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Genetics Topic 4 & 10
Terms to know – use the syllabus first;
Homozygous
crossing over
Heterozygous
multiple alleles
Genotype
haploid, diploid
Phenotype
2n, 1n or n, F1 , F2, P1
Dominant
co dominance
Recessive
test cross
Pure breeding/true breeding
sex-linked
Hybrid
monohybrid
Genome
autosome and somatic
Locus (loci – plural)
karyotype
Mendel’s Law of Segregation and Independent assortment
HL
Chiasma
polyploidy
linked genes
polygenic and pleotrophic
Only reproductive organs use meiosis to produce gametes that fuse to form a zygote.
Each gamete is haploid. Produced after puberty.
Remind – asexual includes budding, binary fusion, self-fertilization.
Genetics – deals with genes and their transmission. The father of genetics is Gregor
Mendel
Gene – an inheritable factor that is composed of DNA. It controls a specific
characteristic by coding (containing the instructions) for building a polypeptide or
protein.
Remember, proteins make up everything including hormones, enzymes etc. If you
compare to words and sentences, a gene is the smallest set of letters needed to make
a recognizable word – some words have a lot of letters, others have few.
Allele – a specific form of a gene, it occupies the same gene locus on the chromosome as
other alleles for one gene, alleles differ from other alleles of other genes by one or more
base pairs.
Mendel a monk in the 1800’s. Overcame financial hardships and serious illness to
excel in high school and received agricultural training. Was eventually assigned as
supply teacher but failed the exam. Did more study, 2 favourite professors –
physicist Doppler and botanist. Had long tradition of growing garden peas in
monastery. He began experimenting with peas- had strict control over mating pairs
– cross-pollination raised true-breeding types and crossed them to get F1. One trait
seemed to disappear but reappeared in F2 in a specific ratio. Eventually lead to
dominant and recessive. Mendel’s worked not believed until 1900’s. Didn’t like law
of segregation, didn’t believe.
Lead to Mendel’s Laws
Law of Segregation – an individual carries genes in pairs, called alleles,that segregate
during meiosis. Each parent can only donate one allele of a pair. **Note - not the
chromosomes that segregate but the alleles
Law of Independent assortment – Alleles of different genes behave independently and
sort independently during meiosis.
Relate to meiosis and variation in sexual reproduction – natural selection and
conditions necessary
Homologous chromosomes – a like pair of chromosomes that carry alleles for the same
genes. The alleles have the same loci but not necessarily the same alleles. Humans have
22 homologous pairs and the sex pair. A gamete should contain one chromosome of each
pair.
Dog – 2n is 78 fern – 2n is 1260 Each chromosome, unpacked and uncoiled would
be about 6 cm long with about 2X108 nucleotides
Genome – the whole genetic information of an organism.
Product rule or rule of multiplication – how to compute the probability of 2 independent
events happening. Eg chance of 2 coins landing heads – ½ X ½ = ¼
Some exceptions – that may not show the 9:3:3:1 ratio, could get 9:3:3 or 9:7
Incomplete dominance – (not used much) Exception to dominance rule – when
certain flowers with red petals were crossed with white – all F1 were pink. New trait
*Codominance – exception, where both alleles affect or are expressed in the phenotype.
Eg. Red coat cow crossed with white coat – offspring has reddish white coat called roan.
Contains both red and white hairs. Human blood type is another example. Except for
blood type, use letter standing for trait and superscript for colour. CR CW
Multiple alleles – a gene that has more than two alleles. Eg ABO blood system. Has
three alleles – IA, IB, i. Any two alleles are present in a person. DO NOT USE HAIR,
EYE OR SKIN COLOUR AS THEY SHOW;
Polygenic inheritance –Characteristics caused by a combine effect of more than one
gene. Phenotype varies in graduation. Most human characteristics are polygenetic. Eg.
Skin colour, body height, muscle development, hair colour, eye colour. (examine eyes
with lens, will see more than 2 separate colours)
Continuous variation – many genes (polygenic inheritance) having an effect over a long
term – skin colour, height
Discontinuous variation – doesn’t vary but has discreet categories – eg blood type, you
are either one or the other but not in-between.
Morgan – modern father of genetics – dealt with fruit flies – prolific breeders, only 4
pairs of chromosomes, short life span, easy to manage and cheap. Now know their
complete genome.
Sex chromosomes – through Morgan’s work, discovered the 23rd pair of chromosomes –
X and y. The X is larger and contains all the genes for female sex characteristics
(primary and secondary). But also contains some non-sex related traits like haemophilia,
colour-blindness and baldness. They are called sex-linked traits. The y chromosome is
smaller and only contains the genes for male sex characteristics. Should not be any
crossing over as the y is too small –not homologous.
Other sex chromosome ratios – crickets and roaches, male is X_ and female XX
Bees and ants – females develop from diploid, fertilized egg and male form haploid,
unfertilized egg (both are social animals) Anatomical evidence of sex begins at 2
months. Before that, embryo could be either but then the y kicks in. If no y, will be
female.
Normal Mendelian rules apply for females in showing a recessive trait. Heterozygous
females, for sex-linked traits, are called carriers. Males need only one allele to show a
recessive trait.
Notation used – haemophilia = XH is normal, Xh is haemophilia
XB is normal vision, Xb is colour blind.
**love linked genes – make sure to use proper notation. Be careful with sex-linked
questions as they could ask about the % of children or the % of girls or boys. In a
pedigree, should be able to determine if it is sex-linked.
Other sex-linked is Duchenne muscular dystrophy – 1 out of 3500 males, don’t live
past 20 years. Sweat glands in women are on X, some of the cells’ X is not working
and gives a mosaic for sweat glands.
