Goal 3 - Test Cross Notes
Test Cross
When you look at someone who has a dominant trait, you cannot tell if they are homozygous or heterozygous for that trait. (i.e. if someone has brown eyes and brown is the dominant trait – they could be BB (homozygous) or Bb (Heterozygous).
Mendel always started his crosses with a purebred homozygous (BB or bb) in the P1 generation.
How did he know his dominant parent was homozygous? He developed the test cross.
He took a recessive (in this case blue eyes, or bb) and mated it with the pea plant showing the dominant trait. If in the F1 generation the offspring were all dominant, then the dominant parent was probably a BB.
If there were any recessives in the F1, then the dominant parent had to be a Bb.
A test cross is a way to determine the genotype of an organism.
A test cross is a cross of an individual of an unknown genotype (dominate) with an individual of a known genotype (homozygous recessive).
The pattern of observed phenotypes in the offspring can determine the unknown genotype of the parent.
If a known parent is homozygous recessive (bb) and an unknown parent is homozygous dominant for a trait then all offspring will be heterozygous and show the dominant trait.
If a known parent is homozygous recessive (bb) and an unknown parent is heterozygous dominant for a trait then 50% of the offspring will be dominant and 50 % recessive.
When you look at someone who has a dominant trait, you cannot tell if they are homozygous or heterozygous for that trait. (i.e. if someone has brown eyes and brown is the dominant trait – they could be BB (homozygous) or Bb (Heterozygous).
Mendel always started his crosses with a purebred homozygous (BB or bb) in the P1 generation.
How did he know his dominant parent was homozygous? He developed the test cross.
He took a recessive (in this case blue eyes, or bb) and mated it with the pea plant showing the dominant trait. If in the F1 generation the offspring were all dominant, then the dominant parent was probably a BB.
If there were any recessives in the F1, then the dominant parent had to be a Bb.
A test cross is a way to determine the genotype of an organism.
A test cross is a cross of an individual of an unknown genotype (dominate) with an individual of a known genotype (homozygous recessive).
The pattern of observed phenotypes in the offspring can determine the unknown genotype of the parent.
If a known parent is homozygous recessive (bb) and an unknown parent is homozygous dominant for a trait then all offspring will be heterozygous and show the dominant trait.
If a known parent is homozygous recessive (bb) and an unknown parent is heterozygous dominant for a trait then 50% of the offspring will be dominant and 50 % recessive.
EXAMPLE PROBLEMS:
In humans, brown eyes (B) are dominant over blue (b). A brown-eyed man marries a blue-eyed woman and they have three children, two of whom are brown-eyed and one of whom is blue-eyed. Draw the Punnett square that illustrates this marriage. What is the man’s genotype? What are the genotypes of the children?
Mom = Dad = Genotypes Children =
In dogs, there is an hereditary deafness caused by a recessive gene, “d.” A kennel owner has a male dog that she wants to use for breeding purposes if possible. The dog can hear, so the owner knows his genotype is either DD or Dd. If the dog’s genotype is Dd, the owner does not wish to use him for breeding so that the deafness gene will not be passed on. This can be tested by breeding the dog to a deaf female (dd). Draw the Punnett squares to illustrate these two possible crosses. In each case, what percentage/how many of the offspring would be expected to be hearing? deaf? How could you tell the genotype of this male dog? Also, using Punnett square(s), show how two hearing dogs could produce deaf offspring.
Parent female = Parent male = Offspring =
Parent female = Parent male = Offspring =
Parent female = Parent male = Offspring =
To identify the genotype of yellow-seeded pea plants as either homozygous dominant (YY) or heterozygous (Yy), you could do a test cross with plants of genotype _______.
A test cross made with a cat that may be heterozygous for a recessive trait produces 10 kittens, none of which has the trait. What is the presumed genotype of the cat? Use a Punnet square to explain.
Parent 1 = Parent 2 = Offspring =
Your pet guinea pig has black hair. This trait is dominant and can be represented by a B allele. Your neighbor has a white guinea pig. This trait is recessive and can be represented by a b allele. You want to breed the two guinea pigs but want all the offspring from the mating to be black. You are not sure however of the genotype of your black guinea pig and want to find out before starting the breeding program.
What are the possible genotype of your black guinea pig?
What is the possible genotype of the white guinea pig?
Complete the test cross …
Parent 1 = Parent 2 = Offspring =
OR>>>>
Parent 1 = Parent 2 = Offspring =
What options do you have for breeding all black offspring if you determine that your guinea pig is heterozygous for black color?
In humans, brown eyes (B) are dominant over blue (b). A brown-eyed man marries a blue-eyed woman and they have three children, two of whom are brown-eyed and one of whom is blue-eyed. Draw the Punnett square that illustrates this marriage. What is the man’s genotype? What are the genotypes of the children?
Mom = Dad = Genotypes Children =
In dogs, there is an hereditary deafness caused by a recessive gene, “d.” A kennel owner has a male dog that she wants to use for breeding purposes if possible. The dog can hear, so the owner knows his genotype is either DD or Dd. If the dog’s genotype is Dd, the owner does not wish to use him for breeding so that the deafness gene will not be passed on. This can be tested by breeding the dog to a deaf female (dd). Draw the Punnett squares to illustrate these two possible crosses. In each case, what percentage/how many of the offspring would be expected to be hearing? deaf? How could you tell the genotype of this male dog? Also, using Punnett square(s), show how two hearing dogs could produce deaf offspring.
Parent female = Parent male = Offspring =
Parent female = Parent male = Offspring =
Parent female = Parent male = Offspring =
To identify the genotype of yellow-seeded pea plants as either homozygous dominant (YY) or heterozygous (Yy), you could do a test cross with plants of genotype _______.
A test cross made with a cat that may be heterozygous for a recessive trait produces 10 kittens, none of which has the trait. What is the presumed genotype of the cat? Use a Punnet square to explain.
Parent 1 = Parent 2 = Offspring =
Your pet guinea pig has black hair. This trait is dominant and can be represented by a B allele. Your neighbor has a white guinea pig. This trait is recessive and can be represented by a b allele. You want to breed the two guinea pigs but want all the offspring from the mating to be black. You are not sure however of the genotype of your black guinea pig and want to find out before starting the breeding program.
What are the possible genotype of your black guinea pig?
What is the possible genotype of the white guinea pig?
Complete the test cross …
Parent 1 = Parent 2 = Offspring =
OR>>>>
Parent 1 = Parent 2 = Offspring =
What options do you have for breeding all black offspring if you determine that your guinea pig is heterozygous for black color?