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Dihybrid Crosses and Law 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 Segregation Review. During gamete formation, alleles segregate (separate)

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Dihybrid crosses and law of independent assortment

Dihybrid Crosses andLaw of Independent Assortment


Monohybrid cross
Monohybrid cross

  • Mendel’s experiments followed the inheritance of single characters

    • E.g. flower color, seed color , etc.

    • monohybrid crosses


Law of segregation review
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
Dihybrid cross

  • Other of Mendel’s experiments followed the inheritance of 2 different characters

    • seed color andseed shape

    • dihybrid crosses

Mendelwas working outmany of the genetic rules!


Dihybrid cross1

F1

generation

(hybrids)

yellow, round peas

100%

F2

generation

Dihybrid cross

P

true-breeding

yellow, round peas

true-breeding

green, wrinkled peas

x

YYRR

yyrr

Y = yellow

R = round

y = green

r = wrinkled

YyRr

self-pollinate

9:3:3:1

9/16

yellow

round

peas

3/16

green

round

peas

3/16

yellow

wrinkled

peas

1/16

green

wrinkled

peas


What s going on here

YR

YR

yR

Yr

yr

yr

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?

Is it this?

Or this?

YyRr

Y and R alleles transmitted independently

Which systemexplains the data?

Y and R alleles transmitted together


Is this the way it works

9/16

yellow

round

YyRr

YyRr

3/16

green

round

YR

yr

YR

yR

Yr

yr

3/16

yellow

wrinkled

1/16

green

wrinkled

Is this the way it works?

or

YyRr

x

YyRr

What was actually observed

YR

yr

Well, that’sNOT right!

YR

YYRR

YyRr

yr

YyRr

yyrr


Dihybrid cross2

9/16

yellow

round

YyRr

YyRr

3/16

green

round

YR

YR

yr

YR

yR

Yr

yr

Yr

3/16

yellow

wrinkled

yR

1/16

green

wrinkled

yr

Dihybrid cross

or

YyRr

x

YyRr

YR

Yr

yR

yr

YYRR

YYRr

YyRR

YyRr

BINGO!

YYRr

YYrr

YyRr

Yyrr

YyRR

YyRr

yyRR

yyRr

YyRr

Yyrr

yyRr

yyrr


Mendel s 2 nd law of heredity

yellow

green

round

wrinkled

Can you thinkof an exceptionto this?

Mendel’s 2nd law of heredity

  • 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)

YyRr

Yr

Yr

yR

yR

YR

YR

yr

yr


Law of independent assortment

Metaphase 1

Law of Independent Assortment

  • Which stage of meiosis creates the law of independent assortment?

RememberMendel didn’teven know DNA—or genes—existed!

EXCEPTION

  • If genes are on same chromosome & close together

    • will usually be inherited together

    • rarely crossover separately

    • “linked”


Sample problem
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.


Solution

gt

gt

gt

gt

Solution

1:1:1:1

GgTt

ggtt

x

4/16 = 1/4

green

tall

GT

Gt

gT

gt

4/16 = 1/4

green

short

GgTt

Ggtt

ggTt

ggtt

Ggtt

GgTt

ggTt

ggtt

4/16 = 1/4

yellow

tall

GgTt

Ggtt

ggTt

ggtt

4/16 = 1/4

yellow

short

GgTt

Ggtt

ggTt

ggtt



Any Questions? heterozygous for both characters?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!


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