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Basic Mendelelian
Genetics
copyright cmassengale
1
INTRODUCTION TO GENETICS
GENETICS – branch of biology that deals with heredity and
variation of organisms.
Chromosomes car...
HUMAN GENOME
Most human cells
contain 46 chromosomes:
2 sex chromosomes (X,Y):
XY – in males.
XX – in females.
22 pairs of...
4
Gregor Mendel
(1822-1884)
• Father of Genetics
• Mendel was the first biologist to
use Mathematics – to explain
his resu...
HUMAN CASE: CYSTIC FIBROSIS
• Mendel‟s Principles of Heredity apply universally to all
organisms.
• Cystic Fibrosis: a let...
Jason’s Story
Today you will use Mendel‟s
principles of inheritance to learn
more about how a boy name
Jason inherited cys...
Mendel Experimented
with pea plants
7
copyright cmassengale
8
9
Mendel‟s Experimental
Methods
Mendel hand-pollinated
flowers using a paintbrush
•He snipped the
stamens to prevent self-
p...
11
How Mendel Began
Mendel
produced
pure strains
by allowing
the plants
to self-
pollinate for
several
generations
12
Mendel‟s Experimental Results
GENETICS TERMS YOU NEED TO KNOW:
• Gene – a unit of heredity;
a section of DNA sequence
encoding a single protein
• Genome...
CHROMOSOME LOGICAL STRUCTURE
Locus – a fixed location on a strand of
DNA where a gene or one of its alleles is
located.
Al...
Homozygous – having identical genes (one from each
parent) for a particular characteristic. (DD or dd)
Heterozygous – havi...
Genotype – the genetic makeup of an organisms (TT, Tt, tt)
Phenotype – the physical appearance (tall, purple, wrinkled)
of...
GENOTYPES DETERMINES PHENOTYPES
At each locus (except for sex chromosomes)
there are 2 genes. These constitute the
individ...
GENOTYPES PHENOTYPES(EXAMPLE)
Eb- dominant allele.
Ew- recessive allele.
genotypes
phenotypes
Jason’s Parent
Both of Jason‟s parents are carriers for the recessive CF
gene, which means their genotype is heterozygous ...
Chromosomes (and genes) occur in pairs
Homologous Chromosomes
New combinations of genes occur in sexual reproduction
• Fer...
Genotype & Phenotype in Flowers
21
Genotype of alleles:
R = red flower
r = yellow flower
All genes occur in pairs, so 2 al...
Following the Generations
22
Cross 2
Pure
Plants
TT x tt
Results
in all
Hybrids
Tt
Cross 2 Hybrids
get
3 Tall & 1 Short
TT...
Monohybrid
Crosses
23
MONOHYBRID CROSS
Parents differ by a single trait.
Crossing two pea plants that differ in stem size, one tall one short
T ...
MONOHYBRID CROSS FOR STEM
LENGTH:
T T  t t
(tall) (dwarf)
T t
(all tall plants)
Parental (P)
Generation
Pure breeds-
homo...
PUNNETT SQUARE
A useful tool to do genetic crosses
For a monohybrid cross, you need a square divided by
four….
Looks like
...
27
USING A PUNNETT SQUARE
STEPS:
1. determine the genotypes of the parent organisms
2. write down your "cross" (mating)
3. dr...
PUNNETT SQUARE
4. "split" the letters of the genotype for each parent
& put them "outside" the p-square
5. determine the p...
MONOHYBRID CROSS: F2
GENERATION
If you let the F1 generation self-fertilize, the
next monohybrid cross would be:
Pp  Pp
(...
SECRET OF THE PUNNETT
SQUARE
Key to the Punnett Square:
Determine the gametes of each parent…
How? By “splitting” the geno...
Once you have the gametes…
P P p p
Pp Pp
Pp Pp

P
P
p p
Shortcut for Punnett Square…
You only need one box!
P P p p
P
p Genotypes:
100% Pp
Phenotypes:
100% Purple flower plan
• ...
Understanding the
shortcut…
P
p
Pp Pp
Pp Pp
P
P
p p
=
Genotypes:
100% Pp
Phenotypes:
100% Purple flower plants
Pp
If you have another
cross…
A heterozygous with a homozygous
T t t t
T
t
t
T t
t t
Genotypes:
50% T t
50 % t t
Phenotypes:...
