Transcript U5 Notes - southbutterfield

Genetics
DNA Review
Mutations: Sex vs Body
Important Science Guy
• Gregor Mendel – father of genetics,
he worked with peas in a monastary
Who was this Mendel and what the heck is
he doing in a monastery?
• born in 1822
• trained himself to be a
naturalist early in life
• worked as a substitute
science teacher
• failed the qualifying exams to be
a regular high school teacher!
• joined a monastery in Brunn,
Austria
• sent to Vienna U. to study
science and math
MendelWeb
Mendel’s first published work:
"Versuche über Pflanzen-Hybriden“
or Experiments in Plant Hybridization
was a landmark in clarity and insight!
Trained as a mathematician
and a biologist, he figured
out the laws of inheritance…
mathematically!!
The work of Gregor Mendel
• worked with pea plants…
…he called
them his
children!
Why pea plants???
There was a long-standing tradition of breeding pea
plants at the monastery where Mendel lived and worked
So…they were readily available and they come in lots
of varieties!
…there were plants with different flower colors, seed
color, flower position etc..
‘Brother Greg... We grow tired of peas again!!!’
And best of all…
Pea plants flowers can
reproduce by themselves
This allowed Mendel to see if
strains were true breeding and to
produce hybrids
What the heck does that mean?
• True-breeding: when plants self pollinate,
all their offspring will be identical to
themselves
• Hybrid: when parents with different traits
are crossed, this is what we call the
offspring
• SO – he could see if the plants would
produce offspring identical to themselves
and how parents with different traits
would affect the offspring
How Mendel made hybrids…
He’d then tie little
bags around the
flowers to prevent
contact with stray
pollen.
Mendel’s hybridization experiments…
Monohybrid crosses:
Parental
Generation
F1 generation
True-breeding
purple flower
x
True-breeding
white flower
All purple flowers (the hybrids)
Allowed F1 offspring
to self-fertilize
F2 generation
705 purple
224 white
The results of Mendel’s monohybrid
crosses led him to propose…
1. All organisms contain two “units of
heredity” for each trait (alleles).
2. Dominant and recessive alleles…
…and organisms can have any combination of
the two alleles (2 dominants, 2 recessives
or a mixture 1 dominant and 1 recessive).
•Alternative versions of genes (called alleles)
account for the variations in inherited
characteristics
•For each character an organism inherits two alleles,
one from each parent
•If the two alleles differ, then one, the dominant
allele, is fully expressed in the organism’s
phenotype– the other (recessive) allele has no
noticeable effect on the organism’s phenotype
•
The Law of Segregation – during gamete
formation, these two alleles separate
randomly into separate gametes, but each
gamete gets one copy of each gene
A bit of genetic jargon…
phenotype vs. genotype
What the
organism
looks like
What alleles
the organism
has - its
genetic makeup
More jargon…
• Genes are usually controlled by two alleles –
expressed by two letters
• These alleles may be dominant or recessive:
– Dominant: allele that is expressed in the
phenotype
• Expressed as a capital letter (A)
– Recessive: alleles that is completely masked in
the phenotype
• Expressed as a lower case letter (a)
More jargon…
homozygous vs. heterozygous
P
2 of the
same alleles:
PP or pp
2 different
alleles:
p
Pp
Sometimes refer to heterozygous individuals as carriers
because they carry both the recessive and dominant
allele and can give either to their offspring, but only
express the dominant allele
A Punnett square…
Gametes from
one parent
p
P
Gametes from
other parent
P
PP
purple
p
Pp
purple
Pp
purple
pp
white
Ratio: 3:1 or ¾ purple, ¼ white
A Punnett
square
Let’s relate Mendel’s findings to what
we now know about gamete formation
True-breeding
purple flower
P
all purple
P
P
P
P
P
True-breeding
white flower
x
P
p
p
p
p
p
p
p
F1 generation
purple hybrid
P
F2
p
P
purple hybrid
p
P P
P
x
p
P
p
P
p
p
p
P
PP (purple) Pp (purple) Pp - purple
pp - white
p
Law of Independent Assortment
• States that each allele pair
segregates independently during
gamete formation
• AKA: Alleles for different genes
separate independently (alone) during
meiosis when egg and sperm cells are
being made – so alleles for seed color
wouldn’t affect alleles for seed coat
Activity:
• Cross the following alleles and determine the phenotypic and
genotypic ratios.
Phenotypic
ratio
Aa x Aa
AA x Aa
aa x Aa
AA x aa
Genotypic
ratio
Activity Answers
Aa x Aa
Phenotypic
ratio
3:1 (dom:rec)
Genotypic
ratio
1:2:1 (hd:h:hr)
AA x Aa
1:0 (dom:rec)
1:1:0 (hd:h:hr)
or all dominant
aa x Aa
1:1 (dom:rec)
0:1:1 (hd:h:hr)
AA x aa
1:0 (dom:rec)
0:1:0 (hd:h:hr)
or all dominant
Pedigrees
• Geneticists tool
• These are family trees that describe
the occurrence of heritable
characters in parents and offspring
across many generations
Pedigree