2nd year

1. Genetics

2. Heredity and Genetics
• Heredity is the passing of traits
from parents to offspring
• Traits are controlled by genes,
so GENETICS is the study of
how traits are inherited through
the action of alleles

3. Gregor Mendel – “Father of Genetics”
• Austrian monk born in
1822 who is responsible
for the laws governing
the inheritance of traits
• Between 1856 and 1863,
Mendel cultivated and
tested over 28,000 pea
plants
• Mendel performed cross-
pollination by
transferring pollen from
one plant to selected ova
of other plants, thereby
controlling which plants
mixed

4. Mendel’s Experiments
• Mendel produced pure strains by allowing
plants to self-pollinate, I.e., pollen (male
gamete) from one plant fertilizes ova (female
gamete) of same plant

5. Mendel’s Peas….
S s
Y y
I i
G g

6. Mendel’s Peas….
P p
A a
T t

7. Mendel’s Results….
(F2 from crossing to heterozygous F1s)

8. Genetics Terminology
• Traits – any characteristic that can be passed
from parents to offspring
• Heredity – the passing of traits from parents to
offspring
• Alleles – one form (dominant or recessive) of a
gene
• Sex cells have ONE form of a gene on their
chromosomes
• Body cells have TWO forms or alleles for a
single gene (you get one from mom and the
other from dad)
• Dominant – is always expressed; masks a
recessive trait
• Recessive – can only be expressed if there are
no dominant alleles present

9. Genetics Terminology
• Dominant alleles are represented by a capital letter
• Recessive alleles are represented by a lower case letter
Example: B = Brown eye color (dominant)
b = Blue eye color (recessive)
• “Purebred” species have two of the same alleles; this is
also called HOMOZYGOUS, e.g., BB (homozygous
dominant) or bb (homozygous recessive)
• Species with two different alleles are called
HETEROZYGOUS, e.g., Bb
• Genotype: the alleles present in the organism, i.e., BB,
Bb, or bb
• Phenotype: the expression of the genes; what is
observed

10. Genotype and
Phenotype in Flower
• All genes occur in pairs, so TWO alleles affect a
trait.
• Possible combinations if:
R = Red flower
r = Yellow flower
• Genotypes RR Rr rr
• Phenotypes Red Red Yellow

11. BrainPop - Heredity
Genetic Crosses
• Monohybrid Cross: cross involving ONE trait,
e.g., eye color
• Dihybrid Cross: cross involving TWO traits, e.g.,
eye color and hair color
• Offspring’s genotype and phenotype is determined
using a Punnett square
B b
B BB Bb
b Bb bb

12. Punnett Square, cont.

13. Generations in Crosses
• Parental Generation (P1) = the parental generation
in a breeding experiment
• First Filial Generation (F1) = the first generation
of offspring in a breeding experiment
• Second Filial Generation (F2) = the second
generation of offspring in a breeding experiment
TT tt Tt Tt TT Tt Tt tt

14. P1 Monohybrid Cross
• Trait: Seed Color
• Alleles: Y – Yellow y – Green
• Cross: Yellow seeds X Green seeds
YY X yy
Crossing two true-breeding (pure) plants
Y Y
Genotype: Yy
y Yy Yy
Phenotype: Yellow
Genotypic Ratio: 100% Yy
Yy Yy Phenotypic Ratio: 100% Yellow
y

15. F1 Monohybrid Cross
• Trait: Seed Color
• Alleles: Y – Yellow y – Green
• Cross: Yellow seeds X Green seeds
Yy X Yy
Crossing to heterozygotes (hybrids)
Y y
Genotype: YY, Yy, yy
Y YY Yy
Phenotype: Yellow and Green
Genotypic Ratio: 25% YY, 50%
Yy yy Yy, 25% yy (1:2:1)
y Phenotypic Ratio: 75% Yellow,
25% Green (3:1)

16. Dihybrid Crosses
• A breeding experiment that tracks the inheritance
of two traits
• Mendel’s “Law of Independent Assortment”
• Each pair of alleles segregates independently
during gamete formation
• Formula: 2n (n = # of heterozygotes)
Example:
1. RrYy: 2n = 22 = 4 possible gametes
RY Ry rY ry
2. AaBbCCDd: 2n = 23 = 8 gametes
ABCD ABCd AbCD AbCd
aBCD aBCd abCD abCD

