Download Lesson 11: - Lake–Sumter State College

Lesson 11:
Patterns of Inheritance
April 8, 2015
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Terms
• Allele – variations of a gene
– Brown eyes vs. Blue eyes
• Homozygous – diploid cell containing two
identical alleles for any one gene (both
Brown eyes or Blue eyes)
• Heterozygous – diploid cell containing two
different alleles for any one gene (Brown
eyes gene and Blue eyes gene)
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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
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Experimental Method
1.Produce true-breeding (homozygous)
strains for each trait he was studying
2.Cross-fertilized true-breeding strains
having alternate forms of a trait
– Also perform reciprocal crosses
3.Allowed the hybrid offspring to self-fertilize
for several generations and count the
number of offspring showing each form of
the trait
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Monohybrid Crosses
• Cross to study only 2 variations of a single
trait
• Mendel produced true-breeding pea
strains for 7 different traits
– Each trait had 2 variants
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F1 Generation
• F1 = First Filial Generation
• Offspring produced by crossing 2 truebreeding strains of differing phenotype
• For every trait Mendel studied, all F1
plants resembled only 1 parent
– Referred to this trait as dominant
– Alternative trait was recessive
• No plants with characteristics intermediate
between the 2 parents were produced
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F2 Generation
• Second filial generation
• Offspring resulting from the selffertilization of F1 plants
• Although hidden in the F1 generation, the
recessive trait had reappeared among
some F2 offspring
• Counted proportions of traits
– 3:1 ratio (phenotypic ratio)
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3:1 is 1:2:1
• F2 plants
– ¾ plants with the dominant form
– ¼ plants with the recessive form
– The dominant to recessive ratio was 3:1
• Mendel discovered the ratio is actually:
– 1 true-breeding dominant plant (homozygous)
– 2 not-true-breeding dominant plants
(heterozygous)
– 1 true-breeding recessive plant (homozygous)
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Conclusions
• His plants did not show intermediate traits
– Each trait is intact, discrete
• For each pair, one trait was dominant, the other
recessive
• Alternative traits were expressed in the F2
generation in the ratio of ¾ dominant to ¼
recessive
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Five Element Model of
Heredity
1. Parents transmit discrete factors (genes)
2. Each individual receives one copy of a
gene (an allele) from each parent
3. Not all copies of a gene are identical
– Allele – alternative form of a gene
– Homozygous – 2 of the same allele
– Heterozygous – different alleles
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4. Alleles remain discrete – no blending
5. Presence of allele does not guarantee
expression
– Dominant allele – expressed
– Recessive allele – hidden by dominant allele
• Genotype – total set of alleles an
individual contains
• Phenotype – physical appearance
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Principle of Segregation
• Two alleles for a gene segregate during
gamete formation (meiosis) and are
rejoined at random, one from each parent,
during fertilization
• Physical basis for allele segregation is the
behavior of homologous chromosomes
during Anaphase I of Meiosis
• Mendel had no knowledge of
chromosomes or meiosis – had not yet
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been described
Punnett Squares
• Cross purple-flowered plant with white-flowered
plant
• P is dominant allele – purple flowers
• p is recessive allele – white flowers
• True-breeding white-flowered plant is pp
– Homozygous recessive
• True-breeding purple-flowered plant is PP
– Homozygous dominant
• Pp is heterozygote purple-flowered plant
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Dihybrid Crosses
• Examination of 2 separate traits in a single
cross
• RR YY x rryy (doubly homozygous)
– R-round; r-wrinkled; Y-yellow; y-green
• The F1 generation of a dihybrid cross
(RrYy) shows only the dominant
phenotypes of each trait
• Allow F1 to self-fertilize to produce F2
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• F1 generation
– RrYy x RrYy (genotype)
– All offspring appear Round and Yellow
(phenotype)
• The F2 generation
– 9:3:3:1 (phenotype)
– 9 Round yellow:3 round green:3 wrinkled
yellow:1 wrinkled green
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Polygenic Inheritance
• Occurs when multiple genes are involved
in controlling the phenotype of a trait
• The phenotype is an accumulation of
contributions by multiple genes
• These traits show continuous variation and
are referred to as quantitative traits
– Ex. Human Height
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Pleiotropy
• Refers to an allele which has more than
one effect on the phenotype
• Pleiotropic effects are difficult to predict,
because a gene that affects one trait often
performs other, unknown functions
• This can be seen in human diseases such
as cystic fibrosis or sickle cell anemia
– Multiple symptoms can be traced back to one
defective allele
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• Incomplete Dominance
– Heterozygote is intermediate phenotype
between the 2 homozygotes
– Red flowers x white flowers = pink flowers
• Codominance
– Heterozygote shows some aspect of the
phenotypes of both homozygotes
– Type AB blood
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