Patterns of Inheritance
Chapter 14, 15
Mendelian Genetics and its Extensions
Mendel,Garden Peas, and Heredity
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Pre-Mendelian theory of hereditary: Blending theory
Mendelian theory of hereditary: Particulate theory
– Law of Segregation/Law of Dominance
– Law of Independent Assortment
Monohybrid Cross on Flower Color
Genotype versus phenotype
The Results of Mendel’s F1 Crosses for Seven Characters in Pea Plants
Alleles, alternative versions of a gene
Mendel’s Law of Segregation/Law of Dominance
Alternative forms of genes
(alleles) are responsible for
variations in phenotypes
For each character, an organism
inherits two alleles, one from
each parent (maternal and
paternal)
If the two alleles differ, one is
fully expressed (dominant
allele); the other is completely
masked (recessive allele)
The two alleles for each
character segregate during
gamete production (meiosis)
Testcross
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To determine
whether an organism
with a dominant
phenotype is
homozygous or
heterozygous, you
use a testcross
Testing Two Hypotheses for
Segregation in a Dihybrid Cross
Mendel’s Lucky Choices of
Characters in Garden Peas
1) Each character is determined by one gene
2) Each gene has only two alleles
3) One allele is completely dominant over the other
4) In dihybrid crosses, the two genes (seed color and seed
shape) are located on different pairs of chromosomes
Theory of Inheritance
Blending Theory
Particulate Theory (discrete heritable factor)
Chromosomal Theory
• Heritable factors are located on chromosomes
• It is the chromosomes that segregate and
independently assort
1860s: Mendel proposed that
1875: cytologists worked out
discrete inherited factors
segregate and assort
independently during gamete
formation
process of mitosis
1890: cytologists worked out
process of meiosis
1902: Cytology and genetics converged as Sutton,
Boveri and others noticed parallels between the behavior
of Mendel’s factors and the behavior of chromosomes:
•chromosomes and genes are both paired in diploid cells
•homologous chromosomes separate and allele pairs segregate
during meiosis
•fertilization restores the paired condition for both
chromosomes and genes
Genes are located on Chromosomes
Comparing chromosome segregation and
allele pairs segregation
Chromosomal Theory of Inheritance
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Mendelian genes have specific loci on
chromosomes, and it is the chromosomes
that undergo segregation and independent
assortment
Theory of segregation
– diploid cells have pairs of genes, on pairs of
homologous chromosomes. During meiosis, the
two genes of each pair segregates from each
other, and end up in different gamete
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Theory of independent assortment
– by the end of meiosis, genes on different pairs of
homologous chromosomes are independently
distributed to gametes
Extending Mendelian Genetics
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Incomplete dominance
Codominance
Multiple alleles for one gene
Polygenic inheritance: multiple gene effect
Pleiotropy: multiple effects of a single gene
Environmental impact
Linked genes
– genetic recombination between linked genes
– sex determination and sex-linked genes
Incomplete
Dominance
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One allele is not
completely dominant
over the other, so the
heterozygote has a
phenotype that is
intermediate between
the phenotypes of the
two homozygotes
It is not support for the
blending theory of
inheritance, because…
Human ABO Blood Types
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Multiple Alleles
– more than two alternative
forms of a gene (IA, IB, i)
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Codominance
– Inheritance characterized
by full expression of both
alleles in the heterozygote
(IAIB)
Complete
dominance
Expression of dominant allele masks
the expression of recessive allele
Incomplete
Dominance
Intermediate phenotype in heterozygote
Codominance
Full expression of both
alleles in heterozygote
Pleiotropic effects of the sickle-cell allele in a homozygote
Polygenic
Inheritance
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Two or more genes
determine a single
phenotypic character
commonly seen in
quantitative characters,
e.g. skin color
Environmental effects
could also affect the
phenotype
Flower Color depends
on acidity;
Extending Mendelian Genetics
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Incomplete dominance
Codominance
Multiple alleles for one gene
Polygenic inheritance: continuous variation
Pleiotropy: multiple effects of a single gene
Environmental impact
Linked genes
– genetic recombination through crossing over
– sex determination and sex-linked genes
Recombination due to crossing over
Recombination Frequency
and Genetic Mapping
An example of Sex Linked Genes
XRY
XrY
XRXr Carrier
XRXR or XRXr
Human Genetics Follows
Mendelian Principles
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Recessively Inherited Disorders
– cystic fibrosis
– sickle-cell disease
– Tay-Sachs disease
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Dominantly Inherited Disorders
– Huntington’s disease
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Multifactorial Disorders
– cancer
– diabetes
– heart disease