17. Dihybrid Crosses
Traits: Seed shape & Seed color
Alleles: R round
r wrinkled
Y yellow
y green
RrYy x RrYy
RY Ry rY ry RY Ry rY ry
All possible gamete combinations

18. Dihybrid Cross
RY Ry rY ry
RY
Ry
rY
ry
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19. Dihybrid Cross
RY Ry rY ry
RY RRYY Round/Yellow: 9
RRYy RrYY RrYy
Ry RRYy Round/green: 3
RRyy RrYy Rryy
wrinkled/Yellow: 3
rY RrYY RrYy rrYY rrYy
wrinkled/green: 1
ry RrYy Rryy rrYy rryy 9:3:3:1 phenotypic
ratio
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20. Dihybrid Cross
Round/Yellow: 9
Round/green: 3
wrinkled/Yellow: 3
wrinkled/green: 1
9:3:3:1
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21. Test Cross
•Test crosses involve breeding the individual in
question with another individual that expresses a
recessive version of the same trait. If all
offspring display the dominant phenotype, the
individual in question is homozygous dominant; if
the offspring display both dominant and recessive
phenotypes, then the individual is heterozygous
Y Y Y y
y Yy Yy
y
Yy yy
Yy Yy Yy yy
y y
Offspring all yellow! ½ Offspring yellow; ½ Offspring
green!

22. Test Cross
• A mating between an individual of unknown
genotype and a homozygous recessive
individual.
• Example: bbC__ x bbcc
• BB = brown eyes
• Bb = brown eyes
• bb = blue eyes
bC b___
• CC = curly hair
bc
• Cc = curly hair
• cc = straight hair
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23. Test Cross
Possible results:
bC b___
C bC b___
c
bc bbCc bbCc or bc bbCc bbcc
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24. Incomplete Dominance
and
Codominance
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25. Incomplete Dominance
• F1 hybrids have an appearance
somewhat in between the phenotypes
of the two parental varieties.
• Example: snapdragons (flower)
red (RR) x white (rr)
(rr
r r
RR = red flower
R
rr = white flower
R
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26. Incomplete Dominance
r r
produces the
R Rr Rr
F1 generation
R Rr Rr All Rr = pink
(heterozygous pink)
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27. Incomplete Dominance
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28. Codominance
• Two alleles are expressed (multiple
alleles) in heterozygous individuals.
• Example: blood type
• 1. type A = IAIA or IAi
• 2. type B = IBIB or IBi
• 3. type AB = IAIB
• 4. type O = ii
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29. Codominance Problem
• Example:
homozygous male Type B (IBIB)
x
heterozygous female Type A (IAi)
IA i
IB IAIB IBi
1/2 = IAIB
1/2 = IBi
IB IAIB IBi
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30. Another Codominance Problem
• Example: male Type O (ii)
x
female type AB (IAIB)
IA IB
i IAi IBi 1/2 = IAi
1/2 = IBi
i IAi IBi
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31. Codominance
• Question:
If a boy has a blood type O and
his sister has blood type AB,
what are the genotypes and
phenotypes of their parents?
boy-type O (ii) X girl-type AB (IAIB)
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32. Codominance
• Answer:
IA i
IB IAIB Parents:
genotypes = IAi and IBi
phenotypes = A and B
i ii
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33. Sex-linked Traits
• Traits (genes) located on the sex
chromosomes
• Sex chromosomes are X and Y
• XX genotype for females
• XY genotype for males
• Many sex-linked traits carried on
X chromosome
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34. Sex-linked Traits
Example: Eye color in fruit flies
Sex Chromosomes
fruit fly
eye color
XX chromosome - female Xy chromosome - male
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35. Sex-linked Trait Problem
• Example: Eye color in fruit flies
• (red-eyed male) x (white-eyed
female) Xr Xr
XR Y x XrXr
• Remember: the Y chromosome in
XR
males does not carry traits.
• RR = red eyed Y
• Rr = red eyed
• rr = white eyed
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36. Sex-linked Trait Solution:
Xr Xr
50% red eyed
XR X X X X
R r R r
female
50% white eyed
Y Xr Y Xr Y male
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37. Female Carriers
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