous crosses
The possible combinations of sperm and egg can be shown using a Punnett square, a diagram for predicting the results of a genetic cross between individuals of known genetic makeup
A capital letter represents a dominant allele, and a lowercase letter represents a recessive allele

22. Two types of “states” Homozygous: Heterozygous:
an individual or a locus carries identical alleles of a given gene.
Heterozygous:
an individual or a locus carries different alleles of a given gene

23. Mendel’s Law of Segregation
Members of a gene pairs (alleles) separate from each other during gamete formation.
The underlying mechanism is separation and then segregation of homologous chromosomes during meiosis.
Key terms:
dominant and recessive traits
genotype versus phenotype

24. Garrod, 1902 - human traits followed Mendelian rules
Inborn errors of metabolism
Hint: much more common in first cousin marriages

25. LE 14-7
Dominant phenotype,
unknown genotype:
PP or Pp?
Recessive phenotype,
known genotype: pp
If PP,
then all offspring
purple:
If Pp,
then 1 2 offspring purple
and 1 2 offspring white: p p p p P P Pp Pp Pp Pp P P Pp Pp pp pp

26. Mendel’s Second Law: The Law of Independent Assortment
Mendel derived the law of segregation by following a single character
The F1 offspring produced in this cross were all heterozygous for that one character
A cross between such heterozygotes is called a monohybrid cross

27. Mendel identified his second law of inheritance by following two characters at the same time
Crossing two, true-breeding parents differing in two characters produces dihybrids in the F1 generation, heterozygous for both characters
A dihybrid cross, a cross between F1 dihybrids, can determine whether two characters are transmitted to offspring as a package or independently

28. LE 14-8 P Generation YYRR yyrr Gametes YR yr YyRr F1 Generation
Hypothesis of
dependent
assortment
Hypothesis of
independent
assortment
Sperm 1 YR 1 Yr yR yr
Sperm 4 4 1 4 1 4
Eggs 1 YR yr 2 1 2 1 YR
Eggs 4
YYRR
YYRr
YyRR
YyRr 1 YR
F2 Generation
(predicted
offspring) 2
YYRR
YyRr 1 Yr 4
YYRr
YYrr
YyRr
Yyrr 1 yr 2
YyRr
yyrr 1 yR 4
YyRR
YyRr
yyRR
yyRr 3 4 1 4 1 yr 4
Phenotypic ratio 3:1
YyRr
Yyrr
yyRr
yyrr 9 16 3 16 3 16 3 16
Phenotypic ratio 9:3:3:1

29. The law of independent assortment states that each pair of alleles segregates independently of other pairs of alleles during gamete formation
Strictly speaking, this law applies only to genes on different, nonhomologous chromosomes
Genes located near each other on the same chromosome tend to be inherited together

30. Probability Ranges from 0 to 1
Probabilities of all possible events must add up to 1
Rule of multiplication: The probability that independent events will occur simultaneously is the product of their individual probabilities.
Rule of addition: The probability of an event that can occur in two or more independent ways is the sum of the different ways.

31. Probability: The likelihood that an event will occur
No chance of event probability = 0 (e.g. chance of rolling 8 on a six-sided die)
Event always occurs probability = 1
(chance of rolling 1,2,3,4,5,or 6 on a six-sided die)
The probabilities of all the possible events add up to 1.
# on die
probability 1
1/6 2 3 4 5 6
The probability of an event
= # of chance of event
total possible events

32. Independent Events
The probability of independent events is calculated by multiplying the probability of each event.
In two rolls of a die, the chance of rolling the number 3 twice:
Probability of rolling 3 with the first die = 1/6
Probability of rolling 3 with the second die = 1/6
Probability of rolling 3 twice = 1/6 x 1/6 or 1/36

33. Independent events
What is the chance of an offspring having the homozygous recessive genotype when both parents are doubly heterozygous?

34. Independent Events

35. Dependent Events The probability of dependent events is calculated
by adding the probability of each event.
In one roll of a die, what is the probability of rolling either
the number 5 or an even number?
Probability of rolling the number = 1/6
Probability of rolling an even number = 3/6
Probability of rolling 5 or an even number = 1/6 + 3/6 or 4/6

36. Dependent Events Parents are heterozygous for a trait, R.
What is the chance that their child carries at least one dominant R allele?
Probability of child carrying RR = 1/4
Probability of child carrying Rr = 1/2
Probability of child carrying R = 1/4 + 1/2 = 3/4

37. Multiplication and Addition Rules Applied to Monohybrid Crosses
The multiplication rule states that the probability that two or more independent events will occur together is the product of their individual probabilities
Probability in an F1 monohybrid cross can be determined using the multiplication rule
Segregation in a heterozygous plant is like flipping a coin: Each gamete has a 1/2 chance of carrying the dominant allele and a 1/2 chance of carrying the recessive allele

38. ½ chance of P and ½ chance of p allele results in ¼ chance of each homozygous genotype.
There are two ways to get the heterozygous genotype so it is
¼ + ¼ = ½
Three genotypes give the same phenotype.