Here’s a problem for
you to try…
36
P1 Monohybrid Cross
Trait: Seed Shape
Alleles: R – Round r – Wrinkled
Cross: Round seeds x Wrinkled seeds
RR x rr
37
R
R
r...
P1 Monohybrid Cross Review
Homozygous dominant x
Homozygous recessive
Offspring all Heterozygous (hybrids)
Offspring ca...
Know do the F1 Monohybrid Cross
Trait: Seed Shape
Alleles: R – Round r – Wrinkled
Cross: Round seeds x Round seeds
Rr x Rr...
F1 Monohybrid Cross Review
Heterozygous x heterozygous
Offspring:
25% Homozygous dominant RR
50% Heterozygous Rr
25% Hom...
Practice…
Work the P1, F1, and both F2
Crosses for each of the other
Seven Pea Plant Traits
41
Mendel‟s Laws
42
Principle of Dominance
43
1. One allele masked another, one allele was
dominant over the other in the F1 generation.
Mendel’s Principles
2. Principle of Segregation:
When gametes are formed, the pairs of
hereditary factors (genes) become s...
45
Applying the Law of Segregation
Use what you have learned so far to
determine the genotypes for Jason’s
parents
46
Did you say both parents have the
genotype…
47
Cc
If so…you are correct!
Now perform a monohybrid cross
using the genotypes of Jason’s
parents to determine the probability
that their children wou...
INHERITANCE PATTERN OF CF
Since both parents carry the recessive gene of Cystic Fibrosis (c), that
is, they are heterozygo...
Probabilities…
Of course, the 1 in 4 probability of getting the disease is just an
expectation, and in reality, any two ca...
LAW OF INDEPENDENT
ASSORTMENT
Alleles for different traits are
distributed to sex cells (&
offspring) independently of one...
Dihybrid Cross
A breeding experiment that tracks the
inheritance of two traits.
Mendel‟s “Law of Independent
Assortment”
a...
Dihybrid Cross
Traits: Seed shape & Seed color
Alleles: R round
r wrinkled
Y yellow
y green
53
RrYy x RrYy
RY Ry rY ry RY ...
Dihybrid Cross
54
RY Ry rY ry
RY
Ry
rY
ry
Dihybrid Cross
55
RRYY
RRYy
RrYY
RrYy
RRYy
RRyy
RrYy
Rryy
RrYY
RrYy
rrYY
rrYy
RrYy
Rryy
rrYy
rryy
Round/Yellow: 9
Round/gr...
Dihybrid Cross
56
Round/Yellow: 9
Round/green: 3
wrinkled/Yellow: 3
wrinkled/green: 1
9:3:3:1
DIHYBRID CROSS: FLOWER
COLOR AND STEM LENGTH
TT PP  tt pp
(tall, purple) (short, white)
Possible Gametes for
parents
T P ...
DIHYBRID CROSS: FLOWER COLOR
AND STEM LENGTH (SHORTCUT)
TT PP  tt pp
(tall, purple) (short, white)
Possible Gametes for
p...
DIHYBRID CROSS F2
If F1 generation is allowed to self pollinate, Mendel observed 4
phenotypes:
Tt Pp  Tt Pp
(tall, purple...
DIHYBRID CROSS
9 Tall purple
3 Tall white
3 Short purple
1 Short white
TTPP TTPp TtPP TtPp
TTPp TTpp TtPp Ttpp
TtPP TtPp t...
Genotype ratios (9): Four
Phenotypes:
1 TTPP
2 TTPp
2 TtPP
4 TtPp
1 TTpp
2 Ttpp
1 ttPP
2 ttPp
1 ttpp
Dihybrid cross: 9 gen...
PRINCIPLE OF INDEPENDENT
ASSORTMENT
Based on these results, Mendel postulated the
Principle of Independent Assortment:
“Me...
Relation of gene segregation to
meiosis…
There‟s a correlation between the movement of
chromosomes in meiosis and the segr...
TEST CROSS
When you have an individual with an unknown genotype, you do a
test cross.
Test cross: Cross with a homozygous ...
TEST CROSS
If you get all 100% purple flowers, then the unknown parent was
PP…
P p P p
P p P p
P P
p
p
P p p p
P p p p
P p...
