Researchers believe that in a fish species, individuals with the recessive genotype aa are predisposed to disease. Homozygous dominant (AA) individuals and heterozygous (Aa) individuals are not believed to be susceptible to this disease. A pond was stocked with 100 fish of the AA genotype and 100 fish of the aa phenotype, and the fish were allowed to breed. In the next generation, 35 percent of the fish had the dominant (AA) phenotype. What does this result indicate?

The homozygous dominant phenotype is higher than expected, indicating that evolution has occurred.

Explanation:

At the start there are 200 fishes in the pond, 100 of them are AA(50%) and 100 of them are aa(50%). Using the Hardy-Weinberg Equilibrium equation we can say that the gene frequency is

A=0.5

a=0.5

With those frequency, the expected percentage of offspring with dominant genotype will be:

AA= 0.5 * 0.5 = 0.25 = 25%

The number of homozygous dominant found is 35% which is higher than expected  (25%). Higher homozygous dominant frequency than expected means the Hardy-Weinberg Equilibrium is changed. In this case, evolution probably the cause that shifts the gene frequency.

fertilization, pictured in figure 24.23a is the process in which gametes (an egg and sperm) fuse to form a zygote. the egg and sperm each contain one set of chromosomes. to ensure that the offspring has only one complete diploid set of chromosomes, only one sperm must fuse with one egg. in mammals, the egg is protected by a layer of extracellular matrix consisting mainly of glycoproteins called the zona pellucida. when a sperm binds to the zona pellucida, a series of biochemical events, called the acrosomal reactions, take place. in placental mammals, the acrosome contains digestive enzymes that initiate the degradation of the glycoprotein matrix protecting the egg and allowing the sperm plasma membrane to fuse with the egg plasma membrane, as illustrated in figure 24.23b. the fusion of these two membranes creates an opening through which the sperm nucleus is transferred into the ovum. the nuclear membranes of the egg and sperm break down and the two haploid genomes condense to form a diploid genome.

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