1. Chapter 15 Genetics 2 H.L.

2. Lesson Objectives
At the end of this sub section you should be able to:
State the Law of Segregation
State the Law of Independent Assortment
Describe the experiments used to formulate these 2 laws.
Complete dihybrid crosses using punnett square
Define linkage
Explain outcome in results with linked genes
Explain sex linked traits
Discuss non nuclear inheritance
Describe DNA structure & protein synthesis

3. Gene
A segment of DNA that contains coding for a polypeptide or protein
A unit of hereditary information.

4. Heredity
The transmission of characteristics from parents to offspring.

6. Gregor Mendel Father of genetics – Gregor Mendel, an Austrian monk
1857 – began collecting pure lines of peas
Chose self-fertilizing peas so that all offspring look exactly like their parent
Mendel chose 7 traits for study

8. Pea Traits used by Mendel

10. Mendel’s Peas – Why Peas?
Pure lines with easily identifiable traits were available
Peas are self-fertilizing with a flower structure that minimizes accidental pollination
Peas can be artificially fertilized which allows specific crosses to be made
Peas have a short growth period
Peas produce large numbers of offspring

11. Mendel’s Experiments
He crossed pure plants with alternative phenotypes for a single trait
He recorded how many offspring were of each type – 1st generation results (F1 generation)
He allowed these offspring to self-fertilize

12. Mendel’s Findings
He again recorded the nature of the offspring – F2 generation
He did a mathematical analysis
He deduced several principles
He published paper in good scientific journal – 1866

13. Example of Results – Seed Coat
(Smooth seeds vs Wrinkled seeds)
Parents:
one parent had smooth seeds
the other wrinkled seeds
Result: F1 all smooth

14. 2nd Generation 2nd generation offspring – F2 Results
Parents – both heterozygous smooth
Offspring:
¾ of offspring were smooth
¼ of offspring were wrinkled
3:1 Ratio

15. For all seven traits he got a 3:1 ratio
Results
For all seven traits he got a 3:1 ratio

17. Summary of Mendel’s Results
F1 showed only one of two parental traits
All crosses were the same; it did not matter which plant the pollen came from
Trait not shown in F1 reappeared in 25% of the plants in the F2 generation

18. Mendel’s Results
Traits are not blended as they are passed from parent to offspring
Each parent makes equal contribution
Genes can be carried but not expressed
Appearance may be similar but genetic make up may differ

19. Why was Mendel successful?
Good luck – he chose peas!
Naturally self-fertilizing – easy to have pure lines but it can be forced to cross with a different line and experimenter can completely control crosses
Analyzed his results quantitatively, large numbers yielded good statistical ratios
Started simply – worked from simple to complex one trait at a time then two at a time, etc

20. Learning Check Who was Gregor Mendel? Why did he choose peas?
Give examples of the pea traits used by Mendel
What types of peas did he cross?
What is meant by F1 generation
What results did he get in the F2 generation?
What conclusions can be drawn form his results?

21. Mendel’s Laws
The Law of Segregation
The Law of Independent Assortment

22. Mendel’s First Experiment
Crossed Pure Tall x Pure Short (Dwarf)
Predictions: The offspring would be either:
Some would be tall and some short
All intermediate
All short
All tall

23. Mendel’s Experiments 1st Experiment: C Pure Tall x Pure Short Results:
All offspring (F1) tall
2nd Experiment:
Bred F1 Plants
Ratio of 787 tall to 277 short (3:1) 1 2

24. Mendel’s First Principle
Mendel assumed that the two “Factors” for each trait must exist in the parental cells producing the gametes
These “Factors” came from the parents’ parents and were united in fertilization
In forming pollen and egg, the two”factors” for any trait must separate and go into different gametes
This became known as Mendel’s “Principle of Segregation”

25. The Law of Segregation
“states that organisms contain 2 factors for each trait. These factors separate in gamete formation producing gametes with only one copy of each factor.”

26. F1 Generation

27. F2 generation

28. How the results are explained
Chromosome movement in meiosis explained Mendel’s results
Chromosomes occur in pairs – genes occur in pairs
Paired chromosomes separate in meiosis just as paired alleles separate in gamete formation

29. Law of Independent Assortment
States that alleles of any one gene are transmitted independently of any other pair of alleles

30. Animation Explaining Independent Assortment of Genes

31. Studying the inheritance of two characters simultaneously
THE DIHYBRID CROSS
Studying the inheritance of two characters simultaneously

32. Mendel’s peas Character Trait Allele Seed shape Round R Wrinkled r
Seed colour
Yellow Y
Green y

33. Possible Combinations
Phenotype
Round Yellow
Round Green
Wrinkled Yellow
Wrinkled Green
Genotype
RRYY
RRYy
RrYY
RrYy
RRyy
Rryy
rrYY
rrYy
rryy

