NonMendelian Genetics
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Transcript of NonMendelian Genetics
NonMendelian Genetics
NonMendelian Genetics
Chapter 14: Mendel and the Gene IdeaChapter 14: Mendel and the Gene Idea
Complex patterns of inheritance
Complex patterns of inheritance
The relationship between genotype and phenotype is rarely as simple as in Mendelian inheritance (controlled by dominant and recessive paired alleles)
Principles of segregation and independent assortment apply to more complex patterns of inheritance
Inheritance may deviate from simple Mendelian patterns in the following situations:Alleles are not completely dominant or recessiveA gene has more than two allelesA gene produces multiple phenotypes
The relationship between genotype and phenotype is rarely as simple as in Mendelian inheritance (controlled by dominant and recessive paired alleles)
Principles of segregation and independent assortment apply to more complex patterns of inheritance
Inheritance may deviate from simple Mendelian patterns in the following situations:Alleles are not completely dominant or recessiveA gene has more than two allelesA gene produces multiple phenotypes
Incomplete dominanceIncomplete dominance In complete dominance,
heterozygous and homozygous dominant individuals have the same phenotype
With incomplete dominance, the phenotype of the heterozygous is intermediate between those of the two homozygotes
This intermediate occurs because neither allele of the pair is completely dominant
In complete dominance, heterozygous and homozygous dominant individuals have the same phenotype
With incomplete dominance, the phenotype of the heterozygous is intermediate between those of the two homozygotes
This intermediate occurs because neither allele of the pair is completely dominant
Incomplete DominanceIncomplete Dominance If you cross a white flower with a red flower that
exhibit incomplete dominance the first generation (heterozygotes) will be all pink.
If you cross two of those heterozygotes you will get 1 red, 2 pink, 1 white flowers. (1:2:1 phenotypic ratio)
If you cross a white flower with a red flower that exhibit incomplete dominance the first generation (heterozygotes) will be all pink.
If you cross two of those heterozygotes you will get 1 red, 2 pink, 1 white flowers. (1:2:1 phenotypic ratio)
We can still use the Punnett Square to solve problems involving incomplete dominance.
The trick is to recognize when you are dealing with a question involving incomplete dominance.
There are two steps to this: 1) Notice that the offspring is showing a 3rd
phenotype. The parents each have one, and the offspring are different from the parents.
2) Notice that the trait in the offspring is a blend (mixing) of the parental traits.
We can still use the Punnett Square to solve problems involving incomplete dominance.
The trick is to recognize when you are dealing with a question involving incomplete dominance.
There are two steps to this: 1) Notice that the offspring is showing a 3rd
phenotype. The parents each have one, and the offspring are different from the parents.
2) Notice that the trait in the offspring is a blend (mixing) of the parental traits.
Incomplete dominance genetic problems
Incomplete dominance genetic problems
Incomplete Dominance Questions
Incomplete Dominance Questions
1. A cross between a black bird & a white bird produces offspring that are grey. The color of birds is determined by just two alleles. a) What are the genotypes of the parent birds in the
original cross? BB = black, BW= grey, WW = whiteBB x WW
b) What is/are the genotype(s) of the grey offspring? BW
c) What would be the phenotypic ratios of offspring produced by two grey birds?
1 black, 2 grey and 1 white
1. A cross between a black bird & a white bird produces offspring that are grey. The color of birds is determined by just two alleles. a) What are the genotypes of the parent birds in the
original cross? BB = black, BW= grey, WW = whiteBB x WW
b) What is/are the genotype(s) of the grey offspring? BW
c) What would be the phenotypic ratios of offspring produced by two grey birds?
1 black, 2 grey and 1 white
B W
B BB BW
W BW WW
Incomplete Dominance Questions
Incomplete Dominance Questions
2. The color of fruit for plant "X" is determined by two alleles. When two plants with orange fruits are crossed the following phenotypic ratios are present in the offspring: 25% red fruit, 50% orange fruit, 25% yellow fruit.What are the genotypes of the parent orange-
fruited plants? RY
2. The color of fruit for plant "X" is determined by two alleles. When two plants with orange fruits are crossed the following phenotypic ratios are present in the offspring: 25% red fruit, 50% orange fruit, 25% yellow fruit.What are the genotypes of the parent orange-
fruited plants? RY
CodominanceCodominance In codominance, two
dominant alleles affect the phenotype in separate, distinguishable ways
Codominant alleles cause the phenotypes of both homozygotes to be produced in heterozygote individuals.