Karyotype – the cells are stopped at metaphase of mitosis, picture of the chromosomes;
the chromosomes are then grouped based on size, position of centromere and banding.
Are given a number. Chromosomes are made up of DNA and histones. When stained,
sections of genes show banding while others don’t. Uses – to determine sex and
chromosomal aberrations.
Note notation for codominant and sickle cell anaemia in syllabus
Genetic Diseases
Many diseases are recessive but not all. (Huntington’s chorea is dominant) Other factors
can affect frequency – religion, geography (Sweden), political, (Hitler) evolution
(malaria)
Can be caused by a gene mutation or a chromosome aberration.
* know sickle cell**Gene mutation – Sickle-cell anaemia – a base substitution. GAG has
mutated to GTG causing glutamic acid to be replaced by valine giving sickle-cell anemia.
* does confer resistance to malaria which alters the frequency- not what you would
expect 1 out of 400 American blacks have it. 1 out of 40 of them is heterozygous as
is codominant. Hetero has some traits
Tay-Sachs – lack of enzyme that breaks down brain lipids, seizures, blindness, recessive,
degeneration of motor and mental abilities. I n 3600 Mediterranean Jews have it. Cultural
influence increasing gene frequency.
Cystic fibrosis – 1 in 2500 Caucasian, 1 out of 12 is carrier, abnormal extra cellular chloride
producing causing mucus to become sticky and thick, and build-up in pancreas, lungs and
digestive tract. If untreated, die before 5, if treated die around 27 years (do you get
married?)
PKU – don’t have enzyme t break down an amino acid that is found in milk, see warning on
some diet sweeteners that contain amino acid as pregnant women who are homo for PKU
would hurt baby
Dwarfism – 1 out of 10000
Huntington’s chorea – degeneration of nervous system, strikes at 35 to 45
Also have multifactorial diseases – diseases that are both genetic and environmental
components like heart disease, cancer, alcoholism, schizophrenia, and manic-depressive.
Gene therapy is used on cystic fibrosis – will discuss later in DNA
Chromosome Aberrations
Some change in the chromosome. Most common is chromosome nondisjunction. This is
where a chromosome does not separate properly during meiosis. A gamete gets one extra
or less.
Major autosomal chromosome nondisjunction is Down’s syndrome or trisomy 21
Sort, round face, short lifespan, sterile, sexual underdeveloped, large tongue, eyefolds,
retardation, heart defects, risk of leukaemia, Alzheimer’s. 0.04% in women less than 30;
1.25% in women of 30-39.
(about 30-40% of spontaneous abortions due to chromosome aberrations)
Cri du chat – deletion of part of chromosome 5, mentally retarded, small head,
meows like cat
Nondisjunction of the sex chromosomeXXy – Klinefelter’s syndrome – usually sterile, mentally retarded, some female sex
characteristics
X_ - Turner’s syndrome – usually normal intelligence, 1 out of 5000 have it
Xyy – super male, sterile, mentally retarded
Genetic screening – testing an individual for the presence or absence of a gene. prenatal
diagnosis, immigration, animal pedigree, insurance purposes, breast mammograms, sex
determination
Amniocentesis – cells form amniotic fluid, need to wait for cells to do mitosis to do
Karyotype, could take weeks
Chorionic villus sampling CVS – take cells from the placenta, which are rapidly dividing
so can do Karyotype in 24 hours.
Cloning – organisms with identical genotypes due to some kind of asexual reproduction.
Some are natural as in identical twins, cuttings for plants etc. Could use totipotency – the
ability of a single cell to produce a whole organism. Take the 8 cells after fertilization
and put into surrogate mother, get identical offspring.
H.L.
Pleiotrophy – ability of a gene to affect an organism in many ways.
Epistasis – a gene at one locus alters the phenotype expressing of agene at another locus.
Egg. Melanin and albino gene.
Transgenic – Biodiversity – need for reservoir for alleles, hybrid vigour
Inbreeding – not cloning, decreases biodiversity
Chi-squared – can establish whether an observed ratio differs from the expected
(Mendelian ratio) Not in syllabus now but sometimes alluded to.
Why wouldn’t you get what you expected?
Could be due to linked genes. The Law of Independent assortment says that alleles and
therefore chromosomes should sort independently. But what if the genes are on the same
chromosome? Linked genes occur on the same chromosome and tend to be inherited
together – that is – do not segregate independently. Therefore, you do not get the ratio of
9:3:3:1.
**Drosophila (fruit fly) shows linked genes. Morgan found that when crossing, didn’t
get expected ratio. First was a pure breeding tan body with long wings crossed with
black body ad short wings (vestigial wings). F1 all had tan bodies and long wings but the
F2 was 75% tan, long wings and 25% black and short wings. Only possible explanation
was that body colour and wing length are found on the same chromosomes.
b+ = tan body b = black
w+ = long wings w = short
**note syllabus and how to write. Another linked is red eye normal wing and purple eye
and short wing Don’t use human eye colour, skin colour or hair as they are
polygenic. Also, do not use sex-linked as they will not show crossing over due to not
being homologous. May 2005.
After discovering this, found that some crosses didn’t give the 75-25 ratio, in fact,
showed offspring with tan bodies and short wings or black and long. How is this
possible? CROSSING OVER.
Occurs during prophase I of meiosis, where chromatids cross is called the chiasma.
Because of this, you get recombinants.
Recombination – the resentment of genes into combinations different from the parents.
Used for linked genes but also can be used for different phenotypes in dihybrid crosses.
Eg. If male is tall and red and female is short and white, recombinant would be tall and
white or short and red.
Because of the physical process of crossing over, linked genes that are found on the ends
of the chromatids, further apart, have a greater tendency to cross over. Lead to mapping
of chromosomes and Centimorgans.