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Chapter 11
Mendel & The
Gene Idea
Mendelian genetics
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Character
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Trait
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parents
F1 generation
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crossing of 2 different true-breds
P generation
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all offspring of same variety
Hybridization
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variant for a character, i.e., brown
True-bred
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heritable feature, i.e., fur color
first filial generation
F2 generation
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second filial generation
The Law of Dominance
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In a cross of parents that are pure for
contrasting traits, only one form of the trait
will appear in the next generation.
Offspring that are hybrid for a trait will
have only the dominant trait in the
phenotype.
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When an organism has two different
alleles for a trait, the allele that is
expressed, overshadowing the
expression of the other allele, is said to
be dominant. The gene whose
expression is overshadowed is said to
be recessive.
Mendel’s Law of Dominance
Leading to the Law of Segregation
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Alternative versions of genes (alleles)
account for variations in inherited
characteristics
For each character, an organism
inherits 2 alleles, one from each
parent
If the two alleles differ, then one, the
dominant allele, is fully expressed in
the organism’s appearance; the
other, the recessive allele, has no
noticeable effect on the organism’s
appearance
The alleles for each character
segregate (separate) during gamete
production (meiosis).
Mendel’s Law of Segregation
The Law of Independent
Assortment
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Law of Segregation
involves 1 character.
What about 2 (or more)
characters?
Monohybrid cross vs.
dihybrid cross
The two pairs of alleles
segregate independently
of each other.
Mendel’s Law of
Independent
Assortment
Genetic vocabulary…….
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Punnett square: predicts the
results of a genetic cross between
individuals of known genotype
Homozygous: pair of identical
alleles for a character
Heterozygous: two different
alleles for a gene
Phenotype: an organism’s traits
Genotype: an organism’s genetic
makeup
Testcross: breeding of a recessive
homozygote X dominate
phenotype (but unknown
genotype)
Alternative Inheritance Patterns
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Incomplete dominance: appearance
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Codominance: two alleles affect the
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Multiple alleles: more than 2 possible
between the phenotypes of the 2
parents. Ex: snapdragons
phenotype in separate, distinguishable
ways. Ex: Tay-Sachs disease
alleles for a gene. Ex: human blood
types
Alternative Inheritance Patterns
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Pleiotropy: genes with multiple
phenotypic effect. Ex: sickle-cell
anemia
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Epistasis: a gene at one locus
(chromosomal location) affects the
phenotypic expression of a gene
at a second locus. Ex: mice coat
color
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Polygenic Inheritance: an additive
effect of two or more genes on a
single phenotypic character Ex:
human skin pigmentation and
height
Human disorders
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The family pedigree
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Recessive disorders:
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Dominant disorders:
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Cystic fibrosis
Tay-Sachs
Sickle-cell
Huntington’s
Testing:
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Amniocentesis
chorionic villus sampling
(CVS)
Chapter 12
The Chromosomal
Basis of Inheritance
Chromosomal Theory of
Inheritance
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Genes have specific
loci on chromosomes
and chromosomes
undergo segregation
and independent
assortment
Chromosomal Linkage
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Morgan
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Drosophilia melanogaster
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XX (female) vs. XY (male)
Sex-linkage: genes located
on a sex chromosome
Linked genes: genes located
on the same chromosome
that tend to be inherited
together
Genetic Recombination
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Crossing over
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Genetic maps
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Genes that DO NOT assort
independently of each other
The further apart 2 genes are, the
higher the probability that a crossover
will occur between them and therefore
the higher the recombination
frequency
Linkage maps
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Genetic map based on recombination
frequencies
Human Sex-Linkage
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SRY gene: gene on Y chromosome that triggers the development of
testes
Fathers= pass X-linked alleles to all daughters only (but not to sons)
Mothers= pass X-linked alleles to both sons & daughters
Sex-Linked Disorders: Color-blindness; Duchenne muscular
dystropy (MD); hemophilia
X-inactivation: 2nd X chromosome in females condenses into a Barr
body (e.g., tortoiseshell gene gene in cats)
Chromosomal Errors
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Nondisjunction:
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members of a pair of homologous
chromosomes do not separate properly
during meiosis I or sister chromatids
fail to separate during meiosis II
Aneuploidy:
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chromosome number is abnormal
Monosomy – missing chromosome
Trisomy - extra chromosome
(ex. Trisomy 21 Down Syndrome)
Polyploidy - extra sets of chromosomes
Chromosomal Errors
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Alterations of chromosomal structure:
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Deletion: removal of a chromosomal segment
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Duplication: repeats a chromosomal segment
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Inversion: segment reversal in a chromosome
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Translocation: movement of a chromosomal segment to another
Genomic Imprinting
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A parental effect on gene
expression
Identical alleles may have
different effects on
offspring, depending on
whether they arrive in the
zygote via the ovum or via
the sperm.
Fragile X syndrome: higher
prevalence of disorder and
mental disability in males