Transcript BIO 103 Genetics Ch.12

Patterns of Inheritance
Chapter 12
Early Ideas of Heredity
Gregor Mendel
-chose to study pea plants because:
1. other research showed that pea hybrids
could be produced
2. many pea varieties were available
3. peas are small plants and easy to grow
4. peas can self-fertilize or be cross-fertilized
5. Produce many “babies”(seeds) FAST!
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Monohybrid Crosses
Monohybrid cross: a cross to study only 2
variations of a single trait
Mendel produced true-breeding pea strains
for 7 different traits
-each trait had 2 alternate forms (variations)
-Mendel cross-fertilized the 2 true-breeding
strains for each trait
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Monohybrid Crosses
F1 generation (1st filial generation):
offspring produced by crossing 2 opposite
(Tall crossed with short) pure-bred strains
All F1 plants resembled only 1 parent
-no plants with intermediate forms
between the 2 parents were produced
(Example: no medium heights)
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Monohybrid Crosses
F2 generation: offspring resulting from the selffertilization of F1 plants
F2 plants exhibited both forms of the trait:
¾ plants with the dominant form
¼ plant with the recessive form
Mendel discovered the ratio is actually:
1 pure-bred dominant plant
2 hybrid dominant plants
1 pure-bred recessive plant
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Monohybrid Crosses
dominant: the form of each trait expressed
in the F1 plants (Capital Letters )
recessive: the form of the trait not seen in
the F1 plants (Small-case Letters)
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Monohybrid Crosses - definitions
gene: information for a trait passed from
parent to offspring
alleles: alternate forms of a gene
homozygous: having 2 of the same allele
(TT – tt – RR – rr – BB – bb)
heterozygous: having 2 different alleles
(Tt – Rr – Bb)
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Monohybrid Crosses - definitions
genotype: total set of alleles of an individual
(Genes - letters)
PP = homozygous dominant
Pp = heterozygous
pp = homozygous recessive
phenotype: outward appearance of an
individual (Physical)
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Monohybrid Crosses –
Mendel’s conclusions
Principle of Segregation:
Two alleles (homologous chromosomes)
separate during gamete (Sperm or Egg)
formation
Meiosis Proves Mendel was correct!
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Monohybrid Crosses –
Mendel’s conclusions
Principle of Independent Assortment:
the alleles of each gene divide into
gametes independently of each other
Mendel was mostly right about this
(sometimes genes are linked on same
chromosome)
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Punnett Squares
• Punnett squares can be used to predict the
outcome of a genetic cross
Probability – Predicting Results
Product Rule: the probability of 2 independent
events occurring is the PRODUCT of their
individual probabilities.
Rr Yy x RrYy,
probability of obtaining rr yy offspring is:
probability of rr = ¼
probability of yy = ¼
probability of rr yy = ¼ x ¼ = 1/16
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Dihybrid Crosses
Dihybrid cross: examination of 2 separate
traits in a single cross
-for example: RR YY x rryy
The F1 generation of a dihybrid cross (RrYy)
shows only the dominant phenotypes for
each trait.
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The F2 generation
shows all 4 possible
phenotypes in a set
ratio:
9:3:3:1
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Extensions to Mendel
Mendel’s model of inheritance assumes
that:
-each trait is controlled by a single gene
-each gene has only 2 alleles
-there is a clear dominant-recessive
relationship between the alleles
Many genes do not meet these criteria!
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Extensions to Mendel
(1) Polygenic inheritance- multiple genes
control the phenotype of a trait.
These traits show continuous variation
Examples: Human height, Eyecolor
and Skin Color
- Lab Retrievers
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Extensions to Mendel
(2) Incomplete dominance: the
heterozygote is intermediate in phenotype
between the 2 homozygotes.
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Red X white
4/
4
pink
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Extensions to Mendel
(3) Codominance: the heterozygote shows
some aspect of the phenotypes of both
homozygotes.
(4) Multiple alleles: more than 2 possible
alleles for a gene
P generation
F1 generation
F1 generation
F2 generation
C.Multiple alleles: gene with more than
2 possible alleles
1. Each individual inherits only 2
2.Examples: Blood types (A B o) +
Rabbit Coats
(C ch h c )
Extensions to Mendel
The human ABO blood group system
demonstrates:
-multiple alleles: there are 3 alleles of the I
gene (IA, IB, and i)
-codominance: IA and IB are dominant to i
but codominant to each other
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