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!