17.1. Sexual Reproduction Flashcards
Sexual reproduction…
Offspring are produced from two genetically different parents.
Why sexual reproduction…
Costs:
- Invest in and maintain the molecular machinery used to do sexual reproduction.
- There is a need to find a mating partner.
- Decreased reproductive capacity (compared to bacteria, who can exponentially increase).
Benefits:
- High genetic variability helps a species to adapt to environmental changes (aiding evolution).
- Brings together advantageous alleles.
- Paired chromosomes allow deleterious mutations to be masked.
Sex determination…
Genetic process, determined by X and Y chromosomes.
The SRY (sex-determining region Y) is a key gene on the Y chromosome that encodes for a transcription factor that induces male testicular development.
Induces expression of the Sox9 transcription factor that drives male development.
XX male: De la Chapelle syndrome. Male develops a female phenotype.
XY gonadal dysgenesis / Swyer syndrome. Female develops a male phenotype.
Environmental sex determination…
Turtle genders are determined based on the temperature.
At higher temperatures, more females are produced.
Diplontic…
Meiosis to produce the gametes.
Fertilisation produces a diploid zygote.
From this, a multicellular organism develops.
Haplontic…
Fertilisation of gametes produces a diploid zygote.
This immediately undergoes meiosis to produce haploid spores.
These haploid spores give rise to a haploid organism.
Alternation of generations…
Used by plants (i.e. fern).
Involves two multicellular stages.
After fertilisation, the zygote produces the phorophyte, the first multicellular stage.
Cells within this undergo meiosis to produce haploid spores, which give rise to a second multicellular stage.
Meiosis I…
Chromatin in the nucleus condenses (chromosomes are visible under a light microscope).
Synapsis aligns homolog and chromosomes condense further (involves homologous chromosomes pairing by adhesion along their lengths).
The chromosomes continue to coil and become shorter. Crossing over occurs which results in a transfer of genetic material. The nuclear membrane also breaks down.
The homologous pairs line up on the metaphase plate.
Homologous chromosomes move to opposite poles of the cell.
The chromosomes gather into nuclei and the original cell divides.
Meiosis II…
The chromosomes condense again.
The centromeres of the paired chromatids line up along the metaphase plate of each cell.
The chromatids finally separate, becoming chromosomes, which are pulled to opposite sides.
Each new cell has a different genetic makeup, due to crossing over in meiosis I.
The chromosomes gather into nuclei again, and the cells divide.
Each of the four cells have a nucleus with a haploid number of chromosomes.
Meiosis and Mendelian inheritance…
Meiosis explains Mendel’s law of inheritance.
The homologous chromosomes line up at the metaphase plate in random orientations and segregate independently of each other, explaining the law of independent assortment.
Genes that are located on the same chromosome do not follow the law of independent assortment, as they are linked.
However, absolute linkage between two genes on the same chromosome is rarely observed.
Chromatid exchange…
During meiosis I, when homologous chromosomes are aligned, chromosomes frequently exchange parts of their DNA.
This is called crossing over and leads to recombinant chromatids that contain a mix of DNA from both homologues.
Crossing over further increases genetic variation of the gametes.
Crossing over…
Crossing over results in incomplete linkage of genes on the same chromosome, which means that alleles on the same chromosome are not always inherited together.
This results in offspring with recombinant phenotypes.
We can calculate the recombinant frequency (number of recombinant offspring / total number of offspring).
Genetic maps…
Allow us to understand which genes are located together on chromosomes.
Allow us to roughly locate them on chromosomes.
This is now not used, as modern sequence technologies allow us to pinpoint genes in DNA.
X-linked inheritance…
Does not follow the usual Mendellian inheritance rules.
Males are hemizygous for genes on the X chromosome (one copy).
Females inherit one X chromosome from each parent, therefore they have both genes on the X chromosome.
The inheritance of colour blindness occurs like this:
- As females have two copies of the X chromosome, they may be carriers and do not know.
- As males have one copy, they are homozygous and will be colour blind.
Aneuploidy…
An abnormal number of chromosomes in an organism.
Can arise due to nondisjunction (failure to separate homologous chromosomes during meiosis).
Autosomal chromosome aneuploidy:
- Down syndrome (trisomy of chromosome 21).
- Edwards syndrome (trisomy of chromosome 18).
- Patau syndrome (trisomy of chromosome 13).
