the same phenotype can result from different genotypes (AA and Aa) that have one copy of the dominant allele (A)
Gregor Mendel
“Father of genetics”
Discovered basic laws of eukaryote genetics by following traits from crosses of plants
People thought that he might’ve made up the data, but this was proven wrong
Genetic Inheritance
It was known that both parents contribute traits
to offspring
Before Mendel’s time it was believed that the units of inheritance blended and could never be separated
Mendel proposed that:
Units of inheritance (genes) are particulate
There are two copies (alleles) of each gene in every parent
One of those alleles is in each sperm or egg
Mendel’s first law of segregation
During gamete formation, one allele for a trait is present in egg or sperm (at a particular locus) and each allele has an equal chance of appearing in offspring
Mendel’s Second Law of independent assortment
From studies of the simultaneous inheritance of two characters, Mendel concluded that alleles of different genes (loci) assort independently
Determining dominance
Mendel could determine if a plant with the dominant
phenotype was homozygous or heterozygous by backcrossing with the recessive
phenotype
Mitosis
producing genetically exact copies
Meiosis
producing genetically different haploid descendent cells
prophase
chromosomes condense and move towards the middle of the cell
Metaphase
chromosomes move to the equatorial plate, then centromeres holding the chromatid pairs together separate
Anaphase
each daughter chromosome migrates to its pole along the spindle fiber/microtubule track
Telophase
the chromosome becomes less condensed. The nuclear envelope and nucleolireform, there is cytokinesis (cell division) producing two identical cells
Chiasma
the point of contact between paired chromatids during meiosis, resulting in a cross-shaped configuration and representing the cytological manifestation of crossing over like the letter X
crossing over
Occurs during prophase during meiosis 1
Crossing over allows for missing genes among homologous (parental) chromosomes
Meiosis 1: the first division
Prophase 1 – homologous chromosomes pair (forming
a tetrad)
Chromatid segments are exchanged by crossing over (at chiasma)
New chromosomes are created
Important Results of Crossing Over
Crossing over breaks up linked loci allowing
independent assortment within a chromosome
It also allows beneficial alleles on different homologs to be on the same chromosome
Crossing over allows the production of chromosomes without deleterious mutations
Crossing Over in Meiosis I
It’s a common process and it produces recombinant chromosomes that combine genes from the mother and father to form new chromosomes
Meiosis II: The reduction division
The sister chromatids separate
No DNA replication precedes this second division
resulting in haploid daughter cells
In sperm production there are 4 daughter cells.
In egg production there is a single cell with polar bodies
Meiosis I gives rise to two lines of cells that then go
through Meiosis II. There is no duplication of DNA between these divisions
Meiosis: two aspects to genetic recombination that areimportant in generating new sexual genotypes
1)The random selection of maternally and
paternally derived homologs contributes to genetically variable daughter cells
2) Crossing over produces novel (recombined)
chromosome types
Meiotic errors – nondisjuction
Sometimes one member of a homologous pair of
chromosomes fails to separate and both go to the same pole