Non-Mendelian Genetics

• Mendel's studies didn't include the fact that there are multiple alleles of the same gene, which makes making punnet squares harder
• Incomplete dominance


  When two alleles may produce an intermediate phenotype which is a mix between both
• Codominance


  When two alleles may be expressed when both present, instead of one determining the entire phenotype
• Pleiotropy


  Some alleles affect multiple characteristics, instead of just one
• Lethal alleles


  Some genes have alleles that can prevent survival when homozygous or heterozygous
• Sex linkage


  Genes on sex chromosomes show difference inheritance patterns than genes on autosomal ones
• Complementary genes


  Recessive alleles of two different genes may give the same phenotype
• Epistasis


  The alleles of one gene may mask or conceal the alleles of another one
• Polygenic inheritance


  When traits form a phenotypic spectrum instead of a clear cut divide, because multiple genes code for one trait
• Environmental effects influence how genotype turns into a phenotype
• Incomplete penetrance


  When some but not all individuals with a specific genotype have the corresponding phenotype (environmental and genetic combo)
• Variable expressivity


  When individuals of a genotype may have stronger or weaker versions of the phenotype
• Chromosome theory of inheritance


  Boveri and Sutton's theory stating that genes are found at specific locations on chromosomes, and behavior of chromosomes during mitosis can explain Mendel's results
• If a recessive gene is on the X chromosome, then it's much more likely to be in males than in females, because males only have one X chromosome, meaning if it has a recessive gene the gene gets expressed. Meanwhile, a female would have to have both parents with the recessive gene, and inherited it from both sides, to express it.
• X linked genes


  Genes on the X chromosome. Have different inheritance patterns than regular genes, and this type of genetic disorder is more likely to be expressed in males than in females
• Male sex chromosomes are not a homologous pair, because they are different shapes and code for different things. Female ones are a bona fide one though
• SRY


  Sex determining region of the Y chromosome which turns on other genes required for male development. Men have it, and women don't. Intersex people either have XX chromosomes with this (so develop as male) or an XY without it (so develop as female)
• Hemizygous


  When you only have one copy of a gene instead of two. Happens most commonly in men, because they only have one X chromosome, so the genes they have there are the genes they get, regardless of dominance
• Carriers


  People who have diseased alleles but do not have any symptoms. Can still pass it down, and it might get activated
• Sons of women who are heterozygous carriers of X-linked recessive diseases, and men who are not carriers have a 50% chance of inheriting the disease. Daughters probably won't get the disease at all, but have a 50% chance of being a carrier
• Hemophilia


  X-linked disease where blood doesn't clot properly. Much more common in males than females
• Crossovers between two very close genes isn't super likely, so they tend to be inherited together from the same parent
• Parental allele configuration


  When two very close genes on a chromosome are inherited together, as they were on one of the parent's chromosomes
• Recombinant allele configuration


  Less common than parental, when two close genes on a chromosome are actually split up and inherited differently (not together)
• Recombination frequency


  How often linked genes are inherited apart from each other. Tested by making sure you have a heterozygote by combining two homogygotes (one dominant and one recessive) then breeding their offspring with a homozygote that has only recessive genes. Then, you count the number of offspring with traits that aren't linked
• Recombination frequency


  Recombinants/total offspring * 100
• Linkage maps


  Chromosomal maps based on recombination frequency
• Recombination frequency caps out at 50% because if it's higher, the genes aren't actually linked
• 1% recombination frequency is equal to one centimorgan or one map unit away from each other on a chromosome map
• Double crossover

  When genes are split down the middle twice, so they are not linked but happen together slightly more than you'd think. This makes people think the genes are actually closer together than they really are
• Pedigree


  Chart that shows the presence or absence of a trait within a family across generations
• If a trait is dominant, then one of the parents at the start of the pedigree must have it. If it is recessive, then it can skip generations
• You cannot determine the genotype of all individuals in a pedigree, because carriers are not explicitly marked off most of the time
• Mitochondrial and chloroplast differences from nuclear DNA:

  • High copy number: each organelle has many copies of the DNA
  • Random segregation: organelles are randomly distributed during cell division, and end up wherever they are
  • Single-parent inheritance: the DNA gets passed down from one parent (like the mitochondria are inherited only from the mother)
• Correns


  Experimented on white, green, and variegated plants, then noticed that the color of the female gamete donor was the determinant of the offspring's, so chloroplasts are maternally inherited
• Variegated plants have a mix of functioning and nonfunctioning chloroplasts, so its offspring could have pure green, pure white, or variegated patterns at unpredictable ratios
• Mitochondrial diseases are passed on from the mother. If all of her mitochondria are affected, then all of her children will have the same disease at the same level. If only some of the mitochondria are mutated, then the proportion of diseased mitochondria the kids get is random: some might get it severe, some less, some not at all