Download Chapter 14 Mendelian Genetics Notes

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Inheritance
Mendel and the Gene
Idea
Gregor Mendel
The passing of traits from parents to offspring
Genetics—the scientific study of inheritance
Helpful Vocabulary
1822-1884
Homozygous—when the two alleles are the same (TT/tt)
Scientist and monk (“Father of Genetics”)
Heterozygous—when the two alleles are different (Tt)
He chose the pea plant because of short lifespan, bisexual, many traits known and
they are capable of cross and self-pollinating
True-breeding means homozygous.
Trait—a heritable feature (ex. Height)
Allele—a variant of a character (ex. Tall or short)
Mendel’s Experimental Design
Mendel’s Experimental Design
Mendel began with true-breeding varieties that contained only one type of gene for
each character.
Mendel called the trait expressed in the F1 plants the dominant trait and the trait not
expressed was recessive.
As first, he only looked at one character at a time—Monohybrid cross.
When F1 were allowed to self-fertilize, Mendel found a 3:1 dominant to recessive
phenotype in the F2 generation.
The first generation of crosses is the P or parental generation.
Their offspring are the F1 generation or first filial generation.
When F2 plants were allowed to self-fertilize, Mendel found a 1:2:1 of true breeding
dominant to not true-breeding dominant to true-breeding recessive (genotypes).
Offspring of two F1 members comprise the F2 generation.
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Today we use these terms:
Each individual has two genes for each trait.
Alternate types of genes for each trait are alleles.
Phenotype refers to the outward expression of the genes.
The actual genetic makeup of an individual is the genotype.
Punnett Squares—easy way to express the probabilities of genotypes.
1. One trait at a time= monohybrid cross
2. Two traits at a time= dihybrid cross
Mendel’s Laws
Mendel’s 1 st Law: Law of Segregation
Only one of a pair of alleles is passed to a gamete
Mendel’s 2 nd Law: Law of Independent Assortment
Mendel determined this when working with dihybrid crosses.
Law states that genes located on different chromosomes are inherited independently.
Ex. Inheritance of height is independent of the inheritance of flower color
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Exceptions to Mendel’s Laws
Incomplete dominance: when offspring exhibit a phenotype intermediate to that of
both parents.
Ex. Red X White Snapdragons
Exceptions to Mendel’s Laws
Multiple alleles: more than two alleles exist for a given trait in a population of
individuals.
Exceptions to Mendel’s Laws
Codominance: two alleles affect the phenotype in separate distinguishable ways
Ex. Tabby cat
Black cat (BB) X Tan cat (TT)
Exceptions to Mendel’s Laws
Pleiotropic Effects: when an allele affects more than one trait
Ex. PKU and CF
Ex. ABO blood groups
Exceptions to Mendel’s Laws
Exceptions to Mendel’s Laws
Epistasis: Interaction between the products of two genes in which one of the genes
modified the phenotypic expression produced by the other.
Polygenic inheritance: when one trait
is determined by the action of several
genes.
Ex. Gerbils
Ex. Skin color, height
1st gene: C=color, c=albino
--skin color is likely controlled by
atleast 4 genes.
2nd
gene: B=brown, b=black
CcBb X CcBb (Brown X Brown)
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Exceptions to Mendel’s Laws
Environmental Effects: the degree to which an allele is expressed can sometimes
depend on the environment
Genetic Studies of Humans
Often done by pedigree charts
Male
Ex. Nature vs. Nurture, Heart Disease
Female
PHENOTYPE= GENOTYPE + ENVIRONMENT
Person with trait
Sample Pedigree
Dominant Trait
Autosomal Recessive Disorders
Recessive Trait
Autosomal Recessive Disorders
Both parents must have the allele to have a child born with the condition.
Cystic Fibrosis
Even if both parents are carriers, they only have a 1-in-4 chance of having an offspring
with the condition
-most common genetic disorder in Caucasian Americans
-1 in 25 is a carrier; 1 in 2500 is affected.
-Causes accumulation of mucus in lungs, pancreas and other organs.
Tay Sachs
-most common in Jews from Eastern & Central Europe
-1 in 30 are carriers, 1 in 3600 are affected
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Autosomal
Recessive Disorders
Sickle-cell anemia
-prevalent in population in
or from Africa (malariaravaged)
-single amino acid
substitution results in
malformed hemoglobin
-Heterozygotes are more
resistant to malarial
infection
Autosomal Dominant Disorders
Single “faulty” allele of a gene causes damage, even with a “good” allele present,
because the “faulty” allele is dominant.
Huntington’s Disease
-dominant lethal condition that doesn’t express itself until later in life
-degenerative neural disease
ALS (Lou Gehrig’s Disease)
-degenerative neural disease
Genetic Counseling and Therapy
Genetic counseling—helps couples predict risk of bearing children with genetic
defects.
Amniocentesis—minute amount of amniotic fluid surround fetus is removed and
checked for genetic defects; happens in 4th month
Chorionic villus sampling—removal of small portion of the chorionic villi of the
placenta for genetic testing; earlier in pregnancy and less invasive
Ultrasound—allows for viewing of the fetus without harming it.
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