inheritance Flashcards
what is genetics?
study of the mechanism by which an organism inherits characteristics from its parents
what is monohybrid inheritance?
inheritance of a single characteristic controlled by one gene
what is mendel’s first law?
characteristics of an organism are determined by factors which occur in pairs
only one of a pair of factors can be present in a single gamete
what is an allele?
variant nucleotide sequence for a particular gene at a given locus which codes for an altered phenotype
what is a dominant or recessive allele?
dominant always expressed when present
recessive only expressed when dominant allele is absent
what is a genotype?
all the alleles that individuals contain
what is a phenotype?
appearance of an individual
how do you test if a dominant phenotype is heterozygous or homozygous?
cross it with homozygous recessive
if homozygous dominant then all offspring will have dominant phenotype
if heterozygous then half of the offspring will show recessive phenotype
what is pure breeding?
variety bred for several generations with itself so all individuals are homozygous (have same phenotype as parents)
what is co-dominance?
both alleles express themselves equally in heterozygote phenotype
shows phenotype intermediate
what is dihybrid inheritance?
simultaneous inheritance of 2 unlinked genes
genes located on different chromosomes
each gene will have 2 alleles
what is the general phenotypic relationship when crossing RrYy with RrYy?
9:3:3:1
what is incomplete dominance?
heterozygote phenotype is intermediate between that of 2 homozygotes
neither allele is completely dominant
what is mendel’s second law?
either one of a pair of contrasted characters may combine with either of another pair
each member of a pair of alleles may combine randomly with either of another pair on a different chromosome
what is the dihybrid test?
test a genotype by crossing it with an individual that is homozygous recessive for both genes
what are multiple alleles?
more than 2 varieties forms a given gene
what is a mutation?
a change in DNA
spontaneous random events that provide a source of material for natural selection and evolution
what must happen for a mutation to be inherited?
must effect DNA of the gametes
what are mutagens?
chemicals/radiation which increases rate of mutation
e.g. ionising radiation, chemicals
what is a carcinogen?
mutagen that causes cancer
what is chromosomal mutation?
large changes in chromosome structure or number
affects many genes
visible under light microscope
what is a gene mutation?
small changes in base sequence, only affects one gene
result of a fault in gene replication
how can substitution mutation affect genes?
maybe be silent if code is degenerate
may be neutral if amino acid altered is not essential to protein structure
what does deletion mutation cause?
more servere
causes ‘frame shift’
what does insertion mutation do?
more likely to result in non-functional protein
what is sickle cell anaemia?
single base substitution in gene for beta haemoglobin
causes quaternary structure of haemoglobin to change
becomes insoluble at low O2 tensions
tend to stick to other haemoglobin molecules
poor at binding to oxygen
homozygous is fatal
heterozygous O2 level reduced by 20%
what is down syndrome caused by?
non-disjunction of chromosome 21 during anaphase
oocytes have extra chromosome
what is polyploidy?
one gamete receives both sets of chromosomes
gamete is fertilised and zygote produces has 3 sets of chromosomes
what is cancer?
uncontrolled division of cells
produces tumor
what happens to malignant tumors?
cells may break off and be carried in blood to form secondary tumors
what can cause cancer?
mutation in gene which would normally control cell division
how does a mutated gene cause cancer?
tumor suppressor genes mutate so lack of proteins which would halt cell division
proto-oncogens mutate and produce oncogene which release excess proteins to stimulate cell division
mutations lead to lack of proteins involved in normal mechanism of cell death
what is the importance of mutations?
increases variation in population
most mutations are recessive to normal allele so must await replication in gene pool before expressed
most gene mutations occur in junk DNA so do not influence phenotype
what is sex linkage?
sex linked genes have loci on X chromosome
what is autosomal linkage?
dihybrid cross where both genes are located close together on the same chromosome
what is the importance of autosomal linkage?
2 genes on same chromosome so likely to be more common in parental combinations (linked)
what is epigenetics?
involves heritable changes in gene function without changes to base sequence of DNA
changes ability of gene to be transcriped
what happens during heritable changes to genes?
change occurs after replication
caused by variation in environment that alters gene expression
how can transcription be inihibited?
increased methylation of DNA
decreased acetylation of associated histones
what is DNA methylation?
enzyme can add methyl groups to DNA base sequence
bases are prevented from being recognised
reduces ability of gene to be expressed
where are methyl groups added to DNA?
cytosine nucleotide
what is gene silencing?
more methyl groups causes DNA to become more tightly wound around histones
transcription is inhibited
what is acetylation?
acetylation of histones leads to gene activation
histones get covered in chemical ‘tags’
tags accumulate through life
determine shapes of histones
increases acetylation causes histones to coil less tightly so increase transcription
what is epigenome (tags)?
reason why identical twins do not stay identical
what are histones?
proteins found in eukaryotic cells
package DNA and help it condense chromosomes
how are inactive genes made?
can be wrapped tightly and become inaccessible
epigentic silencing
