Chapter 18 Flashcards
(10 cards)
Define epistasis and explain how the C gene can mask the P gene in sweet peas.
*Alleles of one gene mask the expression of the alleles of another gene
-Often because 2 or more different proteins involved in a single function
*2 genes involved in sweet pea flower color
*C gene makes protein that is upstream of P gene in making pigment
*A→B is carried out by C protein, then B→ purple pigment is carried out by P protein:
-C (purple) dominant to c (white)
-P (different purple) dominant to p (white)
-cc masks P– producing white flowers in individuals that are ccPP or ccPp
Define polygenic and continuous variation. Describe how skin color in humans is an example of both of these terms.
*Discrete or discontinuous traits
-Clearly defined phenotype: purple or white flowers, red or white eyes
*Continuous or quantitative traits:
-Majority of traits: height, skin color, apples on a tree
-Show continuous variation over a range of phenotypes
-Polygenic- -several genes contribute to the outcome, leading to an additive effect on the phenotype
-Environment also plays a role
Define linkage and explain why linked genes tend to be inherited in parental rather than recombinant combinations. (This connects back to Meiosis). Do linked genes follow the Law of Independent Assortment?
*Genes on different chromosomes follow the Lab of Independent Assortment
*Genes that are close together on the same chromosome do not follow the Law of Independent Assortment
*Linkage - when 2 genes are close on the same chromosomes, they tend to be inherited together:
-Offspring have much higher rates of parental combination of phenotypes than would be expected if the traits followed the Law of Independent Assortment
Explain why recombinant phenotypes arise even when genes are linked. (This connects back to Meiosis).
-Some offspring still end up with recombinant (nonparental) new combinations of the traits due to
crossover during meiosis
-Genes that are far apart in the same chromosome are more likely to cross over; because of this they behave as if they follow independent assortment
Define epigenetic inheritance and dosage compensation. List two examples of epigenetic inheritance
Epigentic inheritance: Chemical modification of a gene or chromosome during egg formation, sperm formation, or early stages of embryo growth that alters gene expression in a way that is fixed during an individual’s lifetime
-Not a mutation in the actual nucleotides
-Causes the affected area of the gene or chromosome to become condensed. Condensed DNA is expressed very little or not at all.
-Permanently affects the phenotype of the individual, but they are not permanent over the course of many generations and they do not change the actual DNA sequence
-Examples of epigenetic inheritance:
X inactivation
Genomic imprinting
Describe the process of X inactivation in female mammals and define a Barr body. Why do females inactivate an X? Is the same X chromosome always inactivated?
*Dosage compensation- inactivation of a gene or chromosome reduces the expression of that gene or area to the appropriate quantity
-Females have two X chromosomes, males have just 1
-The “dose” of X chromosome genes that humans need = 1
-Essentially, this means that two X chromosomes are too much
-So female cells will only use one X chromosome to reduce the “dose” of X
-X inactivation—female cells will randomly turn off one of their X chromosomes
-One X is modified into a highly condensed inactive Barr Body
-The other X remains uncondensed and active for transcription and translation into proteins
Explain how X inactivation produces calico coloration in cats.
*X-linked gene for coat color
-orange allele, XO and a black allele, XB.
In early embryonic development, one X chromosome is randomly inactivated in each of the cat’s somatic cells.
-A female that is heterozygous will have one or the other X inactivated in different groups of cells resulting in patches of black and orange fur
-A calico cat is a Mosaic- female mammals heterozygous for X-linked genes have half of their somatic cells expressing one allele and the other half expressing the other allele.
Define genomic imprinting
*Segment of DNA is imprinted, or marked, in a way that affects gene expression throughout the life of the
individual who inherits the DNA
*Occurs in insects, plants, and mammals
*Can involve a single gene, part of a chromosome, an entire chromosome, or all the chromosomes from one parent
*Imprinted genes do not follow Mendelian patterns of inheritance:
-Offspring distinguish between maternally and paternally inherited chromosomes
-Offspring express either the maternal or paternal allele, but not both
Explain why the imprinted Ifg2 gene shows paternal inheritance in mice. Why do mice inactivate an Igf2 gene? Is the same Igf2 gene always inactivated?
*Normal and dwarf offspring can have the same genotype but different phenotypes In mammals, only the paternal Igf-2 gene is expressed (paternal inheritance):
-The maternal Igf-2 is chemically modified during egg formation and will never be used by the offspring
-The paternal Igf-2 is not modified during sperm formation and will be the only allele used by the
offspring
*All imprinting from the parents is erased when offspring make their own gametes:
-A male can inherit an imprinted copy from his mother that is never transcribed but he can pass on an active, nonmodified copy of this exact same gene to his offspring
Explain why extranuclear (Chloroplast, mitochondrial) genes show maternal inheritance.
*Some genes are not found on the chromosomes in the cell nucleus
*Extranuclear means outside of the nucleus:
-Mitochondrial genome
-Chloroplast genome
*Much less DNA outside the nucleus but can influence some phenotypes
*Follows maternal inheritance patterns:
-Egg contributes cytoplasm and all first organelles to zygote
-Sperm only contributes nuclear DNA
-Mother passes extranuclear traits to all offspring but only daughters pass them on