Summary of Mendel‟s laws
66
LAW
PARENT
CROSS
OFFSPRING
DOMINANCE TT x tt
tall x short
100% Tt
tall
SEGREGATION
Tt x Tt
tal...
Genetic Practice Problems
67
Breed the P1 generation
tall (TT) x dwarf (tt) pea plants
68
T
T
t t
Solution:
69
T
T
t t
Tt
Tt
Tt
Tt All Tt = tall
(heterozygous tall)
produces the
F1 generation
tall (TT) vs. dwarf (tt) pea...
Breed the F1 generation
tall (Tt) vs. tall (Tt) pea plants
70
T
t
T t
Solution:
71
TT
Tt
Tt
tt
T
t
T t
produces the
F2 generation
1/4 (25%) = TT
1/2 (50%) = Tt
1/4 (25%) = tt
1:2:1 genotype
3:...
SUMMARY OF GENETICS
Chromosomes carry hereditary info (genes)
Chromosomes (and genes) occur in pairs
New combinations of g...
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Mendelian Genetics

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Mendelian Genetics

  1. 1. Basic Mendelelian Genetics copyright cmassengale 1
  2. 2. INTRODUCTION TO GENETICS GENETICS – branch of biology that deals with heredity and variation of organisms. Chromosomes carry the hereditary information (genes) • Arrangement of nucleotides in DNA • DNA  RNA  Proteins
  3. 3. HUMAN GENOME Most human cells contain 46 chromosomes: 2 sex chromosomes (X,Y): XY – in males. XX – in females. 22 pairs of chromosomes named autosomes.
  4. 4. 4 Gregor Mendel (1822-1884) • Father of Genetics • Mendel was the first biologist to use Mathematics – to explain his results quantitatively. • Responsible for the Laws governing Inheritance of Traits
  5. 5. HUMAN CASE: CYSTIC FIBROSIS • Mendel‟s Principles of Heredity apply universally to all organisms. • Cystic Fibrosis: a lethal genetic disease affecting Caucasians. • Caused by mutant recessive gene carried by 1 in 20 people of European descent (12M) • One in 400 Caucasian couples will be both carriers of CF – 1 in 4 children will have it. • CF disease affects transport in tissues – mucus is accumulated in lungs, causing infections.
  6. 6. Jason’s Story Today you will use Mendel‟s principles of inheritance to learn more about how a boy name Jason inherited cystic fibrosis. 6
  7. 7. Mendel Experimented with pea plants 7
  8. 8. copyright cmassengale 8
  9. 9. 9
  10. 10. Mendel‟s Experimental Methods Mendel hand-pollinated flowers using a paintbrush •He snipped the stamens to prevent self- pollination •He covered each flower with a cloth bag He followed these traits through the several generations 10
  11. 11. 11 How Mendel Began Mendel produced pure strains by allowing the plants to self- pollinate for several generations
  12. 12. 12 Mendel‟s Experimental Results
  13. 13. GENETICS TERMS YOU NEED TO KNOW: • Gene – a unit of heredity; a section of DNA sequence encoding a single protein • Genome – the entire set of genes in an organism • Locus • Allele
  14. 14. CHROMOSOME LOGICAL STRUCTURE Locus – a fixed location on a strand of DNA where a gene or one of its alleles is located. Allele – two genes that occupy the same position on homologous chromosomes and that cover the same trait (like „flavors‟ of a trait); one variant form of a gene/marker at a particular locus. Locus1 Possible Alleles: A1,A2 Locus2 Possible Alleles: B1,B2,B3
  15. 15. Homozygous – having identical genes (one from each parent) for a particular characteristic. (DD or dd) Heterozygous – having two different genes for a particular characteristic. (Dd) Dominant – the allele of a gene that masks or suppresses the expression of an alternate allele; the trait appears in the heterozygous condition. Recessive – an allele that is masked by a dominant allele; does not appear in the heterozygous condition, only in homozygous. Genetics terms you need to know:
  16. 16. Genotype – the genetic makeup of an organisms (TT, Tt, tt) Phenotype – the physical appearance (tall, purple, wrinkled) of an organism (Genotype + environment) Monohybrid cross: a genetic cross involving a single pair of genes (one trait); parents differ by a single trait. Dihybrid cross: a genetic cross involving two pair of genes (two traits); parents differ by two traits P = Parental generation F1 = First filial generation; offspring from a genetic cross. F2 = Second filial generation of a genetic cross
  17. 17. GENOTYPES DETERMINES PHENOTYPES At each locus (except for sex chromosomes) there are 2 genes. These constitute the individual‟s genotype at the locus. The expression of a genotype is termed a phenotype. For example, hair color, weight, or the presence or absence of a disease.
