In Mendel's monohybrid crosses, the term "homozygous" refers to an organism that has two identical alleles for a specific gene. For example, an organism can be homozygous dominant (AA) or homozygous recessive (aa).
Monohybrid Cross Overview:
- Parental Generation (P): The initial parents in the cross. They can be homozygous for different alleles, such as one being AA (homozygous dominant) and the other aa (homozygous recessive).
- First Filial Generation (F1): The offspring from the parental generation. In a typical monohybrid cross, all F1 offspring will be heterozygous (Aa) if one parent is homozygous dominant and the other is homozygous recessive.
- Second Filial Generation (F2): The offspring produced by crossing two F1 individuals (Aa x Aa). In this generation, you can observe the typical phenotypic ratio of 3:1 (dominant:recessive) in the offspring.
Genetic Cross Notation:
- P (Parent Generation): AA x aa
- F1 Generation: All offspring will be Aa (heterozygous).
- F2 Generation: The cross of F1 generation (Aa x Aa) produces:
- AA (homozygous dominant)
- Aa (heterozygous)
- aa (homozygous recessive)
Punnett Square for F2 Generation:
A A
__________
a | Aa | Aa |
a | Aa | Aa |
In the F2 generation:
- 1 AA (homozygous dominant)
- 2 Aa (heterozygous)
- 1 aa (homozygous recessive)
Phenotypic Ratio in F2 Generation:
- 3 dominant phenotype (AA and Aa) : 1 recessive phenotype (aa)
Summary:
- Homozygous: Organisms with two identical alleles (AA or aa).
- F0: Parental generation (e.g., AA x aa)
- F1: First generation offspring (e.g., 100% Aa)
- F2: Second generation offspring (from F1 cross, e.g., 1 AA : 2 Aa : 1 aa)
Formula:
For a typical monohybrid cross, the genotype frequencies in F2 can be summed up as:
- P = \( p^2 + 2pq + q^2 \) Where:
- \( p \) = frequency of dominant allele
- \( q \) = frequency of recessive allele
- \( p + q = 1 \)
In our case, for P = AA (p=1) and aa (q=1), frequencies in F1 and F2 can be represented in ratios or percentages.
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