Download Mystery of Heredity

Mystery of Heredity
• Before the 20th century, 2 concepts were the basis for
ideas about heredity
– heredity occurs within species
– 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?
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
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
Mendel’s Experimental Method
• Usually 3 stages
1. Produce true-breeding strains for each trait he was
studying
2. Cross-fertilize true-breeding strains having alternate
forms of a trait
– also perform reciprocal crosses
3. Allow the hybrid offspring to self-fertilize for several
generations and count the number of offspring
showing each form of the trait
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
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
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
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
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
Five-Element Model
1. Parents transmit discrete factors (genes)
2. Each individual receives one copy of a gene 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
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
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
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
Human Traits
• Some human traits are controlled by a single gene
– some of these exhibit dominant and recessive
inheritance
• Pedigree analysis is used to track inheritance patterns
in families
• Dominant pedigree
– dominant trait appears in every generation
• Recessive pedigree
– most affected individuals have unaffected parents
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
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
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
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
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
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
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
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
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
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
• 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
Human ABO Blood Group
• The system demonstrates both
– 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
Environmental Influence
• Coat color in
Himalayan rabbits
and Siamese cats
– allele produces
an enzyme that
allows pigment
production only
at temperatures
below 33oC