Ahusband and wife have two sons. one boy has a straight thumb, while the other has a bent thumb. what causes this difference

The answer is; alleles

Most genes have various forms called alleles. In diploid organisms, most alleles are two located at the same loci in homologous chromosomes.  One allele is usually dominant over the other, meaning it makes the effects of the recessive allele. In some occasions, the recessive allele has disadvantageous traits and manifest when the individual is homozygous recessive.

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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