EXAM 2 part 1 : chapter 14.3, 15.2 15.3 Flashcards
What are the 2 laws of Mendelian genetics
- The law of segregation
2. the law of independent assortment
THE LAW OF SEGREGATION
When any individual produces gametes ( ovum and / or sperm) the 2 alleles separate so that each gamete receives only one copy.
Law of Independent Assortment
each pair of alleles segregate independently of each other pair of alleles during gamete formation.
the six assumptions of Mendelian inheritance
- Genotype determines phenotype
- Alleles are either dominant or recessive
- Each trait is affected by one gene ( locus)
- Each gene ( locus) affects one trait
- All gene assort independently
- All traits in a diploid organism are affected by 2 copies of a gene ( b/ c ) chromosomes occur in homologous pairs.
violation of “ genotype determines phenotype”
PHENOTYPIC PLASTICITY
PHENOTYPIC PLASTICITY
The ability of an organism to to change its phenotype in response to its environment.
Phenotype refers to its traits or character and plasticity to variability.
COMPLETE DOMINANCE ( one allele) on one loci
The situation in which the phenotypes of the heterozygote and the dominant homozygote are indistinguishable ( Pp and PP still give a purple flower)
Egs of phenotypic plasticity
- Petal color change in the petals in hydrangea in response to soil ph. This is reversible.
- Pacific Tree frog tadpoles when exposed to different predatory environment they develop different morphology ( different tail shape). ( x- axis= the different treatment groups of predators ( bluegill fish, beatle, and a control). this is an example of INDUCIBLE DEFENSE– exposure to predators influences morphology. When exposed to bluegill that live in open water they have shallower tails that allow them to hide in vegetation. However when exposed to diving beetles that live the vegetation they have larger tails that allow them a quicker escape. This inducible defense is encoded in their genes. The next generation would be affected if the ones that survived long enough to reproduce due to this morphology so the next generation would also exhibit this trait ( not Lamarck ) the environment selected on a morphology. This is a developmental pathway based on their exposure that is not reversible.
- The spadefoot tadpoles’ diet (high protein) determines whether they become omnivores or carnivores( larger, more aggressive and cannibalistic)
INDUCIBLE DEFENSE
An example of phenotypic plasticity where an organism’s exposure to predators influences it’s morphology.
( phenotypic change due to predatory environment)
Violations to the “ alleles are either dominant or recessive, assumption.
PARTIAL DOMINANCE
The spectrum of dominance include
Complete dominance
Incomplete dominance
Codominance
INCOMPLETE DOMINANCE ( of either allele) ( on one loci)
the situation in which the phenotype of heterozygotes is intermediate between the phenotypes of individuals homozygous for either allele ( RR for red flower and rr for white produces pink offspring.
No allele is completely dominant.
. Both alleles determine phenotype but one is blank ( is not being expressed, does,not code,for,anything
eg. Snapdragons: White-No pigment, Red- full pigment; so only red is expressed resulting in a an weak red -a pink
COMPLETE DOMINANCE
One allele determines a phenotype
INCOMPLETE DOMINANCE
Both alleles determine phenotype but one is blank
What makes an allele dominant
An allele is dominant because it is seen in the phenotype ( not because it subdues the recessive gene).
. for any character observed dominance/ recessive allele relationship of allele depends on the level at which we examine phenotype
Eg of COMPLETE DOMINANCE
- Mendelian’s peas where in a dihybrid cross the alleles for yellow and smooth had complete dominance over alleles that were green and wrinkled ( yellow + green = yellow)
Eg for INCOMPLETE DOMINANCE
Snapdragons: when two homozygous snapdragons are crossed ( C^R which codes for red pigment and C^W which codes for no pigment-white) the resulting offspring is a pink. ( white - no pigment + red- full pigment = pink - half pigment)
CODOMINANCE
Both alleles determine phenotype
Eg of CODOMINANCE DOMINANCE
Parakeets that are homozygous for yellow C^Y and blue C^B are crossed they produce offspring where both alleles are expressed resulting in yellow feathers ( yellow + blue = green).
CODOMINANCE ( both allele) of one loci
alleles each affect the phenotype in separate distinguishable ways.
