LECTURE 2: Mendel's Second Law
Mendel's First Law, which is also called as the Law of Segregation states that the characters of an organism are determined by internal factors (genes), which occurs in pairs (homologous chromosomes). Only one of a pair of such factors can be represented in a single gamete.
Mendel's Second Law has been derived from Dihybrid cross (Di = two; hybrid = mix). Mendel considered two characters at a time instead of one (that he considered in the Monohybrid Cross). The law states that any one of a pair of characters may combine with either one of another pair.
Mendel considered seed color and seed shape for his dihybrid cross experiment.
- Yellow and round seed is dominant over green and wrinkled seed.
- Yellow and round is represented by alleles Y and R
- Green and wrinkled seed is represented by y and r
- Genes for seed color and seed shape are present on two different chromosomes
The figure shown on the right demonstrates a dihybrid cross. As mentioned above, the genes for seed color and seed shape are located on two different chromosomes.
YYRR will form YR gamete
yyrr will form yr gamete
In the F1 generation, both gametes fuse to form Y/y R/r genotype plant whose phenotype would be yellow and round because genes for yellow and round are dominant.
Now the F1 is selfed:
Y/y R/r x Y/y R/r
I have kept the boxes blank so that students can make 4 different gametes. 4x4 =16 progeny would be formed out of the dihybrid crosses.
We know that the phenotypic ratio of the dihybrid cross is 9:3:3:1 but the question is how many different types of phenotypes are formed? Well, by looking at the ratio, we can infer that there are 4 different types of phenotypes formed. However, we do not know in what combination they are formed. Well, its quite easy to find out the four different combinations.
TIPS: write down the dominant characters on the left hand side in parentheses, and recessive characters on the right side also in parentheses.
(Round, Wrinkled) x (Yellow, Green)
3 1 3 1
I have written 3 below the round character and 1 below the wrinkled character - this is a monohybrid ratio provided we would have considered only one character. Similarly, yellow and green characters are in the ration of 3:1
- Multiply Round with yellow = 3 x 3 = 9
- Multiply Roybd with green = 3 x 1 = 3
- Multiply Wrinkled with yellow = 1 x 3 = 3
- Multiply Wrinkled wiith green = 1 x 1 =1
You can use this simple technique to avoid Punnet square cross to find out the different combinations.
How would you find a trihybrid phenotypic ratio?
Well, its so simple -- if you know multiplication, you are done.
Multiply 9:3:3:1 by 3:1
Remember: All multiplication should be on the same line.
9:3:3:1
x 3:1
__________________
27:9:9:3:9:3:3:1
Use the same formula to know the genotype ratio of a dihybrid cross -- we know the genotype ratio of a monohybrid cross: 1:2:1
Multiply 1:2:1 by 1:2:1
_________________________________________
1:2:1:2:4:2:1:2:1
Assignment: What would be the test cross ratio of a Dihybrid corss?
See you all in the discussion section of LECTURE 3...
Mendel's Second Law has been derived from Dihybrid cross (Di = two; hybrid = mix). Mendel considered two characters at a time instead of one (that he considered in the Monohybrid Cross). The law states that any one of a pair of characters may combine with either one of another pair.
Mendel considered seed color and seed shape for his dihybrid cross experiment.
- Yellow and round seed is dominant over green and wrinkled seed.
- Yellow and round is represented by alleles Y and R
- Green and wrinkled seed is represented by y and r
- Genes for seed color and seed shape are present on two different chromosomes
The figure shown on the right demonstrates a dihybrid cross. As mentioned above, the genes for seed color and seed shape are located on two different chromosomes.
YYRR will form YR gamete
yyrr will form yr gamete
In the F1 generation, both gametes fuse to form Y/y R/r genotype plant whose phenotype would be yellow and round because genes for yellow and round are dominant.
Now the F1 is selfed:
Y/y R/r x Y/y R/r
I have kept the boxes blank so that students can make 4 different gametes. 4x4 =16 progeny would be formed out of the dihybrid crosses.
We know that the phenotypic ratio of the dihybrid cross is 9:3:3:1 but the question is how many different types of phenotypes are formed? Well, by looking at the ratio, we can infer that there are 4 different types of phenotypes formed. However, we do not know in what combination they are formed. Well, its quite easy to find out the four different combinations.
TIPS: write down the dominant characters on the left hand side in parentheses, and recessive characters on the right side also in parentheses.
(Round, Wrinkled) x (Yellow, Green)
3 1 3 1
I have written 3 below the round character and 1 below the wrinkled character - this is a monohybrid ratio provided we would have considered only one character. Similarly, yellow and green characters are in the ration of 3:1
- Multiply Round with yellow = 3 x 3 = 9
- Multiply Roybd with green = 3 x 1 = 3
- Multiply Wrinkled with yellow = 1 x 3 = 3
- Multiply Wrinkled wiith green = 1 x 1 =1
You can use this simple technique to avoid Punnet square cross to find out the different combinations.
How would you find a trihybrid phenotypic ratio?
Well, its so simple -- if you know multiplication, you are done.
Multiply 9:3:3:1 by 3:1
Remember: All multiplication should be on the same line.
9:3:3:1
x 3:1
__________________
27:9:9:3:9:3:3:1
Use the same formula to know the genotype ratio of a dihybrid cross -- we know the genotype ratio of a monohybrid cross: 1:2:1
Multiply 1:2:1 by 1:2:1
_________________________________________
1:2:1:2:4:2:1:2:1
Assignment: What would be the test cross ratio of a Dihybrid corss?
See you all in the discussion section of LECTURE 3...
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