Download Lecture 7

Genetics
Instructor: Dr. Jihad Abdallah
Topic 7:
Mendelian Genetics
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• In 1866, Gregor Mendel published the results of a
series of experiments that formed the basis of the
modern day genetics.
• He performed his experiments on the garden pea
(Pisum sativum)  simple genetic crosses between
strains of pea.
• Garden pea is self-fertilizing in nature, but easy to
cross-breed experimentally.
• He studied seven characters involving seed shape and
color, pod shape and color, flower color and position,
and plant height.
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The Monohybrid Cross
• Involves a single character with two contrasting
forms ( e.g., seed color: yellow, green). The
monohybrid cross is made by mating individuals
from two parent strains, each strain exhibiting one of
the two contrasting forms of the character under
study.
P1(White flowers) x P2 (Purple flowers)
F1
Selfing
F2
(First filial generation)
All purple
(Second filial generation)
705 Purple and 224 White
(3 : 1 ratio)
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3
1
4
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Mendel’s first three postulates
1. Unit factors in pairs: genetic characters are
controlled by unit factors that exist in pairs in
individual organisms (Diploid).
• These unit factors are now called genes or alleles.
• Allele: an alternative form of the same gene.
• Because the factors occur in pairs, three combinations
are possible:
- two factors for the purple color (PP)
Homozygous
- two factors for the white color (pp)
- one factor for each (Pp) Heterozygous
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2. Dominant/recessive factors: when two unlike unit
factors responsible for a single character are present
in a single individual, one unit factor is dominant
over the other which is said to be recessive.
- PP  purple
- Pp  purple
- pp  white
P (the purple color allele) is the dominant and p (the
white color allele) is the recessive.
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3. Segregation: during the formation of gametes, the
paired unit factors (alleles) separate or segregate
randomly so that each gamete receives one factor
(allele) or the other with equal probability (chance).
• This third postulate is called “Mendel’s first law” 
Law of Segregation
Pp x Pp
(½ P, ½ p) x (½ P, ½ p)
¼ PP, ½ Pp, ¼ pp
1: 2 : 1 Genotypic ratios
¾ purple, ¼ white
3: 1 Phenotypic ratios
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Punnett Square
Female gametes
½P
½p
½P
¼ PP
¼ Pp
½p
¼ pP
¼ pp
Male gametes
Genotypic ratios: ¼ PP : ½Pp : ¼ pp
1 : 2 : 1
Phenotypic ratios: ¾ Purple : ¼ white
3 :
1
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The Test Cross (single character)
• A test designed to reveal the genotype of an individual which
shows the dominant phenotype. In this cross, the individual
with unknown genotype but showing the dominant phenotype
is crossed to a homozygous recessive individual (an individual
showing the recessive phenotype).
• Example:
Tall plant (DD or Dd) x Dwarf plant (dd)
- If the geotype of the tall plant is DD  we expect all offspring plants to be
tall (have Dd genotype)
- If the genotype is Dd  we expect half the offspring to be tall (Dd) and the
other half to be dwarf (dd).
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The Dihybrid Cross
• A cross which involves two characters 
• Mendel crossed two pure strains of pea, one with
round yellow seeds and one with wrinkled green
seeds  all F1 offspring are dihybrids having round
yellow seeds.
- Round seeds allele (R) is dominant over wrinkled seed allele (r)
- Yellow seed allele (Y) is dominant over the green seed allele (y)
• He then allowed the F1 plants to self-fertilize and got
the following results: - 9/16 round, yellow
- 3/16 round, green
- 3/16 wrinkled, yellow
- 1/16 wrinkled, green
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The dihybrid cross
P1 x
P2
(round, yellow) (wrinkled, green)
F1 dihybrids
(all have round, yellow seeds)
Self-fertilization
F2
9/16 round, yellow
3/16 round, green
3/16 wrinkled, yellow
1/16 wrinkled, green
9 : 3 : 3 : 1 phenotypic ratio
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Mendel’s Fourth Postulate
• Independent assortment: during gamete formation,
segregating pairs of unit factors assort independent of
each other (the alleles at one locus segregate into the
gametes independently of the pair of alleles found at a
different locus).
• This is Mendel’s second law and is called the “Law of
Independent Assortment”
• RrYy  4 gamete types with equal proportions:
RY, Ry, rY, ry
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Easy way to obtain phenotypic and genotypic
ratios
Phenotypic
ratios
First character
Second character
(3/4 round: 1/4 wrinkled) x (3/4 yellow:1/4 green)
¾ round x ¾ yellow  9/16 round, yellow
¾ round x ¼ green  3/16 round, green
¼ wrinkled x ¾ yellow  3/16 wrinkled, yellow
¼ wrinkled x ¼ green  1/16 wrinkled, green
Genotypic
ratios
(1/4 RR: 2/4 Rr: 1/4 rr ) x (1/4 YY : 2/4Yy : 1/4 yy)
1/16 RRYY, 2/16RRYy, 1/16 RRyy
2/16 RrYY, 4/16 RrYy, 2/16 Rryy
1/16 rrYY, 2/16 rrYy, 1/16 rryy
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n = number of characters (loci)
monohybrid dihybrid trihybrid
n -hybrid
2
4
8
2n
1/4
1/16
1/64
(1/2n)2
Number of different F2
phenotypes given
complete dominance
2
4
8
2n
Number of different F2
genotypes
3
9
27
3n
F1 gametic genotypes
Proportion of homozygous
recessives in F2
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