In codominance both alleles are expressed.
For example, red cows crossed with white will generate roan cows. Roan refers to cows that have red coats with white blotches.
In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways
Codominant alleles cause the phenotypes of both homozygotes to be produced in heterozygote individuals.
In codominance both alleles are expressed.
For example, red cows crossed with white will generate roan cows. Roan refers to cows that have red coats with white blotches.
CodominanceCodominanceThe genetic gist to codominance is pretty much the
same as incomplete dominance.A hybrid organism shows a third phenotype --- not
the usual "dominant" one & not the "recessive" one.With incomplete dominance we get a blending of
the dominant & recessive traits so that the third phenotype is something in the middle (red x white = pink).
In codominance, the "recessive" & "dominant" traits appear together in the phenotype of hybrid organisms.
red x white ---> red & white spotted
The genetic gist to codominance is pretty much the same as incomplete dominance.
A hybrid organism shows a third phenotype --- not the usual "dominant" one & not the "recessive" one.
With incomplete dominance we get a blending of the dominant & recessive traits so that the third phenotype is something in the middle (red x white = pink).
In codominance, the "recessive" & "dominant" traits appear together in the phenotype of hybrid organisms.
red x white ---> red & white spotted
Codominance Punnett Squares
Codominance Punnett Squares
Some texts use letters & superscripts when dealing with codominance.
Others use different letters, noting the type of nonMendelian cross.Let’s use the second method
for our example R = allele for red flowers W = allele for white flowersred x white --> red & white
spotted flowers RR x WW ----> 100% RWThe symbols you choose to use
don't matter, in the end you end up with hybrid organisms, and rather than one trait (allele) dominating the other, both traits appear together in the phenotype.
Some texts use letters & superscripts when dealing with codominance.
Others use different letters, noting the type of nonMendelian cross.Let’s use the second method
for our example R = allele for red flowers W = allele for white flowersred x white --> red & white
spotted flowers RR x WW ----> 100% RWThe symbols you choose to use
don't matter, in the end you end up with hybrid organisms, and rather than one trait (allele) dominating the other, both traits appear together in the phenotype.
RWRW
RWRW
R R
W
W
Codominance QuestionsCodominance Questions1. Predict the phenotypic ratios of offspring
when a homozygous white cow is crossed with a roan bull. White = WW, roan = RW, red = RRWW x RW = 1/2 white and 1/2 roan
2. A cross between a black cat & a tan cat produces a tabby pattern (black & tan fur together). a) What pattern of inheritance does this illustrate?
Why? Codominance; both showb) What percent of kittens would have tan fur if a
tabby cat is crossed with a black cat?TT=tan, TB = tabby, BB= blackTB x BB = 1/2 tabby and 1/2 black0%
1. Predict the phenotypic ratios of offspring when a homozygous white cow is crossed with a roan bull. White = WW, roan = RW, red = RRWW x RW = 1/2 white and 1/2 roan
2. A cross between a black cat & a tan cat produces a tabby pattern (black & tan fur together). a) What pattern of inheritance does this illustrate?
