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Year 1 Biol
Biol 113
genetic theory
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Gregor Mendel worked with
pea plants
Gregor Mendel
discovered
dominant and recessive traits
concept of
gene
(heritable factor)
formulation of law of
inheritance
a
character
is a
heritable
feature of an individual
a
trait
is a variant form of a character (the
phenotype
)
law of
segregation
2 forms of a gene (
alleles
) present in each parent segregate
independently
law of segregation formulated by
monohybrid crosses
monohybrid cross is
cross between
2
true breeding individuals differing in only
one
character
P generation
is the true breeding parents
F1
generation are the
first
generation of hybrids
Mendel's
observation
f1
progeny
resembled one of the
parents
Mendel's
observation
f1
progeny
resembled one of the parents
f2 generation have both
P
generation traits
ratio in f2 was
3
:
1
heritable factor for the
recessive
trait had not been lost in the
f1
generation, but masked by presence of factor for dominant trait
Mendel's
model:
variation
in inherited characteristics are die to the existence of alternative versions of genes (
alleles
)
organism inherits
2
alleles for each character
dominant allele always presents itself in the
phenotype
alleles remain
discrete
2 alleles
segregate
(separate) during
gamete
formation
how to do a testcross?
Crossing an individual with a
homozygous recessive
individual
will determine the
genotype
of an individual with the dominant
phenotype
Law of
independent assortment
each pair of alleles assorts independently of each other pair of alleles during
gamete formation
how did the law of
independent
assortment occur?
following the
inheritance
of
2
characters at the same time
involves a
dihybrid
cross
results of
f2
generation in dihybrid cross
2 new
phenotypes
(
recombinants
)
9
:
3
:3:1 ratio
meiosis occurs in
germ line
mitosis occurs in
somatic
cells
role of meiosis
produce haploid gametes and introduce
genetic variability
homologous pair
are individual chromosomes that were
inherited
from different parents
homologous pair=
homologues
homologous pair
have different versions of genes at some of their
loci
chromatid
is 1 of
2 identical
strands of a newly replicated chromosome
chromatid
is different to pair of
homologous
chromosomes
sister chromatids are the 2 identical chromatids held together by _ following replication
centromere
chromosome terminology
labels:
A)
homologous pair
B)
sister chromatids
C)
centromere
3
Sutton observed:
chromosomes occur in
pairs
in
somatic
cells
chromosome pairs
segregate
equally into
gametes
different chromosome pairs assort
independently
chromosome
theory of
inheritance
genes
are located at specific positions (AKA
loci
) on chromosomes
chromosomes undergo
segregation
and
independent assortment
genes
are units of
heredity
and determine traits
law of
segregation
explained by
each allele on different member of a homologous pair of chromosomes and moves to
opposite
poles in
anaphase
I
independent assortment
of chromosomes means that there are
4
possible combinations of alleles at 2 loci
law of
independent assortment
happens because of
the arrangement of chromosomes at
metaphase I
is random
heterozygote
is a cell/organism carrying 2 different alleles at a
genetic locus
sources of genetic variability in sexual life cycles:
mutation
independent
assortment
crossing over
random
fertilisation
recombinant phenotype
->
phenotype
differs from either parent
recombinant phenotypes occur from
independent assortment
(for genes on different chromosomes) and
crossing over
(genes on the same chromosome)
when does crossing over occur?
during
pachytene
phase of
prophase
I
how does crossing over happen?
synapsis
(pairing of homologous chromosomes)
synaptonemal complex
(zipper) holding homologous chromosomes together in
tetrad
chiasma
is the point which chromosomes make contact during
crossing
over
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