  18. 18. GENOTYPES PHENOTYPES(EXAMPLE) Eb- dominant allele. Ew- recessive allele. genotypes phenotypes
  19. 19. Jason’s Parent Both of Jason‟s parents are carriers for the recessive CF gene, which means their genotype is heterozygous for CF 19 Inheritance Patterns of CF: CC = normal Cc = carrier, no symptoms cc = has cystic fibrosis
  20. 20. Chromosomes (and genes) occur in pairs Homologous Chromosomes New combinations of genes occur in sexual reproduction • Fertilization from two parents
  21. 21. Genotype & Phenotype in Flowers 21 Genotype of alleles: R = red flower r = yellow flower All genes occur in pairs, so 2 alleles affect a characteristic Possible combinations are: Genotypes RR Rr rr Phenotypes RED RED YELLOW
  22. 22. Following the Generations 22 Cross 2 Pure Plants TT x tt Results in all Hybrids Tt Cross 2 Hybrids get 3 Tall & 1 Short TT, Tt, tt
  23. 23. Monohybrid Crosses 23
  24. 24. MONOHYBRID CROSS Parents differ by a single trait. Crossing two pea plants that differ in stem size, one tall one short T = allele for Tall t = allele for dwarf TT = homozygous tall plant t t = homozygous dwarf plant T T  t t
  25. 25. MONOHYBRID CROSS FOR STEM LENGTH: T T  t t (tall) (dwarf) T t (all tall plants) Parental (P) Generation Pure breeds- homozygous First Filial (F1) Generation Hybrid- Heterozygous
  26. 26. PUNNETT SQUARE A useful tool to do genetic crosses For a monohybrid cross, you need a square divided by four…. Looks like a window pane… We use the Punnett square to predict the genotypes and phenotypes of the offspring.
  27. 27. 27
  28. 28. USING A PUNNETT SQUARE STEPS: 1. determine the genotypes of the parent organisms 2. write down your "cross" (mating) 3. draw a p-square Parent genotypes: PP and pp Cross PP  pp
  29. 29. PUNNETT SQUARE 4. "split" the letters of the genotype for each parent & put them "outside" the p-square 5. determine the possible genotypes of the offspring by filling in the p-square 6. summarize results (genotypes & phenotypes of offspring) Pp Pp Pp Pp P P p p Genotypes: 100% Pp Phenotypes: 100% Purple flowers PP  pp
  30. 30. MONOHYBRID CROSS: F2 GENERATION If you let the F1 generation self-fertilize, the next monohybrid cross would be: Pp  Pp (purple) (purple) PP Pp Pp pp P p P p Genotypes: 1 PP= Purple flowers 2 Pp = Purple flowers 1 pp = white flowers Genotypic ratio= 1:2:1 Phenotype: 3 Purple flowers 1 white flowers Phenotypic ratio= 3:1
  31. 31. SECRET OF THE PUNNETT SQUARE Key to the Punnett Square: Determine the gametes of each parent… How? By “splitting” the genotypes of each parent: If this is your cross P P  p p P P p p The gametes are:
  32. 32. Once you have the gametes… P P p p Pp Pp Pp Pp  P P p p
  33. 33. Shortcut for Punnett Square… You only need one box! P P p p P p Genotypes: 100% Pp Phenotypes: 100% Purple flower plan • If both parents are HOMOZYGOUS Pp
  34. 34. Understanding the shortcut… P p Pp Pp Pp Pp P P p p = Genotypes: 100% Pp Phenotypes: 100% Purple flower plants Pp
  35. 35. If you have another cross… A heterozygous with a homozygous T t t t T t t T t t t Genotypes: 50% T t 50 % t t Phenotypes: 50% Tall plants 50% Dwarf plants You can still use the shortcut!