Why? Codominance; both showb) What percent of kittens would have tan fur if a
tabby cat is crossed with a black cat?TT=tan, TB = tabby, BB= blackTB x BB = 1/2 tabby and 1/2 black0%
W W
R RW RW
W WW WW
T B
B TB BB
B TB BB
Multiple AllelesMultiple Alleles
It is common for more than two alleles to control a trait in a population
Traits controlled by more than two alleles are said to have multiple alleles
A diploid individual can possess only two alleles of each gene
It is common for more than two alleles to control a trait in a population
Traits controlled by more than two alleles are said to have multiple alleles
A diploid individual can possess only two alleles of each gene
Multiple AllelesMultiple Alleles
The number of alleles for any particular trait is not limited to four, there are instances in which more than 100 alleles are known to exist for a single trait
Multiple Alleles & Blood Types
Multiple Alleles & Blood Types
Multiple Alleles govern blood typeHuman blood types are
determined by the presence or absence of certain molecules on the surfaces of red blood cells called antigens
As the determinant of blood type the gene I has three alleles: IA, IB, and iWritten A, B, and OIA (or A) allele produces
antigen A IB (or B) allele produces
antigen Bi (or O) produces no antigensA & B codominant, but both
dominant to O
Multiple Alleles govern blood typeHuman blood types are
determined by the presence or absence of certain molecules on the surfaces of red blood cells called antigens
As the determinant of blood type the gene I has three alleles: IA, IB, and iWritten A, B, and OIA (or A) allele produces
antigen A IB (or B) allele produces
antigen Bi (or O) produces no antigensA & B codominant, but both
dominant to O
Importance of Blood Typing
Importance of Blood Typing
Incompatible blood types could clump together, causing death.
Disputed parentageExample: If a child has type AB blood and
its mother has type A, a man with type O blood could not be the father.
Why?
Incompatible blood types could clump together, causing death.
Disputed parentageExample: If a child has type AB blood and
its mother has type A, a man with type O blood could not be the father.
Why?
Blood Typing PracticeBlood Typing Practice1. A woman with Type O blood and a man who is Type AB
have are expecting a child. What are the possible blood types of the kid? • A, B
2. What are the possible blood types of a child who's parents are both heterozygous for "B" blood type? • B,O
3. What are the chances of a woman with Type AB and a man with Type A having a child with Type O? • None
4. A test was done to determine the biological father of a child.The child's blood Type is A and the mother's is B. Man #1 has a blood type of O, & Man #2 has blood type AB. Which man is the biological father?• Man #2
1. A woman with Type O blood and a man who is Type AB have are expecting a child. What are the possible blood types of the kid? • A, B
2. What are the possible blood types of a child who's parents are both heterozygous for "B" blood type? • B,O
3. What are the chances of a woman with Type AB and a man with Type A having a child with Type O? • None
4. A test was done to determine the biological father of a child.The child's blood Type is A and the mother's is B. Man #1 has a blood type of O, & Man #2 has blood type AB. Which man is the biological father?• Man #2
PleiotropyPleiotropy
Most genes have multiple phenotypic effects, a property called pleiotropy
For example, pleiotropic alleles are responsible for the multiple symptoms of certain hereditary diseases, such as cystic fibrosis and sickle-cell disease
In the garden pea, gene for flower color also affects color of seed coat
Most genes have multiple phenotypic effects, a property called pleiotropy
For example, pleiotropic alleles are responsible for the multiple symptoms of certain hereditary diseases, such as cystic fibrosis and sickle-cell disease
In the garden pea, gene for flower color also affects color of seed coat
EpistasisEpistasis In epistasis, a gene at one
locus alters the phenotypic expression of a gene at a second locus
For example, in mice and many other mammals, coat color depends on two genes
One gene determines the pigment color (B for black and b for brown)
The other gene (C for color and c for no color) determines whether the pigment will be deposited in the hairDominance = B masking bEpistasis = cc masking BB
or Bb or bb
In epistasis, a gene at one locus alters the phenotypic expression of a gene at a second locus
For example, in mice and many other mammals, coat color depends on two genes
One gene determines the pigment color (B for black and b for brown)
The other gene (C for color and c for no color) determines whether the pigment will be deposited in the hairDominance = B masking bEpistasis = cc masking BB
or Bb or bb
Polygenic inheritance
poly = “many” ; genic = “genes”
Polygenic inheritance
poly = “many” ; genic = “genes”
