The dorsal pigment pattern of frogs can be either 'leopard' (white pigment between dark spots) or 'mottled' (pigment between spots appears mottled). The trait is controlled by an autosomal gene. Males and females are selected from pure-breeding populations, and a pair of reciprocal crosses is performed. The cross results are shown below.
    Cross 1:  P₁: Male leopard x male mottled
             F₁: All mottled
             F₂: 70 mottled, 22 leopard
    Cross 2:  P₁: Male mottled x female leopard
             F₁: All mottled
             F₂: 50 mottled, 18 leopard
In the F₂ progeny from both crosses, what proportion is expected to be homozygous? What proportion is expected to be heterozygous?

Verified step by step guidance

1. Step 1: Identify the inheritance pattern of the trait. Since the trait is autosomal and the F₁ generation in both crosses is uniformly mottled, this suggests that the mottled phenotype is dominant over the leopard phenotype.
2. Step 2: Assign genotypes to the parental (P₁) generation. Pure-breeding populations indicate homozygosity. Therefore, the male leopard is homozygous recessive (aa), and the male mottled is homozygous dominant (AA). Similarly, the female leopard is homozygous recessive (aa), and the female mottled is homozygous dominant (AA).
3. Step 3: Determine the genotypes of the F₁ generation. In both crosses, the F₁ offspring are all mottled, which means they are heterozygous (Aa), inheriting one dominant allele (A) from the mottled parent and one recessive allele (a) from the leopard parent.
4. Step 4: Predict the genotypes of the F₂ generation using a Punnett square. Cross two heterozygous individuals (Aa x Aa) to determine the expected genotypic ratios: 1 homozygous dominant (AA), 2 heterozygous (Aa), and 1 homozygous recessive (aa).
5. Step 5: Calculate the proportions of homozygous and heterozygous individuals in the F₂ generation. Based on the Punnett square, 1/4 of the progeny are homozygous dominant (AA), 1/4 are homozygous recessive (aa), and 2/4 (or 1/2) are heterozygous (Aa).

Key Concepts

Autosomal Inheritance

Autosomal inheritance refers to the transmission of genes located on the autosomes, which are the non-sex chromosomes. Traits controlled by autosomal genes can be dominant or recessive. In this case, the dorsal pigment pattern in frogs is determined by an autosomal gene, influencing the phenotypic expression of either leopard or mottled patterns based on the alleles inherited from the parents.

Genotypic Ratios in Mendelian Genetics

Mendelian genetics describes how traits are inherited through specific ratios based on parental genotypes. In a typical monohybrid cross, the expected genotypic ratio for a trait controlled by a single gene is 1 homozygous dominant: 2 heterozygous: 1 homozygous recessive. Understanding these ratios is crucial for predicting the proportions of homozygous and heterozygous individuals in the F₂ generation from the given crosses.

Reciprocal Crosses

Reciprocal crosses involve switching the sexes of the parents in a genetic cross to determine if the inheritance pattern is influenced by sex. In this scenario, both crosses yield the same F₁ generation (all mottled), indicating that the mottled trait is likely dominant. Analyzing the F₂ progeny from both crosses helps confirm the inheritance pattern and calculate the expected proportions of homozygous and heterozygous individuals.