inheritance Flashcards
1
Q
gene
A
a sequence of DNA occupying a specific locus on a chromosome normally coding for a specific polypeptide
2
Q
allele
A
a different form of the same gene coding for a specific polypeptide
3
Q
homozygous
A
where both alleles for a given characteristic are the same
4
Q
heterozygous
A
where both alleles for a given characteristic are different
5
Q
dominant
A
alllels are always expressed ie in both homozygote and heterozygot RR Rr
6
Q
recessive
A
alleles are only expressed in homozygote rr
7
Q
What is a gene
A
- sequence of DNA on a chromosome normally coding for a specific polypeptide which occupies a specific position or locus
8
Q
What do genes normally exist as
A
two or more alleles
9
Q
Give an example where genes normally exist as two or more alleles
A
- rhesus blood group (positive or negative)
- ABO blood group three alleles A,B and O
10
Q
what is a genotype of an organism
A
genetic make up ie the actual alleles it possesses
11
Q
what is the phenotype
A
- characteristics of an organism
- product of its genotype and the environment
12
Q
What are the rules for presenting a genetic cross between two organism
A
- Choose a single letter to represent each characteristic, e.g. R.
- Use upper case letters to represent dominant features (R), lower case for
recessive (r), and state what they represent. - Clearly label PARENTS and circle GAMETES.
- Use a matrix called a Punnett square to calculate crossing.
- State the phenotype and ratios of offspring. First generation is represented
by F1, second by using F2.
- Use upper case letters to represent dominant features (R), lower case for
- Choose a single letter to represent each characteristic, e.g. R.
13
Q
What does monohybrid inheritance involve
A
inheritance of a single gene
14
Q
what is the law of segrragation
A
‘The characteristics of an organism are determined
by factors (alleles) which occur in pairs. Only one of a pair of factors (alleles) can be present in a single gamete.’
15
Q
Why is a test cross used and what does it involve
A
- A test cross is performed to show if a dominant characteristic is determined by one or two dominant alleles,
- i.e. PP or Pp, and involves crossing the
organism with the homozygous recessive.
16
Q
what does co dominance involve
A
In co-dominance both alleles involved are dominant and therefore both are
expressed equally.
17
Q
what is an example of co dominance
A
- found in ABO blood group where A and B are co dominant
18
Q
what is incomplete dominance
A
- when both alleles are present and phenotype is an intermediate rather than both alleles being expressed
19
Q
what is dihybrid inheritance
A
The simultaneous inheritance of two
unlinked genes (genes on different chromosomes)
20
Q
What is the law of independent assortment
A
‘Each member of an allelic pair may combine randomly with either of another pair’.
21
Q
what is autosomal linkage
A
This occurs when two different genes are found on the same chromosome
and therefore cannot segregate independently (because they are on the same
chromosome).
22
Q
What does autosomal linkage apply to
A
autosomes - chromosomes other than sex chromosomes
23
Q
what is the role of autosomal linkage
A
- fewer combination of gametes
- four possible gametes not made in equal proportions
24
Q
How are alleles arranged when autosomal linkage occurs
A
- dominant alleles on one chromosome
- recessive allels on the other chromosome
25
When does crossing over occur and what happens when it does
* if the two genes are not too cloese together
* randmom event so does not always occur on every chromosome pair
* if crossing over does occur the resulting gametes would be one dominant and one recessive
26
what significance level is used in chi squared tests
0.05
27
What is the null hypothesis
no difference between observed and expected
28
what happens if calculated value exceds critical calue
* there would be a signi cant difference between the observed and expected results, so you can reject your null hypothesis, as any differences seen were not due to chance at p = 0.05.
* mendel law do not apply
29
what happens if calculated vaue is less than critical value
there is NOT a signi cant difference between observed and expected results, you
must accept your null hypothesis, as any differences seen were due to chance
at p = 0.05. (Mendel’s laws apply.)
30
how do u calculate degrees of freedom
number of phenotypes - 1
31
How many chromosomes do humans have
46 - 23 pairs
32
How are chromosomes arranged in humans
* 23 pairs
* 22 pairs match - contain the same genes ( not nesscarily the same allels so homologous and autosomes)
* 23rd pair or sex chromosomes
33
What are the sex chromosomes in humans and what is the change of having a male or female offspring
* females - XX
* males - XY
* 50:50 chance offspring male or female
34
what is sex linkage
* Sex linkage is when a gene is carried by a sex chromosome so that a characteristic it encodes is seen predominately in one sex.
35
describe sex linkage in males
As males contain one X and one Y chromosome, the Y must come from his father, the X from
his mother. The Y chromosome is smaller than X, and there are some genes that are only carried on the X chromosome so males only receive one copy.
36
what is haemophilia
* gene for clotting factor VIII
* needed to clot blod for the following injury
* carried on X chromosomes
* recessive sex linked X chromosome disorder
37
Describe which gender more likely to have haemophilia and why
* As males contain
one X and one Y chromosome, the Y must come from his father, the X from
his mother. The Y chromosome is smaller than X, and there are some genes that are only carried on the X chromosome that might carry a normal allele
38
describe the role of females in haemophilia
* Females are almost exclusively carriers of the disorder as they may inherit the defective
allele from their mother or father, or as a result of a new mutation. Only under
rare circumstances do females actually have haemophilia as they would need to
inherit the faulty allele from both parents (homozygous recessive).
