genetics Flashcards
what is a diploid ?
2 copies of each chromosome one from each parent
so you have 46 chromosomes and 23 pairs
what is a haploid ?
one copy of each chromosome
what do gametes produce during fertilisation ?
. gametes must be haploid to begin with
. so during fertilisation the 2 gametes produce a diploid zygote
how are male gametes formed ?
spermatogenesis
where we take cells that have a full complement of genetic material and create sperm cells which have half haploid complement of genetic material
how are female gametes formed ?
oogenesis
produce haploid from cells that have a diploid
what happens in the interphase of the cell cycle ?
- in G0 - normal cell metabolism occurs - duration is variable
- in G1 - prepares the cell for replication by the multiplication of organelles
take 8 hours or more
e.g. centrioles replicate during this phase of the cell cycle - in S phase DNA replicates
the 2 stands of DNA unwind and DNA polymerase moves along the exposed chains adding complimentary nucleotides from a pool within the nucleoplasm , forming 2 identical strands of DNA, histone synthesis also occurs
- takes about 6-8 hrs - in G2 the enzymes needed for cell division are synthesised
. division of nucleus
the cell is ready for meiosis
how are haploid gametes formed ?
formed by meiosis which consists of 2 division cycles
meiosis 1 and meiosis 2
explain the steps that occur during meiosis 1 ?
prophase 1 -
. chromosomes condense
. spindle forms
. homologous chromosomes seek each other ( synapsis )
forming tetrads- matching allele’s of each chromosome are laying on top of each other
. tetrads ( pair of chromosomes )- perform crossing over thus sharing genetic material
metaphase 1 - tetrads line up equator
metaphase runs between chromosome within tetrad
anaphase 1 - tetrads split and each half of cell receives different DNA
cis-chromatid remain together
telophase 1 - nuclear membrane reforms and cytokinesis occurs
2 cells form each cell has haploid amount of genetic material -there is double DNA in each cell
explain the steps that occur during meiosis 2 ?
just like mitosis
prophase 2 - nuclear membrane dissipates and spindle form
metaphase 2 - chromosomes align themselves on equator
anaphase 2 - centromeres break and chromosomes move to opposite ends of cells
telophase 2 and cytokinesis - spindle breaks down and nuclear membrane reforms
you end up with 4 haploid daughter cell each has 23 chromosomes
what is interkinesis ?
meiosis 1 and 2 are separated by interkinesis during which no DNA replication occurs
how does meiosis introduce a lot of genetic variability ?
- independent assortment during metaphase 1-
tetrads line up along metaphase plate you can many different combination of chromosomes from maternal and paternal line lining up along metaphase plate - over 8 million possible combination of genetic material - genetic recombination during prophase 1
- random fertilisation
what is non-disjunction ?
non-disjunction can occur during meiosis 1 resulting in one daughter cell having an extra chromosome and the other one less, pair of chromosomes don’t split up
at the end of meiosis 2 therefore some daughter cells will not contain a particular chromosome, while others will have an extra copy
what happens if fertilisation occurs using a gamete with an extra chromosome ?
fertilisation using gamete with an extra chromosome will result in a zygote with 3 copies of that chromosome ( trisomy )
what is example of trisomy ?
. down’s syndrome - 3 copies of chromosome 21
. trisomy 18 - Edward’s syndrome
trisomy 13 - patau’s syndrome
example of non-disjunction in sex chromosome ?
. turner’s syndrome - one X chromosome
. metafemale syndrome - 3 X chromosome
. kleinfelter’s syndrome - XXY
how is a trait determined ?
trait is determined by a single gene
how is each gene presented ?
as chromosomes are paired , each gene is present as 2 alleles , alleles can be either dominant ( D ) or recessive (d)
what id difference between homozygous and heterozygous ?
.when both alleles are same ( DD or dd ) , the individual is homozygous for that trait
.when two alleles are different (Dd) the individual is heterozygous
why is it wrong to call it a gene for disease ?
.because its a gene with a beneficial function that has mutated
. some genes that cause disease may also have some other beneficial function
e.g. gene that causes sickle cell anaemia also protects against malaria
how are most diseases expressed ?
most diseases are recessive and are only expressed in the homozygous condition
e.g. cystic fibrosis
what is intermediate inheritance ?
. alleles from each parent blend together to form a mixed phenotype in the offspring., heterozygous show phenotype which is somewhere between healthy and complete disease
what is example of disease that displays intermediate inheritance ?
sickle cell anaemia - here a single amino acid is one of the haemoglobin chain is abnormal
SS - homozygous dominant are completely normal
ss - homozygous recessive have severe respiratory problems as none of their haemoglobin can carry enough oxygen
Ss - heterozygous individuals produce both normal and sickled haemoglobin. they are normally symptomless, but in times of increased respiratory demand they have problems
what is sex-linked inheritance ?
involves sex chromosomes
what is the significance of having a longer x chromosome than y in males ?
X linked traits are coded for on the bit of X chromosome for which there is no corresponding piece of Y chromosomes.
one trait inherited this way is possession of a green-sensitive visual pigment
if allele on the X chromosome is dominant G we have the pigment and our colour vision is normal , if it is recessive g we do not and are red/green colour blind deutranopia
why is deutranopia more common in men ?
. heterozygous female will be a carrier for deutanopia but will have normal colour vision as she has one copy of the dominant gene ensuring she has the green pigment
. for a female to be colour blind both her alleles need to be recessive
. a male will express the trait even if he only has one copy of the recessive allele as he has no corresponding allele on the shorter Y chromosome
what are some types of genes?
. some genes are only expressed in certain environmental condition ( freckles and sunlight )
thus both nature ( genotype) and nurture ( environment ) determine the phenotype
. some genes have a controlling function , turning other genes on and off
what do most traits depend on ?
polygenic inheritance which occurs when one characteristic is controlled by two or more genes
what is point mutation ?
.A point mutation is when a single base pair is altered
which occurs by a simple mistake during DNA replication in meiosis
. sometimes such mutations can have severe consequences i.e.sickle cell anaemia
what is frameshift mutation ?
A frameshift mutation is a genetic mutation caused by a deletion or insertion in a DNA sequence that shifts the way the sequence is read.
. this is more serious
