Problem 1: The birds and the bees
Shaffer
Genetic material
- Zygote = a single cell formed at conception from the union of a sperm and ovum
- Chromosome = a threadlike structure made up of genes; in humans there are 46
chromosomes in the nucleus of each body cell.
- Genes = hereditary blueprints for development that are transmitted unchanged from
generation to generation.
- DNA = long, double-stranded molecules that make up chromosomes.
Mitosis and meiosis
1. Mitosis = the process in which a cell duplicates its chromosomes and then divides
into two genetically identical daughter cells.
- Zygote moves through the fallopian tube toward uterus, begins to replicate itself
through mitosis.
- Makes up: muscles, bones, organs, and other bodily structures.
- Generates new cells that enable growth and replacing old ones that are damaged.
- Process:
1) The cell duplicates its 46 chromosomes. These duplicate sets move in opposite
directions.
2) Division of cell proceeds: result = two new cells, each with the identical 23 pairs of
chromosomes (46 in all): thus, the same genetic material as the original cell.
3) With each division, the chromosomes are duplicated > every new cell contains an
exact copy of the 46 chromosomes we inherited at conception.
, - Meiosis = the process in which a germ cell divides, producing gametes (sperm or
ova) that each contain half of the parent cell’s original complement of chromosomes;
in humans, the products of meiosis contain 23 chromosomes.
- Germ cells: produce gametes (sperm and ova) with a special hereditary function.
- Creates a unique cell that will become an unique individual.
- Meiosis: produces germ cells.
- Crossing-over = a process in which genetic material is exchanged between pairs of
chromosomes during meiosis.
- Process:
1) germ cell first duplicates its 46 chromosomes
2) crossing-over: adjacent duplicated chromosomes cross and break at one or more
points along their length, exchanging segments of genetic material.
3) pairs of duplicated chromosomes segregate into two new cells that each contains
46 chromosomes.
4) The new cells divide: result = each of the resulting gametes contains 23 single,
unpaired, chromosomes.
(5) at conception: sperm cell with 23 chromosomes unites with an ovum with 23
chromosomes > producing a zygote with a full set of 46 chromosomes.
Meiosis
Hereditary uniqueness
- When a pair of chromosomes segregates during meiosis: it's a matter of chance
which of the two chromosomes will end up in a particular new cell.
, - Independent assortment = the principle stating that each pair of chromosomes
segregates independently of all other chromosome pairs during meiosis:
- There are many different combinations of chromosomes that could result from
the meiosis of a single germ cell.
- Odds of exact genetic replication very small because of crossing-over process:
- the crossing-over process alters the genetic composition of chromosomes,
and thereby increases the number of possible variations in an individual’s
gametes.
Twins
- Monozygotic (identical) twins = twins who develop from a single zygote that later
divides to form two genetically identical individuals.
- Dizygotic (fraternal) twins = twins that result when a mother releases two ova at
roughly the same time and each is fertilized by a different sperm, producing two
zygotes that are genetically different.
Male/female
- Male: X chromosome (elongated body) and Y chromosome (stubby and short)
- Female: XX
Function of genes
- Promote development; regulate production
- e.g. of a pigment called melanin in the iris of the eye: people with brown eyes
have genes that call for much of this pigment.
- Guide cell differentiation;
- making some cells parts of the brain and central nervous system, and other
parts of the circulatory system, bones, skin, etc.
- The pace and timing of development;
- specific genes are “turned on” or “turned off” by other regulatory genes at
different points in the life span.
- E.g. regulatory genes “turn on” the genes responsible for growth in
adolescence, and then shut these down in adulthood.
- Environmental factors clearly influence how genes function;
- E.g. nutrition: an individual having the genetic potential for becoming tall, but
due to malnutrition, he could end up being only average/below average.
- SO: environmental influences combine with genetic influences to determine
how a genotype is translated into a particular phenotype.
Different levels at which the environment affects the actions of genes:
- Environment within the nucleus (= contains the chromosomes and genes) may affect
the expression of genetic material.
- Internal environment that surrounds the cell may affect the gene’s expression.
- External environment affects the expression of genetic material (nutrition
example)
, Single-Gene inheritance patterns
= About traits that are influenced by a single pair of alleles.
Simple Dominant-Recessive Inheritance
Simple dominant-recessive = a pattern of inheritance in which one allele dominates
another so that only its phenotype is expressed.
- Alleles = alternative forms of a gene that can appear at a particular site on a
chromosome.
- Dominant allele = a relatively powerful gene that is expressed phenotypically and
masks the effect of a less powerful gene.
- Recessive allele = a less powerful gene that is not expressed phenotypically when
paired with a dominant allele.
- Homozygous = having inherited two alleles for an attribute that are identical in their
effects.
- AA or aa
- Heterozygous = having inherited two alleles for an attribute that have different
effects.
- Aa
- Carrier = a heterozygous individual who displays no sign of a recessive allele in his
or her own phenotype but can pass this gene to offspring (Aa).
- Either/or possibilities for phenotype: e.g. colour blind OR not colour blind.
Codominance
Codominance = condition in which two heterozygous but equally powerful alleles produce a
phenotype in which both genes are fully and equally expressed.
- The produced phenotype is a compromise between the two genes.
- E.g.: human blood types: type A and B are equally expressive, and neither
dominates the other. A heterozygous person who inherits an allele for blood
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