Download 1 DTU Systems Biology Mette Voldby Larsen, CBS, Building 208

DTU Systems Biology
Mette Voldby Larsen, CBS, Building 208
February, 2012
Study Guide
Textbook:
Sadava et al.: Life. The Science of Biology, 9. Ed. Vol. I The Cell and Heredity.
NB! The comments are meant to help distinguish between relevant and less relevant material.
Nevertheless, exam questions concerning all material in the chapter can be asked.
Ch. 12:
Inheritance, Genes, and Chromosomes:
The monk, Gregor Mendel, is considered the father of genetics. He studied inheritance in pea plants.
The description of his experiments and results introduces genetics along with many important concepts
and terms.
Mendel’s first law (law of segregation) is important: The two copies of a gene (the alleles) are
separated during the generation of the gametes. The progeny inherits one allele from the father and one
allele from the mother.
Punnett square: Learn how to fill it in and how to use it for deducing geno- and phenotypes in
monohybrid and dihybrid crosses. Probability calculations can be used for the same purpose.
Mendel’s second law (independent assortment): Alleles of different genes are assorted independently of
each other in the gamete (can be shown by dihybrid crosses). It is important to remember that this law
only applies to genes on different chromosomes.
Even though Mendel’s studies of inheritance in pea plants reveal many important relationships, it is –
as always in biology – more complicated than this. Section 12.2 and 12.3 exemplifies this. Understand
the biology behind the examples.
Genes that are located to the same chromosome are linked and are not assorted independently. During
the cross-over event (recombination) of meiosis, recombinant chromosomes are generated (see ch. 11),
resulting in linked genes ending up on different chromatids. The further away from each other two
linked genes are, the higher the probability is that the genes will be separated by a cross-over event.
Based on the frequency of recombinant progeny, a genetic map can be made of the location of the
genes.
The composition of the sex chromosomes specifies the gender of the individual (XX = woman, XY =
man). Many human diseases are sex-linked and preferentially affect men. Genealogical (family) trees
can reveal this.
Bacteria can exchange genetic material from chromosomes or plasmids by conjugation.
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Important terms from ch. 12 that should be remembered by heart:
Allele: Different versions of the same gene.
Autosomes: All chromosomes, except for the sex chromosomes. In humans, chromosome no. 1 -22 are
autosomes. Autosomes are similar for male and female individuals within a species.
Character: An observable feature, for instance eye color or height.
Conjugation: Process that leads to the transfer of DNA between two cells (typically bacteria cells)
through a conjugation tube.
Co-dominant alleles: Sometimes two alleles within an allele pair are equally strong. Both alleles are
expressed in the phenotype of individuals that are heterozygous for such an allele pair. An example is
gprovided by the ABO blood system in which a person with the heterozygous allele pair IAIB has the
phenotype AB.
Dihybrid cross: The crossbreeding of two individuals that differ with respect to two loci (two
characters).
Dominant allele/trait: A dominant allele will be expressed in the phenotype of homozygous as well as
heterozygous individuals (that is, regardless of the other allele in the allele pair). As an example,
humans will always have brown eyes, if they have the allele for brown eyes, regardless of whether the
other allele in the pair encodes blue or brown eyes.
Epistasis: When the phenotype caused by the allele of one locus is determined by the presence of a
particular allele in another locus.
First filial generation: The children. Is also called the F1 generation.
Gene: Area on chromosome that encodes RNA. Often the RNA will be translated to protein. In that
case, the gene is called a protein coding or structural gene. Other genes encode, e.g., rRNA or
microRNA that are not translated to protein.
Genotype: Except for genes of the XY chromosome pair, an individual will have all genes in two
copies (two alleles that can be either identical or different): The genes are located to the chromosomes
and since you inherit one full set of chromosomes from your mother and a full set from your father, you
will also have two copies of each gene. The genotype is the composition of alleles behind a given
phenotype. For instance, the blood type B (example of a phenotype) in the ABO blood system can be
caused by the genotype IBIB (the individual is homozygous) or IBiO (the individual is heterozygous).
Homozygous: Term used about individuals that with regards to a particular genetic character has two
identical alleles. You can be homozygous with regards to certain characters, while heterozygous with
regards to others.
Heterozygous: Term used about individuals that with regards to a particular genetic character has two
different alleles. You can be heterozygous with regards to certain characters, but homozygous with
regards to others.
Hybrid: The progeny of two organisms that differ with regards to one or more characters.
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Incomplete dominance: When neither of the alleles for a particular character is completely dominant
or completely recessive. Instead, the heterozygous individual will display a phenotype that is a mix of
the phenotypes for the two homozygous individuals.
Locus (plural: loci): The area on the chromosome in which the gene is located.
Monohybrid cross: The crossbreeding of two individuals that differ with respect to one loci (one
characters).
Parental generation: The parents. Also called the P generation.
Phenotype: The observable properties of an individual. The trait that is displayed. For instance, the
phenotype of an individual is brown eyes even though the individual has both an allele for brown eyes
and an allele for blue eyes.
Plasmid: Small, circular DNA molecules that many bacteria contain besides the chromosome.
Typically only contain a few genes.
Polymorfic loci: Genetic loci for which two or more different alleles exist in the population as a whole
(obviously, a single individual can only have two different alleles in the same loci).
Recessive allele/trait: A recessive allele will only be expressed in the phenotype of individuals that are
homozygous with regards to the character in question. For instance, you can only have blue eyes if both
alleles in the allele pair encode blue eyes.
Second filial generation: The grand children. Also called the F2 generation.
Sex chromosomes: The chromosomes that contain the genes that control the development of the sexes
in the young fetus. Humans have two different sex chromosomes (chromosome pair no. 23). XX in
chromosome pair 23 will result in a girl, while XY will result in a boy. All egg cells contain an X
chromosome, while half the sperm cells contain an X chromosome and the other half a Y chromosome.
When an egg cell and a sperm cell fuse to from a zygote, there is a fifty-fifty chance for a boy or a girl.
Sex-linked: Genetic traits that are determined by alleles located to the sex chromosomes are inherited
in a sex-linked way.
Trait: The specific form of the character. For instance, eye color is a character, while blue eyes is a
trait.
True breeding: When the same trait is inherited through many generations. The individual is
homozygous for this trait.
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