5.4 Non-Mendelian Genetics

You notice non-Mendelian inheritance when the observed offspring ratios differ from the predicted Mendelian ratios by more than chance can explain. A chi-square test (5.3) is how you check that. If χ² is above the critical value, reject the null hypothesis of Mendelian inheritance and look for an explanation such as linkage or sex linkage.

Genes on the same chromosome are linked. They tend to be inherited together, so they don't assort independently. The only way to separate linked alleles is crossing over in prophase I. The farther apart two genes are, the more likely a crossover falls between them.

Gene mapping uses this. In a test cross of a double heterozygote with a double homozygous recessive, the most common offspring classes are the parental types (allele combinations the parent inherited), and the rarest classes are recombinants. The recombination frequency is (number of recombinant offspring ÷ total offspring) × 100. One percent recombination equals one map unit, so a 19% recombination frequency means the genes are 19 map units apart. A recombination frequency close to 50% looks the same as independent assortment, so very distant genes on one chromosome can act unlinked.

Sex-linked traits are controlled by genes on the sex chromosomes. In humans, XX individuals have two X chromosomes and XY individuals have one X and one Y. The X carries many genes that the much smaller Y doesn't. An XY individual has only one copy of each X-linked gene, so a single recessive allele shows its phenotype. That's why X-linked recessive traits such as red-green color blindness and hemophilia are much more common in XY individuals.

Patterns to know: an XY individual gets their X from their mother, so an affected XY child usually has a carrier (heterozygous) mother; an XY parent passes their X to every XX child and their Y to every XY child, so XY-to-XY transmission of an X-linked trait doesn't happen. Y-linked traits pass from father to every son.

Not every species uses X and Y. Birds use a ZW system (females are ZW and males are ZZ), and in honeybees sex depends on chromosome sets: females develop from fertilized diploid eggs and males from unfertilized haploid eggs (haplodiploidy).

In incomplete dominance, neither allele fully masks the other, so the heterozygote has an in-between phenotype. Red snapdragons crossed with white give pink offspring. Crossing two pinks gives 1 red : 2 pink : 1 white, so the phenotype ratio matches the genotype ratio.

In codominance, both alleles are fully expressed in the heterozygote. Human ABO blood type is the classic case: the Iᴬ allele makes A antigens and the Iᴮ allele makes B antigens, and an IᴬIᴮ person has both on their red blood cells (type AB). A third allele, i, makes neither and is recessive to both. In both patterns the heterozygote's phenotype differs from either homozygote.

Pleiotropy is when one gene affects several traits. The sickle-cell allele changes hemoglobin, which leads to misshapen red blood cells, anemia, pain and organ damage, all from one gene. Because these traits come from the same gene, they're inherited together rather than independently.

Mitochondria and chloroplasts have their own small circular DNA. When a cell divides, these organelles are distributed randomly to daughter cells, so traits they control don't follow Mendel's rules. In animals, the egg supplies almost all of the zygote's mitochondria, so mitochondrial traits are usually inherited from the mother. In plants, mitochondria and chloroplasts come through the ovule rather than the pollen, so those traits are typically maternal too. In a pedigree, a mitochondrial trait typically passes from an affected mother to all of her children, while an affected father passes it to none of them.