Dihybrid Crosses and Law of Independent Assortment.
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Transcript of Dihybrid Crosses and Law of Independent Assortment.
Dihybrid Crosses andLaw of Independent Assortment
Monohybrid cross• Mendel’s experiments followed the
inheritance of single characters – E.g. flower color, seed color , etc.– monohybrid crosses
Law of SegregationReview– During gamete formation,
alleles segregate (separate) • only one of two allelles for a gene
is put into a gamete.
– At fertilization, offspring receive one allele for each gene from each parent.
– alleles are on different chromosomes!
Dihybrid cross• Other of Mendel’s
experiments followed the inheritance of 2 different characters – seed color and
seed shape– dihybrid crosses
Mendelwas working outmany of the genetic rules!
Dihybrid crosstrue-breedingyellow, round peas
true-breedinggreen, wrinkled peasx
YYRR yyrrP
100%F1generation(hybrids)
yellow, round peas
Y = yellowR = round
y = greenr = wrinkled
self-pollinate
9:3:3:19/16yellowround peas
3/16greenround peas
3/16yellowwrinkledpeas
1/16greenwrinkledpeas
F2generation
YyRr
What’s going on here?• If alleles (different characteristics) are on different chromosomes…
– Are alleles transmitted from parents to offspring together or independently?
– Will the Y and R alleles always stay together, generation after generation?
YyRr YyRr
yr
Is it this? Or this?
Which systemexplains the data?
Y and R alleles transmitted independently
Y and R alleles transmitted together
9/16yellowround
3/16greenround
3/16yellowwrinkled
1/16greenwrinkled
Is this the way it works?YyRr YyRr
YR yr
YR
yr
x
or
YYRR YyRr
YyRr yyrr
Well, that’sNOT right!
What was actually observed
Dihybrid crossYyRr YyRr
YR Yr yR yr
YR
Yr
yR
yr
YYRR
x
YYRr YyRR YyRr
YYRr YYrr YyRr Yyrr
YyRR YyRr yyRR yyRr
YyRr Yyrr yyRr yyrr
9/16yellowround
3/16greenround
3/16yellowwrinkled
1/16greenwrinkled
or
BINGO!
Mendel’s 2nd law of heredity
round
wrinkled
• Law of independent assortment– Each pair of alleles (different characteristics)
segregate independently• non-homologous chromosomes align
independently• only true if alleles (different characteristics) are on
separate chromosomes (e.g. gene for colour on separate chromosome as gene for texture)
Yr Yr yR yR YR YR yr yr
YyRr
Can you thinkof an exceptionto this?
Law of Independent Assortment Which stage of meiosis creates
the law of independent assortment?
Metaphase 1
EXCEPTION If genes are on same chromosome &
close together will usually be inherited together rarely crossover separately “linked”
RememberMendel didn’t
even know DNA—or genes—
existed!
Sample Problem
In pea plants, the green colour allele (G) is dominant over the yellow colour allele (g) for seed colour and tall (T) is the dominant plant height. A pea plant that is heterozygous for both seed colour and height is crossed with a plant that is homozygous recessive for both.
Determine the genotypic ratio for their offspring.
SolutionGgTt ggtt
GT Gt gT gt
gt
gt
gt
gt
GgTt
x
Ggtt ggTt ggtt
GgTt ggTt ggtt
GgTt Ggtt ggTt ggtt
GgTt Ggtt ggTt ggtt
4/16 = 1/4greentall
4/16 = 1/4greenshort
4/16 = 1/4yellowtall
4/16 = 1/4yellowshort
1:1:1:1
Ggtt
• What is the probability of getting an offspring that are heterozygous for both characters?A: They must be GgTt
4/16 = ¼ or 25%
2005-2006
Any Questions?
1. Bikini Bottom 2 WS2. Pg. 135 # 10-14 (Mono)3. Pg. 141 #1,2,4-9 (Di)
Homework Quiz next class on Mono and Dihybrid Crosses!