39. Solving Complex Genetics Problems with the Rules of Probability
We can apply the rules of multiplication and addition to predict the outcome of crosses involving multiple characters
A dihybrid or other multicharacter cross is equivalent to two or more independent monohybrid crosses occurring simultaneously
In calculating the chances for various genotypes, each character is considered separately, and then the individual probabilities are multiplied together

40. YYRR yyrr Female Gametes YyRr ¼ ¼ ¼ ¼ YyRr YR Yr yR yr ¼ YR Yr yR yr
¼ ¼ ¼ ¼
YyRr
YR Yr yR yr YR Yr yR yr
YYRR
YYRr
YyRR
YyRr
YyRr
Male gametes
YYrR
YYrr
YyRr
Yyrr
9/16
YyRR
YyRr
yyRR
yyRr
3/16
3/16
YyRr
Yyrr
yyRr
yyrr
1/16

41. For a dihybrid cross – the chance that 2 independent events occur together is the product of their chances of occurring separately.
The chance of yellow (YY or Yy) seeds= 3/4 (the dominant trait)
The chance of round (RR or Rr) seeds = 3/4 (the dominant trait)
The chance of green (yy) seeds= 1/4 (the recessive trait)
The chance of wrinkled (rr) seeds= 1/4 (the recessive trait)
Therefore:
The chance of yellow and round= 3/4 x 3/4 = 9/16
The chance of yellow and wrinkled= 3/4 x 1/4 = 3/16
The chance of green and round= 1/4 x 3/4 = 3/16
The chance of green and wrinkled= 1/4 x 1/4 = 1/16

42. So, Rr genotype = (1/2 x 1/2) x 2 = 1/2 RR genotype is (1/2 x 1/2) = ¼
Add these to get the combined probability.
Can use to solve more complicated problems:
AaBBccDdEeFFGghhIiJJKk x aaBbCCDdEEffggHhIIjjKk

43. Crossing Double Heterozygotes

44. Fig 3.10b

46. Fig 3.11

47. Dihybrid Cross From the 16 possible fertilization events - 9 genotypes
- 4 phenotypes 9:3:3:1

48. Dihybrid cross
Tetrahybrid cross!

49. Statistical Analysis
Simple cross: purple x white
F1: all purple
F2: 2850 purple, 1150 white
10x as many, but
same ratio
Use Χ2 test to determine likelihood of getting this result by chance
Χ2 = total of (observed-expected)2/expected over all classes
"Expected" is from null hypothesis - data fit a 3:1 ratio
( )2/ ( )2/1000 = 30!
P is <<less than 5%, so data are significantly different from null hypothesis

50. Inheritance patterns are often more complex than predicted by Mendel
The relationship between genotype and phenotype is rarely as simple as in the pea plant characters Mendel studied
Many heritable characters are not determined by only one gene with two alleles
However, the basic principles of segregation and independent assortment apply even to more complex patterns of inheritance

51. Extending Mendelian Genetics for a Single Gene
Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following situations:
When alleles are not completely dominant or recessive
When a gene produces multiple phenotypes
When a gene has more than two alleles
The forensic characteristics usually have more than two alleles

52. Mendel’s Third Law: Law of Dominance
Some genes mask the effect of other genes, which then can pass on unchanged to reemerge when they are combined with another recessive gene.

53. The Spectrum of Dominance
Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical
In incomplete dominance, the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties
In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways
Forensic Traits are codominant

54. P Generation Red CRCR White CWCW Gametes CR CW Pink CRCW F1 Generation2 CR 1 2 CW
Sperm 1 2 CR 1 2 CW
Eggs
F2 Generation 1 CR 2
CRCR
CRCW 1 2 CW
CRCW
CWCW

55. The Relation Between Dominance and Phenotype
A dominant allele does not subdue a recessive allele; alleles don’t interact
Alleles are simply variations in a gene’s nucleotide sequence
For any gene, dominance/recessiveness relationships of alleles depend on the level at which we examine the phenotype
If you look directly at DNA, you can always detect codominance.

56. Frequency of Dominant Alleles
Dominant alleles are not always more common in populations than recessive alleles
For example, one baby out of 400 in the USA is born with extra fingers or toes
The allele for this trait is dominant to the allele for the more common trait of five digits per appendage
In this example, the recessive allele is far more prevalent than the dominant allele in the population

57. Multiple Alleles
Most genes exist in populations in more than two allelic forms
For example, the four phenotypes of the ABO blood group in humans are determined by three alleles for the enzyme (I) that attaches A or B carbohydrates to red blood cells: IA, IB, and i.