More than one gene pair contributes to a phenotype
Effects of dominant alleles are additiveMore dominant genes = increased effectNumber of dominant determines
phenotypeHair, eye and skin color (as well as height)
are polygenic traitsMany disorders may be polygenic
Cleft palate, club foot, diabetes, schizophrenia, allergies, cancer
More than one gene pair contributes to a phenotype
Effects of dominant alleles are additiveMore dominant genes = increased effectNumber of dominant determines
phenotypeHair, eye and skin color (as well as height)
are polygenic traitsMany disorders may be polygenic
Cleft palate, club foot, diabetes, schizophrenia, allergies, cancer
Skin color exampleSkin color example If skin color was related to 3 gene
pairsDominant gene A, B or C
produces pigmentIncompletely dominant to a, b or
cSo # of dominant genes
determines how much pigment is produced
AABBCC = lots of pigmentAaBbCc = middle range of
pigmentaabbcc = very little pigment2 heterozygotes (AaBbCc) could
have a child with any pigment range
If skin color was related to 3 gene pairsDominant gene A, B or C
produces pigmentIncompletely dominant to a, b or
cSo # of dominant genes
determines how much pigment is produced
AABBCC = lots of pigmentAaBbCc = middle range of
pigmentaabbcc = very little pigment2 heterozygotes (AaBbCc) could
have a child with any pigment range
Environmental Influences
Environmental Influences
Genes are also influenced by the environment
Temperature and Siamese cats
The darker colors on the extremities are due to a cooler body temperature
Gene that codes for production of the pigment in the Siamese cat only functions under cooler conditions
Many diseases, such as heart disease and cancer, have both genetic and environmental components
Genes are also influenced by the environment
Temperature and Siamese cats
The darker colors on the extremities are due to a cooler body temperature
Gene that codes for production of the pigment in the Siamese cat only functions under cooler conditions
Many diseases, such as heart disease and cancer, have both genetic and environmental components
PedigreePedigree A pedigree is a family tree that
describes the interrelationships of parents and children across generations Inheritance patterns of
particular traits can be traced
Can also be used to make predictions about future offspring
Many genetic disorders are inherited in a recessive manner
Recessively inherited disorders show up only in individuals homozygous for the allele
Carriers are heterozygous individuals who carry the recessive allele but are phenotypically normal
A pedigree is a family tree that describes the interrelationships of parents and children across generations Inheritance patterns of
particular traits can be traced
Can also be used to make predictions about future offspring
Many genetic disorders are inherited in a recessive manner
Recessively inherited disorders show up only in individuals homozygous for the allele
Carriers are heterozygous individuals who carry the recessive allele but are phenotypically normal
Pedigree SymbolsPedigree Symbols
AlbinismAlbinism Albinism is a recessive
condition characterized by a lack of pigmentation in skin and hair
If a recessive allele that causes a disease is rare, then the chance of two carriers meeting and mating is low
Consanguineous matings (i.e., matings between close relatives) increase the chance of mating between two carriers of the same rare allele
Most societies and cultures have laws or taboos against marriages between close relatives
Albinism is a recessive condition characterized by a lack of pigmentation in skin and hair
If a recessive allele that causes a disease is rare, then the chance of two carriers meeting and mating is low
Consanguineous matings (i.e., matings between close relatives) increase the chance of mating between two carriers of the same rare allele
Most societies and cultures have laws or taboos against marriages between close relatives
Cystic FibrosisCystic FibrosisRecessive conditionCystic fibrosis is the most
common lethal genetic disease in the US, striking one out of every 2,500 people of European descent
The cystic fibrosis allele results in defective or absent chloride transport channels in plasma membranes
Symptoms include mucus buildup in some internal organs and abnormal absorption of nutrients in the small intestine
Recessive conditionCystic fibrosis is the most
common lethal genetic disease in the US, striking one out of every 2,500 people of European descent
The cystic fibrosis allele results in defective or absent chloride transport channels in plasma membranes
Symptoms include mucus buildup in some internal organs and abnormal absorption of nutrients in the small intestine
Sickle-cell diseaseSickle-cell disease
Recessive conditionSickle-cell disease
affects one out of 400 African-Americans
The disease is caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells
Symptoms include physical weakness, pain, organ damage, and even paralysis
