Mendelian Genetics
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Mason_ch12_PPTs.pptxChapter 12 Lecture Outline
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Understanding Biology, 2nd edition
Kenneth Mason
Tod Duncan
George Johnson
Jonathan Losos
Susan Singer
Copyright © McGraw-Hill Education. Permission required for reproduction or display.
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Patterns of Inheritance
Chapter 12
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Mystery of heredity
Before the 20th century, 2 concepts were the basis for ideas about heredity
Traits are transmitted directly from parent to offspring
Thought traits were borne through fluid and blended in offspring
Paradox – if blending occurs why don’t all individuals look alike?
Heredity occurs within species
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Early work
Josef Kolreuter – 1760 – crossed tobacco strains to produce hybrids that differed from both parents
Additional variation observed in 2nd generation offspring contradicts direct transmission
T.A. Knight – 1823 – crossed 2 varieties of garden pea, Pisum sativa
Crossed 2 true-breeding strains
1st generation resembled only 1 parent strain
2nd generation resembled both
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Gregor Mendel
Chose to study pea plants because:
Other research showed that pea hybrids could be produced
Many pea varieties were available
Peas are small plants and easy to grow
Peas can self-fertilize or be cross-fertilized
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Figure 12.2
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Mendel’s experimental method
Produce true-breeding strains for each trait he was studying
Cross-fertilize true-breeding strains having alternate forms of a trait
Also perform reciprocal crosses
Allow the hybrid offspring to self-fertilize for several generations
Count the numbers of each variety of F2 plants
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Figure 12.3
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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
First filial generation
Offspring produced by crossing 2 true-breeding strains
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 self-fertilization of F1 plants
Although hidden in the F1 generation, the recessive trait had reappeared among some F2 individuals
Counted proportions of traits
Always found about 3:1 ratio
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Figure 12.4
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3:1 is actually 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
2 not-true-breeding dominant plants
1 true-breeding recessive plant
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Figure 12.5
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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
Pairs of alternative traits examined were segregated among the progeny of a particular cross
Alternative traits were expressed in the F2 generation in the ratio of ¾ dominant to ¼ recessive
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Five-element model
Parents transmit discrete factors (genes)
Each individual receives one copy of a gene from each parent
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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Alleles remain discrete – no blending
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
Alleles
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Principle of Segregation
Two alleles for a gene segregate during gamete formation and are rejoined at random, one from each parent, during fertilization
Physical basis for allele segregation is the behavior of chromosomes during meiosis
Mendel had no knowledge of chromosomes or meiosis – had not yet been described
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Punnett square
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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Figure 12.6a
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Figure 12.6b
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Dihybrid crosses
Examination of 2 separate traits in a single cross
Produced true-breeding lines for 2 traits
RRYY x rryy
The F1 generation of a dihybrid cross (RrYy) shows only the dominant phenotypes for each trait
Allow F1 to self-fertilize to produce F2
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Dihybrid cross
F1 self-fertilizes
RrYy x RrYy
The F2 generation shows all four possible phenotypes in a set ratio
9:3:3:1
R_Y_:R_yy:rrY_:rryy
Round yellow:round green:wrinkled yellow:wrinkled green
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Figure 12.7 top
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Figure 12.7 bottom
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Probability
Rule of addition
Probability of 2 mutually exclusive events occurring simultaneously is the sum of their individual probabilities
When crossing Pp x Pp, the probability of producing Pp offspring is
probability of obtaining Pp (1/4), PLUS probability of obtaining pP (1/4)
¼ + ¼ = ½
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Principle of independent assortment
In a dihybrid cross, the alleles of each gene assort independently
The segregation of different allele pairs is independent
Independent alignment of different homologous chromosome pairs during metaphase I leads to the independent segregation of the different allele pairs
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Probability
Rule of multiplication
Probability of 2 independent events occurring simultaneously is the product of their individual probabilities
When crossing Pp x Pp, the probability of obtaining pp offspring is
Probability of obtaining p from father = ½
Probability of obtaining p from mother = ½
Probability of pp = ½ x ½ = ¼
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Testcross
Cross used to determine the genotype of an individual with dominant phenotype
Cross the individual with unknown genotype (e.g. P_) with a homozygous recessive (pp)
Phenotypic ratios among offspring are different, depending on the genotype of the unknown parent
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Figure 12.8
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Table 12.1
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From DNA to protein
The genotype determines phenotype
A genome is the DNA information for an organism
DNA encodes the amino acid sequence for proteins that determine phenotype
Mutations alter the phenotype
Alters the identity of the encoded amino acid sequence
Natural selection for alternative phenotypes leads to evolution
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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
Most genes do not meet these criteria
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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
For example – human height
Histogram shows normal distribution
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Figure 12.10
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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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Figure 12.11
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Multiple alleles
May be more than 2 alleles for a gene in a population
ABO blood types in humans
3 alleles
Each individual can only have 2 alleles
Number of alleles possible for any gene is constrained, but usually more than two alleles exist for any gene in an outbreeding population
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Incomplete and codominance
Incomplete dominance
Heterozygote is intermediate in 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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Figure 12.12
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Human ABO blood group
The ABO system demonstrates both incomplete dominance and codominance:
Multiple alleles
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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Figure 12.13
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Environmental influence
Coat color in Himalayan rabbits and Siamese cats
Allele produces an enzyme that allows pigment production only at temperatures below 30oC
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Epistasis
Behavior of gene products can change the ratio expected by independent assortment, even if the genes are on different chromosomes that do exhibit independent assortment
R.A. Emerson crossed 2 white varieties of corn
F1 was all purple
F2 was 9 purple:7 white – not expected
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Figure 12.15
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Table 12.2
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