Genes & Traits Gene – section of DNA that encodes a protein, resulting in/affecting a trait ...
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Transcript of Genes & Traits Gene – section of DNA that encodes a protein, resulting in/affecting a trait ...
Genes & Traits Gene – section of DNA that encodes a
protein, resulting in/affecting a trait Genetics – study of heredity, or how
organisms inherit characteristics from parents
Trait – a characteristic of an organism; *genetic traits are inherited*Ex.: hair color, enzymes, size (potential), etc.
Genetics (ch. 9 & 12)
There once was an Austrian monk who liked peas and was
very good at math…
Gregor Mendel’s (1860s)Pea Plant Experiments
Mendel crossed pea plants with different traits and observed the results in the offspring.
Mendel’s Pea Plant Experiments
Dominant
Recessive
When two purebreds were crossed, recessive traits disappeared in the F1, reappeared in the F2.Mendel hypothesized that each trait was controlled by a “factor” andthat there must be at least two forms of each “factor.”
Why pea plants?•short life cycle•traits easily observed, no blending•self-fertilize easily, easy to manipulate/fertilize
Most genetic traits in nature are not so easy to distinguish.
Genes & Traits Allele – one form of a gene
Dominant allele – expressed whenever present, represented by a capital letter; ex.: B for brown hair color
Recessive allele – only expressed when two copies are present; represented by a lower case letter; ex.: b for blonde hair color
A diploid organism inherits two alleles (one from each parent) for every genetic trait
Genotype - the set of alleles for a trait or all traits of an organism
Genotypes
Purebred/Homozygous
Organism that has only one form of a gene/trait
(same allele received from both parents)
Hybrid/Heterozygous
Organism that has two forms of a gene/trait
(different alleles received from each parent)
BB – homozygous dominant genotype
bb – homozygous recessive genotype
Bb – heterozygous genotype
Phenotypes Phenotype - the trait the organism actually
expresses; ex.: having brown hair, or having blonde hair The phenotype may show on the appearance of the
organisms or not; ex.: being able to digest lactose is a phenotype that does not “show”
A phenotype results from the expression of a gene a protein is made that affects the phenotype
Phenotype Possible Genotypes
Dominant Ex.: brown hair
BB orBb
Recessive Ex.: blonde hair
bb
When we look at the genotypes in each generation:
Check yourself:
• Discuss with your partner:– Can you find an example of a dominant trait in
one of you?– Can you find an example of a recessive trait
in one of you?– What is the phenotype for each trait?– What are the possible genotypes for each
trait?
(not in notes)
GG gg
Gg Gg
GG or Gg gg
Mendel’s Laws (modern form) Law of Segregation - Gene pairs separate when
gametes form:Half of an organisms gametes contain one gene from a
homologous pair, half contain the other gene
Law of Independent Assortment - Different traits are inherited independently:
Genes for different traits segregate into gametes randomly and independently from each other new combinations You can inherit your father’s eyes and your mother’s smile!
Law of Dominance - Dominant alleles are expressed, recessive alleles can be hidden:
Hybrid pea plant: Py P (purple flowers, dominant) is expressed, p (white flowers, recessive) is hidden
Walter Sutton’s (1903)Chromosome Theory of Heredity Sutton (American) noticed that
chromosomes behaved like Mendel’s factors
Sutton’s Chromosome Theory of Heredity states that the material of inheritance is carried by the genes in chromosomes
Theodor Boveri (German) reached the same conclusion independently
Predicting the Phenotype of Offspring:Using a Punnett Square for One Trait
*Monohybrid cross
*
Genotypic Ratio:
Phenotypic Ratio:
YY : Yy : yy 1 : 2 : 1
Yellow : Green 3 : 1
Predicting the Phenotypes of Offspring:Using a Punnett Square for Two Traits
*Dihybrid cross
*
Phenotypic Ratio:
9 : 3 : 3 : 1Round & Yellow : Round & Green : Wrinkled & Yellow : Wrinkled & Green
Inheritance Pattern - Autosomal Recessive Trait
Genotypes?
Autosomal Recessive Traits in Humans(know all traits in bold print) Albinism – skin, hair, eyes lack melanin (pigment), sensitive to
sunlight Tay-Sachs – fat deposits prevent normal development of brain in
children Cystic Fibrosis – lungs damaged from excessive mucous, respiratory
problems Sickle Cell Anemia – red blood cells misshaped, clog capillaries
circulation problems; many phenotypic effects, depending on where in the body the blockage occurs
Click for Albino - pictures
Inheritance Pattern - Autosomal DominantTrait
Genotypes? (could also be DD)
Autosomal Dominant Traits in Humans(know all traits in bold print) Dimples, freckles, widow’s peak, farsightedness, broad lips, polydactyly
(extra fingers/toes) Dominant disorders are less common than recessive disorders because
carriers (heterozygotes) are affected and may die before reproducingEx.: Huntington disease – deterioration of nervous system, especially
brain
Inheritantce PatternAutosomal Dominant (not in notes)
Variable Expression:Some people have milder / more severe symptoms than others --Age of onset and organs affected may vary
Reduced Penetrance:Some people have the dominant gene but do not have any symptomsEx.: autosomal dominant cancer susceptibility
Pedigrees
Not always labeled
Pedigrees & Genetic Counseling
People with PKU (phenylketonuria) lack an enzyme needed to break down the amino acid phenylalanine, which is found in milk and other foods. Accumulation of the amino acid in the body leads to developmental disabilities and death if not treated.
Individual II-4 has PKU
From H. Genetics Lab :Pedigree -- Dominant or Recessive trait?Not always clear…
Trait appears in every generation More likely to be a dominant trait, but cannot be sure.
* Notice that the hybrid
individuals are not identified. *
Pedigree -- Dominant or Recessive trait?Not always obvious…
Trait skips a generation or more More likely to be a recessive trait, but cannot be sure.
* Notice that the hybrid
individuals are not identified. *
When can we tell for sure if the trait is dominant or
recessive?Do the Pedigree Analysis
to find out!
Pedigree A
Parents have the trait, child does not.
The trait is dominant: the “non-trait” phenotype is
recessive.
Xx
xx
Xx
Xx xx
xx
xx
xx
xx
xx
xxxx xx
Xx
Xx Xx
Xx or XX
MONOHYBRID CROSS
Pedigree B
Parents don’t have the trait, child does.
The trait is recessive: the allele is “hidden” in
the parents.
xx xx
xx (all)
xx
xx
MONOHYBRID CROSS
XX or Xx
Xx Xx
All Xx
XX or Xx XX or Xx
XX or Xx
XX or XxXX or Xx