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Mendelian Patterns of Inheritance
BIOLOGY
Gregor Mendel (1822-1884)• Austrian monk who formulated
fundamental laws of heredity in early 1860s. – Studied science and
mathematics at University of Vienna.
– Formulated the particulate theory of inheritance.
• Inheritance involves reshuffling of genes from generation to generation.
Gregor Mendel—Important Dates• Experimented with peas,
Pisum sativum,
from 1857-1865
• Reported his findings to the Brunn Society for the Study of Natural Science in 1865
• Published Experiments with Plant Hybrids in 1866
One-Trait Inheritance• Mendel performed cross-breeding
experiments between true-breeding plants (homozygous).– Chose varieties that differed in only one
trait (monohybrid cross)– Performed crosses
• Parental generation = P• First generation offspring = F1 (1st filial)• Second generation offspring = F2
Law of Segregation• Each individual has two
factors for each trait.
• The factors separate when gametes form.
• A gamete contains only one of two factors.
• Fertilization = new individual with 2 factors for each trait.
Genotype Versus Phenotype• Genotype
– Refers to the alleles an individual receives.
• Phenotype – Refers to the
physical appearance of the individual.
Modern Genetics View• Each trait is controlled by two
alleles (alternate forms of a gene).• Dominant allele (D) masks the
expression of the recessive allele (d).
• Alleles occur on a homologous pair of chromosomes at a locus.– Homozygous = identical alleles– Heterozygous = different alleles
Punnett Square• All possible kinds of
sperm are lined up on one axis, and all possible kinds of eggs are lined up on the other axis.– Every possible
combination of alleles is placed within the squares.
Testcross• A monohybrid testcross determines if the phenotype is homozygous dominant (RR) or heterozygous (Rr).
• Those with one dominant gene will express the dominant phenotype whereas individuals with the recessive phenotype must be homozygous (rr)
Two-TraitsDihybrid Cross• Mendel performed cross
using true-breeding plants differing in two traits.
– Observed phenotypes among F2 plants.
– Formulated law of independent assortment:• Each factor segregates
independently of the others• All possible combinations
of factors can occur.
Figure 14.7 Testing two hypotheses for segregation in a dihybrid cross
Two-Trait Testcross
• A two-trait testcross is used to determine if an individual is homozygous dominant or heterozygous for either of the two traits.
Incomplete Dominance• Exhibited when the heterozygote has an
intermediate phenotype between that of either homozygote.
• Most genes have more than two alleles in a population—called multiple alleles.
• The ABO blood groups in humans are determined by three alleles, IA, IB, and i.– Both the IA and IB alleles are dominant to the i
allele – The IA and IB alleles are codominant to each
other.
• Because each individual carries two alleles, there are six possible genotypes and four possible blood types.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Co-Dominance
Polygenic Inheritance• Occurs when a trait is governed by two or
more sets of alleles.– Each dominant allele has a quantitative effect
and these effects are additive like height
Terminology• Pleiotropy - A gene that affects more than one characteristic of an individual.
• Sickle-cell (incomplete dominance)
• Epistasis - A gene at one locus interferes with the expression of a gene at a different locus.– Human skin color (polygenic
inheritance)
Example of Epistasis– In mice, coat color depends on
two genes.– The epistatic gene determines if
pigment will be deposited in hair or not.• Presence (C) is dominant to absence
(c).
– The second determines what pigment is deposited: black (B) or brown (b).• The black allele is dominant to the
brown allele.
– An individual that is cc has a white (albino) coat regardless of the genotype of the second gene.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Environment and Phenotype
• Himalayan Rabbits - Enzyme coding for black fur is active only at low temperatures.– Black fur only occurs on extremities.