genetics

Question 1

what is a diploid ?

Answer 1

2 copies of each chromosome one from each parent

so you have 46 chromosomes and 23 pairs

Question 2

what is a haploid ?

Answer 2

one copy of each chromosome

Question 3

what do gametes produce during fertilisation ?

Answer 3

. gametes must be haploid to begin with

. so during fertilisation the 2 gametes produce a diploid zygote

Question 4

how are male gametes formed ?

Answer 4

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

Question 5

how are female gametes formed ?

Answer 5

oogenesis

produce haploid from cells that have a diploid

Question 6

what happens in the interphase of the cell cycle ?

Answer 6

1. in G0 - normal cell metabolism occurs - duration is variable
2. 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
3. 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
4. in G2 the enzymes needed for cell division are synthesised
   . division of nucleus
   the cell is ready for meiosis

Question 7

how are haploid gametes formed ?

Answer 7

formed by meiosis which consists of 2 division cycles

meiosis 1 and meiosis 2

Question 8

explain the steps that occur during meiosis 1 ?

Answer 8

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

Question 9

explain the steps that occur during meiosis 2 ?

Answer 9

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

Question 10

what is interkinesis ?

Answer 10

meiosis 1 and 2 are separated by interkinesis during which no DNA replication occurs

Question 11

how does meiosis introduce a lot of genetic variability ?

Answer 11

1. 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
2. genetic recombination during prophase 1
3. random fertilisation

Question 12

what is non-disjunction ?

Answer 12

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

Question 13

what happens if fertilisation occurs using a gamete with an extra chromosome ?

Answer 13

fertilisation using gamete with an extra chromosome will result in a zygote with 3 copies of that chromosome ( trisomy )

Question 14

what is example of trisomy ?

Answer 14

. down’s syndrome - 3 copies of chromosome 21
. trisomy 18 - Edward’s syndrome
trisomy 13 - patau’s syndrome

Question 15

example of non-disjunction in sex chromosome ?

Answer 15

. turner’s syndrome - one X chromosome
. metafemale syndrome - 3 X chromosome
. kleinfelter’s syndrome - XXY

Question 16

how is a trait determined ?

Answer 16

trait is determined by a single gene

Question 17

how is each gene presented ?

Answer 17

as chromosomes are paired , each gene is present as 2 alleles , alleles can be either dominant ( D ) or recessive (d)

Question 18

what id difference between homozygous and heterozygous ?

Answer 18

.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

Question 19

why is it wrong to call it a gene for disease ?

Answer 19

.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

Question 20

how are most diseases expressed ?

Answer 20

most diseases are recessive and are only expressed in the homozygous condition
e.g. cystic fibrosis

Question 21

what is intermediate inheritance ?

Answer 21

. 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

Question 22

what is example of disease that displays intermediate inheritance ?

Answer 22

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

Question 23

what is sex-linked inheritance ?

Answer 23

involves sex chromosomes

Question 24

what is the significance of having a longer x chromosome than y in males ?

Answer 24

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

Question 25

why is deutranopia more common in men ?

Answer 25

. 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

Question 26

what are some types of genes?

Answer 26

. 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

Question 27

what do most traits depend on ?

Answer 27

polygenic inheritance which occurs when one characteristic is controlled by two or more genes

Question 28

what is point mutation ?

Answer 28

.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

Question 29

what is frameshift mutation ?

Answer 29

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