Other Laws of Inheritance
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Transcript of Other Laws of Inheritance
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Other Laws of Inheritance
• Mendelian Genetics: characteristics controlled by dominant and recessive paired alleles
– Many traits follow the patterns outlined by Mendel’s Laws, many do not.
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Incomplete Dominance
• Offspring heterozygous for a trait have an intermediate appearance.
• Neither allele is completely dominant over the other
• Ex: Snapdragon flowers– Homozygous Red x Homozygous White– Results in all PINK flowers
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Showing a Cross for Incomplete Dominance– Use a capital letter for each allele since each influence trait
equally
• RR (homozygous red) x WW (homozygous white)• What is phenotype ratio of offspring?
• RW x RW (heterozygous pink)• What is phenotype ratio of offspring?
– How is this ratio different from the offspring
of Mendel’s Monohybrid cross? (Tt xTt)
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Codominance• If individual is heterozygous, both alleles
for gene are expressed.
• Ex: Chickens– Black feathers (BB) x White Feathers (WW)– Offspring (BW) are checkered and have both
black and white feathers
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• Try a codominant cross:
• Checkered (BW) x checkered (BW)
• Checkered (BW) x white (WW)
• Checkered (BW) x black (BB)
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• Ex: Roan Cattle– Offspring of pure red and pure white cows– Have both red and white hair
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Multiple Alleles
• A gene that has more than two alleles.
• Each individual can only have two alleles
• Ex: Human Blood Types
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Possible Blood Types:
A, B, AB, O
– There are 3 alleles for blood type (A, B, O)
– A and B are codominant alleles (IA, IB)
– O is a recessive allele (i)
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Six genotypes are possible for blood:
• IA IA
• IA IB
• IB IB
• IA i • IB i • i i
• What blood types would they have?
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• Ex: Type A (homozygous) x Type B
• What are the blood types of the offspring?
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• Try a Cross: • What blood types do the offspring have?
• IB IB x IA IA
• IA i x i i
• IA IB x i i
• If a Type A mother and a Type B Father produce a type O child, what are the parent’s genotypes?
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– Ex: temperature, nutrition, light etc.
• Ex: Coat color in Himalayan rabbits– Black fur present only on areas of body that are
colder. (ear, nose, feet and tail)
Environmental Influences on Gene Expression
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• Ice Pack placed on fur = fur turns black
• If rabbit kept in warm environment, all fur is white.
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Genetic Research and Testing
• Genetic Research:– To study genes for different traits it is best to
use an organism that grows and reproduces quickly an produces many offspring.
– Can do controlled breeding experiments– Can do forced “inbreeding”
• Ex: Drosophila melanogaster (fruit fly)
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Genetic Tests
• Test Cross: – To determine if an individual is homo or
heterozygous for a trait.– Breed to a homozygous recessive.
– Ex: • Drosophila: red eyes dominant over white.• To see if red eyed fly is hetero/homo, breed to
white eyed fly
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Sex Determination• Humans have 23 pairs of chromosomes• 22 pairs are autosomes• 1 pair are sex chromosomes
I’m a BOY! I’m a GIRL!These pictures are called karyotypes!
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Sex Determination
• XX = female, XY = male• Males can give Y chromosome or X chromosome• Females can only contribute X• There is always a 50% chance of being boy or girl
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Sex Linked Traits• If allele is found on an X or a Y chromosome it is
inherited differently in males and females.
• Ex: – Calico Cats– Color Blindness– Hemophilia– Duchenne Muscular Dystrophy
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Calico Cats: (Codominant X Linked Trait)– X chromosome has gene for black or orange
fur (codominant)– Only females can be “Calico” XBlackXOrange
– Males can only be Black or Orange
they have only one X chromosome and one Y
I’m always a girl!