Recessive conditionSickle-cell disease
affects one out of 400 African-Americans
The disease is caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells
Symptoms include physical weakness, pain, organ damage, and even paralysis
Dominant Genetic Diseases
Dominant Genetic Diseases
Some human disorders are caused by dominant alleles
Dominant alleles that cause a lethal disease are rare and arise by mutation
Achondroplasia is a form of dwarfism caused by a rare dominant allele
Huntington’s disease is a degenerative disease of the nervous system caused by a dominant alleleThe disease has no obvious
phenotypic effects until the individual is about 35 to 40 years of age
Some human disorders are caused by dominant alleles
Dominant alleles that cause a lethal disease are rare and arise by mutation
Achondroplasia is a form of dwarfism caused by a rare dominant allele
Huntington’s disease is a degenerative disease of the nervous system caused by a dominant alleleThe disease has no obvious
phenotypic effects until the individual is about 35 to 40 years of age
Genetic TestsGenetic Tests
There are many genetic diseases that exist (way beyond the scope of what we will discuss)
Genetic counselors can provide information to prospective parents concerned about a family history for a specific disease
Using family histories, they help couples determine the odds that their children will have genetic disorders
For a growing number of diseases, tests are available that identify carriers and help define the odds more accurately
There are many genetic diseases that exist (way beyond the scope of what we will discuss)
Genetic counselors can provide information to prospective parents concerned about a family history for a specific disease
Using family histories, they help couples determine the odds that their children will have genetic disorders
For a growing number of diseases, tests are available that identify carriers and help define the odds more accurately
AmniocentesisAmniocentesis
In amniocentesis, a long thin needle is used to remove amniotic fluid
The amniotic fluid contains fetal cells, which can be tested for genetic diseases
The DNA from fetal cells is karyotyped
In amniocentesis, a long thin needle is used to remove amniotic fluid
The amniotic fluid contains fetal cells, which can be tested for genetic diseases
The DNA from fetal cells is karyotyped
Chorionic Villus SamplingChorionic Villus Sampling
In chorionic villus sampling (CVS), a sample of the chorionic villus (developing placenta) is removed and tested
The chorionic villus cells contain the same genetic material as the fetus, making them fetal cells
The DNA from fetal cells is karyotyped
In chorionic villus sampling (CVS), a sample of the chorionic villus (developing placenta) is removed and tested
The chorionic villus cells contain the same genetic material as the fetus, making them fetal cells
The DNA from fetal cells is karyotyped
KaryotypesKaryotypes
Karyotypes (picture of chromosomes arrested during mitosis) are prepared, which determines:# of chromosomesSex of individualExtra or missing
pieces of chromosomes
Karyotypes (picture of chromosomes arrested during mitosis) are prepared, which determines:# of chromosomesSex of individualExtra or missing
pieces of chromosomes
Other Genetic TestsOther Genetic Tests
Other techniques, such as ultrasound and fetoscopy, allow fetal health to be assessed visually in utero
Some genetic disorders can be detected at birth by simple tests that are now routinely performed in most hospitals in the USPhenylketonuria (PKU)Congential
Hypothyroidism
Other techniques, such as ultrasound and fetoscopy, allow fetal health to be assessed visually in utero
Some genetic disorders can be detected at birth by simple tests that are now routinely performed in most hospitals in the USPhenylketonuria (PKU)Congential
Hypothyroidism
Review QuestionsReview Questions1. Name 3 examples of when inheritance patterns may not
follow Mendelian rules.2. Explain, identify, and solve genetics problems involving
incomplete dominance, codominance, & multiple alleles.3. Complete genetics problems involving blood types.4. Explain, differentiate between, and complete nontraditional
genetics problems involving pleiotropy, epistasis, and polygenic inheritance.
5. Explain the effect of the environment on the expression of our genes.
6. Define and analyze a pedigree in order to answer inheritance questions.
7. Identify the most common pedigree symbols.8. Identify the inheritance patterns and major characteristics
of the following genetic conditions: albinism, cystic fibrosis, sickle-cell disease, achondroplasia, & Huntington’s disease.
9. Explain the purpose, benefits, and risks of genetic testing.10. Differentiate between amniocentesis and chorionic villus
sampling.11. Explain the purpose and use of a karyotype.12. List 3 pieces of information that can be obtained from a
karyotype.