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Genetics - Coggle Diagram
Genetics
Definitions
genetics
study of heredity
way in which characteristics pass
from parents to offspring
genes
short sections on chromosome
code for production
of specific polypeptide
heterozygous
alleles of same gene
DIFFERENT
homozygous
alleles of same gene
IDENTICAL
alleles
particular form of gene
located at the same locus
on homologous chromosomes
in diploid organisms
genotype
2 alleles for a particular characteristic
phenotype
outward appearance of individual
in terms of particular characteristic
gene locus
position of gene
on chromosome
dominant
in heterozygous condition
DA OVERRIDE non-dominant allele
recessive
will be dominated by DA
only show itself in phenotype if 2 R
autosomes
(not sex chromosome)
homologous chromosomes
identical
in appearance
heterosomes
homologous chromosomes
not identical
in appearance
carrier
has recessive allele
but is not affected
as also carries D
History of Genetics
Gregor Mendel
work on breeding peas
formed basis of genetics
in pea plants -
many characteristics varied in discrete way
from plant to plant
height
pea shape + size
flower colour
why peas?
easy to cultivate
plant self-pollinating
easily observed differences
hybrids = completely fertile
Monohybrid Inheritance
inheritance of alleles @ 1 gene locus
M investigated
pea height
normal
tall
Mendel's Experiment
1.allowed tall plants to
interbreed
ensure produce ALL tall
eg pure breeding (homozygous)
**did same w/ small plants
2.
crossbred
tall + small (parental generation)
ALL
offspring =
TALL
(F1 generation = offspring of cross between 2 parental genotypes)
3. two F1 plants crossed
offspring contain MIX SHORT + TALL
(
F2 generation - offspring resulting from cross 2 F1 individuals)
tall:short - 3:1 ratio
Conclusions
only 1 of alleles in parental genotype pass to gamete
during meiosis
1 homologous chromosome from each pair
into gamete
totally random which allele enter gamete
Law of Segregation
MENDEL'S FIRST LAW
2 alleles of each gene
separate during meiosis
with only 1 entering each gamete
D: inheritance of the alleles
of a single gene
at 1 gene locus
3:1 RATIO
F2 generation
parental genotype
homozygous dominant
homozygous recessive
F1 generation genotype
all heterozygous
1:1 RATIO
F1 generation
parental genotypes
heterozygous (Tt)
homozygous recessive (tt)
Mutations
causes 2 or more
alternative alleles
most often mutated
allele = recessive
Test/Back Cross
to determine
GENOTYPE
homozygous dominant + heterozygous = SAME PHENOTYPE
= DIFFERENT GENOTYPE
method
cross unknown individual
w/ individual known genotype
homozygous recessive
conclusions
if ALL offspring is tall
unknown = TT
if 50% tall + 50% short
unknown = Tt
Co-dominance
2 alleles @ same gene locus
neither dominant/masking effect
both co-dominant alleles make
contributions to heterozygous phenotype
1:2:2 RATIO
phenotype ratio
1M:2MN:1NN
Multiple Alleles
more than 2 alleles of particular gene
eg ABO blood groups
A and B
dominant to O
A co-dominant
with B
1:1:1:1 RATIO
heterozygous group A = IAIO
heterozygous group B = IBIO
Sex Determination
Y chromosome
smaller than X
ONLY
carries gene for maleness
codes for protein to switch on direct development of testes in embryo
female = homogametic
XX
sex chromosomes identical
male = heterogametic
XY
sex chromosomes non-identical
50:50 RATIO
Dihybrid Inheritance
inheritance of alleles @ 2 gene loci
simultaneous inheritance
of 2 characteristics
Law of Independent Assortment
MENDEL'S SECOND LAW
during the formation of gametes
segregation of alleles of 1 gene
INDEPENDENT
to segregation of alleles of other gene
explained during
metaphase I (meiosis)
homologous pairs of chro (bivalents)
arrange themselves independently
on equator of spindle
9:3:3:1 RATIO
homozygous dominant - TTRR
homozygous recessive - ttrr
1:1:1:1 RATIO
heterozygous - TtRr
homozygous recessive - ttrr
independent inheritance
if genes on different chromosomes
Polygenic Inheritance
(gene interaction)
is complete range of
intermediate phenotypes
eg. height
CONTINUOUS
VARIATION
(bell-shaped curve=normal distribution)
CUMULATIVE effect of
more than 1 gene
D: refers to characteristic
controlled by many genes
with individual slight effect on phenotype
features:
characters controlled
by no. of genes
genes individual small
effects
phenotype subject
to
environmental variation
EFFECT OF ENVIRONMENT
height = diet etc
genes switched on =
epigenetics
characters show continuous
range of variation
Epistasis
(gene interaction)
D: gene interaction when 1 gene
masks/modifies action of other gene
results in unusual
genetic ratio
examples
2 loci INTERACT
create NEW PHENOTYPE
comb type = chickens
OR
flower colour = sweet pea
2 genes DUPLICATE work of each other
wheat kernel colour
15:1 ratio
allele @ 1 locus MASK allele @ other locus
squash colour
Lethal Alleles
eg house mouse
coat colour
EXPECTED RATIO
3 (yellow):1 (agouti)
OBSERVED RATIO
2 (yellow):1 (agouti)
homozygous dominant
yellow (YY)
always dies
2 alleles coding for yellow
lethal combination
fail to develop properly
die as embryo
result in:
spontaneous abortions
embryos/foetus
OR
death
affected individuals in
adulthood
(with no prior warning) - eg Huntington's
require:
both recessive
OR
single dominant autosomal
(eg )Huntington's
Sex Linkage
sons NEVER inherit father's X chro
sons NEVER inherit father's sex linked character.
daughters ALWAYS inherit father's X chr
∴ 50% chance passing sex linked to offspring
conditions caused by
single RECESSIVE ALLELE
on X chromosome
examples
red-green colour blindness
haemophilia
(abnormally long time clot blood)
Duchenne muscular dystrophy
recessive in females
(not "recessive in males" = only recessive to dominant & males only have 1 allele)
why more common
in males?
males only need 1 recessive allele
only need 1 from mum
(carrier/have condition)
females need 2 recessive alleles
1 from each parent
dad must have condition
mum = carrier/have condition
WORKING OUT PROBABILITY
"colour blind" son - P (son) x P (colour blind)
"son that is colour blind"