. 2 different alleles are expressed toghether in a phenotype
Why does codominance and incomplete dominance exist
Because an allele is a piece of DNA that gets transcribed, except in white snapdragons, into protein for example a pigment protein
The violation to “ each trait is affected by ONE GENE /LOCI
- POLYGENIC INHERITANCE
2. EPISTASIS
Eg of EPISTASIS
In mice and Labrador retrievers, whether or not any color is express is determined by gene locus CC or Cc which acts upon the gene locus B which determines the hair pigment ( brown or black)
Eg of EPISTASIS AND PARTIAL DOMINANCE combined
human Eye color: alleles at 2 ( one loci for brown D and the other for green G )loci modify the expression at the B locus ( EPISTASIS portion : is where the gene B at one loci is affecting expression at the D locus. If turned off results in green eyes to blue eyes, if turn on eye color May range from dark brown to hazel( a mixture of green and brown).
Eg of POLYGENIC INHERITANCE
For human skin color ( a simplified eg of this is there are 3 gene loci that code for the amount of melanin the skin is producing. When heterozygotes for these 3 sets of alleles have offspring there are many combinations /64 ( 7 possible allele combinations )of which the alleles can be inherited leading to a variation in skin color among siblings. The combinations are additive ranging com no melanin to high melanin
POLYGENIC INHERITANCE
An additive effect of two or more genes on a single phenotype
. the opposite of pleiotropy
PLEIOTROPY
One gene affects many traits.
Violation of the “ each gene affect only one trait” assumption.
PLEIOTROPY
TESTCROSS
Used to determine the genotype of an organism from its phenotype by crossing it with a recessive homozygote.
EPISTASIS (a type of POLYGENIC INHERITANCE)
The genotype at one locus determines the expression of a gene at another locus. This is not additive as in polygenic inheritance.
Eg of PLEIOTROPY
- Sickle cell disease : Recessive alleles ( single recessive = carrier , double = infected) at one locus affects both blood oxygen and severity or susceptibility of malaria infection. ( affects 2 phenotypes) Sickled red blood cells are less susceptible it the plasmodium that carries malaria.
- Siamese cats: One gene locus ( protein codes for ) determines the phenotypes of crossed eyes AND abnormal coloration of fur.
Violator to the “ all gene assort independently “ assumption
LINKAGE
POLYGENIC INHERITANCE
That a trait is affected by many gene loci
LINKED GENES
2 or more Genes located near each other on the same chromosome that tend to be inherited together in genetic crosses; such genes are said to be genetically linked and are called linked genes
. Linked genes are carried on the same chromosome and are therefore inherited together.
.. The traits that Mendel followed peas where on separate chromosomes.
. In independent assortment each pair alleles are separated independently of other pairs of alleles during gamete formation. However this would occur if pairs of alleles where on separate chromosomes. In linked genes, during meiosis, prophase 1, homologs ( of sister chromatids) align, then cross over at the end points of non sister chromatids ( of maternal and paternal chromosomes) resulting in varieties unlike the parent.
Eg of linked genes
Morgan’s fruit flies:if wing and body color are linked then a dihybrid ( with gray body and normal wings ) crossed with a homozygote recessive for black body and vestigial wings, the offspring should mostly be like the parent. This is considered a test cross. Since recessive homozygote contributes only one type of gamete it easier to determine the genotype from the phenotype. However if not linked a mendelian 1:1:1:1 ratio should result.
Morgan’ s result didn’t fit either prediction instead the offspring were a majority like the parent type with a few mixed. He concluded that during meiosis crossing over occurs homologous chromosomes when next to each other they can swap pieces of chromosomes. He believed that the 2 gene loci is on the same chromosome that is why he got more of the parent type over the mixed. But crossing over can also result in mixing of the alleles
Violation of the “ all diploid organisms are affected by 2 copies of a gene” assumption.
.GENOMIC IMPRINTING -
.Sex- linked traits
Eg of sex linked traits
Tortoiseshell cats are always heterozygous female. Males are always one color. This is due to x activation.
X inactivation ( codominance )
Relative to males female mammals have twice as many X chromosome.. Cells balance this inequality by x inactivation, in which a cell shuts off all but one chromosome in each cell.. This process happens early in the embryonic development of a mammal. Which X chromosome that gets turned off ( maternal or paternal ) is at random so a female will express paternal x - chromosome alleles in some cells and maternal x- chromosome alleles in others.
In cats the x- chromosome carries a coat color gene with alleles for black or orange coloration calico and tortoiseshell cats are heterozygous for this gene, one of the two chromosome is inactivated in each patch. If male all the color is expressed but if female, to prevent a double dose of protein being expressed one of the x is turned off. Which is turned off is at random. Sine the female is heterozygote for the color gene carried on the X chromosome,
GENOMIC IMPRINTING
Offspring inherit gene copies from mom and dad but one copy is silenced( “ imprinted in some or all tissues)
Eg of genomic imprinting
Genomic imprinting in the gene for insulin like growth factor 2 in mice where only the paternal alleles are expressed.
