4.3 Inheritance Flashcards
Define a gene
A sequence of DNA that codes for a polypeptide and occupies a specific locus on a chromosome
Define an allele
A variant nucleotide sequence for a particular gene at a given locus which codes for an altered phenotype
What is the test cross used for?
- identification of an unknown genotype by crossing with homozygous recessive
What is meant by co-dominance?
- neither allele is dominant and so both are expressed
—> ie speckled hen
What is meant by incomplete dominance?
- phenotype is a blend of the two parent traits
—> ie grey hen
Define locus
Position of a gene on a chromosome
What is the ratio for dihybrid inheritance?
9:3:3:1
What is the ratio for monohybrid inheritance
3:1
What is meant by linked genes?
- pairs of genes that tend to be inherited together
- same chromosome so don’t segregate independently at meiosis
Cross over value equation
number of recombinants
———————————— x100
number of offspring
What is the null hypothesis?
That there is no significant difference between the over served and expected results
What assumptions could be the cause of any deviation between observed and expected results?
Inheritance assumptions:
- number of different types of gamete is equal
- probability of each gamete fusing with another gamete is equal
- viability of embryos is not impacted by the genotype
- genes are not linked
How do you calculate degrees of freedom in chi squared?
Number of categories - 1
Monoecious plants
Separate male and female flowers on the same plant
Diecious plants
Separate male and female plants
What are hermaphrodites?
Organisms with male and female sex organs
What are some factors that determine sex?
- temperature (reptiles)
- sequential hermaphroditism
—> male and female but not at the same time - ploidy level
—> males can be haploid whilst females diploid - chromosome structure
—> hetero or homo
Differentiate between heterogametic and homogametic
Hetero: XY
Homo: XX
What is meant by an autosome?
Matching genes that have a homologous pair and are not sex chromosomes
What is meant by a heterosome?
Sex chromosome
What gene does the Y chromosome have that X doesn’t, and what is its role?
- SRY gene (sex determining region)
- switch on genes on other chromosomes which are responsible for expression of male characteristics
What is PAR1 and PAR2?
- human pseudo autosomal region
- behave like autosomal
- regions of homology in X and Y chromosomes
Define sex linkage
A gene is carried by a sex chromosome so that a characteristic it encodes is seen predominantly in one sex
Define a carrier
A phenotypically normal individual with one normal dominant allele and one mutant recessive allele
Describe sex inheritance of DMD
- X-linked recessive allele of the dystrophin gene
- carrier female and normal male = sons 50% affected and 50% unaffected, daughters 50% unaffected and 50% carrier
- affected males cannot pass their mutant allele onto their sons who will all be unaffected
- if mother is unaffected, daughters will all be carriers
- for female to be affected she must inherit 2x Xd (mutant)
Draw the cross of carrier female and normal male of DMD
Describe haemophilia
- gene on X chromosome
- individual cannot produce enough clotting proteins
- normal is XH and mutant is Xh
Possible genotypes for males and females (haemophilia)
Females
- XHXH - unaffected
- XHXh - carrier
- XhXh - haemophilic
Males
- XHY - unaffected
- XhY - haemophilic
Define a mutation
A change in the amount, arrangement or structure in the hereditary material of an organism
—> spontaneous and random
Why are mutations described as random?
Happen with equal probability anywhere in the genome of diploid organisms
What mutations are inherited?
Ones that occur in gametes
How can mutation rates be increased?
- ionising radiation
—> joins adjacent Pyrimidine bases in a DNA strand so that at replication, DNA polymerase may insert an incorrect nucleotide - mutagenic chemicals
—> have flat molecules which slide between base pairs in a double helix and prevent DNA polymerase inserting the correct nucleotide at replication
Describe gene or point mutation
- DNA is not copied accurately in S phase before cell division
- involve one or small number of bases
Describe chromosome mutation
- chromosomes may get damaged or break
- broken chromosomes may repaired themselves and the DNA and protein rejoin, but they can repair incorrectly which alters structure and potentially a lot of genes
Describe aneuploidy
- whole chromosome or small number of chromosomes may be lost or added in non-disjunction
—> chromosomes fail to separate to the poles of dividing cells at anaphase I or fail to separate at anaphase II
Describe polyploidy
- number of chromosomes may double if cell fails to divide following the first nuclear division after fertilisation
What does it mean if DNA polymerase changes?