34. Probability
Probability: the likelihood that a specific event will occur.
May be expressed as a decimal, %, fraction, or ratio.
Probability= # of actual times
# of opportunities

35. The expected probability of each type of seed can be calculated:
Probability of an F2 seed being round = 75% or ¾
Probability of an F2 seed being wrinkled = 25% or ¼
Probability of an F2 seed being yellow = 75% or ¾
Probability of an F2 seed being green = 25% or ¼

36. THE LAW OF INDEPENDENT ASSORTMENT
It appears that the inheritance of seed shape has no influence over the inheritance of seed colour
The two characters are inherited INDEPENDENTLY
The pairs of alleles that control these two characters assort themselves independently

37. Learning Check What are Mendel’s two Laws?
State the Law of Segregation
State the Law of Independent Assortment
What is the difference between a monohybrid and a dihybrid cross?

39. Homozygous x Homozygous

40. Homozygous x Heterozygous

41. Heterozygous x Heterozygous

42. Learning Check
If R is dominant to r, what the offspring of the cross of RR with rr be?
According to Mendel, what kind of genes "disappear" in F1 pea plants?
Assuming complete dominance, the F2 generation following the cross Aa x Aa will show a phenotypic ratio of _____ .
In meiosis what happens to chromosome from each homologous pair?

43. Dihybrid test cross
In a dihybrid cross the test cross is made with an individual which is homozygous recessive for both characters RRYY and rryy

44. Parents Phenotypes: X Parents Genotypes: X Gametes: F1 Genotypes: F1 Phenotypes: Ratio

46. X
Phenotype
Round, yellow
Wrinkled, green
rryy
RRYY
Genotype
all RY
all ry
Gametes
all RrYy F1
All of the F1 generation would be heterozygous for both characteristics, meaning that they would all be round and yellow. Mendel then crossed two of the F1 generation together…

47. X Use a punnett square RrYy RrYy RY, Ry, rY, ry RY, Ry, rY, ry
Phenotype
Round, yellow
Round, yellow
RrYy
RrYy
Genotype
RY, Ry, rY, ry
RY, Ry, rY, ry
Gametes F2
Use a punnett square

48. RY Ry rY ry
RRYY
RRYy
RrYY
RrYy
RRyy
Rryy
rrYY
rrYy
rryy

49. This is the typical ratio expected in a dihybrid cross.F2 : : : 9 3 3 1
Round
Yellow
Round
Green
Wrinkled
Yellow
Wrinkled
Green
This is the typical ratio expected in a dihybrid cross.

50. Parents Phenotypes: X
Parents Genotypes: X
Gametes
Genotypes
Phenotypes
Ratio

51. Dihybrid test cross Phenotypes Round Yellow X Wrinkled Green Genotypes
RrYy
rryy
Gametes
RY, Ry, rY, ry ry
Genotypes RY Ry rY ry
RrYy
Rryy
rrYy
rryy
Phenotypes
Round Yellow
Round Green
Wrinkled Yellow
Wrinkled Green
Proportions
25%

52. Learning Check What is a dihybrid test cross?
In pea plants, tall plant (T) is dominant over a short plant(t). Purple flower (P) is dominant over Short white(p) plant.
A homozygous Tall purple flower (P) plant is crossed with a Short white(p) plant.
State the genotypes of the parent plants and of the F1 generation by doing out a cross.

53. Parents Phenotypes: Tall purple X Short white Parents Genotypes: TTPP X ttpp Gametes: TP tp F1 Genotype: TtPp F1 Phenotype: Tall purple

54. 4. If the F1 plants are self fertilised what will the genotypes of the F2 generation be?
Use a punnett square

55. Parents Phenotypes: Tall purple X Tall purple
Parents Genotype: TtPp X TtPp
Gametes: TP Tp tP tp X TP Tp tP tp
Ratio 9:3:3:1 TP Tp tP tp
TTPP
TTPp
TtPP
TtPp
TTpp
Ttpp
ttPP
ttPp
ttpp

56. Linkage
Linked Genes are genes are contained in the same chromosome which tend to be inherited together.
In a dihybrid heterozygote, if the genes are not linked, 4 gamete types are produced in equal ratio.
If the genes are linked only 2 types of gametes are formed in equal ratio.