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Colorblindness: (Recessive X linked trait)
• Certain colors cannot be distinguished from others (usually red or green)
• Most often seen males
• Females are more likely to be “carriers”
• Father cannot pass gene to son, only mother
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Hemophilia: (Recessive X-linked trait)– Blood has trouble clotting
• XHXh Carrier female
• XHY Normal Male
• XhY Affected Male
• XhXh Affected Female
• XHXHNormal Female
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• Examples of Crosses for Hemophilia
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• Queen Victoria of England passed this gene onto some of the royal family offspring
She must have been a “carrier”
Queen Victoria
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Chromosomal Disorders
• Due to the presence of absence of an entire chromosome or part of a chromosome.
• Can be determined by doing a Karyotype from blood or amniotic fluid
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• Amniocentesis: – Prenatal test
– Grow cells from amniotic fluid around fetus.
– Helps to rule out major chromosomal abnormalities
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Nondisjuction
• Nondisjunction: – homologous
chromosomes fail to separate properly during meiosis.
– Results in egg or sperm with one or more extra chromosomes.
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Normal Meiosis Nondisjunction
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What is Wrong?
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Nondisjunction of Sex Chromosomes
• Klinefelter’s Syndrome (XXY)– 2 X’s from mother, Y from father– Often tall, long arms and legs– May have some gynomastia and small
testicles
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• Turner’s Syndrome (XO)– Offspring has only one X (always female)– Usually sterile, short, thick neck
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Nondisjunction of Autosomes
• Trisomy 21 (Down’s Syndrome) (47, +21)– Most commonly seen chromosomal disorder
resulting in live birth– Extra Chromosome 21
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• Other trisomys can happen as well
• Babies may make it to term but usually don’t live very long (nonviable)– Ex: Trisomy 18, Trisomy 13
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Gene Linkage• When alleles for different traits are located
on the same chromosome
• They tend get inherited together.– They are “linked”– They do no “assort independently”
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• Ex: Red hair and freckles
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Crossing Over and Linked Genes
Crossing Over: homologous chromosomes
line up during synapsis and exchange
material.– Increases variation in offspring.
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Sometimes linked
genes will separate
when crossing over
occurs.
The closer genes are
on the chromosome,
the less chance they
will be separated during
crossing over
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Autosomal Genetic Disorders
• Caused by defective alleles on autosomes
• Can be recessive or dominant
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Sickle Cell Anemia: (Autosomal recessive)
• Red blood cells have a sickle shape
• Very fragile and break easily
• Clump up on blood vessels (causing pain)
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• Reduces oxygen carrying capacity of red blood cells
• Caused by a single flaw in gene
• One nitrogenous base in DNA sequence is different
• Cause wrong amino acid in protein chain for hemoglobin, changing it’s shape
• Hemoglobin doesn’t transport oxygen as well
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• Mostly found in people of African American descent – (1 in 500 births heterozygous)
• If disorder has negative effects why so common in population?– Malaria is a deadly disease
common in Africa
– Heterozygous individuals for sickle cell have malarial resistance, more likely to survive than people who aren’t carriers.
I spread malaria!
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Phenylketonuria (PKU):(Autosomal recessive)
• Enzyme that breaks down amino acid phenylalanine doesn’t function
• Chemical builds up in system and eventually forms substances that can damage the brain and cause mental retardation
• Can test for it at birth and treat with a low phenylalanine diet
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Tay-Sachs Disease: (Autosomal Recessive)
• Incurable inherited disorder that damages the brain
• Enzyme doesn’t function that helps breakdown lipid in brain.
• Brain tissue deteriorates
• Most often found in eastern European Jews
• Death occurs several years after birth
• http://www.youtube.com/watch?v=SeoPF74QSms&safe=active
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Gene-Chromosome Theory
• Explains the hereditary patterns that Mendel and others observed.
• The genetic characteristics of an organism are determined by the genes for different traits present on their chromosomes.
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Mutations
• Sometimes a genetic disorder is caused by a mutation of a gene.
• The mutation can only be passed to the next generation if it happens in a sex cell.
• Mutagenic Agents:– Exposure to x-rays– Radiation – Certain chemical toxins