58. Codominance is based on the carbohydrates being expressed differently on the red blood cells

59. Polygenic Inheritance
Quantitative characters are those that vary in the population along a continuum
Quantitative variation usually indicates polygenic inheritance, an additive effect of two or more genes on a single phenotype
Skin color in humans is an example of polygenic inheritance

60. 20/64 15/64 6/64 1/64 LE 14-12 AaBbCc AaBbCc aabbcc Aabbcc AaBbcc
Fraction of progeny
6/64
1/64

61. The Environmental Impact on Phenotype
Nature and Nurture:
The Environmental Impact on Phenotype

62. Relating Mendel’s Laws to Cells
Law of Segregation
Pairs of characteristics (alleles) separate during gamete formation
Each cell has two sets of chromosomes that are divided to one set per gamete.
Law of Independent Assortment
The inheritance of an allele of one gene does not influence the allele inherited at a second gene.
Genes on different chromosomes segregate their alleles independently.

63. Offspring acquire genes from parents by inheriting chromosomes
Children do not inherit particular physical traits from their parents
It is genes that are actually inherited
Genes are carried on chromosomes.
Mendel identified 7 sets of characters- One per each of the 7 chromosomes in peas, so his law worked out perfectly.
Two characters on the same chromosome are linked together and would have messed up his law.

64. Terminology Review Genes are the units of heredity
Genes are segments of DNA
Each gene has a specific locus on a certain chromosome
Genetic variants at the same locus are alleles of one another.
SNPs (Single Nucleotide Polymorphisms) are alleles of the same gene.

65. Key
Maternal set of
chromosomes
Possibility 1
Possibility 2
Paternal set of
chromosomes
Two equally probable
arrangements of
chromosomes at
metaphase I
Metaphase II
Daughter
cells
Combination 1
Combination 2
Combination 3
Combination 4

66. 8 Gamete Combinations vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv Maternal set of
chromosomes (n = 3)
2n = 6
Paternal set of
chromosomes (n = 3)
Two sister chromatids
of one replicated
chromosomes
Centromere
Two nonsister
chromatids in
a homologous pair
Pair of homologous
chromosomes
(one from each set)

67. LE 13-5
Key
Haploid gametes (n = 23)
Haploid (n)
Ovum (n)
Diploid (2n)
Sperm
cell (n)
MEIOSIS
FERTILIZATION
Ovary
Testis
Diploid
zygote
(2n = 46)
Mitosis and
development
Multicellular diploid
adults (2n = 46)

68. Recall Meiosis
Homologous pairs of chromosomes orient randomly at metaphase I of meiosis
In independent assortment, each pair of chromosomes sorts maternal and paternal homologues into daughter cells independently of the other pairs
The number of combinations possible when chromosomes assort independently into gametes is 2n, where n is the haploid number
For humans (n = 23), there are more than 8 million (223) possible combinations of chromosomes

69. Random Fertilization
Random fertilization adds to genetic variation because any sperm can fuse with any ovum (unfertilized egg)
The fusion of gametes produces a zygote with any of about 64 trillion diploid combinations
Crossing over adds even more variation
Each zygote has a unique genetic identity

70. LE 13-11
Prophase I
of meiosis
Nonsister
chromatids
Tetrad
Chiasma,
site of
crossing
over
Metaphase I
Metaphase II
Daughter
cells
Recombinant
chromosomes

71. Monohybrid Cross: - cross involving only one character.
Monohybrid Cross: - cross involving only one character.

72. Results from Crosses F1 offspring - F2 offspring CONCLUSION
the trait expressed was the same as that of one of the parental lines
traits did not blend
F2 offspring
the traits from both parents were expressed in a 3:1 ratio
while the trait had not been expressed in the F1, it remained unchanged as it was passed from the P1 to the F1 and then to the F2 generation. 
CONCLUSION
Traits are inherited as discrete, separate units.

73. Mendel’s Conclusions
Factors (genes) that determine traits can be hidden or unexpressed.
Dominant traits have a factor (gene) that is expressed in the F1 offspring
Recessive traits have a factor (gene) that is not expressed in the F1 offspring

74. Mendel’s Conclusions
2. Despite P1 and F1 generations appearing identical, they must be genetically different.
Phenotype- observed properties of a trait
Genotype- the genetic makeup of a trait
PP1 and F1 seeds have the same phenotype but different genotypes

75. Mendel’s Conclusions
3. Since the F1 offspring had factors (genes) for both smooth and wrinkled - then there must be at least 2 factors for every trait.
Alleles- alternative forms of a gene
Genotype- indicates the combination of alleles present
Phenotype- indicates the trait observed
These terms differentiate the observed form and the underlying alleles present at a particular gene.