39
What is duchenne muscular dystrophy DMD
* x linked recessive allele
* involves gene codes for dystrophin a component of glycoprotein that stabilises the cell membranes of muscle fibres
* progressive muscle loss and weakness
40
what are the alleles for DMD and where are they passed from
* the defective allele is passed to
children from their mothers (as boys receive the Y chromosome from their father).
XD is normal dystrophin allele
Xd is allele coding for faulty dystrophin resulting in DMD.
The outcome of a carrier female having children with a normal male is shown
41
what are pedigree diagrams
These are family trees that show the instances of a particular inherited
condition within a family.
42
what is a mutation
A mutation is a change in the amount, arrangement or structure of the DNA
in an organism.
43
what are the two types of mutation
* Gene or point mutations, e.g. sickle cell disease, caused during DNA
replication.
* Chromosome mutations cause changes in the structure or number of
whole chromosomes in cells. They result in an entire chromosome being
added or lost (called aneuploidy, e.g. trisomy 21 or Down’s syndrome), or
when one chromosome breaks off and attaches to another chromosome
called translocation, e.g. translocation Down’s.
44
When do mutations occur
* can be spontaneous
* random events
* occurring with equal probability anywhere in the genome of diploid organisms
45
What may mutations contribute to and why
* evolution
* provides advantages which can be selected for
46
What are the rates of mutation and how are they increased
* mutations rates are very low
* ionising radiation
* x rays
* polycyclic hydrocarbons in tabacco smoke
* some chemicals eg benzene
47
when are mutations common
* organisms which hve short life cycles eg bacteria
48
when do mutations occur in the cell cycle
* during crossing over in prophase I
* and non disjunction in anaphase I and anaphase II of meiosis
49
when are mutations non heritable
mutations that occur in somatic (body) cells
50
How do point mutations occur
■ Addition or subtraction: where a base is added/deleted. Both result in a
frame shift, where the reading frame moves one place and usually results in
a non-functional protein as the amino acid sequence changes signi cantly.
■ Substitution: where one base is ‘swapped’ for another, which may result in
a change of codon, and hence amino acid.
51
Why do point mutations may not have any effects
■ Is silent, i.e. the base changes but the amino acid for which the codon codes
does not.
■ May be in a non-coding region or intron.
■ May be in a recessive allele and so is not expressed.
■ Alters an amino acid but may not result in a change to the functioning of
the protein.
52
What is an example of point mutations
* An example of a point mutation is sickle cell disease, which is common in
Afro-Caribbean, Middle Eastern, Eastern Mediterranean and Asian populations
* that evolved in malarial habitats.
53
How does sickle cell disease come about and
* In sickle cell, a mutation involves a
substitution of adenine for thymine which does result in the change of one
amino acid to valine
* causing red cells to deform and block capillaries under low
partial pressures of oxygen.
* The alleles for normal and affected haemoglobin are co-dominant, so people who have one copy of the faulty allele do have
symptoms, but not as severe as sufferers,
54
What is an advantage of sickle cell disease
increased resistance to
malaria which is an example of heterozygote advantage.
54
What is down syndrome
* Down’s syndrome is a chromosomal disorder occurring in around 1 in 800
births
* results when a person inherits all or part of an extra copy of chromosome 21.
54
How does downs syndrome come about
* This occurs most commonly following non-disjunction of chromosome 21 during anaphase 1 or 2 of meiosis, where both copies of chromosome 21 enter the gamete.
* When this is then fertilised by a normal
gamete, three copies of chromosome 21 result (trisomy 21).
* The oocyte containing no copies of chromosome 21 fails to develop further
55
What is the risk of downs syndrome related to
Down’s syndrome is related to the mother’s age:
18 yr old = 1 in 2100 births
30 yr old = 1 in 1000 births
40 yr old = 1 in 100 births
56
is there a treatment to downs syndrome
There is no treatment, but it can be diagnosed by prenatal tests, e.g.
amniocentesis.
57
What causes cancer
a mutagen known as a carcinogen
58
How does cancer form
* Cancer results from mutations in protooncogenes producing oncogenes that cause the rate of cell division to increase either by resulting in the production of excess growth factor
* or by mutated receptor proteins that do not require growth factor to
initiate cell division.
59
How is cell division slowed
* tumour suppressor genes
* anti oncogenes
60
How is cell division increased
* mutated suppressor gene
* gene is inactivated
61
epigenetics
* is the control of gene expression by
modifying DNA or histones, without modification of the base sequence.
62
How is variation with a population created
* allelic variations
* environment
63
how do epigenetic modifications occur
* result of environmental condisiotns
* causes changes to how genes are transcribed
64
What is the result of visible differences seen in identiacal twins
epigenetic modfications
65
What are epigenetic modifications caused by and explain each point
* DNA methylation, which involves the addition of a methyl or hydroxymethyl group to cytosine, reducing transcription of the gene.
* Histone modification post-translation, resulting in changes to the way
the histones interact with DNA by causing a looser arrangement of the
nucleosomes
* This causes increased transcription because RNA polymerase and other transcription factors have easier access to the DNA
* histonemodification can involve the addition of an acetyl or methyl group to
lysine, and arginine or phosphate groups to serine and threonine.
66
Describe the link between stem cells and epigenetic changes
* As stem cells differentiate, epigenetic changes result in the production
of different proteins, e.g. melanin within skin cells, and amylase within
pancreatic cells.
67
how can some epigenetic modifications be heritable and give an example
* if changes occur in gametes which is referred to as genomic imprinting, and whole
chromosomes can be inactivated,
* e.g. X chromosome inactivation which
results in the patchwork of tortoiseshell cats.