  36. 36. Here’s a problem for you to try… 36
  37. 37. P1 Monohybrid Cross Trait: Seed Shape Alleles: R – Round r – Wrinkled Cross: Round seeds x Wrinkled seeds RR x rr 37 R R rr Rr RrRr Rr Genotype: Rr Phenotype: Round Genotypic Ratio: All alike Phenotypic Ratio: All alike
  38. 38. P1 Monohybrid Cross Review Homozygous dominant x Homozygous recessive Offspring all Heterozygous (hybrids) Offspring called F1 generation Genotypic & Phenotypic ratio is ALL ALIKE 38
  39. 39. Know do the F1 Monohybrid Cross Trait: Seed Shape Alleles: R – Round r – Wrinkled Cross: Round seeds x Round seeds Rr x Rr 39 R r rR RR rrRr Rr Genotype: RR, Rr, rr Phenotype: Round & wrinkled G.Ratio: 1:2:1 P.Ratio: 3:1
  40. 40. F1 Monohybrid Cross Review Heterozygous x heterozygous Offspring: 25% Homozygous dominant RR 50% Heterozygous Rr 25% Homozygous Recessive rr Offspring called F2 generation Genotypic ratio is 1:2:1 Phenotypic Ratio is 3:1 40
  41. 41. Practice… Work the P1, F1, and both F2 Crosses for each of the other Seven Pea Plant Traits 41
  42. 42. Mendel‟s Laws 42
  43. 43. Principle of Dominance 43 1. One allele masked another, one allele was dominant over the other in the F1 generation.
  44. 44. Mendel’s Principles 2. Principle of Segregation: When gametes are formed, the pairs of hereditary factors (genes) become separated, so that each sex cell (egg/sperm) receives only one kind of gene.
  45. 45. 45 Applying the Law of Segregation
  46. 46. Use what you have learned so far to determine the genotypes for Jason’s parents 46
  47. 47. Did you say both parents have the genotype… 47 Cc If so…you are correct!
  48. 48. Now perform a monohybrid cross using the genotypes of Jason’s parents to determine the probability that their children would not have CF, and the probability that Jason would have CF. 48
  49. 49. INHERITANCE PATTERN OF CF Since both parents carry the recessive gene of Cystic Fibrosis (c), that is, they are heterozygous (Cc), one in four of their children is expected to be homozygous for cf and have the disease: C C C c C c c c C c C c CC = 25% chance of being normal Cc = 50% chance carrier, no symptoms cc = 25% chance that Jason would have cystic fibrosis
  50. 50. Probabilities… Of course, the 1 in 4 probability of getting the disease is just an expectation, and in reality, any two carriers may have normal children. However, the greatest probability is for 1 in 4 children to be affected. Important factor when prospective parents are concerned about their chances of having affected children. Now, 1 in 29 Americans is a symptom-less carrier (Cf cf) of the gene.
  51. 51. LAW OF INDEPENDENT ASSORTMENT Alleles for different traits are distributed to sex cells (& offspring) independently of one another. This law can be illustrated using dihybrid crosses. 51
  52. 52. Dihybrid Cross A breeding experiment that tracks the inheritance of two traits. Mendel‟s “Law of Independent Assortment” a. Each pair of alleles segregates independently during gamete formation b. Formula: 2n (n = # of heterozygotes) 52
  53. 53. Dihybrid Cross Traits: Seed shape & Seed color Alleles: R round r wrinkled Y yellow y green 53 RrYy x RrYy RY Ry rY ry RY Ry rY ry All possible gamete combinations
  54. 54. Dihybrid Cross 54 RY Ry rY ry RY Ry rY ry
  55. 55. Dihybrid Cross 55 RRYY RRYy RrYY RrYy RRYy RRyy RrYy Rryy RrYY RrYy rrYY rrYy RrYy Rryy rrYy rryy Round/Yellow: 9 Round/green: 3 wrinkled/Yellow: 3 wrinkled/green: 1 9:3:3:1 phenotypic ratio RY Ry rY ry RY Ry rY ry
  56. 56. Dihybrid Cross 56 Round/Yellow: 9 Round/green: 3 wrinkled/Yellow: 3 wrinkled/green: 1 9:3:3:1
