Review from Basic Heredity

So how do the genes described in Table 1 (found here) interact to produce varied phenotypes of dogs? Let’s start with a review from the Basic Heredity unit.

Four cartoon dogs, one with short black fur, one with short brown fur, one with long black fur, and one with long brown fur.
Figure 3: Two separate genes control fur length (black/brown) and fur color (black/brown) in dogs. These act independently, so a dog may have short and black fur, short and brown fur, long and black fur, or long and brown fur.

Test Your Understanding

Remember, the Law of Independent Assortment states that inheritance of alleles from one gene is independent of inheritance of alleles from another. For example, two different genes affect coat length and black/brown coat color: a dog may have long and black, long and brown, short and black, or short and brown fur (Figure 3). A dog that is heterozygous for both genes (dihybrid genotype) will likely have short, black fur.

A Weimaraner dog standing on gravel
Figure 4.2 A Weimaraner appears silvery-brown, with genotype bbdd. This color is sometimes called “Isabella” by breeders.
A blue greyhound lying in the grass
Figure 4.1 A greyhound with genotype B_dd appears silvery gray, a color that is sometimes called “blue” by breeders.

If multiple genes affect the same phenotype, like the fur color loci shown in Table 1, things get more complicated. Those genes can interact in multiple ways. Some pairs of genes still produce distinct phenotypes in patterns that obey the laws of independent assortment. For example, the B locus determines whether a dog produces a black pigment or a brown pigment; black is dominant to brown. The D locus determines the saturation of color: dogs with a homozygous recessive genotype (dd) have “dilute” or lightened pigmentation compared to the dominant phenotypes. A black (B_) and dilute (dd) dog appears silvery blue-gray in color like the greyhound shown in Figure 4.1, and a brown (bb) and dilute (dd) dog appears silvery brown like the Weimaraner shown in in Figure 4.2. Black and brown dogs with the dominant D allele (D_) are darker.

Test Your Understanding

Whether or not a dog is dilute does not affect whether the dog is black or brown. Because the B and D genes act independent of one another and independently assort, a dihybrid cross gives the expected offspring ratio for a dihybrid cross with independent assortment: 9 Black (black and undilute): 3 silver (black and dilute): 3 brown (brown and undilute): 1 silvery brown. This is shown in Figure 5, with a 4×4 Punnett square that shows all 16 possible offspring and their phenotypic class.

Test Your Understanding

Figure 5 A Dihybrid cross gives a 9:3:3:1 phenotypic offspring ratio. For a dihybrid cross involving the B and D loci, this gives a ratio of 9 black : 3 silver : 3 brown : 1 silvery brown.

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