A point or gene mutation occurs
Describe the types of point or gene mutations
- addition - a base is added
- duplication - the same base is incorporated twice
- subtraction - a base is deleted
- inversion - adjacent bases in the same DNA strand exchange position
Describe how point mutation can influence the polypeptide produced
- new codon may code for same amino acid so no change - silent mutation
- if amino acid with similar chemical nature is substituted then the effect may be small
- if mutation is at significant site on the protein molecule it may make a significant difference to the activity of the protein ie enzymes
- if one or two bases are added/deleted, a frame shift mutation occurs and all subsequent amino acids incorporated will be altered
Describe and explain sickle cell disease
- substitution in the gene producing the beta polypeptide of haemoglobin
- CTC (glutamate) swapped for CTC (valine)
- side chains differ in size and hydrophobic/hydrophilic properties
- when oxygen tension is low, the affected Hb within the red blood cell aggregates and the cell membrane collapses, causing the blood cell to become sickle shaped
- the cells become fragile and may break in the capillaries
Genotype of those with sickle cell anaemia
HbS HbS - severe
HbA HbS - symptoms but less severe
Define endomitosis
Chromosome replication not followed by cytokinesis
What is meant by non-disjunction?
- a faulty cell division in meiosis following which one of the daughter cells receives two copies of a chromosome and the other receives none
How does Down syndrome occur?
Non-disjunction
What is apoptosis?
Programmed cell death
What are tumour suppressor genes?
- genes that regulate mitosis and prevent cells dividing too quickly
- growth inhibiting proteins
Describe the functions of normal p53
- activates repair of damaged DNA
- prevents cells from entering S phase while damage DNA is repaired
- initiates apoptosis if damaged DNA cannot be repaired
Describe the role of mutant p53
- no DNA repair
- cell with damaged DNA enters S phase and DNA is replicated
- mutant cells survive and undergo mitosis
What is the role of proto-oncogenes and how do they become oncogenes?
- contributes to cell division
- mutation may switch gene so that the protein is made excessively, leading to rapid and uncontrolled division which causes cancer
How may a proto-oncogene become activated?
- mutation causes chromosomes to rearrange and places the proto-oncogene next to a DNA sequence that permanently activates it
- extra copy of proto-oncogene resulting in excessive mitosis
Define epigenetics
The control of gene expression by modifying DNA or histones, but not by affecting the DNA nucleotide sequence
Describe DNA methylation and its impact
- cytosine can have a methyl or hydroxymethyl group added
- methylated cytosine can be read as cytosine, pairing with guanine at transcription
- if regions of DNA are heavily methylated, they are less likely to be transcribed
Describe histone modification and the impact
- add an acetyl group, methyl group or phosphate group
- alter the interaction with DNA which changes the arrangement of nucleosomes
- when unmodified, nucleosomes pack tightly so DNA is less susceptible to enzymes so transcription is reduced
- when modified, coiling is relaxed and transcription factors and RNA polymerase have access to the DNA, so transcription is increased
What are the consequences of epigenetic changes
- genomic imprinting
—> gene may be permanently switched off by DNA methylation on the chromosome derived from one parent
—> generational impact that could cause medical issues - X inactivation
—> can switch off a whole chromosome
—> becomes a mass of densely staining chromatin called the Barr body - implicated in autoimmune conditions, mental illness, diabetes and many cancers
What causes common phenotypes?
Linked genes
What causes rare phenotypes?
Crossing over at chiasmata
