Genetic crosses question ?
When a white ram was mated with black ewes, it sired only white lambs. When this ram was mated with a certain white ewe, all lambs were white. When one of thes lambs reached maturity, it was back-crossed with its mother, which produced a black lamb. Account for these results by making diagrams of each of these crosses mentioned, including the genotypes of the parents, gametes and offspring and the phenotypes of the parents and offspring. Hint: you should show three different crosses
Please show me how to do it so that I understand for my other questions which are like this one.
Thanks.
To solve this genetic crosses question, we need to understand the basic principles of inheritance and use Punnett squares to determine the possible genotypes and phenotypes of the parents and offspring.
First, let's identify the key information given in the question:
1. When a white ram (unknown genotype) was mated with black ewes (known genotype: bb), all lambs were white.
2. When the white ram was mated with a certain white ewe (unknown genotype), all lambs were white.
3. When one of these white lambs reached maturity (known genotype: unknown), it was back-crossed with its white mother (known genotype: unknown), and a black lamb was produced.
Based on this information, we can deduce that the black color is recessive since it only appeared when both parents carried the black allele.
Now, let's proceed with each cross one by one and create Punnett squares to visualize the possible genetic combinations and resulting phenotypes.
1. Cross between white ram and black ewe:
- The black ewe genotype: bb
- The ram genotype: unknown
- Offspring phenotype: all white lambs
Punnett square:
| b | b |
--------------------------
R | Rb | Rb |
--------------------------
R | Rb | Rb |
From this Punnett square, we can see that all offspring will be white (Rb), confirming the given information.
2. Cross between white ram and white ewe:
- The white ram genotype: unknown
- The white ewe genotype: unknown
- Offspring phenotype: all white lambs
Punnett square:
| W | W |
--------------------------
R | RW | RW |
--------------------------
R | RW | RW |
From this Punnett square, we can see that all offspring will be white (RW), in accordance with the given information.
3. Back-cross between a white lamb (unknown genotype) and its white mother (unknown genotype):
- The white lamb genotype: unknown
- The white ewe genotype: unknown
- Offspring phenotype: one black lamb
Since we know that the white lamb produced a black lamb when back-crossed with its mother, it suggests that the white lamb must be heterozygous for black color (carrying the recessive allele 'b').
Punnett square:
| R | b |
--------------------------
b | Rb | bb |
From this Punnett square, we can see that the offspring have a 50% chance of being black (bb) and 50% chance of being white (Rb).
To summarize, the possible genotypes and phenotypes for each cross are as follows:
1. Cross between white ram and black ewe:
- Ram genotype: unknown
- Ewe genotype: bb
- Offspring genotype: Rb (all white lambs)
2. Cross between white ram and white ewe:
- Ram genotype: unknown
- Ewe genotype: unknown
- Offspring genotype: RW (all white lambs)
3. Back-cross between a white lamb and its white mother:
- White lamb genotype: Rb
- White ewe genotype: unknown
- Offspring genotype: 50% chance of being bb (black lamb) and 50% chance of being Rb (white lamb)
Please note that the genotypes of the ram and white ewe are unknown, so we cannot determine their specific genetic makeup without further information.