  57. 57. DIHYBRID CROSS: FLOWER COLOR AND STEM LENGTH TT PP  tt pp (tall, purple) (short, white) Possible Gametes for parents T P and t p F1 Generation: All tall, purple flowers (Tt Pp) TtPp TtPp TtPp TtPp TtPp TtPp TtPp TtPp TtPp TtPp TtPp TtPp TtPp TtPp TtPp TtPp tp tp tp tp TP TP TP TP
  58. 58. DIHYBRID CROSS: FLOWER COLOR AND STEM LENGTH (SHORTCUT) TT PP  tt pp (tall, purple) (short, white) Possible Gametes for parents F1 Generation: All tall, purple flowers (Tt Pp) T t P pTP t p T P t p
  59. 59. DIHYBRID CROSS F2 If F1 generation is allowed to self pollinate, Mendel observed 4 phenotypes: Tt Pp  Tt Pp (tall, purple) (tall, purple) Possible gametes: TP Tp tP tp Four phenotypes observed Tall, purple (9); Tall, white (3); Short, purple (3); Short white (1) TTPP TTPp TtPP TtPp TTPp TTpp TtPp Ttpp TtPP TtPp ttPP ttPp TtPp Ttpp ttPp ttpp TP Tp tP tp TP Tp tP tp
  60. 60. DIHYBRID CROSS 9 Tall purple 3 Tall white 3 Short purple 1 Short white TTPP TTPp TtPP TtPp TTPp TTpp TtPp Ttpp TtPP TtPp ttPP ttPp TtPp Ttpp ttPp ttpp TP Tp tP tp TP Tp tP tp Phenotype Ratio = 9:3:3:1
  61. 61. Genotype ratios (9): Four Phenotypes: 1 TTPP 2 TTPp 2 TtPP 4 TtPp 1 TTpp 2 Ttpp 1 ttPP 2 ttPp 1 ttpp Dihybrid cross: 9 genotypes Tall, purple (9) Tall, white (3) Short, purple (3) Short, white (1)
  62. 62. PRINCIPLE OF INDEPENDENT ASSORTMENT Based on these results, Mendel postulated the Principle of Independent Assortment: “Members of one gene pair segregate independently from other gene pairs during gamete formation” Genes get shuffled – these many combinations are one of the advantages of sexual reproduction
  63. 63. Relation of gene segregation to meiosis… There‟s a correlation between the movement of chromosomes in meiosis and the segregation of alleles that occurs in meiosis
  64. 64. TEST CROSS When you have an individual with an unknown genotype, you do a test cross. Test cross: Cross with a homozygous recessive individual. For example, a plant with purple flowers can either be PP or Pp… therefore, you cross the plant with a pp (white flowers, homozygous recessive) P ?  pp
  65. 65. TEST CROSS If you get all 100% purple flowers, then the unknown parent was PP… P p P p P p P p P P p p P p p p P p p p P p p p •If you get 50% white, 50% purple flowers, then the unknown parent was Pp…
  66. 66. Summary of Mendel‟s laws 66 LAW PARENT CROSS OFFSPRING DOMINANCE TT x tt tall x short 100% Tt tall SEGREGATION Tt x Tt tall x tall 75% tall 25% short INDEPENDENT ASSORTMENT RrGg x RrGg round & green x round & green 9/16 round seeds & green pods 3/16 round seeds & yellow pods 3/16 wrinkled seeds & green pods 1/16 wrinkled seeds & yellow pods
  67. 67. Genetic Practice Problems 67
  68. 68. Breed the P1 generation tall (TT) x dwarf (tt) pea plants 68 T T t t
  69. 69. Solution: 69 T T t t Tt Tt Tt Tt All Tt = tall (heterozygous tall) produces the F1 generation tall (TT) vs. dwarf (tt) pea plants
  70. 70. Breed the F1 generation tall (Tt) vs. tall (Tt) pea plants 70 T t T t
  71. 71. Solution: 71 TT Tt Tt tt T t T t produces the F2 generation 1/4 (25%) = TT 1/2 (50%) = Tt 1/4 (25%) = tt 1:2:1 genotype 3:1 phenotype tall (Tt) x tall (Tt) pea plants
  72. 72. SUMMARY OF GENETICS Chromosomes carry hereditary info (genes) Chromosomes (and genes) occur in pairs New combinations of genes occur in sexual reproduction Monohybrid vs. Dihybrid crosses Mendel‟s Principles: • Dominance: one allele masks another • Segregation: genes become separated in gamete formation • Independent Assortment: Members of one gene pair segregate independently from other gene pairs during gamete formation

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