58. The genes A and B are linked as are the genes a and b
Linkage
The genes A and B are linked as are the genes a and b A a b B

59. Locus The locus of a gene is its position on a chromosome Cell

60. Activity sheet 3 -Linkage
In Drosophila Straight wing (S) is dominant to curled wing (s) and grey body (G) is dominant to ebony body (g). S and G are linked.
 Parents: Ss Gg x ssgg
 Gametes: SG, sg x sg
 F1 genotypes: SsGg and ssgg
 F1 phenotypes: Straight wing curled wing
Grey body ebony body

61. Sex Linked Genes
Are genes found on the X chromosome without a corresponding gene on theY chromosome.

62. Human Chromosomes We have 46 chromosomes, or 23 pairs.
44 of them are called autosomes and are numbered 1 through 22. Chromosome 1 is the longest, 22 is the shortest.
The other 2 chromosomes are the sex chromosomes: the X chromosome and the Y chromosome.
Males have and X and a Y;
females have 2 X’s

63. Male Karyotype

64. Female Karyotype

65. Sex-linked Genes
Genes on the X chromosome are called “sex-linked”, because they expressed more often in males than in females
There are very few genes on the Y chromosome.
Since males only have one X chromosome, all genes on it, whether dominant or recessive, are expressed.

66. Mutations on the X Chromosome
In contrast, a mutant gene on an X chromosome in a female is usually covered up by the normal allele on the other X.
Most mutations are recessive. So, most people with sex-linked genetic conditions are male.

67. Boy or Girl?
Males produce sperm with X chromosome, and sperm with their Y chromosome.
The X-bearing sperm lead to daughters
The Y-bearing sperm lead to sons.

68. Sons or Daughters? Sons get their only X chromosome from their mothers
The father’s X chromosome goes only to daughters.
The Y chromosome is passed from father to son.

69. Historically, women have been held responsible (blamed
Historically, women have been held responsible (blamed!) for not producing the male heir.
King Henry VIII blamed his wife and subsequently had her beheaded for not producing a male heir.
Are women responsible for the sex of the child?
Which parent actually determines the gender of the child?

70. Learning Check What is the difference between linkage and sex linkage?
What is a locus of a gene?
What is the sex of an individual determined by?
What can we predict about the phenotype of offspring when genes are linked?

71. Color blindness We have 3 color receptors in the retinas of our eyes.
They respond best to red, green, and blue light.
Each receptor is made by a gene.
The blue receptor is on an autosome
The red and green receptors are on the X chromosome (sex-linked).

73. Inheritance of Colourblindness
A heterozygous female has normal color vision.
Sons get their only X from their mother.
So, ½ of the sons of a heterozygous mother are colorblind, and ½ are normal.

74. Colour-blindness
A colourblind male will give his X chromosome to his daughters only.
If the mother is homozygous normal, all of the children will be normal.
However, the daughters will be heterozygous carriers of the trait, and ½ of their sons will be colorblind.

75. Haemophilia blood does not clot when exposed to air.
People with hemophilia can easily bleed to death from very minor wounds.
Hemophilia is another sex-linked trait.

76. Treatment of Haemophilia
Hemophilia is treated by injecting the proper clotting proteins
These are isolated from the blood of normal people.

78. Non Nuclear Inheritance

79. Non Nuclear Inheritance
Non nuclear DNA found in mitochondria and chloroplasts.
When these organelles replicate during cell division, they pass on their DNA.

80. Learning Check What is the difference between linkage and sex linkage?
Give an example of a sex linked condition
What is meant by non nuclear inheritance?

81. Tutorials The Biology Project – Monohybrid Crosses
The Biology Project – Dihybrid Crosses
Slide Show
Drag and Drop Genetics
Java Genetics problems
BBC Bitesize tutorial

82. It was made in the nucleus by transcription from a DNA molecule.
This is a molecule of messenger RNA.
It was made in the nucleus by transcription from a DNA molecule.
codon
A U G G G C U U A A A G C A G U G C A C G U U
mRNA molecule

83. A ribosome attaches to the mRNA molecule.
A U G G G C U U A A A G C A G U G C A C G U U

84. Amino acid
U A C
tRNA molecule
A transfer RNA molecule arrives.
It brings an amino acid to the first three bases (codon) on the mRNA.
anticodon
The three unpaired bases (anticodon) on the tRNA link up with the codon.
A U G G G C U U A A A G C A G U G C A C G U U

85. U A C
C C G
Another tRNA molecule comes into place, bringing a second amino acid.
Its anticodon links up with the second codon on the mRNA.
A U G G G C U U A A A G C A G U G C A C G U U

86. C C G
A A U
Another tRNA molecule brings the next amino acid into place.
A U G G G C U U A A A G C A G U G C A C G U U

87. C C G
C C G
A peptide bond joins the second and third amino acids to form a polypeptide chain.
A U G G G C U U A A A G C A G U G C A C G U U

88. A C G
G U C
The process continues.
The polypeptide chain gets longer.
This continues until a termination (stop) codon is reached.
The polypeptide is then complete.
A U G G G C U U A A A G C A G U G C A C G U U

89. End